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socat(1) socat(1)
NAME
socat - Multipurpose relay (SOcket CAT)
SYNOPSIS
socat [options] <address> <address>
socat -V
socat -h[h[h]] | -?[?[?]]
filan
procan
DESCRIPTION
Socat is a command line based utility that establishes two bidirectional byte streams and
transfers data between them. Because the streams can be constructed from a large set of
different types of data sinks and sources (see address types), and because lots of address
options may be applied to the streams, socat can be used for many different purposes.
Filan is a utility that prints information about its active file descriptors to stdout. It
has been written for debugging socat, but might be useful for other purposes too. Use the
-h option to find more infos.
Procan is a utility that prints information about process parameters to stdout. It has
been written to better understand some UNIX process properties and for debugging socat,
but might be useful for other purposes too.
The life cycle of a socat instance typically consists of four phases.
In the init phase, the command line options are parsed and logging is initialized.
During the open phase, socat opens the first address and afterwards the second address.
These steps are usually blocking; thus, especially for complex address types like socks,
connection requests or authentication dialogs must be completed before the next step is
started.
In the transfer phase, socat watches both streams' read and write file descriptors via se-
lect() , and, when data is available on one side and can be written to the other side, so-
cat reads it, performs newline character conversions if required, and writes the data to
the write file descriptor of the other stream, then continues waiting for more data in
both directions.
When one of the streams effectively reaches EOF, the closing phase begins. Socat transfers
the EOF condition to the other stream, i.e. tries to shutdown only its write stream, giv-
ing it a chance to terminate gracefully. For a defined time socat continues to transfer
data in the other direction, but then closes all remaining channels and terminates.
OPTIONS
Socat provides some command line options that modify the behaviour of the program. They
have nothing to do with so called address options that are used as parts of address speci-
fications.
-V Print version and available feature information to stdout, and exit.
-h | -?
Print a help text to stdout describing command line options and available address
types, and exit.
-hh | -??
Like -h, plus a list of the short names of all available address options. Some op-
tions are platform dependend, so this output is helpful for checking the particular
implementation.
-hhh | -???
Like -hh, plus a list of all available address option names.
-d Without this option, only fatal and error messages are generated; applying this op-
tion also prints warning messages. See DIAGNOSTICS for more information.
-d -d Prints fatal, error, warning, and notice messages.
-d -d -d
Prints fatal, error, warning, notice, and info messages.
-d -d -d -d
Prints fatal, error, warning, notice, info, and debug messages.
-D Logs information about file descriptors before starting the transfer phase.
-ly[<facility>]
Writes messages to syslog instead of stderr; severity as defined with -d option.
With optional <facility>, the syslog type can be selected, default is "daemon".
Third party libraries might not obey this option.
-lf<logfile>
Writes messages to <logfile> [filename] instead of stderr. Some third party li-
braries, in particular libwrap, might not obey this option.
-ls Writes messages to stderr (this is the default). Some third party libraries might
not obey this option, in particular libwrap appears to only log to syslog.
-lp<progname>
Overrides the program name printed in error messages and used for constructing en-
vironment variable names.
-lu Extends the timestamp of error messages to microsecond resolution. Does not work
when logging to syslog.
-lm[<facility>]
Mixed log mode. During startup messages are printed to stderr; when socat starts
the transfer phase loop or daemon mode (i.e. after opening all streams and before
starting data transfer, or, with listening sockets with fork option, before the
first accept call), it switches logging to syslog. With optional <facility>, the
syslog type can be selected, default is "daemon".
-lh Adds hostname to log messages. Uses the value from environment variable HOSTNAME or
the value retrieved with uname() if HOSTNAME is not set.
-v Writes the transferred data not only to their target streams, but also to stderr.
The output format is text with some conversions for readability, and prefixed with
"> " or "< " indicating flow directions.
-x Writes the transferred data not only to their target streams, but also to stderr.
The output format is hexadecimal, prefixed with "> " or "< " indicating flow direc-
tions. Can be combined with -v .
-r <file>
Dumps the raw (binary) data flowing from left to right address to the given file.
-R <file>
Dumps the raw (binary) data flowing from right to left address to the given file.
-b<size>
Sets the data transfer block <size> [size_t]. At most <size> bytes are transferred
per step. Default is 8192 bytes.
-s By default, socat terminates when an error occurred to prevent the process from
running when some option could not be applied. With this option, socat is sloppy
with errors and tries to continue. Even with this option, socat will exit on fa-
tals, and will abort connection attempts when security checks failed.
-t<timeout>
When one channel has reached EOF, the write part of the other channel is shut down.
Then, socat waits <timeout> [timeval] seconds before terminating. Default is 0.5
seconds. This timeout only applies to addresses where write and read part can be
closed independently. When during the timeout interval the read part gives EOF, so-
cat terminates without awaiting the timeout.
-T<timeout>
Total inactivity timeout: when socat is already in the transfer loop and nothing
has happened for <timeout> [timeval] seconds (no data arrived, no interrupt oc-
curred...) then it terminates. Useful with protocols like UDP that cannot transfer
EOF.
-u Uses unidirectional mode. The first address is only used for reading, and the sec-
ond address is only used for writing (example).
-U Uses unidirectional mode in reverse direction. The first address is only used for
writing, and the second address is only used for reading.
-g During address option parsing, don't check if the option is considered useful in
the given address environment. Use it if you want to force, e.g., appliance of a
socket option to a serial device.
-L<lockfile>
If lockfile exists, exits with error. If lockfile does not exist, creates it and
continues, unlinks lockfile on exit.
-W<lockfile>
If lockfile exists, waits until it disappears. When lockfile does not exist, cre-
ates it and continues, unlinks lockfile on exit.
-4 Use IP version 4 in case that the addresses do not implicitly or explicitly specify
a version; this is the default.
-6 Use IP version 6 in case that the addresses do not implicitly or explicitly specify
a version.
ADDRESS SPECIFICATIONS
With the address command line arguments, the user gives socat instructions and the neces-
sary information for establishing the byte streams.
An address specification usually consists of an address type keyword, zero or more re-
quired address parameters separated by ':' from the keyword and from each other, and zero
or more address options separated by ','.
The keyword specifies the address type (e.g., TCP4, OPEN, EXEC). For some keywords there
exist synonyms ('-' for STDIO, TCP for TCP4). Keywords are case insensitive. For a few
special address types, the keyword may be omitted: Address specifications starting with a
number are assumed to be FD (raw file descriptor) addresses; if a '/' is found before the
first ':' or ',', GOPEN (generic file open) is assumed.
The required number and type of address parameters depend on the address type. E.g., TCP4
requires a server specification (name or address), and a port specification (number or
service name).
Zero or more address options may be given with each address. They influence the address in
some ways. Options consist of an option keyword or an option keyword and a value, sepa-
rated by '='. Option keywords are case insensitive. For filtering the options that are
useful with an address type, each option is member of one option group. For each address
type there is a set of option groups allowed. Only options belonging to one of these ad-
dress groups may be used (except with option -g).
Address specifications following the above schema are also called single address specifi-
cations. Two single addresses can be combined with "!!" to form a dual type address for
one channel. Here, the first address is used by socat for reading data, and the second ad-
dress for writing data. There is no way to specify an option only once for being applied
to both single addresses.
Usually, addresses are opened in read/write mode. When an address is part of a dual ad-
dress specification, or when option -u or -U is used, an address might be used only for
reading or for writing. Considering this is important with some address types.
With socat version 1.5.0 and higher, the lexical analysis tries to handle quotes and
parenthesis meaningfully and allows escaping of special characters. If one of the charac-
ters ( { [ ' is found, the corresponding closing character - ) } ] ' - is looked for; they
may also be nested. Within these constructs, socats special characters and strings : , !!
are not handled specially. All those characters and strings can be escaped with \ or
within ""
ADDRESS TYPES
This section describes the available address types with their keywords, parameters, and
semantics.
CREATE:<filename>
Opens <filename> with creat() and uses the file descriptor for writing. This ad-
dress type requires write-only context, because a file opened with creat cannot be
read from.
Flags like O_LARGEFILE cannot be applied. If you need them use OPEN with options
create,create.
<filename> must be a valid existing or not existing path. If <filename> is a named
pipe, creat() might block; if <filename> refers to a socket, this is an error.
Option groups: FD,REG,NAMED
Useful options: mode, user, group, unlink-early, unlink-late, append
See also: OPEN, GOPEN
EXEC:<command-line>
Forks a sub process that establishes communication with its parent process and in-
vokes the specified program with execvp() . <command-line> is a simple command
with arguments separated by single spaces. If the program name contains a '/', the
part after the last '/' is taken as ARGV[0]. If the program name is a relative
path, the execvp() semantics for finding the program via $PATH apply. After suc-
cessful program start, socat writes data to stdin of the process and reads from its
stdout using a UNIX domain socket generated by socketpair() per default. (example)
Option groups: FD,SOCKET,EXEC,FORK,TERMIOS
Useful options: path, fdin, fdout, chroot, su, su-d, nofork, pty, stderr, ctty,
setsid, pipes, login, sigint, sigquit
See also: SYSTEM
FD:<fdnum>
Uses the file descriptor <fdnum>. It must already exist as valid UN*X file descrip-
tor.
Option groups: FD (TERMIOS,REG,SOCKET)
See also: STDIO, STDIN, STDOUT, STDERR
GOPEN:<filename>
(Generic open) This address type tries to handle any file system entry except di-
rectories usefully. <filename> may be a relative or absolute path. If it already
exists, its type is checked. In case of a UNIX domain socket, socat connects; if
connecting fails, socat assumes a datagram socket and uses sendto() calls. If the
entry is not a socket, socat opens it applying the O_APPEND flag. If it does not
exist, it is opened with flag O_CREAT as a regular file (example).
Option groups: FD,REG,SOCKET,NAMED,OPEN
See also: OPEN, CREATE, UNIX-CONNECT
IP-SENDTO:<host>:<protocol>
Opens a raw IP socket. Depending on host specification or option pf, IP protocol
version 4 or 6 is used. It uses <protocol> to send packets to <host> [IP address]
and receives packets from host, ignores packets from other hosts. Protocol 255
uses the raw socket with the IP header being part of the data.
Option groups: FD,SOCKET,IP4,IP6
Useful options: pf, ttl
See also: IP4-SENDTO, IP6-SENDTO, IP-RECVFROM, IP-RECV, UDP-SENDTO, UNIX-SENDTO
INTERFACE:<interface>
Communicates with a network connected on an interface using raw packets including
link level data. <interface> is the name of the network interface. Currently only
available on Linux. Option groups: FD,SOCKET
Useful options: pf, type
See also: ip-recv
IP4-SENDTO:<host>:<protocol>
Like IP-SENDTO, but always uses IPv4.
Option groups: FD,SOCKET,IP4
IP6-SENDTO:<host>:<protocol>
Like IP-SENDTO, but always uses IPv6.
Option groups: FD,SOCKET,IP6
IP-DATAGRAM:<address>:<protocol>
Sends outgoing data to the specified address which may in particular be a broadcast
or multicast address. Packets arriving on the local socket are checked if their
source addresses match RANGE or TCPWRAP options. This address type can for example
be used for implementing symmetric or asymmetric broadcast or multicast communica-
tions.
Option groups: FD, SOCKET, IP4, IP6, RANGE
Useful options: bind, range, tcpwrap, broadcast, ip-multicast-loop, ip-multi-
cast-ttl, ip-multicast-if, ip-add-membership, ip-add-source-membership, ttl, tos,
pf
See also: IP4-DATAGRAM, IP6-DATAGRAM, IP-SENDTO, IP-RECVFROM, IP-RECV, UDP-DATAGRAM
IP4-DATAGRAM:<host>:<protocol>
Like IP-DATAGRAM, but always uses IPv4. (example)
Option groups: FD,SOCKET,IP4,RANGE
IP6-DATAGRAM:<host>:<protocol>
Like IP-DATAGRAM, but always uses IPv6. Please note that IPv6 does not know broad-
casts.
Option groups: FD,SOCKET,IP6,RANGE
IP-RECVFROM:<protocol>
Opens a raw IP socket of <protocol>. Depending on option pf, IP protocol version 4
or 6 is used. It receives one packet from an unspecified peer and may send one or
more answer packets to that peer. This mode is particularly useful with fork op-
tion where each arriving packet - from arbitrary peers - is handled by its own sub
process. This allows a behaviour similar to typical UDP based servers like ntpd or
named.
Please note that the reply packets might be fetched as incoming traffic when sender
and receiver IP address are identical because there is no port number to distin-
guish the sockets.
This address works well with IP-SENDTO address peers (see above). Protocol 255
uses the raw socket with the IP header being part of the data.
See the note about RECVFROM addresses.
Option groups: FD,SOCKET,IP4,IP6,CHILD,RANGE
Useful options: pf, fork, range, ttl, broadcast
See also: IP4-RECVFROM, IP6-RECVFROM, IP-SENDTO, IP-RECV, UDP-RECVFROM,
UNIX-RECVFROM
IP4-RECVFROM:<protocol>
Like IP-RECVFROM, but always uses IPv4.
Option groups: FD,SOCKET,IP4,CHILD,RANGE
IP6-RECVFROM:<protocol>
Like IP-RECVFROM, but always uses IPv6.
Option groups: FD,SOCKET,IP6,CHILD,RANGE
IP-RECV:<protocol>
Opens a raw IP socket of <protocol>. Depending on option pf, IP protocol version 4
or 6 is used. It receives packets from multiple unspecified peers and merges the
data. No replies are possible. It can be, e.g., addressed by socat IP-SENDTO ad-
dress peers. Protocol 255 uses the raw socket with the IP header being part of the
data.
Option groups: FD,SOCKET,IP4,IP6,RANGE
Useful options: pf, range
See also: IP4-RECV, IP6-RECV, IP-SENDTO, IP-RECVFROM, UDP-RECV, UNIX-RECV
IP4-RECV:<protocol>
Like IP-RECV, but always uses IPv4.
Option groups: FD,SOCKET,IP4,RANGE
IP6-RECV:<protocol>
Like IP-RECV, but always uses IPv6.
Option groups: FD,SOCKET,IP6,RANGE
OPEN:<filename>
Opens <filename> using the open() system call (example). This operation fails on
UNIX domain sockets.
Note: This address type is rarely useful in bidirectional mode.
Option groups: FD,REG,NAMED,OPEN
Useful options: creat, excl, noatime, nofollow, append, rdonly, wronly, lock, read-
bytes, ignoreeof
See also: CREATE, GOPEN, UNIX-CONNECT
OPENSSL:<host>:<port>
Tries to establish a SSL connection to <port> [TCP service] on <host> [IP address]
using TCP/IP version 4 or 6 depending on address specification, name resolution, or
option pf.
NOTE: Up to version 1.7.2.4 the server certificate was only checked for validity
against the system certificate store or cafile or capath, but not for match with
the server's name or its IP address. Since version 1.7.3.0 socat checks the peer
certificate for match with the <host> parameter or the value of the openssl-common-
name option. Socat tries to match it against the certificates subject commonName,
and the certificates extension subjectAltName DNS names. Wildcards in the certifi-
cate are supported.
Option groups: FD,SOCKET,IP4,IP6,TCP,OPENSSL,RETRY
Useful options: cipher, verify, commonname, cafile, capath, certificate, key, com-
press, bind, pf, connect-timeout, sourceport, retry
See also: OPENSSL-LISTEN, TCP
OPENSSL-LISTEN:<port>
Listens on tcp <port> [TCP service]. The IP version is 4 or the one specified with
pf. When a connection is accepted, this address behaves as SSL server.
Note: You probably want to use the certificate option with this address.
NOTE: The client certificate is only checked for validity against cafile or capath,
but not for match with the client's name or its IP address!
Option groups: FD,SOCKET,IP4,IP6,TCP,LISTEN,OPENSSL,CHILD,RANGE,RETRY
Useful options: pf, cipher, verify, commonname, cafile, capath, certificate, key,
compress, fork, bind, range, tcpwrap, su, reuseaddr, retry
See also: OPENSSL, TCP-LISTEN
OPENSSL-DTLS-CLIENT:<host>:<port>
Tries to establish a DTLS connection to <port> [UDP service] on <host> [IP address]
using UDP/IP version 4 or 6 depending on address specification, name resolution, or
option pf.
Socat checks the peer certificates subjectAltName or commonName against the ad-
dresses option openssl-commonname or the host name. Wildcards in the certificate
are supported.
Use socat option -b to make datagrams small enough to fit with overhead on the net-
work. Use option -T to prevent indefinite hanging when peer went down quietly.
Option groups: FD,SOCKET,IP4,IP6,OPENSSL,RETRY
Useful options: cipher, verify, commonname, cafile, capath, certificate, key, com-
press, bind, pf, sourceport, retry
See also: OPENSSL-DTLS-SERVER, OPENSSL-CONNECT, UDP-CONNECT
OPENSSL-DTLS-SERVER:<port>
Listens on UDP <port> [UDP service]. The IP version is 4 or the one specified with
pf. When a connection is accepted, this address behaves as DTLS server.
Note: You probably want to use the certificate option with this address.
NOTE: The client certificate is only checked for validity against cafile or capath,
but not for match with the client's name or its IP address! Use socat option -b to
make datagrams small enough to fit with overhead on the network. Use option -T to
prevent indefinite hanging when peer went down quietly.
Option groups: FD,SOCKET,IP4,IP6,LISTEN,OPENSSL,CHILD,RANGE,RETRY
Useful options: pf, cipher, verify, commonname, cafile, capath, certificate, key,
compress, fork, bind, range, tcpwrap, su, reuseaddr, retry
See also: OPENSSL-DTLS-CLIENT, OPENSSL-LISTEN, UDP-LISTEN
PIPE:<filename>
If <filename> already exists, it is opened. If it does not exist, a named pipe is
created and opened. Beginning with socat version 1.4.3, the named pipe is removed
when the address is closed (but see option unlink-close
Note: When a pipe is used for both reading and writing, it works as echo service.
Note: When a pipe is used for both reading and writing, and socat tries to write
more bytes than the pipe can buffer (Linux 2.4: 2048 bytes), socat might block.
Consider using socat option, e.g., -b 2048
Option groups: FD,NAMED,OPEN
Useful options: rdonly, nonblock, group, user, mode, unlink-early
See also: unnamed pipe
PIPE Creates an unnamed pipe and uses it for reading and writing. It works as an echo,
because everything written to it appeares immediately as read data.
Note: When socat tries to write more bytes than the pipe can queue (Linux 2.4: 2048
bytes), socat might block. Consider, e.g., using option -b 2048
Option groups: FD
See also: named pipe
PROXY:<proxy>:<hostname>:<port>
Connects to an HTTP proxy server on port 8080 using TCP/IP version 4 or 6 depend-
ing on address specification, name resolution, or option pf, and sends a CONNECT
request for hostname:port. If the proxy grants access and succeeds to connect to
the target, data transfer between socat and the target can start. Note that the
traffic need not be HTTP but can be an arbitrary protocol.
Option groups: FD,SOCKET,IP4,IP6,TCP,HTTP,RETRY
Useful options: proxyport, ignorecr, proxyauth, resolve, crnl, bind, connect-time-
out, mss, sourceport, retry
See also: SOCKS, TCP
PTY Generates a pseudo terminal (pty) and uses its master side. Another process may
open the pty's slave side using it like a serial line or terminal. (example). If
both the ptmx and the openpty mechanisms are available, ptmx is used (POSIX).
Option groups: FD,NAMED,PTY,TERMIOS
Useful options: link, openpty, wait-slave, mode, user, group
See also: UNIX-LISTEN, PIPE, EXEC, SYSTEM
READLINE
Uses GNU readline and history on stdio to allow editing and reusing input lines
(example).
Due to licensing restrictions the readline feature is disabled in Debian. See
BUGS.
You can use STDIO instead.
SCTP-CONNECT:<host>:<port>
Establishes an SCTP stream connection to the specified <host> [IP address] and
<port> [TCP service] using IP version 4 or 6 depending on address specification,
name resolution, or option pf.
Option groups: FD,SOCKET,IP4,IP6,SCTP,CHILD,RETRY
Useful options: bind, pf, connect-timeout, tos, mtudiscover, sctp-maxseg,
sctp-nodelay, nonblock, sourceport, retry, readbytes
See also: SCTP4-CONNECT, SCTP6-CONNECT, SCTP-LISTEN, TCP-CONNECT
SCTP4-CONNECT:<host>:<port>
Like SCTP-CONNECT, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4,SCTP,CHILD,RETRY
SCTP6-CONNECT:<host>:<port>
Like SCTP-CONNECT, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6,SCTP,CHILD,RETRY
SCTP-LISTEN:<port>
Listens on <port> [TCP service] and accepts an SCTP connection. The IP version is 4
or the one specified with address option pf, socat option (-4, -6), or environment
variable SOCAT_DEFAULT_LISTEN_IP. Note that opening this address usually blocks
until a client connects.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4,IP6,SCTP,RETRY
Useful options: crnl, fork, bind, range, tcpwrap, pf, max-children, backlog, ac-
cept-timeout, sctp-maxseg, sctp-nodelay, su, reuseaddr, retry, cool-write
See also: SCTP4-LISTEN, SCTP6-LISTEN, TCP-LISTEN, SCTP-CONNECT
SCTP4-LISTEN:<port>
Like SCTP-LISTEN, but only supports IPv4 protocol.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4,SCTP,RETRY
SCTP6-LISTEN:<port>
Like SCTP-LISTEN, but only supports IPv6 protocol.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP6,SCTP,RETRY
SOCKET-CONNECT:<domain>:<protocol>:<remote-address>
Creates a stream socket using the first and second given socket parameters and
SOCK_STREAM (see man socket(2)) and connects to the remote-address. The two socket
parameters have to be specified by int numbers. Consult your OS documentation and
include files to find the appropriate values. The remote-address must be the data
representation of a sockaddr structure without sa_family and (BSD) sa_len compo-
nents.
Please note that you can - beyond the options of the specified groups - also use
options of higher level protocols when you apply socat option -g.
Option groups: FD,SOCKET,CHILD,RETRY
Useful options: bind, setsockopt,
See also: TCP, UDP-CONNECT, UNIX-CONNECT, SOCKET-LISTEN, SOCKET-SENDTO
SOCKET-DATAGRAM:<domain>:<type>:<protocol>:<remote-address>
Creates a datagram socket using the first three given socket parameters (see man
socket(2)) and sends outgoing data to the remote-address. The three socket parame-
ters have to be specified by int numbers. Consult your OS documentation and include
files to find the appropriate values. The remote-address must be the data represen-
tation of a sockaddr structure without sa_family and (BSD) sa_len components.
Please note that you can - beyond the options of the specified groups - also use
options of higher level protocols when you apply socat option -g.
Option groups: FD,SOCKET,RANGE
Useful options: bind, range, setsockopt,
See also: UDP-DATAGRAM, IP-DATAGRAM, SOCKET-SENDTO, SOCKET-RECV, SOCKET-RECVFROM
SOCKET-LISTEN:<domain>:<protocol>:<local-address>
Creates a stream socket using the first and second given socket parameters and
SOCK_STREAM (see man socket(2)) and waits for incoming connections on local-ad-
dress. The two socket parameters have to be specified by int numbers. Consult your
OS documentation and include files to find the appropriate values. The local-ad-
dress must be the data representation of a sockaddr structure without sa_family and
(BSD) sa_len components.
Please note that you can - beyond the options of the specified groups - also use
options of higher level protocols when you apply socat option -g.
Option groups: FD,SOCKET,LISTEN,RANGE,CHILD,RETRY
Useful options: setsockopt, setsockopt-listen,
See also: TCP, UDP-CONNECT, UNIX-CONNECT, SOCKET-LISTEN, SOCKET-SENDTO,
SOCKET-SENDTO
SOCKET-RECV:<domain>:<type>:<protocol>:<local-address>
Creates a socket using the three given socket parameters (see man socket(2)) and
binds it to <local-address>. Receives arriving data. The three parameters have to
be specified by int numbers. Consult your OS documentation and include files to
find the appropriate values. The local-address must be the data representation of a
sockaddr structure without sa_family and (BSD) sa_len components.
Option groups: FD,SOCKET,RANGE
Useful options: range, setsockopt, setsockopt-listen
See also: UDP-RECV, IP-RECV, UNIX-RECV, SOCKET-DATAGRAM, SOCKET-SENDTO,
SOCKET-RECVFROM
SOCKET-RECVFROM:<domain>:<type>:<protocol>:<local-address>
Creates a socket using the three given socket parameters (see man socket(2)) and
binds it to <local-address>. Receives arriving data and sends replies back to the
sender. The first three parameters have to be specified as int numbers. Consult
your OS documentation and include files to find the appropriate values. The lo-
cal-address must be the data representation of a sockaddr structure without sa_fam-
ily and (BSD) sa_len components.
See the note about RECVFROM addresses.
Option groups: FD,SOCKET,CHILD,RANGE
Useful options: fork, range, setsockopt, setsockopt-listen
See also: UDP-RECVFROM, IP-RECVFROM, UNIX-RECVFROM, SOCKET-DATAGRAM, SOCKET-SENDTO,
SOCKET-RECV
SOCKET-SENDTO:<domain>:<type>:<protocol>:<remote-address>
Creates a socket using the three given socket parameters (see man socket(2)). Sends
outgoing data to the given address and receives replies. The three parameters have
to be specified as int numbers. Consult your OS documentation and include files to
find the appropriate values. The remote-address must be the data representation of
a sockaddr structure without sa_family and (BSD) sa_len components.
Option groups: FD,SOCKET
Useful options: bind, setsockopt, setsockopt-listen
See also: UDP-SENDTO, IP-SENDTO, UNIX-SENDTO, SOCKET-DATAGRAM, SOCKET-RECV
SOCKET-RECVFROM
SOCKS4:<socks-server>:<host>:<port>
Connects via <socks-server> [IP address] to <host> [IPv4 address] on <port> [TCP
service], using socks version 4 protocol over IP version 4 or 6 depending on ad-
dress specification, name resolution, or option pf (example).
Option groups: FD,SOCKET,IP4,IP6,TCP,SOCKS4,RETRY
Useful options: socksuser, socksport, sourceport, pf, retry
See also: SOCKS4A, PROXY, TCP
SOCKS4A:<socks-server>:<host>:<port>
like SOCKS4, but uses socks protocol version 4a, thus leaving host name resolution
to the socks server.
Option groups: FD,SOCKET,IP4,IP6,TCP,SOCKS4,RETRY
STDERR Uses file descriptor 2.
Option groups: FD (TERMIOS,REG,SOCKET)
See also: FD
STDIN Uses file descriptor 0.
Option groups: FD (TERMIOS,REG,SOCKET)
Useful options: readbytes
See also: FD
STDIO Uses file descriptor 0 for reading, and 1 for writing.
Option groups: FD (TERMIOS,REG,SOCKET)
Useful options: readbytes
See also: FD
STDOUT Uses file descriptor 1.
Option groups: FD (TERMIOS,REG,SOCKET)
See also: FD
SYSTEM:<shell-command>
Forks a sub process that establishes communication with its parent process and in-
vokes the specified program with system() . Please note that <shell-command>
[string] must not contain ',' or "!!", and that shell meta characters may have to
be protected. After successful program start, socat writes data to stdin of the
process and reads from its stdout.
Option groups: FD,SOCKET,EXEC,FORK,TERMIOS
Useful options: path, fdin, fdout, chroot, su, su-d, nofork, pty, stderr, ctty,
setsid, pipes, sigint, sigquit
See also: EXEC
TCP:<host>:<port>
Connects to <port> [TCP service] on <host> [IP address] using TCP/IP version 4 or 6
depending on address specification, name resolution, or option pf.
Option groups: FD,SOCKET,IP4,IP6,TCP,RETRY
Useful options: crnl, bind, pf, connect-timeout, tos, mtudiscover, mss, nodelay,
nonblock, sourceport, retry, readbytes
See also: TCP4, TCP6, TCP-LISTEN, UDP, SCTP-CONNECT, UNIX-CONNECT
TCP4:<host>:<port>
Like TCP, but only supports IPv4 protocol (example).
Option groups: FD,SOCKET,IP4,TCP,RETRY
TCP6:<host>:<port>
Like TCP, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6,TCP,RETRY
TCP-LISTEN:<port>
Listens on <port> [TCP service] and accepts a TCP/IP connection. The IP version is
4 or the one specified with address option pf, socat option (-4, -6), or environ-
ment variable SOCAT_DEFAULT_LISTEN_IP. Note that opening this address usually
blocks until a client connects.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4,IP6,TCP,RETRY
Useful options: crnl, fork, bind, range, tcpwrap, pf, max-children, backlog, ac-
cept-timeout, mss, su, reuseaddr, retry, cool-write
See also: TCP4-LISTEN, TCP6-LISTEN, UDP-LISTEN, SCTP-LISTEN, UNIX-LISTEN,
OPENSSL-LISTEN, TCP-CONNECT
TCP4-LISTEN:<port>
Like TCP-LISTEN, but only supports IPv4 protocol (example).
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4,TCP,RETRY
TCP6-LISTEN:<port>
Like TCP-LISTEN, but only supports IPv6 protocol.
Additional useful option: ipv6only
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP6,TCP,RETRY
TUN[:<if-addr>/<bits>]
Creates a Linux TUN/TAP device and optionally assignes it the address and netmask
given by the parameters. The resulting network interface is almost ready for use by
other processes; socat serves its "wire side". This address requires read and write
access to the tunnel cloning device, usually /dev/net/tun , as well as permission
to set some ioctl()s. Option iff-up is required to immediately activate the inter-
face!
Note: If you intend to transfer packets between two Socat "wire sides" you need a
protocol that keeps packet boundaries, e.g.UDP; TCP might work with option nodelay.
Option groups: FD,NAMED,OPEN,TUN
Useful options: iff-up, tun-device, tun-name, tun-type, iff-no-pi
See also: ip-recv
UDP:<host>:<port>
Connects to <port> [UDP service] on <host> [IP address] using UDP/IP version 4 or 6
depending on address specification, name resolution, or option pf.
Please note that, due to UDP protocol properties, no real connection is estab-
lished; data has to be sent for `connecting' to the server, and no end-of-file con-
dition can be transported.
Option groups: FD,SOCKET,IP4,IP6
Useful options: ttl, tos, bind, sourceport, pf
See also: UDP4, UDP6, UDP-LISTEN, TCP, IP
UDP4:<host>:<port>
Like UDP, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4
UDP6:<host>:<port>
Like UDP, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6
UDP-DATAGRAM:<address>:<port>
Sends outgoing data to the specified address which may in particular be a broadcast
or multicast address. Packets arriving on the local socket are checked for the cor-
rect remote port only when option sourceport is used (this is a change with Socat
version 1.7.4.0) and if their source addresses match RANGE or TCPWRAP options. This
address type can for example be used for implementing symmetric or asymmetric
broadcast or multicast communications.
Option groups: FD,SOCKET,IP4,IP6,RANGE
Useful options: bind, range, tcpwrap, broadcast, ip-multicast-loop, ip-multi-
cast-ttl, ip-multicast-if, ip-add-membership, ip-add-source-membership, ttl, tos,
sourceport, pf
See also: UDP4-DATAGRAM, UDP6-DATAGRAM, UDP-SENDTO, UDP-RECVFROM, UDP-RECV,
UDP-CONNECT, UDP-LISTEN, IP-DATAGRAM
UDP4-DATAGRAM:<address>:<port>
Like UDP-DATAGRAM, but only supports IPv4 protocol (example1, example2).
Option groups: FD,SOCKET,IP4, RANGE
UDP6-DATAGRAM:<address>:<port>
Like UDP-DATAGRAM, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6,RANGE
UDP-LISTEN:<port>
Waits for a UDP/IP packet arriving on <port> [UDP service] and `connects' back to
sender. The accepted IP version is 4 or the one specified with option pf. Please
note that, due to UDP protocol properties, no real connection is established; data
has to arrive from the peer first, and no end-of-file condition can be transported.
Note that opening this address usually blocks until a client connects.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4,IP6
Useful options: fork, bind, range, pf
See also: UDP, UDP4-LISTEN, UDP6-LISTEN, TCP-LISTEN
UDP4-LISTEN:<port>
Like UDP-LISTEN, but only support IPv4 protocol.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4
UDP6-LISTEN:<port>
Like UDP-LISTEN, but only support IPv6 protocol.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP6
UDP-SENDTO:<host>:<port>
Communicates with the specified peer socket, defined by <port> [UDP service] on
<host> [IP address], using UDP/IP version 4 or 6 depending on address specifica-
tion, name resolution, or option pf. It sends packets to and receives packets from
that peer socket only. This address effectively implements a datagram client. It
works well with socat UDP-RECVFROM and UDP-RECV address peers.
Option groups: FD,SOCKET,IP4,IP6
Useful options: ttl, tos, bind, sourceport, pf
See also: UDP4-SENDTO, UDP6-SENDTO, UDP-RECVFROM, UDP-RECV, UDP-CONNECT, UDP-LIS-
TEN, IP-SENDTO
UDP4-SENDTO:<host>:<port>
Like UDP-SENDTO, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4
UDP6-SENDTO:<host>:<port>
Like UDP-SENDTO, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6
UDP-RECVFROM:<port>
Creates a UDP socket on <port> [UDP service] using UDP/IP version 4 or 6 depending
on option pf. It receives one packet from an unspecified peer and may send one or
more answer packets to that peer. This mode is particularly useful with fork option
where each arriving packet - from arbitrary peers - is handled by its own sub
process. This allows a behaviour similar to typical UDP based servers like ntpd or
named. This address works well with socat UDP-SENDTO address peers.
Note: When the second address fails before entering the transfer loop the packet is
dropped. Use option retry or forever on the second address to avoid data loss.
Option groups: FD,SOCKET,IP4,IP6,CHILD,RANGE
Useful options: fork, ttl, tos, bind, sourceport, pf
See also: UDP4-RECVFROM, UDP6-RECVFROM, UDP-SENDTO, UDP-RECV, UDP-CONNECT, UDP-LIS-
TEN, IP-RECVFROM, UNIX-RECVFROM
UDP4-RECVFROM:<port>
Like UDP-RECVFROM, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4,CHILD,RANGE
UDP6-RECVFROM:<port>
Like UDP-RECVFROM, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6,CHILD,RANGE
UDP-RECV:<port>
Creates a UDP socket on <port> [UDP service] using UDP/IP version 4 or 6 depending
on option pf. It receives packets from multiple unspecified peers and merges the
data. No replies are possible. It works well with, e.g., socat UDP-SENDTO address
peers; it behaves similar to a syslog server.
Note: if you need the fork option, use UDP-RECVFROM in unidirectional mode (with
option -u) instead.
Option groups: FD,SOCKET,IP4,IP6,RANGE
Useful options: pf, bind, sourceport, ttl, tos
See also: UDP4-RECV, UDP6-RECV, UDP-SENDTO, UDP-RECVFROM, UDP-CONNECT, UDP-LISTEN,
IP-RECV, UNIX-RECV
UDP4-RECV:<port>
Like UDP-RECV, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4,RANGE
UDP6-RECV:<port>
Like UDP-RECV, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6,RANGE
UNIX-CONNECT:<filename>
Connects to <filename> assuming it is a UNIX domain socket. If <filename> does not
exist, this is an error; if <filename> is not a UNIX domain socket, this is an er-
ror; if <filename> is a UNIX domain socket, but no process is listening, this is an
error.
Option groups: FD,SOCKET,NAMED,RETRY,UNIX
) Useful options: bind
See also: UNIX-LISTEN, UNIX-SENDTO, TCP
UNIX-LISTEN:<filename>
Listens on <filename> using a UNIX domain stream socket and accepts a connection.
If <filename> exists and is not a socket, this is an error. If <filename> exists
and is a UNIX domain socket, binding to the address fails (use option un-
link-early!). Note that opening this address usually blocks until a client con-
nects. Beginning with socat version 1.4.3, the file system entry is removed when
this address is closed (but see option unlink-close) (example).
Option groups: FD,SOCKET,NAMED,LISTEN,CHILD,RETRY,UNIX
Useful options: fork, umask, mode, user, group, unlink-early
See also: UNIX-CONNECT, UNIX-RECVFROM, UNIX-RECV, TCP-LISTEN
UNIX-SENDTO:<filename>
Communicates with the specified peer socket, defined by [<filename>] assuming it is
a UNIX domain datagram socket. It sends packets to and receives packets from that
peer socket only. Please note that it might be necessary to bind the local socket
to an address (e.g. /tmp/sock1, which must not exist before). This address type
works well with socat UNIX-RECVFROM and UNIX-RECV address peers.
Option groups: FD,SOCKET,NAMED,UNIX
Useful options: bind
See also: UNIX-RECVFROM, UNIX-RECV, UNIX-CONNECT, UDP-SENDTO, IP-SENDTO
UNIX-RECVFROM:<filename>
Creates a UNIX domain datagram socket [<filename>]. Receives one packet and may
send one or more answer packets to that peer. This mode is particularly useful
with fork option where each arriving packet - from arbitrary peers - is handled by
its own sub process. This address works well with socat UNIX-SENDTO address peers.
Option groups: FD,SOCKET,NAMED,CHILD,UNIX
See the note about RECVFROM addresses.
Useful options: fork
See also: UNIX-SENDTO, UNIX-RECV, UNIX-LISTEN, UDP-RECVFROM, IP-RECVFROM
UNIX-RECV:<filename>
Creates a UNIX domain datagram socket [<filename>]. Receives packets from multiple
unspecified peers and merges the data. No replies are possible. It can be, e.g.,
addressed by socat UNIX-SENDTO address peers. It behaves similar to a syslog
server.
Option groups: FD,SOCKET,NAMED,UNIX
See also: UNIX-SENDTO, UNIX-RECVFROM, UNIX-LISTEN, UDP-RECV, IP-RECV
UNIX-CLIENT:<filename>
Communicates with the specified peer socket, defined by [<filename>] assuming it is
a UNIX domain socket. It first tries to connect and, if that fails, assumes it is
a datagram socket, thus supporting both types.
Option groups: FD,SOCKET,NAMED,UNIX
Useful options: bind
See also: UNIX-CONNECT, UNIX-SENDTO, GOPEN
VSOCK-CONNECT:<cid>:<port>
Establishes a VSOCK stream connection to the specified <cid> [VSOCK cid] and <port>
[VSOCK port].
Option groups: FD,SOCKET,CHILD,RETRY
Useful options: bind, pf, connect-timeout, retry, readbytes
See also: VSOCK-LISTEN,
VSOCK-LISTEN:<port>
Listens on <port> [VSOCK port] and accepts a VSOCK connection. Note that opening
this address usually blocks until a client connects.
Option groups: FD,SOCKET,LISTEN,CHILD,RETRY
Useful options: fork, bind, pf, max-children, backlog, su, reuseaddr, retry,
cool-write
See also: VSOCK-CONNECT
ABSTRACT-CONNECT:<string>
ABSTRACT-LISTEN:<string>
ABSTRACT-SENDTO:<string>
ABSTRACT-RECVFROM:<string>
ABSTRACT-RECV:<string>
ABSTRACT-CLIENT:<string>
The ABSTRACT addresses are almost identical to the related UNIX addresses except
that they do not address file system based sockets but an alternate UNIX domain ad-
dress space. To achieve this the socket address strings are prefixed with "\0" in-
ternally. This feature is available (only?) on Linux. Option groups are the same
as with the related UNIX addresses, except that the ABSTRACT addresses are not mem-
ber of the NAMED group.
ADDRESS OPTIONS
Address options can be applied to address specifications to influence the process of open-
ing the addresses and the properties of the resulting data channels.
For technical reasons not every option can be applied to every address type; e.g., apply-
ing a socket option to a regular file will fail. To catch most useless combinations as
early as in the open phase, the concept of option groups was introduced. Each option be-
longs to one or more option groups. Options can be used only with address types that sup-
port at least one of their option groups (but see option -g).
Address options have data types that their values must conform to. Every address option
consists of just a keyword or a keyword followed by "=value", where value must conform to
the options type. Some address options manipulate parameters of system calls; e.g., op-
tion sync sets the O_SYNC flag with the open() call. Other options cause a system or li-
brary call; e.g., with option `ttl=value' the setsockopt(fd, SOL_IP, IP_TTL, value,
sizeof(int)) call is applied. Other options set internal socat variables that are used
during data transfer; e.g., `crnl' causes explicit character conversions. A few options
have more complex implementations; e.g., su-d (substuser-delayed) inquires some user and
group infos, stores them, and applies them later after a possible chroot() call.
If multiple options are given to an address, their sequence in the address specification
has (almost) no effect on the sequence of their execution/application. Instead, socat has
built in an option phase model that tries to bring the options in a useful order. Some op-
tions exist in different forms (e.g., unlink, unlink-early, unlink-late) to control the
time of their execution.
If the same option is specified more than once within one address specification, with
equal or different values, the effect depends on the kind of option. Options resulting in
function calls like setsockopt() cause multiple invocations. With options that set parame-
ters for a required call like open() or set internal flags, the value of the last option
occurrence is effective.
The existence or semantics of many options are system dependent. Socat usually does NOT
try to emulate missing libc or kernel features, it just provides an interface to the un-
derlying system. So, if an operating system lacks a feature, the related option is simply
not available on this platform.
The following paragraphs introduce just the more common address options. For a more com-
prehensive reference and to find information about canonical option names, alias names,
option phases, and platforms see file xio.help.
FD option group
This option group contains options that are applied to a UN*X style file descriptor, no
matter how it was generated. Because all current socat address types are file descriptor
based, these options may be applied to any address.
Note: Some of these options are also member of another option group, that provides an-
other, non-fd based mechanism. For these options, it depends on the actual address type
and its option groups which mechanism is used. The second, non-fd based mechanism is pri-
oritized.
cloexec=<bool>
Sets the FD_CLOEXEC flag with the fcntl() system call to value <bool>. If set, the
file descriptor is closed on exec() family function calls. Socat internally handles
this flag for the fds it controls, so in most cases there will be no need to apply
this option.
setlk Tries to set a discretionary write lock to the whole file using the fcntl(fd,
F_SETLK, ...) system call. If the file is already locked, this call results in an
error. On Linux, when the file permissions for group are "S" (g-x,g+s), and the
file system is locally mounted with the "mand" option, the lock is mandatory, i.e.
prevents other processes from opening the file.
setlkw Tries to set a discretionary waiting write lock to the whole file using the fc-
ntl(fd, F_SETLKW, ...) system call. If the file is already locked, this call
blocks. See option setlk for information about making this lock mandatory.
setlk-rd
Tries to set a discretionary read lock to the whole file using the fcntl(fd,
F_SETLK, ...) system call. If the file is already write locked, this call results
in an error. See option setlk for information about making this lock mandatory.
setlkw-rd
Tries to set a discretionary waiting read lock to the whole file using the fc-
ntl(fd, F_SETLKW, ...) system call. If the file is already write locked, this call
blocks. See option setlk for information about making this lock mandatory.
flock-ex
Tries to set a blocking exclusive advisory lock to the file using the flock(fd,
LOCK_EX) system call. Socat hangs in this call if the file is locked by another
process.
flock-ex-nb
Tries to set a nonblocking exclusive advisory lock to the file using the flock(fd,
LOCK_EX|LOCK_NB) system call. If the file is already locked, this option results in
an error.
flock-sh
Tries to set a blocking shared advisory lock to the file using the flock(fd,
LOCK_SH) system call. Socat hangs in this call if the file is locked by another
process.
flock-sh-nb
Tries to set a nonblocking shared advisory lock to the file using the flock(fd,
LOCK_SH|LOCK_NB) system call. If the file is already locked, this option results in
an error.
lock Sets a blocking lock on the file. Uses the setlk or flock mechanism depending on
availability on the particular platform. If both are available, the POSIX variant
(setlkw) is used.
user=<user>
Sets the <user> (owner) of the stream. If the address is member of the NAMED op-
tion group, socat uses the chown() system call after opening the file or binding to
the UNIX domain socket (race condition!). Without filesystem entry, socat sets the
user of the stream using the fchown() system call. These calls might require root
privilege.
user-late=<user>
Sets the owner of the fd to <user> with the fchown() system call after opening or
connecting the channel. This is useful only on file system entries.
group=<group>
Sets the <group> of the stream. If the address is member of the NAMED option
group, socat uses the chown() system call after opening the file or binding to the
UNIX domain socket (race condition!). Without filesystem entry, socat sets the
group of the stream with the fchown() system call. These calls might require group
membership or root privilege.
group-late=<group>
Sets the group of the fd to <group> with the fchown() system call after opening or
connecting the channel. This is useful only on file system entries.
mode=<mode>
Sets the <mode> [mode_t] (permissions) of the stream. If the address is member of
the NAMED option group and uses the open() or creat() call, the mode is applied
with these. If the address is member of the NAMED option group without using these
system calls, socat uses the chmod() system call after opening the filesystem entry
or binding to the UNIX domain socket (race condition!). Otherwise, socat sets the
mode of the stream using fchmod() . These calls might require ownership or root
privilege.
perm-late=<mode>
Sets the permissions of the fd to value <mode> [mode_t] using the fchmod() system
call after opening or connecting the channel. This is useful only on file system
entries.
append=<bool>
Always writes data to the actual end of file. If the address is member of the OPEN
option group, socat uses the O_APPEND flag with the open() system call (example).
Otherwise, socat applies the fcntl(fd, F_SETFL, O_APPEND) call.
nonblock=<bool>
Tries to open or use file in nonblocking mode. Its only effects are that the con-
nect() call of TCP addresses does not block, and that opening a named pipe for
reading does not block. If the address is member of the OPEN option group, socat
uses the O_NONBLOCK flag with the open() system call. Otherwise, socat applies the
fcntl(fd, F_SETFL, O_NONBLOCK) call.
binary Opens the file in binary mode to avoid implicit line terminator conversions (Cyg-
win).
text Opens the file in text mode to force implicit line terminator conversions (Cygwin).
noinherit
Does not keep this file open in a spawned process (Cygwin).
cool-write
Takes it easy when write fails with EPIPE or ECONNRESET and logs the message with
notice level instead of error. This prevents the log file from being filled with
useless error messages when socat is used as a high volume server or proxy where
clients often abort the connection.
This option is experimental.
end-close
Changes the (address dependent) method of ending a connection to just close the
file descriptors. This is useful when the connection is to be reused by or shared
with other processes (example).
Normally, socket connections will be ended with shutdown(2) which terminates the
socket even if it is shared by multiple processes. close(2) "unlinks" the socket
from the process but keeps it active as long as there are still links from other
processes.
Similarly, when an address of type EXEC or SYSTEM is ended, socat usually will ex-
plicitly kill the sub process. With this option, it will just close the file de-
scriptors.
shut-none
Changes the (address dependent) method of shutting down the write part of a connec-
tion to not do anything.
shut-down
Changes the (address dependent) method of shutting down the write part of a connec-
tion to shutdown(fd, SHUT_WR). Is only useful with sockets.
shut-close
Changes the (address dependent) method of shutting down the write part of a connec-
tion to close(fd).
shut-null
When one address indicates EOF, socat will send a zero sized packet to the write
channel of the other address to transfer the EOF condition. This is useful with UDP
and other datagram protocols. Has been tested against netcat and socat with option
null-eof.
null-eof
Normally socat will ignore empty (zero size payload) packets arriving on datagram
sockets, so it survives port scans. With this option socat interprets empty data-
gram packets as EOF indicator (see shut-null).
ioctl-void=<request>
Calls ioctl() with the request value as second argument and NULL as third argument.
This option allows utilizing ioctls that are not explicitly implemented in socat.
ioctl-int=<request>:<value>
Calls ioctl() with the request value as second argument and the integer value as
third argument.
ioctl-intp=<request>:<value>
Calls ioctl() with the request value as second argument and a pointer to the inte-
ger value as third argument.
ioctl-bin=<request>:<value>
Calls ioctl() with the request value as second argument and a pointer to the given
data value as third argument. This data must be specified in <dalan> form.
ioctl-string=<request>:<value>
Calls ioctl() with the request value as second argument and a pointer to the given
string as third argument. <dalan> form.
NAMED option group
These options work on file system entries.
Please note that, with UNIX domain client addresses, this means the bind entry, not the
target/peer entry.
See also options user, group, and mode.
user-early=<user>
Changes the <user> (owner) of the file system entry before accessing it, using the
chown() system call. This call might require root privilege.
group-early=<group>
Changes the <group> of the file system entry before accessing it, using the chown()
system call. This call might require group membership or root privilege.
perm-early=<mode>
Changes the <mode> [mode_t] of the file system entry before accessing it, using the
chmod() system call. This call might require ownership or root privilege.
umask=<mode>
Sets the umask of the process to <mode> [mode_t] before accessing the file system
entry (useful with UNIX domain sockets!). This call might affect all further opera-
tions of the socat process!
unlink-early
Unlinks (removes) the file before opening it and even before applying user-early
etc.
unlink Unlinks (removes) the file before accessing it, but after user-early etc.
unlink-late
Unlinks (removes) the file after opening it to make it inaccessible for other pro-
cesses after a short race condition.
unlink-close
Removes the addresses file system entry when closing the address. For named pipes,
UNIX domain sockets, and the symbolic links of pty addresses, the default is 1; for
created files, opened files, and generic opened files the default is 0.
OPEN option group
The OPEN group options allow setting flags with the open() system call. E.g., option
`creat' sets the O_CREAT flag.
See also options append and nonblock.
creat=<bool>
Creates the file if it does not exist (example).
dsync=<bool>
Blocks write() calls until metainfo is physically written to media.
excl=<bool>
With option creat, if file exists this is an error.
largefile=<bool>
On 32 bit systems, allows a file larger than 2^31 bytes.
noatime
Sets the O_NOATIME options, so reads do not change the access timestamp.
noctty=<bool>
Does not make this file the controlling terminal.
nofollow=<bool>
Does not follow symbolic links.
nshare=<bool>
Does not allow sharing this file with other processes.
rshare=<bool>
Does not allow other processes to open this file for writing.
rsync=<bool>
Blocks write() until metainfo is physically written to media.
sync=<bool>
Blocks write() until data is physically written to media.
rdonly=<bool>
Opens the file for reading only.
wronly=<bool>
Opens the file for writing only.
trunc Truncates the file to size 0 during opening it.
REG and BLK option group
These options are usually applied to a UN*X file descriptor, but their semantics make
sense only on a file supporting random access.
seek=<offset>
Applies the lseek(fd, <offset>, SEEK_SET) (or lseek64 ) system call, thus position-
ing the file pointer absolutely to <offset> [off_t or off64_t]. Please note that a
missing value defaults to 1, not 0.
seek-cur=<offset>
Applies the lseek(fd, <offset>, SEEK_CUR) (or lseek64 ) system call, thus position-
ing the file pointer <offset> [off_t or off64_t] bytes relatively to its current
position (which is usually 0). Please note that a missing value defaults to 1, not
0.
seek-end=<offset>
Applies the lseek(fd, <offset>, SEEK_END) (or lseek64 ) system call, thus position-
ing the file pointer <offset> [off_t or off64_t] bytes relatively to the files cur-
rent end. Please note that a missing value defaults to 1, not 0.
ftruncate=<offset>
Applies the ftruncate(fd, <offset>) (or ftruncate64 if available) system call, thus
truncating the file at the position <offset> [off_t or off64_t]. Please note that a
missing value defaults to 1, not 0.
secrm=<bool>
unrm=<bool>
compr=<bool>
fs-sync=<bool>
immutable=<bool>
fs-append=<bool>
nodump=<bool>
fs-noatime=<bool>
journal-data=<bool>
notail=<bool>
dirsync=<bool>
These options change non standard file attributes on operating systems and file
systems that support these features, like Linux with ext2fs and successors, xfs, or
reiserfs. See man 1 chattr for information on these options. Please note that there
might be a race condition between creating the file and applying these options.
PROCESS option group
Options of this group change the process properties instead of just affecting one data
channel. For EXEC and SYSTEM addresses and for LISTEN and CONNECT type addresses with op-
tion FORK, these options apply to the child processes instead of the main socat process.
chroot=<directory>
Performs a chroot() operation to <directory> after processing the address (exam-
ple). This call might require root privilege.
chroot-early=<directory>
Performs a chroot() operation to <directory> before opening the address. This call
might require root privilege.
setgid=<group>
Changes the primary <group> of the process after processing the address. This call
might require root privilege. Please note that this option does not drop other
group related privileges.
setgid-early=<group>
Like setgit but is performed before opening the address.
setuid=<user>
Changes the <user> (owner) of the process after processing the address. This call
might require root privilege. Please note that this option does not drop group re-
lated privileges. Check if option su better fits your needs.
setuid-early=<user>
Like setuid but is performed before opening the address.
su=<user>
Changes the <user> (owner) and groups of the process after processing the address
(example). This call might require root privilege.
su-d=<user>
Short name for substuser-delayed. Changes the <user> (owner) and groups of the
process after processing the address (example). The user and his groups are re-
trieved before a possible chroot() . This call might require root privilege.
setpgid=<pid_t>
Makes the process a member of the specified process group <pid_t>. If no value is
given, or if the value is 0 or 1, the process becomes leader of a new process
group.
setsid Makes the process the leader of a new session (example).
READLINE option group
Due to licensing restrictions the readline feature is disabled in Debian (see BUGS).
These options apply to the readline address type.
history=<filename>
Reads and writes history from/to <filename> (example).
noprompt
Since version 1.4.0, socat per default tries to determine a prompt - that is then
passed to the readline call - by remembering the last incomplete line of the out-
put. With this option, socat does not pass a prompt to readline, so it begins line
editing in the first column of the terminal.
noecho=<pattern>
Specifies a regular pattern for a prompt that prevents the following input line
from being displayed on the screen and from being added to the history. The prompt
is defined as the text that was output to the readline address after the lastest
newline character and before an input character was typed. The pattern is a regular
expression, e.g. "^[Pp]assword:.*$" or "([Uu]ser:|[Pp]assword:)". See regex(7) for
details. (example)
prompt=<string>
Passes the string as prompt to the readline function. readline prints this prompt
when stepping through the history. If this string matches a constant prompt issued
by an interactive program on the other socat address, consistent look and feel can
be achieved.
APPLICATION option group
This group contains options that work at data level. Note that these options only apply
to the "raw" data transferred by socat, but not to protocol data used by addresses like
PROXY.
cr Converts the default line termination character NL ('\n', 0x0a) to/from CR ('\r',
0x0d) when writing/reading on this channel.
crnl Converts the default line termination character NL ('\n', 0x0a) to/from CRNL
("\r\n", 0x0d0a) when writing/reading on this channel (example). Note: socat sim-
ply strips all CR characters.
ignoreeof
When EOF occurs on this channel, socat ignores it and tries to read more data (like
"tail -f") (example).
readbytes=<bytes>
socat reads only so many bytes from this address (the address provides only so many
bytes for transfer and pretends to be at EOF afterwards). Must be greater than 0.
lockfile=<filename>
If lockfile exists, exits with error. If lockfile does not exist, creates it and
continues, unlinks lockfile on exit.
waitlock=<filename>
If lockfile exists, waits until it disappears. When lockfile does not exist, cre-
ates it and continues, unlinks lockfile on exit.
escape=<int>
Specifies the numeric code of a character that triggers EOF on the input stream. It
is useful with a terminal in raw mode (example).
SOCKET option group
These options are intended for all kinds of sockets, e.g. IP or UNIX domain. Most are ap-
plied with a setsockopt() call.
bind=<sockname>
Binds the socket to the given socket address using the bind() system call. The form
of <sockname> is socket domain dependent: IP4 and IP6 allow the form [host-
name|hostaddress][:(service|port)] (example), UNIX domain sockets require <file-
name>, VSOCK allow the form [cid][:(port)].
connect-timeout=<seconds>
Abort the connection attempt after <seconds> [timeval] with error status.
so-bindtodevice=<interface>
Binds the socket to the given <interface>. This option might require root privi-
lege.
broadcast
For datagram sockets, allows sending to broadcast addresses and receiving packets
addressed to broadcast addresses.
debug Enables socket debugging.
dontroute
Only communicates with directly connected peers, does not use routers.
keepalive
Enables sending keepalives on the socket.
linger=<seconds>
Blocks shutdown() or close() until data transfers have finished or the given time-
out [int] expired.
oobinline
Places out-of-band data in the input data stream.
priority=<priority>
Sets the protocol defined <priority> [<int>] for outgoing packets.
rcvbuf=<bytes>
Sets the size of the receive buffer after the socket() call to <bytes> [int]. With
TCP sockets, this value corresponds to the socket's maximal window size.
rcvbuf-late=<bytes>
Sets the size of the receive buffer when the socket is already connected to <bytes>
[int]. With TCP sockets, this value corresponds to the socket's maximal window
size.
rcvlowat=<bytes>
Specifies the minimum number of received bytes [int] until the socket layer will
pass the buffered data to socat.
reuseaddr
Allows other sockets to bind to an address even if parts of it (e.g. the local
port) are already in use by socat (example).
sndbuf=<bytes>
Sets the size of the send buffer after the socket() call to <bytes> [int].
sndbuf-late=<bytes>
Sets the size of the send buffer when the socket is connected to <bytes> [int].
sndlowat=<bytes>
Specifies the minimum number of bytes in the send buffer until the socket layer
will send the data to <bytes> [int].
pf=<string>
Forces the use of the specified IP version or protocol. <string> can be something
like "ip4" or "ip6". The resulting value is used as first argument to the socket()
or socketpair() calls. This option affects address resolution and the required
syntax of bind and range options.
type=<type>
Sets the type of the socket, specified as second argument to the socket() or sock-
etpair() calls, to <type> [int]. Address resolution is not affected by this option.
Under Linux, 1 means stream oriented socket, 2 means datagram socket, and 3 means
raw socket.
protocol
Sets the protocol of the socket, specified as third argument to the socket() or
socketpair() calls, to <protocol> [int]. Address resolution is not affected by this
option. 6 means TCP, 17 means UDP.
reuseport
Set the SO_REUSEPORT socket option.
so-timestamp
Sets the SO_TIMESTAMP socket option. This enables receiving and logging of time-
stamp ancillary messages.
setsockopt=<level>:<optname>:<optval>
Invokes setsockopt() for the socket with the given parameters. level [int] is used
as second argument to setsockopt() and specifies the layer, e.g. SOL_TCP for TCP (6
on Linux), or SOL_SOCKET for the socket layer (1 on Linux). optname [int] is the
third argument to setsockopt() and tells which socket option is to be set. For the
actual numbers you might have to look up the appropriate include files of your sys-
tem. For the 4th and 5th setsockopt() parameters, value [dalan] specifies an arbi-
trary sequence of bytes that are passed to the function per pointer, with the auto-
matically derived length parameter.
setsockopt-int=<level>:<optname>:<optval>
Like setsockopt, but <optval> is a pointer to int [int]
setsockopt-listen=<level>:<optname>:<optval>
Like setsockopt, but for listen type addresses it is applied to the listening
socket instead of the connected socket.
setsockopt-string=<level>:<optname>:<optval>
Like setsockopt, but <optval> is a string. This string is passed to the function
with trailing null character, and the length parameter is automatically derived
from the data.
UNIX option group
These options apply to UNIX domain based addresses.
unix-tightsocklen=[0|1]
On socket operations, pass a socket address length that does not include the whole
struct sockaddr_un record but (besides other components) only the relevant part of
the filename or abstract string. Default is 1.
IP4 and IP6 option groups
These options can be used with IPv4 and IPv6 based sockets.
tos=<tos>
Sets the TOS (type of service) field of outgoing packets to <tos> [byte] (see RFC
791).
ttl=<ttl>
Sets the TTL (time to live) field of outgoing packets to <ttl> [byte].
ip-options=<data>
Sets IP options like source routing. Must be given in binary form, recommended for-
mat is a leading "x" followed by an even number of hex digits. This option may be
used multiple times, data are appended. E.g., to connect to host 10.0.0.1 via some
gateway using a loose source route, use the gateway as address parameter and set a
loose source route using the option ip-options=x8307040a000001 .
IP options are defined in RFC 791.
mtudiscover=<0|1|2>
Takes 0, 1, 2 to never, want, or always use path MTU discover on this socket.
ip-pktinfo
Sets the IP_PKTINFO socket option. This enables receiving and logging of ancillary
messages containing destination address and interface (Linux) (example).
ip-recverr
Sets the IP_RECVERR socket option. This enables receiving and logging of ancillary
messages containing detailed error information.
ip-recvopts
Sets the IP_RECVOPTS socket option. This enables receiving and logging of IP op-
tions ancillary messages (Linux, *BSD).
ip-recvtos
Sets the IP_RECVTOS socket option. This enables receiving and logging of TOS (type
of service) ancillary messages (Linux).
ip-recvttl
Sets the IP_RECVTTL socket option. This enables receiving and logging of TTL (time
to live) ancillary messages (Linux, *BSD).
ip-recvdstaddr
Sets the IP_RECVDSTADDR socket option. This enables receiving and logging of ancil-
lary messages containing destination address (*BSD) (example).
ip-recvif
Sets the IP_RECVIF socket option. This enables receiving and logging of interface
ancillary messages (*BSD) (example).
ip-add-membership=<multicast-address:interface-address>
ip-add-membership=<multicast-address:interface-name>
ip-add-membership=<multicast-address:interface-index>
ip-add-membership=<multicast-address:interface-address:interface-name>
ip-add-membership=<multicast-address:interface-address:interface-index>
Makes the socket member of the specified multicast group. This is currently only
implemented for IPv4. The option takes the IP address of the multicast group and
info about the desired network interface. The most common syntax is the first one,
while the others are only available on systems that provide struct mreqn (Linux).
The indices of active network interfaces can be shown using the utility procan.
ip-add-source-membership=<multicast-address:interface-address:source-address>
Makes the socket member of the specified multicast group for the specified source,
i.e. only multicast traffic from this address is to be delivered. This is cur-
rently only implemented for IPv4.
ip-multicast-if=<hostname>
Specifies hostname or address of the network interface to be used for multicast
traffic.
ip-multicast-loop=<bool>
Specifies if outgoing multicast traffic should loop back to the interface.
ip-multicast-ttl=<byte>
Sets the TTL used for outgoing multicast traffic. Default is 1.
ip-transparent
Sets the IP_TRANSPARENT socket option. This option might require root privilege.
res-debug
res-aaonly
res-usevc
res-primary
res-igntc
res-recurse
res-defnames
res-stayopen
res-dnsrch
These options set the corresponding resolver (name resolution) option flags. Ap-
pend "=0" to clear a default option. See man resolver(5) for more information on
these options. Note: these options are valid only for the address they are applied
to.
IP6 option group
These options can only be used on IPv6 based sockets. See IP options for options that can
be applied to both IPv4 and IPv6 sockets.
ipv6only=<bool>
Sets the IPV6_V6ONLY socket option. If 0, the TCP stack will also accept connec-
tions using IPv4 protocol on the same port. The default is system dependent.
ipv6-recvdstopts
Sets the IPV6_RECVDSTOPTS socket option. This enables receiving and logging of an-
cillary messages containing the destination options.
ipv6-recvhoplimit
Sets the IPV6_RECVHOPLIMIT socket option. This enables receiving and logging of an-
cillary messages containing the hoplimit.
ipv6-recvhopopts
Sets the IPV6_RECVHOPOPTS socket option. This enables receiving and logging of an-
cillary messages containing the hop options.
ipv6-recvpktinfo
Sets the IPV6_RECVPKTINFO socket option. This enables receiving and logging of an-
cillary messages containing destination address and interface.
ipv6-unicast-hops=link(TYPE_INT)(<int>)
Sets the IPV6_UNICAST_HOPS socket option. This sets the hop count limit (TTL) for
outgoing unicast packets.
ipv6-recvrthdr
Sets the IPV6_RECVRTHDR socket option. This enables receiving and logging of ancil-
lary messages containing routing information.
ipv6-tclass
Sets the IPV6_TCLASS socket option. This sets the transfer class of outgoing pack-
ets.
ipv6-recvtclass
Sets the IPV6_RECVTCLASS socket option. This enables receiving and logging of an-
cillary messages containing the transfer class.
TCP option group
These options may be applied to TCP sockets. They work by invoking setsockopt() with the
appropriate parameters.
cork Doesn't send packets smaller than MSS (maximal segment size).
defer-accept
While listening, accepts connections only when data from the peer arrived.
keepcnt=<count>
Sets the number of keepalives before shutting down the socket to <count> [int].
keepidle=<seconds>
Sets the idle time before sending the first keepalive to <seconds> [int].
keepintvl=<seconds>
Sets the interval between two keepalives to <seconds> [int].
linger2=<seconds>
Sets the time to keep the socket in FIN-WAIT-2 state to <seconds> [int].
mss=<bytes>
Sets the MSS (maximum segment size) after the socket() call to <bytes> [int]. This
value is then proposed to the peer with the SYN or SYN/ACK packet (example).
mss-late=<bytes>
Sets the MSS of the socket after connection has been established to <bytes> [int].
nodelay
Turns off the Nagle algorithm for measuring the RTT (round trip time).
rfc1323
Enables RFC1323 TCP options: TCP window scale, round-trip time measurement (RTTM),
and protect against wrapped sequence numbers (PAWS) (AIX).
stdurg Enables RFC1122 compliant urgent pointer handling (AIX).
syncnt=<count>
Sets the maximal number of SYN retransmits during connect to <count> [int].
md5sig Enables generation of MD5 digests on the packets (FreeBSD).
noopt Disables use of TCP options (FreeBSD, MacOSX).
nopush sets the TCP_NOPUSH socket option (FreeBSD, MacOSX).
sack-disable
Disables use the selective acknowledge feature (OpenBSD).
signature-enable
Enables generation of MD5 digests on the packets (OpenBSD).
abort-threshold=<milliseconds>
Sets the time to wait for an answer of the peer on an established connection
(HP-UX).
conn-abort-threshold=<milliseconds>
Sets the time to wait for an answer of the server during the initial connect
(HP-UX).
keepinit
Sets the time to wait for an answer of the server during connect() before giving
up. Value in half seconds, default is 150 (75s) (Tru64).
paws Enables the "protect against wrapped sequence numbers" feature (Tru64).
sackena
Enables selective acknowledge (Tru64).
tsoptena
Enables the time stamp option that allows RTT recalculation on existing connections
(Tru64).
UDP option group
This option may be applied to UDP datagram sockets.
udp-ignore-peerport>
Address UDP-DATAGRAM expects incoming responses to come from the port specified in
its second parameter. With this option, it accepts packets coming from any port.
SCTP option group
These options may be applied to SCTP stream sockets.
sctp-nodelay
Sets the SCTP_NODELAY socket option that disables the Nagle algorithm.
sctp-maxseg=<bytes>
Sets the SCTP_MAXSEG socket option to <bytes> [int]. This value is then proposed
to the peer with the SYN or SYN/ACK packet.
UDP, TCP, and SCTP option group
Here we find options that are related to the network port mechanism and thus can be used
with UDP, TCP, and SCTP client and server addresses.
sourceport=<port>
For outgoing (client) TCP and UDP connections, it sets the source <port> using an
extra bind() call. With TCP or UDP listen addresses, socat immediately shuts down
the connection if the client does not use this sourceport. UDP-RECV, UDP-RECVFROM,
UDP-SENDTO, and UDP-DATAGRAM addresses ignore the packet when it does not match.
(example).
lowport
Outgoing (client) TCP and UDP connections with this option use an unused random
source port between 640 and 1023 incl. On UNIX class operating systems, this re-
quires root privilege, and thus indicates that the client process is authorized by
local root. TCP and UDP listen addresses with this option immediately shut down
the connection if the client does not use a sourceport <= 1023. This mechanism can
provide limited authorization under some circumstances.
SOCKS option group
When using SOCKS type addresses, some socks specific options can be set.
socksport=<tcp service>
Overrides the default "socks" service or port 1080 for the socks server port with
<TCP service>.
socksuser=<user>
Sends the <user> [string] in the username field to the socks server. Default is the
actual user name ($LOGNAME or $USER) (example).
HTTP option group
Options that can be provided with HTTP type addresses. The only HTTP address currently im-
plemented is proxy-connect.
proxyport=<TCP service>
Overrides the default HTTP proxy port 8080 with <TCP service>.
ignorecr
The HTTP protocol requires the use of CR+NL as line terminator. When a proxy server
violates this standard, socat might not understand its answer. This option directs
socat to interprete NL as line terminator and to ignore CR in the answer. Neverthe-
less, socat sends CR+NL to the proxy.
proxy-authorization=<username>:<password>
Provide "basic" authentication to the proxy server. The argument to the option is
used with a "Proxy-Authorization: Basic" header in base64 encoded form.
Note: username and password are visible for every user on the local machine in the
process list; username and password are transferred to the proxy server unencrypted
(base64 encoded) and might be sniffed.
proxy-authorization-file=<filename>
Like option proxy-authorization, but the credentials are read from the file and
therefore not visible in the process list.
resolve
Per default, socat sends to the proxy a CONNECT request containing the target host-
name. With this option, socat resolves the hostname locally and sends the IP ad-
dress. Please note that, according to RFC 2396, only name resolution to IPv4 ad-
dresses is implemented.
RANGE option group
These options check if a connecting client should be granted access. They can be applied
to listening and receiving network sockets. tcp-wrappers options fall into this group.
range=<address-range>
After accepting a connection, tests if the peer is within range. For IPv4 ad-
dresses, address-range takes the form address/bits, e.g. 10.0.0.0/8, or ad-
dress:mask, e.g. 10.0.0.0:255.0.0.0 (example); for IPv6, it is [ip6-address]/bits,
e.g. [::1]/128. If the client address does not match, socat refuses the connection
attempt, issues a warning, and keeps listening/receiving.
tcpwrap[=<name>]
Uses Wietse Venema's libwrap (tcpd) library to determine if the client is allowed
to connect. The configuration files are /etc/hosts.allow and /etc/hosts.deny per
default, see "man 5 hosts_access" for more information. The optional <name> (type
string) is passed to the wrapper functions as daemon process name (example). If
omitted, the basename of socats invocation (argv[0]) is passed. If both tcpwrap
and range options are applied to an address, both conditions must be fulfilled to
allow the connection.
allow-table=<filename>
Takes the specified file instead of /etc/hosts.allow.
deny-table=<filename>
Takes the specified file instead of /etc/hosts.deny.
tcpwrap-etc=<directoryname>
Looks for hosts.allow and hosts.deny in the specified directory. Is overridden by
options hosts-allow and hosts-deny.
LISTEN option group
Options specific to listening sockets.
backlog=<count>
Sets the backlog value passed with the listen() system call to <count> [int]. De-
fault is 5.
accept-timeout=<seconds>
End waiting for a connection after <seconds> [timeval] with error status.
max-children=<count>
Limits the number of concurrent child processes [int]. Default is no limit.
CHILD option group
Options for addresses with multiple connections via child processes.
fork After establishing a connection, handles its channel in a child process and keeps
the parent process attempting to produce more connections, either by listening or
by connecting in a loop (example).
OPENSSL-CONNECT and OPENSSL-LISTEN differ in when they actually fork off the child:
OPENSSL-LISTEN forks before the SSL handshake, while OPENSSL-CONNECT forks after-
wards. retry and forever options are not inherited by the child process.
On some operating systems (e.g. FreeBSD) this option does not work for UDP-LISTEN
addresses.
EXEC option group
Options for addresses that invoke a program.
path=<string>
Overrides the PATH environment variable for searching the program with <string>.
This $PATH value is effective in the child process too.
login Prefixes argv[0] for the execvp() call with '-', thus making a shell behave as lo-
gin shell.
FORK option group
EXEC or SYSTEM addresses invoke a program using a child process and transfer data between
socat and the program. The interprocess communication mechanism can be influenced with the
following options. Per default, a socketpair() is created and assigned to stdin and stdout
of the child process, while stderr is inherited from the socat process, and the child
process uses file descriptors 0 and 1 for communicating with the main socat process.
nofork Does not fork a subprocess for executing the program, instead calls execvp() or
system() directly from the actual socat instance. This avoids the overhead of an-
other process between the program and its peer, but introduces a lot of restric-
tions:
o this option can only be applied to the second socat address.
o it cannot be applied to a part of a dual address.
o the first socat address cannot be OPENSSL or READLINE
o socat options -b, -t, -D, -l, -v, -x become useless
o for both addresses, options ignoreeof, cr, and crnl become useless
o for the second address (the one with option nofork), options append, cloexec,
flock, user, group, mode, nonblock, perm-late, setlk, and setpgid cannot be ap-
plied. Some of these could be used on the first address though.
pipes Creates a pair of unnamed pipes for interprocess communication instead of a socket
pair.
openpty
Establishes communication with the sub process using a pseudo terminal created with
openpty() instead of the default (socketpair or ptmx).
ptmx Establishes communication with the sub process using a pseudo terminal created by
opening /dev/ptmx or /dev/ptc instead of the default (socketpair).
pty Establishes communication with the sub process using a pseudo terminal instead of a
socket pair. Creates the pty with an available mechanism. If openpty and ptmx are
both available, it uses ptmx because this is POSIX compliant (example).
ctty Makes the pty the controlling tty of the sub process (example).
stderr Directs stderr of the sub process to its output channel by making stderr a dup() of
stdout (example).
fdin=<fdnum>
Assigns the sub processes input channel to its file descriptor <fdnum> instead of
stdin (0). The program started from the subprocess has to use this fd for reading
data from socat (example).
fdout=<fdnum>
Assigns the sub processes output channel to its file descriptor <fdnum> instead of
stdout (1). The program started from the subprocess has to use this fd for writing
data to socat (example).
sighup, sigint, sigquit
Has socat pass signals of this type to the sub process. If no address has this op-
tion, socat terminates on these signals.
TERMIOS option group
For addresses that work on a tty (e.g., stdio, file:/dev/tty, exec:...,pty), the terminal
parameters defined in the UN*X termios mechanism are made available as address option pa-
rameters. Please note that changes of the parameters of your interactive terminal remain
effective after socat's termination, so you might have to enter "reset" or "stty sane" in
your shell afterwards. For EXEC and SYSTEM addresses with option PTY, these options apply
to the pty by the child processes.
b0 Disconnects the terminal.
b19200 Sets the serial line speed to 19200 baud. Some other rates are possible; use some-
thing like socat -hh |grep ' b[1-9]' to find all speeds supported by your implemen-
tation.
Note: On some operating systems, these options may not be available. Use ispeed or
ospeed instead.
echo=<bool>
Enables or disables local echo.
icanon=<bool>
Sets or clears canonical mode, enabling line buffering and some special characters.
raw Sets raw mode, thus passing input and output almost unprocessed. This option is ob-
solete, use option rawer or cfmakeraw instead.
rawer Makes terminal rawer than raw option. This option implicitly turns off echo. (exam-
ple).
cfmakeraw
Sets raw mode by invoking cfmakeraw() or by simulating this call. This option im-
plicitly turns off echo.
ignbrk=<bool>
Ignores or interpretes the BREAK character (e.g., ^C)
brkint=<bool>
bs0
bs1
bsdly=<0|1>
clocal=<bool>
cr0
cr1
cr2
cr3
Sets the carriage return delay to 0, 1, 2, or 3, respectively. 0 means no delay,
the other values are terminal dependent.
crdly=<0|1|2|3>
cread=<bool>
crtscts=<bool>
cs5
cs6
cs7
cs8
Sets the character size to 5, 6, 7, or 8 bits, respectively.
csize=<0|1|2|3>
cstopb=<bool>
Sets two stop bits, rather than one.
dsusp=<byte>
Sets the value for the VDSUSP character that suspends the current foreground
process and reactivates the shell (all except Linux).
echoctl=<bool>
Echos control characters in hat notation (e.g. ^A)
echoe=<bool>
echok=<bool>
echoke=<bool>
echonl=<bool>
echoprt=<bool>
eof=<byte>
eol=<byte>
eol2=<byte>
erase=<byte>
discard=<byte>
ff0
ff1
ffdly=<bool>
flusho=<bool>
hupcl=<bool>
icrnl=<bool>
iexten=<bool>
igncr=<bool>
ignpar=<bool>
imaxbel=<bool>
inlcr=<bool>
inpck=<bool>
intr=<byte>
isig=<bool>
ispeed=<unsigned-int>
Set the baud rate for incoming data on this line.
See also: ospeed, b19200
istrip=<bool>
iuclc=<bool>
ixany=<bool>
ixoff=<bool>
ixon=<bool>
kill=<byte>
lnext=<byte>
min=<byte>
nl0 Sets the newline delay to 0.
nl1
nldly=<bool>
noflsh=<bool>
ocrnl=<bool>
ofdel=<bool>
ofill=<bool>
olcuc=<bool>
onlcr=<bool>
onlret=<bool>
onocr=<bool>
opost=<bool>
Enables or disables output processing; e.g., converts NL to CR-NL.
ospeed=<unsigned-int>
Set the baud rate for outgoing data on this line.
See also: ispeed, b19200
parenb=<bool>
Enable parity generation on output and parity checking for input.
parmrk=<bool>
parodd=<bool>
pendin=<bool>
quit=<byte>
reprint=<byte>
sane Brings the terminal to something like a useful default state.
start=<byte>
stop=<byte>
susp=<byte>
swtc=<byte>
tab0
tab1
tab2
tab3
tabdly=<unsigned-int>
time=<byte>
tostop=<bool>
vt0
vt1
vtdly=<bool>
werase=<byte>
xcase=<bool>
xtabs
i-pop-all
With UNIX System V STREAMS, removes all drivers from the stack.
i-push=<string>
With UNIX System V STREAMS, pushes the driver (module) with the given name (string)
onto the stack. For example, to make sure that a character device on Solaris sup-
ports termios etc, use the following options:
i-pop-all,i-push=ptem,i-push=ldterm,i-push=ttcompat
PTY option group
These options are intended for use with the pty address type.
link=<filename>
Generates a symbolic link that points to the actual pseudo terminal (pty). This
might help to solve the problem that ptys are generated with more or less unpre-
dictable names, making it difficult to directly access the socat generated pty au-
tomatically. With this option, the user can specify a "fix" point in the file hier-
archy that helps him to access the actual pty (example). Beginning with socat ver-
sion 1.4.3, the symbolic link is removed when the address is closed (but see option
unlink-close).
wait-slave
Blocks the open phase until a process opens the slave side of the pty. Usually,
socat continues after generating the pty with opening the next address or with en-
tering the transfer loop. With the wait-slave option, socat waits until some
process opens the slave side of the pty before continuing. This option only works
if the operating system provides the poll() system call. And it depends on an un-
documented behaviour of pty's, so it does not work on all operating systems. It has
successfully been tested on Linux, FreeBSD, NetBSD, and on Tru64 with openpty.
pty-interval=<seconds>
When the wait-slave option is set, socat periodically checks the HUP condition us-
ing poll() to find if the pty's slave side has been opened. The default polling in-
terval is 1s. Use the pty-interval option [timeval] to change this value.
OPENSSL option group
These options apply to the openssl and openssl-listen address types.
cipher=<cipherlist>
Selects the list of ciphers that may be used for the connection. See the man page
of ciphers , section CIPHER LIST FORMAT, for detailed information about syntax,
values, and default of <cipherlist>.
Several cipher strings may be given, separated by ':'. Some simple cipher strings:
3DES Uses a cipher suite with triple DES.
MD5 Uses a cipher suite with MD5.
aNULL Uses a cipher suite without authentication.
NULL Does not use encryption.
HIGH Uses a cipher suite with "high" encryption. Note that the peer must support the
selected property, or the negotiation will fail.
method=<ssl-method>
This option is based on deprecated functions and is only available when socat was
build with option --with-openssl-method. Sets the protocol version to be used.
Valid strings (not case sensitive) are:
SSL2 Select SSL protocol version 2.
SSL3 Select SSL protocol version 3.
SSL23 Select the best available SSL or TLS protocol.
TLS1 Select TLS protocol version 1.
TLS1.1 Select TLS protocol version 1.1.
TLS1.2 Select TLS protocol version 1.2. When this option is not provided OpenSSL negoti-
ates the mothod with its peer.
verify=<bool>
Controls check of the peer's certificate. Default is 1 (true). Disabling verify
might open your socket for everyone, making the encryption useless!
cert=<filename>
Specifies the file with the certificate and private key for authentication. The
certificate must be in OpenSSL format (*.pem). With openssl-listen, use of this
option is strongly recommended. Except with cipher aNULL, "no shared ciphers" error
will occur when no certificate is given.
key=<filename>
Specifies the file with the private key. The private key may be in this file or in
the file given with the cert option. The party that has to proof that it is the
owner of a certificate needs the private key.
dhparams=<filename>
Specifies the file with the Diffie Hellman parameters. These parameters may also be
in the file given with the cert option in which case the dhparams option is not
needed.
cafile=<filename>
Specifies the file with the trusted (root) authority certificates. The file must be
in PEM format and should contain one or more certificates. The party that checks
the authentication of its peer trusts only certificates that are in this file.
capath=<dirname>
Specifies the directory with the trusted (root) certificates. The directory must
contain certificates in PEM format and their hashes (see OpenSSL documentation)
egd=<filename>
On some systems, openssl requires an explicit source of random data. Specify the
socket name where an entropy gathering daemon like egd provides random data, e.g.
/dev/egd-pool.
pseudo On systems where openssl cannot find an entropy source and where no entropy gather-
ing daemon can be utilized, this option activates a mechanism for providing pseudo
entropy. This is achieved by taking the current time in microseconds for feeding
the libc pseudo random number generator with an initial value. openssl is then
feeded with output from random() calls.
NOTE:This mechanism is not sufficient for generation of secure keys!
compress
Enable or disable the use of compression for a connection. Setting this to "none"
disables compression, setting it to "auto" lets OpenSSL choose the best available
algorithm supported by both parties. The default is to not touch any compres-
sion-related settings. NOTE: Requires OpenSSL 0.9.8 or higher and disabling com-
pression with OpenSSL 0.9.8 affects all new connections in the process.
commonname=<string>
Specify the commonname that the peer certificate must match. With OPENSSL-CONNECT
address this overrides the given hostname or IP target address; with OPENSSL-LISTEN
this turns on check of peer certificates commonname. This option has only meaning
when option verify is not disabled and the chosen cipher provides a peer certifi-
cate.
no-sni=<bool>
Do not use the client side Server Name Indication (SNI) feature that selects the
desired server certificate.
Note: SNI is automatically used since socat version 1.7.4.0 and uses commonname or
the given host name.
snihost=<string>
Set the client side Server Name Indication (SNI) host name different from the ad-
dressed server name or common name. This might be useful when the server certifi-
cate has multiple host names or wildcard names because the SNI host name is passed
in cleartext to the server and might be eavesdropped; with this option a mock name
of the desired certificate may be transferred.
fips Enables FIPS mode if compiled in. For info about the FIPS encryption implementation
standard see http://oss-institute.org/fips-faq.html. This mode might require that
the involved certificates are generated with a FIPS enabled version of openssl.
Setting or clearing this option on one socat address affects all OpenSSL addresses
of this process.
RETRY option group
Options that control retry of some system calls, especially connection attempts.
retry=<num>
Number of retries before the connection or listen attempt is aborted. Default is
0, which means just one attempt.
interval=<timespec>
Time between consecutive attempts (seconds, [timespec]). Default is 1 second.
forever
Performs an unlimited number of retry attempts.
TUN option group
Options that control Linux TUN/TAP interface device addresses.
tun-device=<device-file>
Instructs socat to take another path for the TUN clone device. Default is
/dev/net/tun.
tun-name=<if-name>
Gives the resulting network interface a specific name instead of the system gener-
ated (tun0, tun1, etc.)
tun-type=[tun|tap]
Sets the type of the TUN device; use this option to generate a TAP device. See the
Linux docu for the difference between these types. When you try to establish a
tunnel between two TUN devices, their types should be the same.
iff-no-pi
Sets the IFF_NO_PI flag which controls if the device includes additional packet in-
formation in the tunnel. When you try to establish a tunnel between two TUN de-
vices, these flags should have the same values.
iff-up Sets the TUN network interface status UP. Strongly recommended.
iff-broadcast
Sets the BROADCAST flag of the TUN network interface.
iff-debug
Sets the DEBUG flag of the TUN network interface.
iff-loopback
Sets the LOOPBACK flag of the TUN network interface.
iff-pointopoint
Sets the POINTOPOINT flag of the TUN device.
iff-notrailers
Sets the NOTRAILERS flag of the TUN device.
iff-running
Sets the RUNNING flag of the TUN device.
iff-noarp
Sets the NOARP flag of the TUN device.
iff-promisc
Sets the PROMISC flag of the TUN device.
iff-allmulti
Sets the ALLMULTI flag of the TUN device.
iff-master
Sets the MASTER flag of the TUN device.
iff-slave
Sets the SLAVE flag of the TUN device.
iff-multicast
Sets the MULTICAST flag of the TUN device.
iff-portsel
Sets the PORTSEL flag of the TUN device.
iff-automedia
Sets the AUTOMEDIA flag of the TUN device.
iff-dynamic
Sets the DYNAMIC flag of the TUN device.
DATA VALUES
This section explains the different data types that address parameters and address options
can take.
address-range
Is currently only implemented for IPv4 and IPv6. See address-option `range'
bool "0" or "1"; if value is omitted, "1" is taken.
byte An unsigned int number, read with strtoul() , lower or equal to UCHAR_MAX .
command-line
A string specifying a program name and its arguments, separated by single spaces.
data This is a more general data specification. The given text string contains informa-
tion about the target data type and value. Generally a leading character specifies
the type of the following data item. In its specific context a default data type
may exist.
Currently only the following specifications are implemented:
i A signed integer number, stored in host byte order.
Example: i-1000 (Integer number -1000)
I An unsigned integer number, stored in host byte order.
l A signed long integer number, stored in host byte order.
L An unsigned long integer number, stored in host byte order.
s A signed short integer number, stored in host byte order.
S An unsigned short integer number, stored in host byte order.
b A signed byte (signed char).
B An unsigned byte (unsigned char).
x Following is an even number of hex digits, stored as sequence of bytes.
Example: x7f000001 (IP address 127.0.0.1)
" Following is a string that is used with the common conversions \n \r \t \f \b \a \e
\0; the string must be closed with '"'. Please note that the quotes and backslashes
need to be escaped from shell and socat conversion.
Example: "Hello world!\n"
' A single char, with the usual conversions. Please note that the quotes and back-
slashes need to be escaped from shell and socat conversion.
Example: 'a' Data items may be separated with white space without need to repeat
the type specifier again.
directory
A string with usual UN*X directory name semantics.
facility
The name of a syslog facility in lower case characters.
fdnum An unsigned int type, read with strtoul() , specifying a UN*X file descriptor.
filename
A string with usual UN*X filename semantics.
group If the first character is a decimal digit, the value is read with strtoul() as un-
signed integer specifying a group id. Otherwise, it must be an existing group name.
int A number following the rules of the strtol() function with base "0", i.e. decimal
number, octal number with leading "0", or hexadecimal number with leading "0x". The
value must fit into a C int.
interface
A string specifying the device name of a network interface as shown by ifconfig or
procan, e.g. "eth0".
IP address
An IPv4 address in numbers-and-dots notation, an IPv6 address in hex notation en-
closed in brackets, or a hostname that resolves to an IPv4 or an IPv6 address.
Examples: 127.0.0.1, [::1], www.dest-unreach.org, dns1
IPv4 address
An IPv4 address in numbers-and-dots notation or a hostname that resolves to an IPv4
address.
Examples: 127.0.0.1, www.dest-unreach.org, dns2
IPv6 address
An IPv6 address in hexnumbers-and-colons notation enclosed in brackets, or a host-
name that resolves to an IPv6 address.
Examples: [::1], [1234:5678:9abc:def0:1234:5678:9abc:def0], ip6name.domain.org
long A number read with strtol() . The value must fit into a C long.
long long
A number read with strtoll() . The value must fit into a C long long.
off_t An implementation dependend signed number, usually 32 bits, read with strtol or
strtoll.
off64_t
An implementation dependend signed number, usually 64 bits, read with strtol or
strtoll.
mode_t An unsigned integer, read with strtoul() , specifying mode (permission) bits.
pid_t A number, read with strtol() , specifying a process id.
port A uint16_t (16 bit unsigned number) specifying a TCP or UDP port, read with str-
toul() .
protocol
An unsigned 8 bit number, read with strtoul() .
size_t An unsigned number with size_t limitations, read with strtoul .
sockname
A socket address. See address-option `bind'
string A sequence of characters, not containing '\0' and, depending on the position within
the command line, ':', ',', or "!!". Note that you might have to escape shell meta
characters in the command line.
TCP service
A service name, not starting with a digit, that is resolved by getservbyname() , or
an unsigned int 16 bit number read with strtoul() .
timeval
A double float specifying seconds; the number is mapped into a struct timeval, con-
sisting of seconds and microseconds.
timespec
A double float specifying seconds; the number is mapped into a struct timespec,
consisting of seconds and nanoseconds.
UDP service
A service name, not starting with a digit, that is resolved by getservbyname() , or
an unsigned int 16 bit number read with strtoul() .
unsigned int
A number read with strtoul() . The value must fit into a C unsigned int.
user If the first character is a decimal digit, the value is read with strtoul() as un-
signed integer specifying a user id. Otherwise, it must be an existing user name.
VSOCK cid
A uint32_t (32 bit unsigned number) specifying a VSOCK Context Identifier (CID),
read with strtoul() . There are several special addresses: VMADDR_CID_ANY (-1U)
means any address for binding; VMADDR_CID_HOST (2) is the well-known address of the
host.
VSOCK port
A uint32_t (32 bit unsigned number) specifying a VSOCK port, read with strtoul() .
EXAMPLES
socat - TCP4:www.domain.org:80
transfers data between STDIO (-) and a TCP4 connection to port 80 of host www.do-
main.org. This example results in an interactive connection similar to telnet or
netcat. The stdin terminal parameters are not changed, so you may close the relay
with ^D or abort it with ^C.
socat -d -d READLINE,history=$HOME/.http_history \
TCP4:www.domain.org:www,crnl
this is similar to the previous example, but you can edit the current line in a
bash like manner (READLINE) and use the history file .http_history; socat prints
messages about progress (-d -d). The port is specified by service name (www), and
correct network line termination characters (crnl) instead of NL are used.
socat TCP4-LISTEN:www TCP4:www.domain.org:www
installs a simple TCP port forwarder. With TCP4-LISTEN it listens on local port
"www" until a connection comes in, accepts it, then connects to the remote host
(TCP4) and starts data transfer. It will not accept a second connection.
socat -d -d -lmlocal2 \
TCP4-LISTEN:80,bind=myaddr1,reuseaddr,fork,su=nobody,range=10.0.0.0/8 \
TCP4:www.domain.org:80,bind=myaddr2
TCP port forwarder, each side bound to another local IP address (bind). This exam-
ple handles an almost arbitrary number of parallel or consecutive connections by
fork'ing a new process after each accept() . It provides a little security by
su'ing to user nobody after forking; it only permits connections from the private
10 network (range); due to reuseaddr, it allows immediate restart after master
process's termination, even if some child sockets are not completely shut down.
With -lmlocal2, socat logs to stderr until successfully reaching the accept loop.
Further logging is directed to syslog with facility local2.
socat TCP4-LISTEN:5555,fork,tcpwrap=script \
EXEC:/bin/myscript,chroot=/home/sandbox,su-d=sandbox,pty,stderr
a simple server that accepts connections (TCP4-LISTEN) and fork's a new child
process for each connection; every child acts as single relay. The client must
match the rules for daemon process name "script" in /etc/hosts.allow and
/etc/hosts.deny, otherwise it is refused access (see "man 5 hosts_access"). For
EXEC'uting the program, the child process chroot's to /home/sandbox, su's to user
sandbox, and then starts the program /home/sandbox/bin/myscript. Socat and myscript
communicate via a pseudo tty (pty); myscript's stderr is redirected to stdout, so
its error messages are transferred via socat to the connected client.
socat EXEC:"mail.sh target AT domain.com",fdin=3,fdout=4 \
TCP4:mail.relay.org:25,crnl,bind=alias1.server.org,mss=512
mail.sh is a shell script, distributed with socat, that implements a simple SMTP
client. It is programmed to "speak" SMTP on its FDs 3 (in) and 4 (out). The fdin
and fdout options tell socat to use these FDs for communication with the program.
Because mail.sh inherits stdin and stdout while socat does not use them, the script
can read a mail body from stdin. Socat makes alias1 your local source address
(bind), cares for correct network line termination (crnl) and sends at most 512
data bytes per packet (mss).
socat -,escape=0x0f /dev/ttyS0,rawer,crnl
opens an interactive connection via the serial line, e.g. for talking with a modem.
rawer sets the console's and ttyS0's terminal parameters to practicable values,
crnl converts to correct newline characters. escape allows terminating the socat
process with character control-O.
socat UNIX-LISTEN:/tmp/.X11-unix/X1,fork \
SOCKS4:host.victim.org:127.0.0.1:6000,socksuser=nobody,sourceport=20
with UNIX-LISTEN, socat opens a listening UNIX domain socket /tmp/.X11-unix/X1.
This path corresponds to local XWindow display :1 on your machine, so XWindow
client connections to DISPLAY=:1 are accepted. Socat then speaks with the SOCKS4
server host.victim.org that might permit sourceport 20 based connections due to an
FTP related weakness in its static IP filters. Socat pretends to be invoked by
socksuser nobody, and requests to be connected to loopback port 6000 (only weak
sockd configurations will allow this). So we get a connection to the victims XWin-
dow server and, if it does not require MIT cookies or Kerberos authentication, we
can start work. Please note that there can only be one connection at a time, be-
cause TCP can establish only one session with a given set of addresses and ports.
socat -u /tmp/readdata,seek-end=0,ignoreeof -
this is an example for unidirectional data transfer (-u). Socat transfers data from
file /tmp/readdata (implicit address GOPEN), starting at its current end
(seek-end=0 lets socat start reading at current end of file; use seek=0 or no seek
option to first read the existing data) in a "tail -f" like mode (ignoreeof). The
"file" might also be a listening UNIX domain socket (do not use a seek option
then).
(sleep 5; echo PASSWORD; sleep 5; echo ls; sleep 1) |
socat - EXEC:'ssh -l user server',pty,setsid,ctty
EXEC'utes an ssh session to server. Uses a pty for communication between socat and
ssh, makes it ssh's controlling tty (ctty), and makes this pty the owner of a new
process group (setsid), so ssh accepts the password from socat.
socat -u TCP4-LISTEN:3334,reuseaddr,fork \
OPEN:/tmp/in.log,creat,append
implements a simple network based message collector. For each client connecting to
port 3334, a new child process is generated (option fork). All data sent by the
clients are append'ed to the file /tmp/in.log. If the file does not exist, socat
creat's it. Option reuseaddr allows immediate restart of the server process.
socat PTY,link=$HOME/dev/vmodem0,rawer,wait-slave \
EXEC:"ssh modemserver.us.org socat - /dev/ttyS0,nonblock,rawer"
generates a pseudo terminal device (PTY) on the client that can be reached under
the symbolic link $HOME/dev/vmodem0. An application that expects a serial line or
modem can be configured to use $HOME/dev/vmodem0; its traffic will be directed to a
modemserver via ssh where another socat instance links it to /dev/ttyS0.
socat TCP4-LISTEN:2022,reuseaddr,fork \
PROXY:proxy:www.domain.org:22,proxyport=3128,proxyauth=user:pass
starts a forwarder that accepts connections on port 2022, and directs them through
the proxy daemon listening on port 3128 (proxyport) on host proxy, using the CON-
NECT method, where they are authenticated as "user" with "pass" (proxyauth). The
proxy should establish connections to host www.domain.org on port 22 then.
socat - OPENSSL:server:4443,cafile=server.crt,cert=client.pem
is an OpenSSL client that tries to establish a secure connection to an SSL server.
Option cafile specifies a file that contains trust certificates: we trust the
server only when it presents one of these certificates and proofs that it owns the
related private key. Otherwise the connection is terminated. With cert a file
containing the client certificate and the associated private key is specified. This
is required in case the server wishes a client authentication; many Internet
servers do not.
The first address ('-') can be replaced by almost any other socat address.
socat OPENSSL-LISTEN:4443,reuseaddr,pf=ip4,fork,cert=server.pem,cafile=client.crt PIPE
is an OpenSSL server that accepts TCP connections, presents the certificate from
the file server.pem and forces the client to present a certificate that is verified
against cafile.crt.
The second address ('PIPE') can be replaced by almost any other socat address.
For instructions on generating and distributing OpenSSL keys and certificates see
the additional socat docu socat-openssl.txt.
echo |socat -u - file:/tmp/bigfile,create,largefile,seek=100000000000
creates a 100GB sparse file; this requires a file system type that supports this
(ext2, ext3, reiserfs, jfs; not minix, vfat). The operation of writing 1 byte might
take long (reiserfs: some minutes; ext2: "no" time), and the resulting file can
consume some disk space with just its inodes (reiserfs: 2MB; ext2: 16KB).
socat tcp-l:7777,reuseaddr,fork system:'filan -i 0 -s >&2',nofork
listens for incoming TCP connections on port 7777. For each accepted connection,
invokes a shell. This shell has its stdin and stdout directly connected to the TCP
socket (nofork). The shell starts filan and lets it print the socket addresses to
stderr (your terminal window).
echo -en "\0\14\0\0\c" |socat -u - file:/usr/bin/squid.exe,seek=0x00074420
functions as primitive binary editor: it writes the 4 bytes 000 014 000 000 to the
executable /usr/bin/squid at offset 0x00074420 (this is a real world patch to make
the squid executable from Cygwin run under Windows, actual per May 2004).
socat - tcp:www.blackhat.org:31337,readbytes=1000
connects to an unknown service and prevents being flooded.
socat -U TCP:target:9999,end-close TCP-L:8888,reuseaddr,fork
merges data arriving from different TCP streams on port 8888 to just one stream to
target:9999. The end-close option prevents the child processes forked off by the
second address from terminating the shared connection to 9999 (close(2) just un-
links the inode which stays active as long as the parent process lives; shutdown(2)
would actively terminate the connection).
socat - UDP4-DATAGRAM:192.168.1.0:123,sp=123,broadcast,range=192.168.1.0/24
sends a broadcast to the network 192.168.1.0/24 and receives the replies of the
timeservers there. Ignores NTP packets from hosts outside this network.
socat - SOCKET-DATAGRAM:2:2:17:x007bxc0a80100x0000000000000000,b-
ind=x007bx00000000x0000000000000000,setsockopt-int=1:6:1,r-
ange=x0000xc0a80100x0000000000000000:x0000xffffff00x0000000000000000
is semantically equivalent to the previous example, but all parameters are speci-
fied in generic form. the value 6 of setsockopt-int is the Linux value for
SO_BROADCAST.
socat - IP4-DATAGRAM:255.255.255.255:44,broadcast,range=10.0.0.0/8
sends a broadcast to the local network(s) using protocol 44. Accepts replies from
the private address range only.
socat - UDP4-DATAGRAM:224.255.0.1:6666,bind=:6666,ip-add-membership=224.255.0.1:eth0
transfers data from stdin to the specified multicast address using UDP. Both local
and remote ports are 6666. Tells the interface eth0 to also accept multicast pack-
ets of the given group. Multiple hosts on the local network can run this command,
so all data sent by any of the hosts will be received by all the other ones. Note
that there are many possible reasons for failure, including IP-filters, routing is-
sues, wrong interface selection by the operating system, bridges, or a badly con-
figured switch.
socat UDP:host2:4443 TUN:192.168.255.1/24,up
establishes one side of a virtual (but not private!) network with host2 where a
similar process might run, with UDP-L and tun address 192.168.255.2. They can reach
each other using the addresses 192.168.255.1 and 192.168.255.2. Note that streaming
eg.via TCP or SSL does not guarantee to retain packet boundaries and might thus
cause packet loss.
socat - VSOCK-CONNECT:2:1234
establishes a VSOCK connection with the host (host is always reachable with the
well-know CID=2) on 1234 port.
socat - VSOCK-LISTEN:1234
listens for a VSOCK connection on 1234 port.
socat - VSOCK-CONNECT:31:4321,bind:5555
establishes a VSOCK connection with the guest that have CID=31 on 1234 port, bind-
ing the local socket to the 5555 port.
socat VSOCK-LISTEN:3333,reuseaddr,fork VSOCK-CONNECT:42,3333
starts a forwarder that accepts VSOCK connections on port 3333, and directs them to
the guest with CID=42 on the same port.
socat VSOCK-LISTEN:22,reuseaddr,fork TCP:localhost:22
forwards VSOCK connections from 22 port to the local SSH server. Running this in a
VM allows you to connect via SSH from the host using VSOCK, as in the example be-
low.
socat TCP4-LISTEN:22222,reuseaddr,fork VSOCK-CONNECT:33:22
forwards TCP connections from 22222 port to the guest with CID=33 listening on
VSOCK port 22. Running this in the host, allows you to connect via SSH running
"ssh -p 22222 user@localhost", if the guest runs the example above.
socat PTY,link=/var/run/ppp,rawer INTERFACE:hdlc0
circumvents the problem that pppd requires a serial device and thus might not be
able to work on a synchronous line that is represented by a network device. socat
creates a PTY to make pppd happy, binds to the network interface hdlc0, and can
transfer data between both devices. Use pppd on device /var/run/ppp then.
socat -T 1 -d -d TCP-L:10081,reuseaddr,fork,crlf SYSTEM:"echo -e \"\\\"HTTP/1.0 200
OK\\\nDocumentType: text/plain\\\n\\\ndate: \$\(date\)\\\nserver:\$SOCAT_SOCKADDR:\$SO-
CAT_SOCKPORT\\\nclient: \$SOCAT_PEERADDR:\$SOCAT_PEERPORT\\\n\\\"\"; cat; echo -e
\"\\\"\\\n\\\"\""
creates a simple HTTP echo server: each HTTP client that connects gets a valid HTTP
reply that contains information about the client address and port as it is seen by
the server host, the host address (which might vary on multihomed servers), and the
original client request.
socat -d -d UDP4-RECVFROM:9999,so-broadcast,so-timestamp,ip-pkt-
info,ip-recverr,ip-recvopts,ip-recvtos,ip-recvttl!!- SYSTEM:'export; sleep 1' |grep SOCAT
waits for an incoming UDP packet on port 9999 and prints the environment variables
provided by socat. On BSD based systems you have to replace ip-pktinfo with
ip-recvdstaddr,ip-recvif. Especially interesting is SOCAT_IP_DSTADDR: it contains
the target address of the packet which may be a unicast, multicast, or broadcast
address.
echo -e "M-SEARCH * HTTP/1.1\nHOST: 239.255.255.250:1900\nMAN: \"ssdp:discover\"\nMX:
4\nST: \"ssdp:all\"\n" |./socat - UDP-DATAGRAM:239.255.255.250:1900,crlf
sends an SSDP (Simple Service Discovery Protocol) query to the local network and
collects and outputs the answers received.
DIAGNOSTICS
Socat uses a logging mechanism that allows filtering messages by severity. The severities
provided are more or less compatible to the appropriate syslog priority. With one or up to
four occurrences of the -d command line option, the lowest priority of messages that are
issued can be selected. Each message contains a single uppercase character specifying the
messages severity (one of F, E, W, N, I, or D)
FATAL: Conditions that require unconditional and immediate program termination.
ERROR: Conditions that prevent proper program processing. Usually the program is termi-
nated (see option -s).
WARNING:
Something did not function correctly or is in a state where correct further pro-
cessing cannot be guaranteed, but might be possible.
NOTICE:
Interesting actions of the program, e.g. for supervising socat in some kind of
server mode.
INFO: Description of what the program does, and maybe why it happens. Allows monitoring
the lifecycles of file descriptors.
DEBUG: Description of how the program works, all system or library calls and their re-
sults.
Log messages can be written to stderr, to a file, or to syslog.
On exit, socat gives status 0 if it terminated due to EOF or inactivity timeout, with a
positive value on error, and with a negative value on fatal error.
FILES
/usr/bin/socat
/usr/bin/filan
/usr/bin/procan
ENVIRONMENT VARIABLES
Input variables carry information from the environment to socat, output variables are set
by socat for use in executed scripts and programs.
In the output variables beginning with "SOCAT" this prefix is actually replaced by the up-
per case name of the executable or the value of option -lp.
SOCAT_DEFAULT_LISTEN_IP (input)
(Values 4 or 6) Sets the IP version to be used for listen, recv, and recvfrom ad-
dresses if no pf (protocol-family) option is given. Is overridden by socat options
-4 or -6.
SOCAT_PREFERRED_RESOLVE_IP (input)
(Values 0, 4, or 6) Sets the IP version to be used when resolving target host names
when version is not specified by address type, option pf (protocol-family), or ad-
dress format. If name resolution does not return a matching entry, the first result
(with differing IP version) is taken. With value 0, socat always selects the first
record and its IP version.
SOCAT_FORK_WAIT (input)
Specifies the time (seconds) to sleep the parent and child processes after success-
ful fork(). Useful for debugging.
SOCAT_VERSION (output)
Socat sets this variable to its version string, e.g. "1.7.0.0" for released ver-
sions or e.g. "1.6.0.1+envvar" for temporary versions; can be used in scripts in-
voked by socat.
SOCAT_PID (output)
Socat sets this variable to its process id. In case of fork address option, SO-
CAT_PID gets the child processes id. Forking for exec and system does not change
SOCAT_PID.
SOCAT_PPID (output)
Socat sets this variable to its process id. In case of fork, SOCAT_PPID keeps the
pid of the master process.
SOCAT_PEERADDR (output)
With passive socket addresses (all LISTEN and RECVFROM addresses), this variable is
set to a string describing the peers socket address. Port information is not in-
cluded.
SOCAT_PEERPORT (output)
With appropriate passive socket addresses (TCP, UDP, and SCTP - LISTEN and
RECVFROM), this variable is set to a string containing the number of the peer port.
SOCAT_SOCKADDR (output)
With all LISTEN addresses, this variable is set to a string describing the local
socket address. Port information is not included example
SOCAT_SOCKPORT (output)
With TCP-LISTEN, UDP-LISTEN, and SCTP-LISTEN addresses, this variable is set to the
local port.
SOCAT_TIMESTAMP (output)
With all RECVFROM addresses where address option so-timestamp is applied, socat
sets this variable to the resulting timestamp.
SOCAT_IP_OPTIONS (output)
With all IPv4 based RECVFROM addresses where address option ip-recvopts is applied,
socat fills this variable with the IP options of the received packet.
SOCAT_IP_DSTADDR (output)
With all IPv4 based RECVFROM addresses where address option ip-recvdstaddr (BSD) or
ip-pktinfo (other platforms) is applied, socat sets this variable to the destina-
tion address of the received packet. This is particularly useful to identify broad-
cast and multicast addressed packets.
SOCAT_IP_IF (output)
With all IPv4 based RECVFROM addresses where address option ip-recvif (BSD) or
ip-pktinfo (other platforms) is applied, socat sets this variable to the name of
the interface where the packet was received.
SOCAT_IP_LOCADDR (output)
With all IPv4 based RECVFROM addresses where address option ip-pktinfo is applied,
socat sets this variable to the address of the interface where the packet was re-
ceived.
SOCAT_IP_TOS (output)
With all IPv4 based RECVFROM addresses where address option ip-recvtos is applied,
socat sets this variable to the TOS (type of service) of the received packet.
SOCAT_IP_TTL (output)
With all IPv4 based RECVFROM addresses where address option ip-recvttl is applied,
socat sets this variable to the TTL (time to live) of the received packet.
SOCAT_IPV6_HOPLIMIT (output)
With all IPv6 based RECVFROM addresses where address option ipv6-recvhoplimit is
applied, socat sets this variable to the hoplimit value of the received packet.
SOCAT_IPV6_DSTADDR (output)
With all IPv6 based RECVFROM addresses where address option ipv6-recvpktinfo is ap-
plied, socat sets this variable to the destination address of the received packet.
SOCAT_IPV6_TCLASS (output)
With all IPv6 based RECVFROM addresses where address option ipv6-recvtclass is ap-
plied, socat sets this variable to the transfer class of the received packet.
SOCAT_OPENSSL_X509_ISSUER (output)
Issuer field from peer certificate
SOCAT_OPENSSL_X509_SUBJECT (output)
Subject field from peer certificate
SOCAT_OPENSSL_X509_COMMONNAME (output)
commonName entries from peer certificates subject. Multiple values are separated by
" // ".
SOCAT_OPENSSL_X509_* (output)
all other entries from peer certificates subject
SOCAT_OPENSSL_X509V3_DNS (output)
DNS entries from peer certificates extensions - subjectAltName field. Multiple val-
ues are separated by " // ".
HOSTNAME (input)
Is used to determine the hostname for logging (see -lh).
LOGNAME (input)
Is used as name for the socks client user name if no socksuser is given.
With options su and su-d, LOGNAME is set to the given user name.
USER (input)
Is used as name for the socks client user name if no socksuser is given and LOGNAME
is empty.
With options su and su-d, USER is set to the given user name.
SHELL (output)
With options su and su-d, SHELL is set to the login shell of the given user.
PATH (output)
Can be set with option path for exec and system addresses.
HOME (output)
With options su and su-d, HOME is set to the home directory of the given user.
CREDITS
The work of the following groups and organizations was invaluable for this project:
The FSF (GNU, http://www.fsf.org/ project with their free and portable development soft-
ware and lots of other useful tools and libraries.
The Linux developers community (http://www.linux.org/) for providing a free, open source
operating system.
The Open Group (http://www.unix-systems.org/) for making their standard specifications
available on the Internet for free.
VERSION
This man page describes version 1.7.4 of socat.
BUGS
Addresses cannot be nested, so a single socat process cannot, e.g., drive ssl over socks.
Address option ftruncate without value uses default 1 instead of 0.
Verbose modes (-x and/or -v) display line termination characters inconsistently when ad-
dress options cr or crnl are used: They show the data after conversion in either direc-
tion.
The licenses of OpenSSL and GNU Readline are incompatible. Therefore readline support is
disabled in Debian.
Send bug reports to <socat AT dest-unreach.org>
SEE ALSO
nc(1), rinetd(8), openssl(1), stunnel(8), rlwrap(1), setsid(1)
Socat home page http://www.dest-unreach.org/socat/
AUTHOR
Gerhard Rieger <rieger AT dest-unreach.org> and contributors
socat(1)
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