On Apache/1.3.41 (Unix) PHP/5.2.5 mod_perl/1.30 mod_gzip/1.3.26.1a
Under GNU General Public License
2008-12-02 07:40 @38.103.63.58 CrawledBy CCBot/1.0 (+http://www.commoncrawl.org/bot.html)
HOSTS_ACCESS(5) HOSTS_ACCESS(5)
NAME
hosts_access - format of host access control files
DESCRIPTION
This manual page describes a simple access control language that is based on client
(host name/address, user name), and server (process name, host name/address) pat-
terns. Examples are given at the end. The impatient reader is encouraged to skip
to the EXAMPLES section for a quick introduction.
An extended version of the access control language is described in the
hosts_options(5) document. The extensions are turned on at program build time by
building with -DPROCESS_OPTIONS.
In the following text, daemon is the the process name of a network daemon process,
and client is the name and/or address of a host requesting service. Network daemon
process names are specified in the inetd configuration file.
ACCESS CONTROL FILES
The access control software consults two files. The search stops at the first
match:
· Access will be granted when a (daemon,client) pair matches an entry in the
/etc/hosts.allow file.
· Otherwise, access will be denied when a (daemon,client) pair matches an
entry in the /etc/hosts.deny file.
· Otherwise, access will be granted.
A non-existing access control file is treated as if it were an empty file. Thus,
access control can be turned off by providing no access control files.
ACCESS CONTROL RULES
Each access control file consists of zero or more lines of text. These lines are
processed in order of appearance. The search terminates when a match is found.
· A newline character is ignored when it is preceded by a backslash character.
This permits you to break up long lines so that they are easier to edit.
· Blank lines or lines that begin with a ‘#´ character are ignored. This per-
mits you to insert comments and whitespace so that the tables are easier to
read.
· All other lines should satisfy the following format, things between [] being
optional:
daemon_list : client_list [ : shell_command ]
daemon_list is a list of one or more daemon process names (argv[0] values) or wild-
cards (see below).
client_list is a list of one or more host names, host addresses, patterns or wild-
cards (see below) that will be matched against the client host name or address.
The more complex forms daemon@host and user@host are explained in the sections on
server endpoint patterns and on client username lookups, respectively.
List elements should be separated by blanks and/or commas.
With the exception of NIS (YP) netgroup lookups, all access control checks are case
insensitive.
PATTERNS
The access control language implements the following patterns:
· A string that begins with a ‘.´ character. A host name is matched if the
last components of its name match the specified pattern. For example, the
pattern ‘.tue.nl´ matches the host name ‘wzv.win.tue.nl´.
· A string that ends with a ‘.´ character. A host address is matched if its
first numeric fields match the given string. For example, the pattern
‘131.155.´ matches the address of (almost) every host on the Eindhoven Uni-
versity network (131.155.x.x).
· A string that begins with an ‘@´ character is treated as an NIS (formerly
YP) netgroup name. A host name is matched if it is a host member of the
specified netgroup. Netgroup matches are not supported for daemon process
names or for client user names.
· An expression of the form ‘n.n.n.n/m.m.m.m´ is interpreted as a ‘net/mask´
pair. An IPv4 host address is matched if ‘net´ is equal to the bitwise AND
of the address and the ‘mask´. For example, the net/mask pattern
‘131.155.72.0/255.255.254.0´ matches every address in the range
‘131.155.72.0´ through ‘131.155.73.255´.
· An expression of the form ‘[n:n:n:n:n:n:n:n]/m´ is interpreted as a
‘[net]/prefixlen´ pair. An IPv6 host address is matched if ‘prefixlen´ bits
of ‘net´ is equal to the ‘prefixlen´ bits of the address. For example, the
[net]/prefixlen pattern ‘[3ffe:505:2:1::]/64´ matches every address in the
range ‘3ffe:505:2:1::´ through ‘3ffe:505:2:1:ffff:ffff:ffff:ffff´.
· A string that begins with a ‘/´ character is treated as a file name. A host
name or address is matched if it matches any host name or address pattern
listed in the named file. The file format is zero or more lines with zero or
more host name or address patterns separated by whitespace. A file name
pattern can be used anywhere a host name or address pattern can be used.
· Wildcards ‘*´ and ‘?´ can be used to match hostnames or IP addresses. This
method of matching cannot be used in conjunction with ‘net/mask´ matching,
hostname matching beginning with ‘.´ or IP address matching ending with ‘.´.
WILDCARDS
The access control language supports explicit wildcards:
ALL The universal wildcard, always matches.
LOCAL Matches any host whose name does not contain a dot character.
UNKNOWN
Matches any user whose name is unknown, and matches any host whose name or
address are unknown. This pattern should be used with care: host names may
be unavailable due to temporary name server problems. A network address will
be unavailable when the software cannot figure out what type of network it
is talking to.
KNOWN Matches any user whose name is known, and matches any host whose name and
address are known. This pattern should be used with care: host names may be
unavailable due to temporary name server problems. A network address will
be unavailable when the software cannot figure out what type of network it
is talking to.
PARANOID
Matches any host whose name does not match its address. When tcpd is built
with -DPARANOID (default mode), it drops requests from such clients even
before looking at the access control tables. Build without -DPARANOID when
you want more control over such requests.
OPERATORS
EXCEPT Intended use is of the form: ‘list_1 EXCEPT list_2´; this construct matches
anything that matches list_1 unless it matches list_2. The EXCEPT operator
can be used in daemon_lists and in client_lists. The EXCEPT operator can be
nested: if the control language would permit the use of parentheses, ‘a
EXCEPT b EXCEPT c´ would parse as ‘(a EXCEPT (b EXCEPT c))´.
SHELL COMMANDS
If the first-matched access control rule contains a shell command, that command is
subjected to %<letter> substitutions (see next section). The result is executed by
a /bin/sh child process with standard input, output and error connected to
/dev/null. Specify an ‘&´ at the end of the command if you do not want to wait
until it has completed.
Shell commands should not rely on the PATH setting of the inetd. Instead, they
should use absolute path names, or they should begin with an explicit PATH=whatever
statement.
The hosts_options(5) document describes an alternative language that uses the shell
command field in a different and incompatible way.
% EXPANSIONS
The following expansions are available within shell commands:
%a (%A)
The client (server) host address.
%c Client information: user@host, user@address, a host name, or just an
address, depending on how much information is available.
%d The daemon process name (argv[0] value).
%h (%H)
The client (server) host name or address, if the host name is unavailable.
%n (%N)
The client (server) host name (or "unknown" or "paranoid").
%p The daemon process id.
%s Server information: daemon@host, daemon@address, or just a daemon name,
depending on how much information is available.
%u The client user name (or "unknown").
%% Expands to a single ‘%´ character.
Characters in % expansions that may confuse the shell are replaced by underscores.
SERVER ENDPOINT PATTERNS
In order to distinguish clients by the network address that they connect to, use
patterns of the form:
process_name@host_pattern : client_list ...
Patterns like these can be used when the machine has different internet addresses
with different internet hostnames. Service providers can use this facility to
offer FTP, GOPHER or WWW archives with internet names that may even belong to dif-
ferent organizations. See also the ‘twist’ option in the hosts_options(5) document.
Some systems (Solaris, FreeBSD) can have more than one internet address on one
physical interface; with other systems you may have to resort to SLIP or PPP pseudo
interfaces that live in a dedicated network address space.
The host_pattern obeys the same syntax rules as host names and addresses in
client_list context. Usually, server endpoint information is available only with
connection-oriented services.
CLIENT USERNAME LOOKUP
When the client host supports the RFC 931 protocol or one of its descendants (TAP,
IDENT, RFC 1413) the wrapper programs can retrieve additional information about the
owner of a connection. Client username information, when available, is logged
together with the client host name, and can be used to match patterns like:
daemon_list : ... user_pattern@host_pattern ...
The daemon wrappers can be configured at compile time to perform rule-driven user-
name lookups (default) or to always interrogate the client host. In the case of
rule-driven username lookups, the above rule would cause username lookup only when
both the daemon_list and the host_pattern match.
A user pattern has the same syntax as a daemon process pattern, so the same wild-
cards apply (netgroup membership is not supported). One should not get carried
away with username lookups, though.
· The client username information cannot be trusted when it is needed most,
i.e. when the client system has been compromised. In general, ALL and
(UN)KNOWN are the only user name patterns that make sense.
· Username lookups are possible only with TCP-based services, and only when
the client host runs a suitable daemon; in all other cases the result is
"unknown".
· A well-known UNIX kernel bug may cause loss of service when username lookups
are blocked by a firewall. The wrapper README document describes a procedure
to find out if your kernel has this bug.
· Username lookups may cause noticeable delays for non-UNIX users. The
default timeout for username lookups is 10 seconds: too short to cope with
slow networks, but long enough to irritate PC users.
Selective username lookups can alleviate the last problem. For example, a rule
like:
daemon_list : @pcnetgroup ALL@ALL
would match members of the pc netgroup without doing username lookups, but would
perform username lookups with all other systems.
DETECTING ADDRESS SPOOFING ATTACKS
A flaw in the sequence number generator of many TCP/IP implementations allows
intruders to easily impersonate trusted hosts and to break in via, for example, the
remote shell service. The IDENT (RFC931 etc.) service can be used to detect such
and other host address spoofing attacks.
Before accepting a client request, the wrappers can use the IDENT service to find
out that the client did not send the request at all. When the client host provides
IDENT service, a negative IDENT lookup result (the client matches ‘UNKNOWN@host’)
is strong evidence of a host spoofing attack.
A positive IDENT lookup result (the client matches ‘KNOWN@host’) is less trustwor-
thy. It is possible for an intruder to spoof both the client connection and the
IDENT lookup, although doing so is much harder than spoofing just a client connec-
tion. It may also be that the client´s IDENT server is lying.
Note: IDENT lookups don´t work with UDP services.
EXAMPLES
The language is flexible enough that different types of access control policy can
be expressed with a minimum of fuss. Although the language uses two access control
tables, the most common policies can be implemented with one of the tables being
trivial or even empty.
When reading the examples below it is important to realize that the allow table is
scanned before the deny table, that the search terminates when a match is found,
and that access is granted when no match is found at all.
The examples use host and domain names. They can be improved by including address
and/or network/netmask information, to reduce the impact of temporary name server
lookup failures.
MOSTLY CLOSED
In this case, access is denied by default. Only explicitly authorized hosts are
permitted access.
The default policy (no access) is implemented with a trivial deny file:
/etc/hosts.deny:
ALL: ALL
This denies all service to all hosts, unless they are permitted access by entries
in the allow file.
The explicitly authorized hosts are listed in the allow file. For example:
/etc/hosts.allow:
ALL: LOCAL @some_netgroup
ALL: .foobar.edu EXCEPT terminalserver.foobar.edu
The first rule permits access from hosts in the local domain (no ‘.´ in the host
name) and from members of the some_netgroup netgroup. The second rule permits
access from all hosts in the foobar.edu domain (notice the leading dot), with the
exception of terminalserver.foobar.edu.
MOSTLY OPEN
Here, access is granted by default; only explicitly specified hosts are refused
service.
The default policy (access granted) makes the allow file redundant so that it can
be omitted. The explicitly non-authorized hosts are listed in the deny file. For
example:
/etc/hosts.deny:
ALL: some.host.name, .some.domain
ALL EXCEPT in.fingerd: other.host.name, .other.domain
The first rule denies some hosts and domains all services; the second rule still
permits finger requests from other hosts and domains.
BOOBY TRAPS
The next example permits tftp requests from hosts in the local domain (notice the
leading dot). Requests from any other hosts are denied. Instead of the requested
file, a finger probe is sent to the offending host. The result is mailed to the
superuser.
/etc/hosts.allow:
in.tftpd: LOCAL, .my.domain
/etc/hosts.deny:
in.tftpd: ALL: spawn (/some/where/safe_finger -l @%h | \
/usr/ucb/mail -s %d-%h root) &
The safe_finger command comes with the tcpd wrapper and should be installed in a
suitable place. It limits possible damage from data sent by the remote finger
server. It gives better protection than the standard finger command.
The expansion of the %h (client host) and %d (service name) sequences is described
in the section on shell commands.
Warning: do not booby-trap your finger daemon, unless you are prepared for infinite
finger loops.
On network firewall systems this trick can be carried even further. The typical
network firewall only provides a limited set of services to the outer world. All
other services can be "bugged" just like the above tftp example. The result is an
excellent early-warning system.
DIAGNOSTICS
An error is reported when a syntax error is found in a host access control rule;
when the length of an access control rule exceeds the capacity of an internal
buffer; when an access control rule is not terminated by a newline character; when
the result of %<letter> expansion would overflow an internal buffer; when a system
call fails that shouldn´t. All problems are reported via the syslog daemon.
FILES
/etc/hosts.allow, (daemon,client) pairs that are granted access.
/etc/hosts.deny, (daemon,client) pairs that are denied access.
SEE ALSO
tcpd(8) tcp/ip daemon wrapper program.
tcpdchk(8), tcpdmatch(8), test programs.
BUGS
If a name server lookup times out, the host name will not be available to the
access control software, even though the host is registered.
Domain name server lookups are case insensitive; NIS (formerly YP) netgroup lookups
are case sensitive.
AUTHOR
Wietse Venema (wietse AT wzv.nl)
Department of Mathematics and Computing Science
Eindhoven University of Technology
Den Dolech 2, P.O. Box 513,
5600 MB Eindhoven, The Netherlands
HOSTS_ACCESS(5)
Generated by $Id: phpMan.php,v 4.55 2007/09/05 04:42:51 chedong Exp $ Author: Che Dong
On Apache/1.3.41 (Unix) PHP/5.2.5 mod_perl/1.30 mod_gzip/1.3.26.1a
Under GNU General Public License
2008-12-02 07:40 @38.103.63.58 CrawledBy CCBot/1.0 (+http://www.commoncrawl.org/bot.html)