# phpman > man > sshd_config(5) [SSHD_CONFIG(5)](https://www.chedong.com/phpMan.php/man/SSHDCONFIG/5/markdown) BSD File Formats Manual [SSHD_CONFIG(5)](https://www.chedong.com/phpMan.php/man/SSHDCONFIG/5/markdown) ## NAME **sshd**___**config** — OpenSSH daemon configuration file ## DESCRIPTION [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) reads configuration data from _/etc/ssh/sshd_config_ (or the file specified with **-f** on the command line). The file contains keyword-argument pairs, one per line. For each keyword, the first obtained value will be used. Lines starting with ‘#’ and empty lines are interpreted as comments. Arguments may optionally be enclosed in double quotes (") in order to represent arguments containing spaces. Note that the Debian **openssh-server** package sets several options as standard in _/etc/ssh/sshd_config_ which are not the default in [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown): •• **Include** **/etc/ssh/sshd**___**config.d/*.conf** •• **KbdInteractiveAuthentication** no •• **X11Forwarding** yes •• **PrintMotd** no •• **AcceptEnv** LANG LC_* •• **Subsystem** sftp /usr/lib/openssh/sftp-server •• **UsePAM** yes _/etc/ssh/sshd_config.d/*.conf_ files are included at the start of the configuration file, so op‐ tions set there will override those in _/etc/ssh/sshd_config._ The possible keywords and their meanings are as follows (note that keywords are case-insensi‐ tive and arguments are case-sensitive): ### AcceptEnv Specifies what environment variables sent by the client will be copied into the ses‐ sion's [environ(7)](https://www.chedong.com/phpMan.php/man/environ/7/markdown). See **SendEnv** and **SetEnv** in [ssh_config(5)](https://www.chedong.com/phpMan.php/man/sshconfig/5/markdown) for how to configure the client. The TERM environment variable is always accepted whenever the client requests a pseudo-terminal as it is required by the protocol. Variables are specified by name, which may contain the wildcard characters ‘*’ and ‘?’. Multiple environment variables may be separated by whitespace or spread across multiple **AcceptEnv** directives. Be warned that some environment variables could be used to bypass restricted user environ‐ ments. For this reason, care should be taken in the use of this directive. The de‐ fault is not to accept any environment variables. ### AddressFamily Specifies which address family should be used by [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown). Valid arguments are **any** (the default), **inet** (use IPv4 only), or **inet6** (use IPv6 only). ### AllowAgentForwarding Specifies whether [ssh-agent(1)](https://www.chedong.com/phpMan.php/man/ssh-agent/1/markdown) forwarding is permitted. The default is **yes**. Note that disabling agent forwarding does not improve security unless users are also denied shell access, as they can always install their own forwarders. ### AllowGroups This keyword can be followed by a list of group name patterns, separated by spaces. If specified, login is allowed only for users whose primary group or supplementary group list matches one of the patterns. Only group names are valid; a numerical group ID is not recognized. By default, login is allowed for all groups. The allow/deny groups directives are processed in the following order: **DenyGroups**, **AllowGroups**. See PATTERNS in [ssh_config(5)](https://www.chedong.com/phpMan.php/man/sshconfig/5/markdown) for more information on patterns. ### AllowStreamLocalForwarding Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted. The avail‐ able options are **yes** (the default) or **all** to allow StreamLocal forwarding, **no** to pre‐ vent all StreamLocal forwarding, **local** to allow local (from the perspective of [ssh(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown)) forwarding only or **remote** to allow remote forwarding only. Note that disabling Stream‐ Local forwarding does not improve security unless users are also denied shell access, as they can always install their own forwarders. ### AllowTcpForwarding Specifies whether TCP forwarding is permitted. The available options are **yes** (the de‐ fault) or **all** to allow TCP forwarding, **no** to prevent all TCP forwarding, **local** to allow local (from the perspective of [ssh(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown)) forwarding only or **remote** to allow remote for‐ warding only. Note that disabling TCP forwarding does not improve security unless users are also denied shell access, as they can always install their own forwarders. ### AllowUsers This keyword can be followed by a list of user name patterns, separated by spaces. If specified, login is allowed only for user names that match one of the patterns. Only user names are valid; a numerical user ID is not recognized. By default, login is al‐ lowed for all users. If the pattern takes the form USER@HOST then USER and HOST are separately checked, restricting logins to particular users from particular hosts. HOST criteria may additionally contain addresses to match in CIDR address/masklen format. The allow/deny users directives are processed in the following order: **DenyUsers**, **AllowUsers**. See PATTERNS in [ssh_config(5)](https://www.chedong.com/phpMan.php/man/sshconfig/5/markdown) for more information on patterns. ### AuthenticationMethods Specifies the authentication methods that must be successfully completed for a user to be granted access. This option must be followed by one or more lists of comma-sepa‐ rated authentication method names, or by the single string **any** to indicate the default behaviour of accepting any single authentication method. If the default is overridden, then successful authentication requires completion of every method in at least one of these lists. For example, "publickey,password publickey,keyboard-interactive" would require the user to complete public key authentication, followed by either password or keyboard interac‐ tive authentication. Only methods that are next in one or more lists are offered at each stage, so for this example it would not be possible to attempt password or key‐ board-interactive authentication before public key. For keyboard interactive authentication it is also possible to restrict authentication to a specific device by appending a colon followed by the device identifier **bsdauth** or **pam**. depending on the server configuration. For example, "keyboard-interactive:bsdauth" would restrict keyboard interactive authentication to the **bsdauth** device. If the publickey method is listed more than once, [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) verifies that keys that have been used successfully are not reused for subsequent authentications. For example, "publickey,publickey" requires successful authentication using two different public keys. Note that each authentication method listed should also be explicitly enabled in the configuration. The available authentication methods are: "gssapi-with-mic", "hostbased", "keyboard-interactive", "none" (used for access to password-less accounts when **PermitEmptyPasswords** is enabled), "password" and "publickey". ### AuthorizedKeysCommand Specifies a program to be used to look up the user's public keys. The program must be owned by root, not writable by group or others and specified by an absolute path. Ar‐ guments to **AuthorizedKeysCommand** accept the tokens described in the _TOKENS_ section. If no arguments are specified then the username of the target user is used. The program should produce on standard output zero or more lines of authorized_keys output (see _AUTHORIZED_KEYS_ in [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown)). **AuthorizedKeysCommand** is tried after the usual **AuthorizedKeysFile** files and will not be executed if a matching key is found there. By default, no **AuthorizedKeysCommand** is run. ### AuthorizedKeysCommandUser Specifies the user under whose account the **AuthorizedKeysCommand** is run. It is recom‐ mended to use a dedicated user that has no other role on the host than running autho‐ rized keys commands. If **AuthorizedKeysCommand** is specified but **AuthorizedKeysCommandUser** is not, then [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) will refuse to start. ### AuthorizedKeysFile Specifies the file that contains the public keys used for user authentication. The format is described in the AUTHORIZED_KEYS FILE FORMAT section of [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown). Arguments to **AuthorizedKeysFile** accept the tokens described in the _TOKENS_ section. After expan‐ sion, **AuthorizedKeysFile** is taken to be an absolute path or one relative to the user's home directory. Multiple files may be listed, separated by whitespace. Alternately this option may be set to **none** to skip checking for user keys in files. The default is ".ssh/authorized_keys .ssh/authorized_keys2". ### AuthorizedPrincipalsCommand Specifies a program to be used to generate the list of allowed certificate principals as per **AuthorizedPrincipalsFile**. The program must be owned by root, not writable by group or others and specified by an absolute path. Arguments to **AuthorizedPrincipalsCommand** accept the tokens described in the _TOKENS_ section. If no arguments are specified then the username of the target user is used. The program should produce on standard output zero or more lines of **AuthorizedPrincipalsFile** output. If either **AuthorizedPrincipalsCommand** or **AuthorizedPrincipalsFile** is specified, then certificates offered by the client for au‐ thentication must contain a principal that is listed. By default, no **AuthorizedPrincipalsCommand** is run. ### AuthorizedPrincipalsCommandUser Specifies the user under whose account the **AuthorizedPrincipalsCommand** is run. It is recommended to use a dedicated user that has no other role on the host than running au‐ thorized principals commands. If **AuthorizedPrincipalsCommand** is specified but **AuthorizedPrincipalsCommandUser** is not, then [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) will refuse to start. ### AuthorizedPrincipalsFile Specifies a file that lists principal names that are accepted for certificate authenti‐ cation. When using certificates signed by a key listed in **TrustedUserCAKeys**, this file lists names, one of which must appear in the certificate for it to be accepted for au‐ thentication. Names are listed one per line preceded by key options (as described in _AUTHORIZED_KEYS_ _FILE_ _FORMAT_ in [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown)). Empty lines and comments starting with ‘#’ are ignored. Arguments to **AuthorizedPrincipalsFile** accept the tokens described in the _TOKENS_ sec‐ tion. After expansion, **AuthorizedPrincipalsFile** is taken to be an absolute path or one relative to the user's home directory. The default is **none**, i.e. not to use a princi‐ pals file – in this case, the username of the user must appear in a certificate's prin‐ cipals list for it to be accepted. Note that **AuthorizedPrincipalsFile** is only used when authentication proceeds using a CA listed in **TrustedUserCAKeys** and is not consulted for certification authorities trusted via _~/.ssh/authorized_keys_, though the **principals=** key option offers a similar facility (see [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) for details). **Banner** The contents of the specified file are sent to the remote user before authentication is allowed. If the argument is **none** then no banner is displayed. By default, no banner is displayed. ### CASignatureAlgorithms Specifies which algorithms are allowed for signing of certificates by certificate au‐ thorities (CAs). The default is: ssh-ed25519,ecdsa-sha2-nistp256, ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, , , rsa-sha2-512,rsa-sha2-256 If the specified list begins with a ‘+’ character, then the specified algorithms will be appended to the default set instead of replacing them. If the specified list begins with a ‘-’ character, then the specified algorithms (including wildcards) will be re‐ moved from the default set instead of replacing them. Certificates signed using other algorithms will not be accepted for public key or host- based authentication. ### ChrootDirectory Specifies the pathname of a directory to [chroot(2)](https://www.chedong.com/phpMan.php/man/chroot/2/markdown) to after authentication. At session startup [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) checks that all components of the pathname are root-owned directories which are not writable by any other user or group. After the chroot, [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) changes the working directory to the user's home directory. Arguments to **ChrootDirectory** ac‐ cept the tokens described in the _TOKENS_ section. The **ChrootDirectory** must contain the necessary files and directories to support the user's session. For an interactive session this requires at least a shell, typically [sh(1)](https://www.chedong.com/phpMan.php/man/sh/1/markdown), and basic _/dev_ nodes such as [null(4)](https://www.chedong.com/phpMan.php/man/null/4/markdown), [zero(4)](https://www.chedong.com/phpMan.php/man/zero/4/markdown), [stdin(4)](https://www.chedong.com/phpMan.php/man/stdin/4/markdown), [stdout(4)](https://www.chedong.com/phpMan.php/man/stdout/4/markdown), [stderr(4)](https://www.chedong.com/phpMan.php/man/stderr/4/markdown), and [tty(4)](https://www.chedong.com/phpMan.php/man/tty/4/markdown) devices. For file transfer sessions using SFTP no additional configuration of the environment is necessary if the in-process sftp-server is used, though sessions which use logging may require _/dev/log_ inside the chroot directory on some operating systems (see [sftp-server(8)](https://www.chedong.com/phpMan.php/man/sftp-server/8/markdown) for details). For safety, it is very important that the directory hierarchy be prevented from modifi‐ cation by other processes on the system (especially those outside the jail). Miscon‐ figuration can lead to unsafe environments which [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) cannot detect. The default is **none**, indicating not to [chroot(2)](https://www.chedong.com/phpMan.php/man/chroot/2/markdown). ### Ciphers Specifies the ciphers allowed. Multiple ciphers must be comma-separated. If the spec‐ ified list begins with a ‘+’ character, then the specified ciphers will be appended to the default set instead of replacing them. If the specified list begins with a ‘-’ character, then the specified ciphers (including wildcards) will be removed from the default set instead of replacing them. If the specified list begins with a ‘^’ charac‐ ter, then the specified ciphers will be placed at the head of the default set. The supported ciphers are: 3des-cbc aes128-cbc aes192-cbc aes256-cbc aes128-ctr aes192-ctr aes256-ctr The default is: , aes128-ctr,aes192-ctr,aes256-ctr, , The list of available ciphers may also be obtained using "ssh -Q cipher". ### ClientAliveCountMax Sets the number of client alive messages which may be sent without [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) receiving any messages back from the client. If this threshold is reached while client alive messages are being sent, sshd will disconnect the client, terminating the session. It is important to note that the use of client alive messages is very different from **TCPKeepAlive**. The client alive messages are sent through the encrypted channel and therefore will not be spoofable. The TCP keepalive option enabled by **TCPKeepAlive** is spoofable. The client alive mechanism is valuable when the client or server depend on knowing when a connection has become unresponsive. The default value is 3. If **ClientAliveInterval** is set to 15, and **ClientAliveCountMax** is left at the default, unresponsive SSH clients will be disconnected after approxi‐ mately 45 seconds. Setting a zero **ClientAliveCountMax** disables connection termination. ### ClientAliveInterval Sets a timeout interval in seconds after which if no data has been received from the client, [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) will send a message through the encrypted channel to request a response from the client. The default is 0, indicating that these messages will not be sent to the client. ### Compression Specifies whether compression is enabled after the user has authenticated successfully. The argument must be **yes**, **delayed** (a legacy synonym for **yes**) or **no**. The default is **yes**. ### DebianBanner Specifies whether the distribution-specified extra version suffix is included during initial protocol handshake. The default is **yes**. ### DenyGroups This keyword can be followed by a list of group name patterns, separated by spaces. Login is disallowed for users whose primary group or supplementary group list matches one of the patterns. Only group names are valid; a numerical group ID is not recog‐ nized. By default, login is allowed for all groups. The allow/deny groups directives are processed in the following order: **DenyGroups**, **AllowGroups**. See PATTERNS in [ssh_config(5)](https://www.chedong.com/phpMan.php/man/sshconfig/5/markdown) for more information on patterns. ### DenyUsers This keyword can be followed by a list of user name patterns, separated by spaces. Lo‐ gin is disallowed for user names that match one of the patterns. Only user names are valid; a numerical user ID is not recognized. By default, login is allowed for all users. If the pattern takes the form USER@HOST then USER and HOST are separately checked, restricting logins to particular users from particular hosts. HOST criteria may additionally contain addresses to match in CIDR address/masklen format. The al‐ low/deny users directives are processed in the following order: **DenyUsers**, **AllowUsers**. See PATTERNS in [ssh_config(5)](https://www.chedong.com/phpMan.php/man/sshconfig/5/markdown) for more information on patterns. ### DisableForwarding Disables all forwarding features, including X11, [ssh-agent(1)](https://www.chedong.com/phpMan.php/man/ssh-agent/1/markdown), TCP and StreamLocal. This option overrides all other forwarding-related options and may simplify restricted configurations. ### ExposeAuthInfo Writes a temporary file containing a list of authentication methods and public creden‐ tials (e.g. keys) used to authenticate the user. The location of the file is exposed to the user session through the SSH_USER_AUTH environment variable. The default is **no**. ### FingerprintHash Specifies the hash algorithm used when logging key fingerprints. Valid options are: **md5** and **sha256**. The default is **sha256**. ### ForceCommand Forces the execution of the command specified by **ForceCommand**, ignoring any command supplied by the client and _~/.ssh/rc_ if present. The command is invoked by using the user's login shell with the -c option. This applies to shell, command, or subsystem execution. It is most useful inside a **Match** block. The command originally supplied by the client is available in the SSH_ORIGINAL_COMMAND environment variable. Specifying a command of **internal-sftp** will force the use of an in-process SFTP server that requires no support files when used with **ChrootDirectory**. The default is **none**. ### GatewayPorts Specifies whether remote hosts are allowed to connect to ports forwarded for the client. By default, [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) binds remote port forwardings to the loopback address. This prevents other remote hosts from connecting to forwarded ports. **GatewayPorts** can be used to specify that sshd should allow remote port forwardings to bind to non-loop‐ back addresses, thus allowing other hosts to connect. The argument may be **no** to force remote port forwardings to be available to the local host only, **yes** to force remote port forwardings to bind to the wildcard address, or **clientspecified** to allow the client to select the address to which the forwarding is bound. The default is **no**. ### GSSAPIAuthentication Specifies whether user authentication based on GSSAPI is allowed. The default is **no**. ### GSSAPICleanupCredentials Specifies whether to automatically destroy the user's credentials cache on logout. The default is **yes**. ### GSSAPIKeyExchange Specifies whether key exchange based on GSSAPI is allowed. GSSAPI key exchange doesn't rely on ssh keys to verify host identity. The default is **no**. ### GSSAPIStrictAcceptorCheck Determines whether to be strict about the identity of the GSSAPI acceptor a client au‐ thenticates against. If set to **yes** then the client must authenticate against the host service on the current hostname. If set to **no** then the client may authenticate against any service key stored in the machine's default store. This facility is provided to assist with operation on multi homed machines. The default is **yes**. ### GSSAPIStoreCredentialsOnRekey Controls whether the user's GSSAPI credentials should be updated following a successful connection rekeying. This option can be used to accepted renewed or updated credentials from a compatible client. The default is “no”. For this to work **GSSAPIKeyExchange** needs to be enabled in the server and also used by the client. ### GSSAPIKexAlgorithms The list of key exchange algorithms that are accepted by GSSAPI key exchange. Possible values are gss-gex-sha1-, gss-group1-sha1-, gss-group14-sha1-, gss-group14-sha256-, gss-group16-sha512-, gss-nistp256-sha256-, gss-curve25519-sha256- The default is “gss-group14-sha256-,gss-group16-sha512-,gss-nistp256-sha256-,gss-curve25519-sha256-,gss-gex-sha1-,gss-group14-sha1-”. This option only applies to connections using GSSAPI. ### HostbasedAcceptedAlgorithms Specifies the signature algorithms that will be accepted for hostbased authentication as a list of comma-separated patterns. Alternately if the specified list begins with a ‘+’ character, then the specified signature algorithms will be appended to the default set instead of replacing them. If the specified list begins with a ‘-’ character, then the specified signature algorithms (including wildcards) will be removed from the de‐ fault set instead of replacing them. If the specified list begins with a ‘^’ charac‐ ter, then the specified signature algorithms will be placed at the head of the default set. The default for this option is: , , , , , , , , ssh-ed25519, ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, , , rsa-sha2-512,rsa-sha2-256 The list of available signature algorithms may also be obtained using "ssh -Q HostbasedAcceptedAlgorithms". This was formerly named HostbasedAcceptedKeyTypes. ### HostbasedAuthentication Specifies whether rhosts or /etc/hosts.equiv authentication together with successful public key client host authentication is allowed (host-based authentication). The de‐ fault is **no**. ### HostbasedUsesNameFromPacketOnly Specifies whether or not the server will attempt to perform a reverse name lookup when matching the name in the _~/.shosts_, _~/.rhosts_, and _/etc/hosts.equiv_ files during **HostbasedAuthentication**. A setting of **yes** means that [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) uses the name supplied by the client rather than attempting to resolve the name from the TCP connection itself. The default is **no**. ### HostCertificate Specifies a file containing a public host certificate. The certificate's public key must match a private host key already specified by **HostKey**. The default behaviour of [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) is not to load any certificates. ### HostKey Specifies a file containing a private host key used by SSH. The defaults are _/etc/ssh/ssh_host_ecdsa_key_, _/etc/ssh/ssh_host_ed25519_key_ and _/etc/ssh/ssh_host_rsa_key_. Note that [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) will refuse to use a file if it is group/world-accessible and that the **HostKeyAlgorithms** option restricts which of the keys are actually used by [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown). It is possible to have multiple host key files. It is also possible to specify public host key files instead. In this case operations on the private key will be delegated to an [ssh-agent(1)](https://www.chedong.com/phpMan.php/man/ssh-agent/1/markdown). ### HostKeyAgent Identifies the UNIX-domain socket used to communicate with an agent that has access to the private host keys. If the string "SSH_AUTH_SOCK" is specified, the location of the socket will be read from the SSH_AUTH_SOCK environment variable. ### HostKeyAlgorithms Specifies the host key signature algorithms that the server offers. The default for this option is: , , , , , , , , ssh-ed25519, ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, , , rsa-sha2-512,rsa-sha2-256 The list of available signature algorithms may also be obtained using "ssh -Q HostKeyAlgorithms". ### IgnoreRhosts Specifies whether to ignore per-user _.rhosts_ and _.shosts_ files during **HostbasedAuthentication**. The system-wide _/etc/hosts.equiv_ and _/etc/ssh/shosts.equiv_ are still used regardless of this setting. Accepted values are **yes** (the default) to ignore all per-user files, **shosts-only** to al‐ low the use of _.shosts_ but to ignore _.rhosts_ or **no** to allow both _.shosts_ and _rhosts_. ### IgnoreUserKnownHosts Specifies whether [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) should ignore the user's _~/.ssh/known_hosts_ during **HostbasedAuthentication** and use only the system-wide known hosts file _/etc/ssh/known_hosts_. The default is “no”. ### Include Include the specified configuration file(s). Multiple pathnames may be specified and each pathname may contain [glob(7)](https://www.chedong.com/phpMan.php/man/glob/7/markdown) wildcards that will be expanded and processed in lex‐ ical order. Files without absolute paths are assumed to be in _/etc/ssh_. An **Include** directive may appear inside a **Match** block to perform conditional inclusion. **IPQoS** Specifies the IPv4 type-of-service or DSCP class for the connection. Accepted values are **af11**, **af12**, **af13**, **af21**, **af22**, **af23**, **af31**, **af32**, **af33**, **af41**, **af42**, **af43**, **cs0**, **cs1**, **cs2**, **cs3**, **cs4**, **cs5**, **cs6**, **cs7**, **ef**, **le**, **lowdelay**, **throughput**, **reliability**, a numeric value, or **none** to use the operating system default. This option may take one or two arguments, separated by whitespace. If one argument is specified, it is used as the packet class unconditionally. If two values are specified, the first is automatically selected for interactive sessions and the second for non-interactive sessions. The de‐ fault is **lowdelay** for interactive sessions and **throughput** for non-interactive sessions. ### KbdInteractiveAuthentication Specifies whether to allow keyboard-interactive authentication. The default is **yes**. The argument to this keyword must be **yes** or **no**. **ChallengeResponseAuthentication** is a deprecated alias for this. ### KerberosAuthentication Specifies whether the password provided by the user for **PasswordAuthentication** will be validated through the Kerberos KDC. To use this option, the server needs a Kerberos servtab which allows the verification of the KDC's identity. The default is **no**. ### KerberosGetAFSToken If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire an AFS token be‐ fore accessing the user's home directory. The default is **no**. ### KerberosOrLocalPasswd If password authentication through Kerberos fails then the password will be validated via any additional local mechanism such as _/etc/passwd_. The default is **yes**. ### KerberosTicketCleanup Specifies whether to automatically destroy the user's ticket cache file on logout. The default is **yes**. ### KexAlgorithms Specifies the available KEX (Key Exchange) algorithms. Multiple algorithms must be comma-separated. Alternately if the specified list begins with a ‘+’ character, then the specified algorithms will be appended to the default set instead of replacing them. If the specified list begins with a ‘-’ character, then the specified algorithms (in‐ cluding wildcards) will be removed from the default set instead of replacing them. If the specified list begins with a ‘^’ character, then the specified algorithms will be placed at the head of the default set. The supported algorithms are: curve25519-sha256 diffie-hellman-group1-sha1 diffie-hellman-group14-sha1 diffie-hellman-group14-sha256 diffie-hellman-group16-sha512 diffie-hellman-group18-sha512 diffie-hellman-group-exchange-sha1 diffie-hellman-group-exchange-sha256 ecdh-sha2-nistp256 ecdh-sha2-nistp384 ecdh-sha2-nistp521 The default is: curve25519-sha256,, ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, , diffie-hellman-group-exchange-sha256, diffie-hellman-group16-sha512,diffie-hellman-group18-sha512, diffie-hellman-group14-sha256 The list of available key exchange algorithms may also be obtained using "ssh -Q KexAlgorithms". ### ListenAddress Specifies the local addresses [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) should listen on. The following forms may be used: **ListenAddress** _hostname_|_address_ **ListenAddress** _hostname_:_port_ **ListenAddress** _IPv4_address_:_port_ **ListenAddress** [_hostname_|_address_]:_port_ If _port_ is not specified, sshd will listen on the address and all **Port** options speci‐ fied. The default is to listen on all local addresses. Multiple **ListenAddress** options are permitted. ### LoginGraceTime The server disconnects after this time if the user has not successfully logged in. If the value is 0, there is no time limit. The default is 120 seconds. ### LogLevel Gives the verbosity level that is used when logging messages from [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown). The possi‐ ble values are: QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify higher levels of debugging output. Logging with a DEBUG level violates the privacy of users and is not recommended. ### LogVerbose Specify one or more overrides to LogLevel. An override consists of a pattern lists that matches the source file, function and line number to force detailed logging for. For example, an override pattern of: kex.c:*:1000,*:kex_exchange_identification():*,packet.c:* would enable detailed logging for line 1000 of _kex.c_, everything in the **kex**___**exchange**___**identification**() function, and all code in the _packet.c_ file. This option is intended for debugging and no overrides are enabled by default. **MACs** Specifies the available MAC (message authentication code) algorithms. The MAC algo‐ rithm is used for data integrity protection. Multiple algorithms must be comma-sepa‐ rated. If the specified list begins with a ‘+’ character, then the specified algo‐ rithms will be appended to the default set instead of replacing them. If the specified list begins with a ‘-’ character, then the specified algorithms (including wildcards) will be removed from the default set instead of replacing them. If the specified list begins with a ‘^’ character, then the specified algorithms will be placed at the head of the default set. The algorithms that contain "-etm" calculate the MAC after encryption (encrypt-then- mac). These are considered safer and their use recommended. The supported MACs are: hmac-md5 hmac-md5-96 hmac-sha1 hmac-sha1-96 hmac-sha2-256 hmac-sha2-512 The default is: ,, ,, , ,, hmac-sha2-256,hmac-sha2-512,hmac-sha1 The list of available MAC algorithms may also be obtained using "ssh -Q mac". **Match** Introduces a conditional block. If all of the criteria on the **Match** line are satis‐ fied, the keywords on the following lines override those set in the global section of the config file, until either another **Match** line or the end of the file. If a keyword appears in multiple **Match** blocks that are satisfied, only the first instance of the keyword is applied. The arguments to **Match** are one or more criteria-pattern pairs or the single token **All** which matches all criteria. The available criteria are **User**, **Group**, **Host**, **LocalAddress**, **LocalPort**, and **Address**. The match patterns may consist of single entries or comma-separated lists and may use the wildcard and negation operators described in the _PATTERNS_ section of [ssh_config(5)](https://www.chedong.com/phpMan.php/man/sshconfig/5/markdown). The patterns in an **Address** criteria may additionally contain addresses to match in CIDR address/masklen format, such as 192.0.2.0/24 or 2001:db8::/32. Note that the mask length provided must be consistent with the address - it is an error to specify a mask length that is too long for the address or one with bits set in this host portion of the address. For example, 192.0.2.0/33 and 192.0.2.0/8, respectively. Only a subset of keywords may be used on the lines following a **Match** keyword. Avail‐ able keywords are **AcceptEnv**, **AllowAgentForwarding**, **AllowGroups**, **AllowStreamLocalForwarding**, **AllowTcpForwarding**, **AllowUsers**, **AuthenticationMethods**, **AuthorizedKeysCommand**, **AuthorizedKeysCommandUser**, **AuthorizedKeysFile**, **AuthorizedPrincipalsCommand**, **AuthorizedPrincipalsCommandUser**, **AuthorizedPrincipalsFile**, **Banner**, **CASignatureAlgorithms**, **ChrootDirectory**, **ClientAliveCountMax**, **ClientAliveInterval**, **DenyGroups**, **DenyUsers**, **DisableForwarding**, **ExposeAuthInfo**, **ForceCommand**, **GatewayPorts**, **GSSAPIAuthentication**, **HostbasedAcceptedAlgorithms**, **HostbasedAuthentication**, **HostbasedUsesNameFromPacketOnly**, **IgnoreRhosts**, **Include**, **IPQoS**, **KbdInteractiveAuthentication**, **KerberosAuthentication**, **LogLevel**, **MaxAuthTries**, **MaxSessions**, **PasswordAuthentication**, **PermitEmptyPasswords**, **PermitListen**, **PermitOpen**, **PermitRootLogin**, **PermitTTY**, **PermitTunnel**, **PermitUserRC**, **PubkeyAcceptedAlgorithms**, **PubkeyAuthentication**, **PubkeyAuthOptions**, **RekeyLimit**, **RevokedKeys**, **SetEnv**, **StreamLocalBindMask**, **StreamLocalBindUnlink**, **TrustedUserCAKeys**, **X11DisplayOffset**, **X11Forwarding** and **X11UseLocalhost**. ### MaxAuthTries Specifies the maximum number of authentication attempts permitted per connection. Once the number of failures reaches half this value, additional failures are logged. The default is 6. ### MaxSessions Specifies the maximum number of open shell, login or subsystem (e.g. sftp) sessions permitted per network connection. Multiple sessions may be established by clients that support connection multiplexing. Setting **MaxSessions** to 1 will effectively disable session multiplexing, whereas setting it to 0 will prevent all shell, login and subsys‐ tem sessions while still permitting forwarding. The default is 10. ### MaxStartups Specifies the maximum number of concurrent unauthenticated connections to the SSH dae‐ mon. Additional connections will be dropped until authentication succeeds or the **LoginGraceTime** expires for a connection. The default is 10:30:100. Alternatively, random early drop can be enabled by specifying the three colon separated values start:rate:full (e.g. "10:30:60"). [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) will refuse connection attempts with a probability of rate/100 (30%) if there are currently start (10) unauthenticated con‐ nections. The probability increases linearly and all connection attempts are refused if the number of unauthenticated connections reaches full (60). ### ModuliFile Specifies the [moduli(5)](https://www.chedong.com/phpMan.php/man/moduli/5/markdown) file that contains the Diffie-Hellman groups used for the “diffie-hellman-group-exchange-sha1” and “diffie-hellman-group-exchange-sha256” key ex‐ change methods. The default is _/etc/ssh/moduli_. ### PasswordAuthentication Specifies whether password authentication is allowed. The default is **yes**. ### PermitEmptyPasswords When password authentication is allowed, it specifies whether the server allows login to accounts with empty password strings. The default is **no**. ### PermitListen Specifies the addresses/ports on which a remote TCP port forwarding may listen. The listen specification must be one of the following forms: **PermitListen** _port_ **PermitListen** _host_:_port_ Multiple permissions may be specified by separating them with whitespace. An argument of **any** can be used to remove all restrictions and permit any listen requests. An argu‐ ment of **none** can be used to prohibit all listen requests. The host name may contain wildcards as described in the PATTERNS section in [ssh_config(5)](https://www.chedong.com/phpMan.php/man/sshconfig/5/markdown). The wildcard ‘*’ can also be used in place of a port number to allow all ports. By default all port for‐ warding listen requests are permitted. Note that the **GatewayPorts** option may further restrict which addresses may be listened on. Note also that [ssh(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) will request a lis‐ ten host of “localhost” if no listen host was specifically requested, and this name is treated differently to explicit localhost addresses of “127.0.0.1” and “::1”. ### PermitOpen Specifies the destinations to which TCP port forwarding is permitted. The forwarding specification must be one of the following forms: **PermitOpen** _host_:_port_ **PermitOpen** _IPv4_addr_:_port_ **PermitOpen** _[IPv6_addr]_:_port_ Multiple forwards may be specified by separating them with whitespace. An argument of **any** can be used to remove all restrictions and permit any forwarding requests. An ar‐ gument of **none** can be used to prohibit all forwarding requests. The wildcard ‘*’ can be used for host or port to allow all hosts or ports respectively. Otherwise, no pat‐ tern matching or address lookups are performed on supplied names. By default all port forwarding requests are permitted. ### PermitRootLogin Specifies whether root can log in using [ssh(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown). The argument must be **yes**, **prohibit-password**, **forced-commands-only**, or **no**. The default is **prohibit-password**. If this option is set to **prohibit-password** (or its deprecated alias, **without-password**), password and keyboard-interactive authentication are disabled for root. If this option is set to **forced-commands-only**, root login with public key authentica‐ tion will be allowed, but only if the _command_ option has been specified (which may be useful for taking remote backups even if root login is normally not allowed). All other authentication methods are disabled for root. If this option is set to **no**, root is not allowed to log in. ### PermitTTY Specifies whether [pty(4)](https://www.chedong.com/phpMan.php/man/pty/4/markdown) allocation is permitted. The default is **yes**. ### PermitTunnel Specifies whether [tun(4)](https://www.chedong.com/phpMan.php/man/tun/4/markdown) device forwarding is allowed. The argument must be **yes**, **point-to-point** (layer 3), **ethernet** (layer 2), or **no**. Specifying **yes** permits both **point-to-point** and **ethernet**. The default is **no**. Independent of this setting, the permissions of the selected [tun(4)](https://www.chedong.com/phpMan.php/man/tun/4/markdown) device must allow access to the user. ### PermitUserEnvironment Specifies whether _~/.ssh/environment_ and **environment=** options in _~/.ssh/authorized_keys_ are processed by [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown). Valid options are **yes**, **no** or a pattern-list specifying which environment variable names to accept (for example "LANG,LC_*"). The default is **no**. Enabling environment processing may enable users to bypass access restrictions in some configurations using mechanisms such as LD_PRELOAD. ### PermitUserRC Specifies whether any _~/.ssh/rc_ file is executed. The default is **yes**. ### PerSourceMaxStartups Specifies the number of unauthenticated connections allowed from a given source ad‐ dress, or “none” if there is no limit. This limit is applied in addition to **MaxStartups**, whichever is lower. The default is **none**. ### PerSourceNetBlockSize Specifies the number of bits of source address that are grouped together for the pur‐ poses of applying PerSourceMaxStartups limits. Values for IPv4 and optionally IPv6 may be specified, separated by a colon. The default is **32:128**, which means each address is considered individually. ### PidFile Specifies the file that contains the process ID of the SSH daemon, or **none** to not write one. The default is _/run/sshd.pid_. **Port** Specifies the port number that [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) listens on. The default is 22. Multiple op‐ tions of this type are permitted. See also **ListenAddress**. ### PrintLastLog Specifies whether [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) should print the date and time of the last user login when a user logs in interactively. The default is **yes**. ### PrintMotd Specifies whether [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) should print _/etc/motd_ when a user logs in interactively. (On some systems it is also printed by the shell, _/etc/profile_, or equivalent.) The default is **yes**. ### PubkeyAcceptedAlgorithms Specifies the signature algorithms that will be accepted for public key authentication as a list of comma-separated patterns. Alternately if the specified list begins with a ‘+’ character, then the specified algorithms will be appended to the default set in‐ stead of replacing them. If the specified list begins with a ‘-’ character, then the specified algorithms (including wildcards) will be removed from the default set instead of replacing them. If the specified list begins with a ‘^’ character, then the speci‐ fied algorithms will be placed at the head of the default set. The default for this option is: , , , , , , , , ssh-ed25519, ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, , , rsa-sha2-512,rsa-sha2-256 The list of available signature algorithms may also be obtained using "ssh -Q PubkeyAcceptedAlgorithms". ### PubkeyAuthOptions Sets one or more public key authentication options. The supported keywords are: **none** (the default; indicating no additional options are enabled), **touch-required** and **verify-required**. The **touch-required** option causes public key authentication using a FIDO authenticator algorithm (i.e. **ecdsa-sk** or **ed25519-sk**) to always require the signature to attest that a physically present user explicitly confirmed the authentication (usually by touching the authenticator). By default, [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) requires user presence unless overridden with an authorized_keys option. The **touch-required** flag disables this override. The **verify-required** option requires a FIDO key signature attest that the user was veri‐ fied, e.g. via a PIN. Neither the **touch-required** or **verify-required** options have any effect for other, non- FIDO, public key types. ### PubkeyAuthentication Specifies whether public key authentication is allowed. The default is **yes**. ### RekeyLimit Specifies the maximum amount of data that may be transmitted before the session key is renegotiated, optionally followed by a maximum amount of time that may pass before the session key is renegotiated. The first argument is specified in bytes and may have a suffix of ‘K’, ‘M’, or ‘G’ to indicate Kilobytes, Megabytes, or Gigabytes, respec‐ tively. The default is between ‘1G’ and ‘4G’, depending on the cipher. The optional second value is specified in seconds and may use any of the units documented in the _TIME_ _FORMATS_ section. The default value for **RekeyLimit** is **default** **none**, which means that rekeying is performed after the cipher's default amount of data has been sent or received and no time based rekeying is done. ### RevokedKeys Specifies revoked public keys file, or **none** to not use one. Keys listed in this file will be refused for public key authentication. Note that if this file is not readable, then public key authentication will be refused for all users. Keys may be specified as a text file, listing one public key per line, or as an OpenSSH Key Revocation List (KRL) as generated by [ssh-keygen(1)](https://www.chedong.com/phpMan.php/man/ssh-keygen/1/markdown). For more information on KRLs, see the KEY REVOCA‐ TION LISTS section in [ssh-keygen(1)](https://www.chedong.com/phpMan.php/man/ssh-keygen/1/markdown). ### SecurityKeyProvider Specifies a path to a library that will be used when loading FIDO authenticator-hosted keys, overriding the default of using the built-in USB HID support. **SetEnv** Specifies one or more environment variables to set in child sessions started by [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) as “NAME=VALUE”. The environment value may be quoted (e.g. if it contains whitespace characters). Environment variables set by **SetEnv** override the default environment and any variables specified by the user via **AcceptEnv** or **PermitUserEnvironment**. ### StreamLocalBindMask Sets the octal file creation mode mask (umask) used when creating a Unix-domain socket file for local or remote port forwarding. This option is only used for port forwarding to a Unix-domain socket file. The default value is 0177, which creates a Unix-domain socket file that is readable and writable only by the owner. Note that not all operating systems honor the file mode on Unix-domain socket files. ### StreamLocalBindUnlink Specifies whether to remove an existing Unix-domain socket file for local or remote port forwarding before creating a new one. If the socket file already exists and **StreamLocalBindUnlink** is not enabled, **sshd** will be unable to forward the port to the Unix-domain socket file. This option is only used for port forwarding to a Unix-domain socket file. The argument must be **yes** or **no**. The default is **no**. ### StrictModes Specifies whether [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) should check file modes and ownership of the user's files and home directory before accepting login. This is normally desirable because novices sometimes accidentally leave their directory or files world-writable. The default is **yes**. Note that this does not apply to **ChrootDirectory**, whose permissions and ownership are checked unconditionally. ### Subsystem Configures an external subsystem (e.g. file transfer daemon). Arguments should be a subsystem name and a command (with optional arguments) to execute upon subsystem re‐ quest. The command **sftp-server** implements the SFTP file transfer subsystem. Alternately the name **internal-sftp** implements an in-process SFTP server. This may sim‐ plify configurations using **ChrootDirectory** to force a different filesystem root on clients. By default no subsystems are defined. ### SyslogFacility Gives the facility code that is used when logging messages from [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown). The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The default is AUTH. ### TCPKeepAlive Specifies whether the system should send TCP keepalive messages to the other side. If they are sent, death of the connection or crash of one of the machines will be properly noticed. However, this means that connections will die if the route is down temporar‐ ily, and some people find it annoying. On the other hand, if TCP keepalives are not sent, sessions may hang indefinitely on the server, leaving "ghost" users and consuming server resources. The default is **yes** (to send TCP keepalive messages), and the server will notice if the network goes down or the client host crashes. This avoids infinitely hanging sessions. To disable TCP keepalive messages, the value should be set to **no**. This option was formerly called **KeepAlive**. ### TrustedUserCAKeys Specifies a file containing public keys of certificate authorities that are trusted to sign user certificates for authentication, or **none** to not use one. Keys are listed one per line; empty lines and comments starting with ‘#’ are allowed. If a certificate is presented for authentication and has its signing CA key listed in this file, then it may be used for authentication for any user listed in the certificate's principals list. Note that certificates that lack a list of principals will not be permitted for authentication using **TrustedUserCAKeys**. For more details on certificates, see the CER‐ TIFICATES section in [ssh-keygen(1)](https://www.chedong.com/phpMan.php/man/ssh-keygen/1/markdown). **UseDNS** Specifies whether [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) should look up the remote host name, and to check that the resolved host name for the remote IP address maps back to the very same IP address. If this option is set to **no** (the default) then only addresses and not host names may be used in _~/.ssh/authorized_keys_ **from** and **sshd**___**config** **Match** **Host** directives. **UsePAM** Enables the Pluggable Authentication Module interface. If set to **yes** this will enable PAM authentication using **KbdInteractiveAuthentication** and **PasswordAuthentication** in ad‐ dition to PAM account and session module processing for all authentication types. Because PAM keyboard-interactive authentication usually serves an equivalent role to password authentication, you should disable either **PasswordAuthentication** or **KbdInteractiveAuthentication**. If **UsePAM** is enabled, you will not be able to run [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) as a non-root user. The de‐ fault is **no**. ### VersionAddendum Optionally specifies additional text to append to the SSH protocol banner sent by the server upon connection. The default is **none**. ### X11DisplayOffset Specifies the first display number available for [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown)'s X11 forwarding. This pre‐ vents sshd from interfering with real X11 servers. The default is 10. ### X11Forwarding Specifies whether X11 forwarding is permitted. The argument must be **yes** or **no**. The default is **no**. When X11 forwarding is enabled, there may be additional exposure to the server and to client displays if the [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) proxy display is configured to listen on the wildcard address (see **X11UseLocalhost**), though this is not the default. Additionally, the au‐ thentication spoofing and authentication data verification and substitution occur on the client side. The security risk of using X11 forwarding is that the client's X11 display server may be exposed to attack when the SSH client requests forwarding (see the warnings for **ForwardX11** in [ssh_config(5)](https://www.chedong.com/phpMan.php/man/sshconfig/5/markdown)). A system administrator may have a stance in which they want to protect clients that may expose themselves to attack by unwittingly requesting X11 forwarding, which can warrant a **no** setting. Note that disabling X11 forwarding does not prevent users from forwarding X11 traffic, as users can always install their own forwarders. ### X11UseLocalhost Specifies whether [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) should bind the X11 forwarding server to the loopback address or to the wildcard address. By default, sshd binds the forwarding server to the loop‐ back address and sets the hostname part of the DISPLAY environment variable to **localhost**. This prevents remote hosts from connecting to the proxy display. However, some older X11 clients may not function with this configuration. **X11UseLocalhost** may be set to **no** to specify that the forwarding server should be bound to the wildcard ad‐ dress. The argument must be **yes** or **no**. The default is **yes**. ### XAuthLocation Specifies the full pathname of the [xauth(1)](https://www.chedong.com/phpMan.php/man/xauth/1/markdown) program, or **none** to not use one. The de‐ fault is _/usr/bin/xauth_. ## TIME FORMATS [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) command-line arguments and configuration file options that specify time may be ex‐ pressed using a sequence of the form: _time_[_qualifier_], where _time_ is a positive integer value and _qualifier_ is one of the following: ⟨**none**⟩ seconds **s** | **S** seconds **m** | **M** minutes **h** | **H** hours **d** | **D** days **w** | **W** weeks Each member of the sequence is added together to calculate the total time value. Time format examples: 600 600 seconds (10 minutes) 10m 10 minutes 1h30m 1 hour 30 minutes (90 minutes) ## TOKENS Arguments to some keywords can make use of tokens, which are expanded at runtime: %% A literal ‘%’. %F The fingerprint of the CA key. %f The fingerprint of the key or certificate. %h The home directory of the user. %i The key ID in the certificate. %K The base64-encoded CA key. %k The base64-encoded key or certificate for authentication. %s The serial number of the certificate. %T The type of the CA key. %t The key or certificate type. %U The numeric user ID of the target user. %u The username. **AuthorizedKeysCommand** accepts the tokens %%, %f, %h, %k, %t, %U, and %u. **AuthorizedKeysFile** accepts the tokens %%, %h, %U, and %u. **AuthorizedPrincipalsCommand** accepts the tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u. **AuthorizedPrincipalsFile** accepts the tokens %%, %h, %U, and %u. **ChrootDirectory** accepts the tokens %%, %h, %U, and %u. ## FILES /etc/ssh/sshd_config Contains configuration data for [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown). This file should be writable by root only, but it is recommended (though not necessary) that it be world-readable. ## SEE ALSO [sftp-server(8)](https://www.chedong.com/phpMan.php/man/sftp-server/8/markdown), [sshd(8)](https://www.chedong.com/phpMan.php/man/sshd/8/markdown) ## AUTHORS OpenSSH is a derivative of the original and free ssh 1.2.12 release by Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo de Raadt and Dug Song removed many bugs, re-added newer features and created OpenSSH. Markus Friedl contributed the support for SSH protocol versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support for privi‐ lege separation. BSD December 4, 2021 BSD