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sshd_config — OpenSSH SSH daemon configuration file
sshd(8) 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. 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). The exact list depends on whether the package was installed fresh or upgraded from various possible previous versions, but includes at least the following: · Protocol 2 · ChallengeResponseAuthentication no · X11Forwarding yes · PrintMotd no · AcceptEnv LANG LC_* · Subsystem sftp /usr/lib/openssh/sftp-server · UsePAM yes The possible keywords and their meanings are as follows (note that keywords are case- insensitive and arguments are case-sensitive): AcceptEnv Specifies what environment variables sent by the client will be copied into the session's environ(7). See SendEnv in ssh_config(5) for how to configure the client. Note that environment passing is only supported for protocol 2. 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 environments. For this reason, care should be taken in the use of this directive. The default is not to accept any environment variables. AddressFamily Specifies which address family should be used by sshd(8). Valid arguments are “any”, “inet” (use IPv4 only), or “inet6” (use IPv6 only). The default is “any”. AllowAgentForwarding Specifies whether ssh-agent(1) 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 directives are processed in the following order: DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups. See PATTERNS in ssh_config(5) for more information on patterns. AllowTcpForwarding Specifies whether TCP forwarding is permitted. The default is “yes”. 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 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. The allow/deny directives are processed in the following order: DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups. See PATTERNS in ssh_config(5) for more information on patterns. AuthorizedKeysFile Specifies the file that contains the public keys that can be used for user authentication. The format is described in the AUTHORIZED_KEYS FILE FORMAT section of sshd(8). AuthorizedKeysFile may contain tokens of the form %T which are substituted during connection setup. The following tokens are defined: %% is replaced by a literal '%', %h is replaced by the home directory of the user being authenticated, and %u is replaced by the username of that user. After expansion, 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. The default is “.ssh/authorized_keys .ssh/authorized_keys2”. AuthorizedPrincipalsFile Specifies a file that lists principal names that are accepted for certificate authentication. 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 authentication. Names are listed one per line preceded by key options (as described in AUTHORIZED_KEYS FILE FORMAT in sshd(8)). Empty lines and comments starting with ‘#’ are ignored. AuthorizedPrincipalsFile may contain tokens of the form %T which are substituted during connection setup. The following tokens are defined: %% is replaced by a literal '%', %h is replaced by the home directory of the user being authenticated, and %u is replaced by the username of that user. After expansion, AuthorizedPrincipalsFile is taken to be an absolute path or one relative to the user's home directory. The default is not to use a principals file – in this case, the username of the user must appear in a certificate's principals 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) 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. This option is only available for protocol version 2. By default, no banner is displayed. ChallengeResponseAuthentication Specifies whether challenge-response authentication is allowed (e.g. via PAM). The default is “yes”. ChrootDirectory Specifies the pathname of a directory to chroot(2) to after authentication. All components of the pathname must be root-owned directories that are not writable by any other user or group. After the chroot, sshd(8) changes the working directory to the user's home directory. The pathname may contain the following tokens that are expanded at runtime once the connecting user has been authenticated: %% is replaced by a literal '%', %h is replaced by the home directory of the user being authenticated, and %u is replaced by the username of that user. 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), and basic /dev nodes such as null(4), zero(4), stdin(4), stdout(4), stderr(4), arandom(4) and tty(4) 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 do require /dev/log inside the chroot directory (see sftp-server(8) for details). The default is not to chroot(2). Ciphers Specifies the ciphers allowed for protocol version 2. Multiple ciphers must be comma-separated. The supported ciphers are “3des-cbc”, “aes128-cbc”, “aes192-cbc”, “aes256-cbc”, “aes128-ctr”, “aes192-ctr”, “aes256-ctr”, “arcfour128”, “arcfour256”, “arcfour”, “blowfish-cbc”, and “cast128-cbc”. The default is: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128, aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc, aes256-cbc,arcfour ClientAliveCountMax Sets the number of client alive messages (see below) which may be sent without sshd(8) 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 (below). 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 inactive. The default value is 3. If ClientAliveInterval (see below) is set to 15, and ClientAliveCountMax is left at the default, unresponsive SSH clients will be disconnected after approximately 45 seconds. This option applies to protocol version 2 only. ClientAliveInterval Sets a timeout interval in seconds after which if no data has been received from the client, sshd(8) 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. This option applies to protocol version 2 only. Compression Specifies whether compression is allowed, or delayed until the user has authenticated successfully. The argument must be “yes”, “delayed”, or “no”. The default is “delayed”. 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 recognized. By default, login is allowed for all groups. The allow/deny directives are processed in the following order: DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups. See PATTERNS in ssh_config(5) for more information on patterns. DenyUsers This keyword can be followed by a list of user name patterns, separated by spaces. Login 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. The allow/deny directives are processed in the following order: DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups. See PATTERNS in ssh_config(5) for more information on patterns. 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. GatewayPorts Specifies whether remote hosts are allowed to connect to ports forwarded for the client. By default, sshd(8) 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-loopback 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”. Note that this option applies to protocol version 2 only. 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”. Note that this option applies to protocol version 2 only. GSSAPICleanupCredentials Specifies whether to automatically destroy the user's credentials cache on logout. The default is “yes”. Note that this option applies to protocol version 2 only. GSSAPIStrictAcceptorCheck Determines whether to be strict about the identity of the GSSAPI acceptor a client authenticates against. If “yes” then the client must authenticate against the host service on the current hostname. If “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”. Note that this option applies only to protocol version 2 GSSAPI connections, and setting it to “no” may only work with recent Kerberos GSSAPI libraries. 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”. HostbasedAuthentication Specifies whether rhosts or /etc/hosts.equiv authentication together with successful public key client host authentication is allowed (host-based authentication). This option is similar to RhostsRSAAuthentication and applies to protocol version 2 only. The default 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) 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) is not to load any certificates. HostKey Specifies a file containing a private host key used by SSH. The default is /etc/ssh/ssh_host_key for protocol version 1, and /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_ecdsa_key and /etc/ssh/ssh_host_rsa_key for protocol version 2. Note that sshd(8) will refuse to use a file if it is group/world-accessible. It is possible to have multiple host key files. “rsa1” keys are used for version 1 and “dsa”, “ecdsa” or “rsa” are used for version 2 of the SSH protocol. IgnoreRhosts Specifies that .rhosts and .shosts files will not be used in RhostsRSAAuthentication or HostbasedAuthentication. /etc/hosts.equiv and /etc/ssh/shosts.equiv are still used. The default is “yes”. IgnoreUserKnownHosts Specifies whether sshd(8) should ignore the user's ~/.ssh/known_hosts during RhostsRSAAuthentication or HostbasedAuthentication. The default is “no”. IPQoS Specifies the IPv4 type-of-service or DSCP class for the connection. Accepted values are “af11”, “af12”, “af13”, “af14”, “af22”, “af23”, “af31”, “af32”, “af33”, “af41”, “af42”, “af43”, “cs0”, “cs1”, “cs2”, “cs3”, “cs4”, “cs5”, “cs6”, “cs7”, “ef”, “lowdelay”, “throughput”, “reliability”, or a numeric value. 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 default is “lowdelay” for interactive sessions and “throughput” for non-interactive sessions. 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 before 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. The default is “ecdh-sha2-nistp256”, “ecdh-sha2-nistp384”, “ecdh-sha2-nistp521”, “diffie-hellman-group-exchange-sha256”, “diffie-hellman-group-exchange-sha1”, “diffie-hellman-group14-sha1”, “diffie-hellman-group1-sha1”. KeyRegenerationInterval In protocol version 1, the ephemeral server key is automatically regenerated after this many seconds (if it has been used). The purpose of regeneration is to prevent decrypting captured sessions by later breaking into the machine and stealing the keys. The key is never stored anywhere. If the value is 0, the key is never regenerated. The default is 3600 (seconds). ListenAddress Specifies the local addresses sshd(8) should listen on. The following forms may be used: ListenAddress host|IPv4_addr|IPv6_addr ListenAddress host|IPv4_addr:port ListenAddress [host|IPv6_addr]:port If port is not specified, sshd will listen on the address and all prior Port options specified. The default is to listen on all local addresses. Multiple ListenAddress options are permitted. Additionally, any Port options must precede this option for non-port qualified addresses. 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). The possible 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. MACs Specifies the available MAC (message authentication code) algorithms. The MAC algorithm is used in protocol version 2 for data integrity protection. Multiple algorithms must be comma-separated. The default is: hmac-md5,hmac-sha1,firstname.lastname@example.org, hmac-ripemd160,hmac-sha1-96,hmac-md5-96, hmac-sha2-256,hmac-sha256-96,hmac-sha2-512, hmac-sha2-512-96 Match Introduces a conditional block. If all of the criteria on the Match line are satisfied, 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. The arguments to Match are one or more criteria-pattern pairs. The available criteria are User, Group, Host, 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). The patterns in an Address criteria may additionally contain addresses to match in CIDR address/masklen format, e.g. “192.0.2.0/24” or “3ffe:ffff::/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. Available keywords are AllowAgentForwarding, AllowTcpForwarding, AuthorizedKeysFile, AuthorizedPrincipalsFile, Banner, ChrootDirectory, ForceCommand, GatewayPorts, GSSAPIAuthentication, HostbasedAuthentication, HostbasedUsesNameFromPacketOnly, KbdInteractiveAuthentication, KerberosAuthentication, MaxAuthTries, MaxSessions, PasswordAuthentication, PermitEmptyPasswords, PermitOpen, PermitRootLogin, PermitTunnel, PubkeyAuthentication, RhostsRSAAuthentication, RSAAuthentication, 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 sessions permitted per network connection. The default is 10. MaxStartups Specifies the maximum number of concurrent unauthenticated connections to the SSH daemon. Additional connections will be dropped until authentication succeeds or the LoginGraceTime expires for a connection. The default is 10. Alternatively, random early drop can be enabled by specifying the three colon separated values “start:rate:full” (e.g. "10:30:60"). sshd(8) will refuse connection attempts with a probability of “rate/100” (30%) if there are currently “start” (10) unauthenticated connections. The probability increases linearly and all connection attempts are refused if the number of unauthenticated connections reaches “full” (60). PasswordAuthentication Specifies whether password authentication is allowed. The default is “yes”. PermitBlacklistedKeys Specifies whether sshd(8) should allow keys recorded in its blacklist of known- compromised keys (see ssh-vulnkey(1)). If “yes”, then attempts to authenticate with compromised keys will be logged but accepted. If “no”, then attempts to authenticate with compromised keys will be rejected. The default is “no”. PermitEmptyPasswords When password authentication is allowed, it specifies whether the server allows login to accounts with empty password strings. The default is “no”. 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. By default all port forwarding requests are permitted. PermitRootLogin Specifies whether root can log in using ssh(1). The argument must be “yes”, “without-password”, “forced-commands-only”, or “no”. The default is “yes”. If this option is set to “without-password”, password authentication is disabled for root. If this option is set to “forced-commands-only”, root login with public key authentication 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. PermitTunnel Specifies whether tun(4) 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”. PermitUserEnvironment Specifies whether ~/.ssh/environment and environment= options in ~/.ssh/authorized_keys are processed by sshd(8). The default is “no”. Enabling environment processing may enable users to bypass access restrictions in some configurations using mechanisms such as LD_PRELOAD. PidFile Specifies the file that contains the process ID of the SSH daemon. The default is /var/run/sshd.pid. Port Specifies the port number that sshd(8) listens on. The default is 22. Multiple options of this type are permitted. See also ListenAddress. PrintLastLog Specifies whether sshd(8) 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) 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”. Protocol Specifies the protocol versions sshd(8) supports. The possible values are ‘1’ and ‘2’. Multiple versions must be comma-separated. The default is ‘2’. Note that the order of the protocol list does not indicate preference, because the client selects among multiple protocol versions offered by the server. Specifying “2,1” is identical to “1,2”. PubkeyAuthentication Specifies whether public key authentication is allowed. The default is “yes”. Note that this option applies to protocol version 2 only. RevokedKeys Specifies a list of revoked public keys. 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. RhostsRSAAuthentication Specifies whether rhosts or /etc/hosts.equiv authentication together with successful RSA host authentication is allowed. The default is “no”. This option applies to protocol version 1 only. RSAAuthentication Specifies whether pure RSA authentication is allowed. The default is “yes”. This option applies to protocol version 1 only. ServerKeyBits Defines the number of bits in the ephemeral protocol version 1 server key. The minimum value is 512, and the default is 1024. StrictModes Specifies whether sshd(8) 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 request. The command sftp-server(8) implements the “sftp” file transfer subsystem. Alternately the name “internal-sftp” implements an in-process “sftp” server. This may simplify configurations using ChrootDirectory to force a different filesystem root on clients. By default no subsystems are defined. Note that this option applies to protocol version 2 only. SyslogFacility Gives the facility code that is used when logging messages from sshd(8). 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 temporarily, 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. 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 CERTIFICATES section in ssh-keygen(1). UseDNS Specifies whether sshd(8) should look up the remote host name and check that the resolved host name for the remote IP address maps back to the very same IP address. The default is “yes”. UseLogin Specifies whether login(1) is used for interactive login sessions. The default is “no”. Note that login(1) is never used for remote command execution. Note also, that if this is enabled, X11Forwarding will be disabled because login(1) does not know how to handle xauth(1) cookies. If UsePrivilegeSeparation is specified, it will be disabled after authentication. UsePAM Enables the Pluggable Authentication Module interface. If set to “yes” this will enable PAM authentication using ChallengeResponseAuthentication and PasswordAuthentication in addition to PAM account and session module processing for all authentication types. Because PAM challenge-response authentication usually serves an equivalent role to password authentication, you should disable either PasswordAuthentication or ChallengeResponseAuthentication. If UsePAM is enabled, you will not be able to run sshd(8) as a non-root user. The default is “no”. UsePrivilegeSeparation Specifies whether sshd(8) separates privileges by creating an unprivileged child process to deal with incoming network traffic. After successful authentication, another process will be created that has the privilege of the authenticated user. The goal of privilege separation is to prevent privilege escalation by containing any corruption within the unprivileged processes. The default is “yes”. If UsePrivilegeSeparation is set to “sandbox” then the pre-authentication unprivileged process is subject to additional restrictions. X11DisplayOffset Specifies the first display number available for sshd(8)'s X11 forwarding. This prevents 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) proxy display is configured to listen on the wildcard address (see X11UseLocalhost below), though this is not the default. Additionally, the authentication 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)). 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. X11 forwarding is automatically disabled if UseLogin is enabled. X11UseLocalhost Specifies whether sshd(8) should bind the X11 forwarding server to the loopback address or to the wildcard address. By default, sshd binds the forwarding server to the loopback 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 address. The argument must be “yes” or “no”. The default is “yes”. XAuthLocation Specifies the full pathname of the xauth(1) program. The default is /usr/bin/xauth.
sshd(8) command-line arguments and configuration file options that specify time may be expressed 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)
/etc/ssh/sshd_config Contains configuration data for sshd(8). This file should be writable by root only, but it is recommended (though not necessary) that it be world-readable.
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 privilege separation.