SSHD_CONFIG(5) File Formats Manual SSHD_CONFIG(5) NAME sshd_config -- OpenSSH SSH daemon configuration file SYNOPSIS /etc/ssh/sshd_config DESCRIPTION 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 inter- preted as comments. Arguments may optionally be enclosed in double quotes (") in order to represent arguments containing spaces. 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. The TERM environment variable is always sent 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 environ- ment variables may be separated by whitespace or spread across mul- tiple AcceptEnv directives. Be warned that some environment vari- ables 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 de- fault is "yes". Note that disabling agent forwarding does not im- prove 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, sep- arated 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 al- low/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 available op- tions are "yes" or "all" to allow TCP forwarding, "no" to prevent all TCP forwarding, "local" to allow local (from the perspective of ssh(1)) forwarding only or "remote" to allow remote forwarding only. The default is "yes". Note that disabling TCP forwarding does not improve security unless users are also denied shell ac- cess, as they can always install their own forwarders. AllowStreamLocalForwarding Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted. The available options are "yes" or "all" to allow StreamLocal forwarding, "no" to prevent all StreamLocal forwarding, "local" to allow local (from the perspective of ssh(1)) forwarding only or "remote" to allow remote forwarding only. The default is "yes". Note that disabling StreamLocal forwarding does not improve security unless users are also denied shell access, as they can al- ways install their own forwarders. AllowUsers This keyword can be followed by a list of user name patterns, sepa- rated 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. AuthenticationMethods Specifies the authentication methods that must be successfully com- pleted for a user to be granted access. This option must be fol- lowed by one or more comma-separated lists of authentication method names. Successful authentication requires completion of every method in at least one of these lists. For example, an argument of "publickey,password publickey,keyboard-interactive" would require the user to complete public key authentication, followed by either password or keyboard interactive 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 keyboard-interactive authen- tication before public key. For keyboard interactive authentication it is also possible to re- strict authentication to a specific device by appending a colon followed by the device identifier "bsdauth", "pam", or "skey", de- pending 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) veri- fies that keys that have been used successfully are not reused for subsequent authentications. For example, an AuthenticationMethods of "publickey,publickey" will require successful authentication us- ing two different public keys. This option will yield a fatal error if enabled if protocol 1 is also enabled. Note that each authentication method listed should also be explicitly enabled in the configuration. The default is not to require multiple authentication; successful completion of a single authentication method is sufficient. 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. Arguments to AuthorizedKeysCommand may be provided using the fol- lowing tokens, which will be expanded at runtime: %% is replaced by a literal '%', %u is replaced by the username being authenticated, %h is replaced by the home directory of the user being authenti- cated, %t is replaced with the key type offered for authentication, %f is replaced with the fingerprint of the key, and %k is replaced with the key being offered for authentication. If no arguments are specified then the username of the target user will be supplied. The program should produce on standard output zero or more lines of authorized_keys output (see AUTHORIZED_KEYS in sshd(8)). If a key supplied by AuthorizedKeysCommand does not successfully authenti- cate and authorize the user then public key authentication contin- ues using the usual AuthorizedKeysFile files. By default, no Au- thorizedKeysCommand is run. AuthorizedKeysCommandUser Specifies the user under whose account the AuthorizedKeysCommand is run. It is recommended to use a dedicated user that has no other role on the host than running authorized keys commands. If AuthorizedKeysCommand is specified but AuthorizedKeysCommandUser is not, then sshd(8) will refuse to start. AuthorizedKeysFile Specifies the file that contains the public keys that can be used for user authentication. The format is described in the AUTHO- RIZED_KEYS FILE FORMAT section of sshd(8). AuthorizedKeysFile may contain tokens of the form %T which are substituted during connec- tion 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. 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 cer- tificate principals as per AuthorizedPrincipalsFile. The program must be owned by root, not writable by group or others and speci- fied by an absolute path. Arguments to AuthorizedPrincipalsCommand may be provided using the following tokens, which will be expanded at runtime: %% is replaced by a literal '%', %u is replaced by the username being authenti- cated and %h is replaced by the home directory of the user being authenticated. The program should produce on standard output zero or more lines of AuthorizedPrincipalsFile output. If either AuthorizedPrincipalsCommand or AuthorizedPrincipalsFile is speci- fied, then certificates offered by the client for authentication must contain a principal that is listed. By default, no Autho- rizedPrincipalsCommand is run. AuthorizedPrincipalsCommandUser Specifies the user under whose account the AuthorizedPrincipalsCom- mand is run. It is recommended to use a dedicated user that has no other role on the host than running authorized principals commands. If AuthorizedPrincipalsCommand is specified but AuthorizedPrincipalsCommandUser is not, then sshd(8) will refuse to start. 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 au- thentication. Names are listed one per line preceded by key op- tions (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 "none", i.e. not to use a principals 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) for details). Banner The contents of the specified file are sent to the remote user be- fore authentication is allowed. If the argument is "none" then no banner is displayed. By default, no banner is displayed. ChallengeResponseAuthentication Specifies whether challenge-response authentication is allowed (e.g. via PAM or through authentication styles supported in lo- gin.conf(5)) The default is "yes". ChrootDirectory Specifies the pathname of a directory to chroot(2) to after authen- tication. At session startup sshd(8) 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) 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 re- placed 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 directo- ries 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), and tty(4) devices. For file transfer sessions using "sftp", no addi- tional 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 sys- tems (see sftp-server(8) for details). For safety, it is very important that the directory hierarchy be prevented from modification by other processes on the system (espe- cially those outside the jail). Misconfiguration can lead to un- safe environments which sshd(8) cannot detect. The default is "none", indicating not to chroot(2). Ciphers Specifies the ciphers allowed. Multiple ciphers must be comma-sep- arated. If the specified value begins with a `+' character, then the specified ciphers will be appended to the default set instead of replacing them. The supported ciphers are: 3des-cbc aes128-cbc aes192-cbc aes256-cbc aes128-ctr aes192-ctr aes256-ctr aes128-gcm@openssh.com aes256-gcm@openssh.com arcfour arcfour128 arcfour256 blowfish-cbc cast128-cbc chacha20-poly1305@openssh.com The default is: chacha20-poly1305@openssh.com, aes128-ctr,aes192-ctr,aes256-ctr, aes128-gcm@openssh.com,aes256-gcm@openssh.com, aes128-cbc,aes192-cbc,aes256-cbc The list of available ciphers may also be obtained using the -Q op- tion of ssh(1) with an argument of "cipher". 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. 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 de- fault is 0, indicating that these messages will not be sent to the client. 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". DenyGroups This keyword can be followed by a list of group name patterns, sep- arated 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 direc- tives 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, sepa- rated 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 sepa- rately checked, restricting logins to particular users from partic- ular hosts. The allow/deny directives are processed in the follow- ing order: DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups. See PATTERNS in ssh_config(5) for more information on patterns. 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, ig- noring 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 en- vironment 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 for- warded for the client. By default, sshd(8) binds remote port for- wardings 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 avail- able 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". GSSAPIStrictAcceptorCheck Determines whether to be strict about the identity of the GSSAPI acceptor a client authenticates 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 facil- ity is provided to assist with operation on multi homed machines. The default is "yes". HostbasedAcceptedKeyTypes Specifies the key types that will be accepted for hostbased authen- tication as a comma-separated pattern list. Alternately if the specified value begins with a `+' character, then the specified key types will be appended to the default set instead of replacing them. The default for this option is: ecdsa-sha2-nistp256-cert-v01@openssh.com, ecdsa-sha2-nistp384-cert-v01@openssh.com, ecdsa-sha2-nistp521-cert-v01@openssh.com, ssh-ed25519-cert-v01@openssh.com, ssh-rsa-cert-v01@openssh.com, ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, ssh-ed25519,ssh-rsa The -Q option of ssh(1) may be used to list supported key types. HostbasedAuthentication Specifies whether rhosts or /etc/hosts.equiv authentication to- gether with successful public key client host authentication is al- lowed (host-based authentication). The default is "no". HostbasedUsesNameFromPacketOnly Specifies whether or not the server will attempt to perform a re- verse 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 re- solve the name from the TCP connection itself. The default is "no". HostCertificate Specifies a file containing a public host certificate. The cer- tificate's public key must match a private host key already speci- fied 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, /etc/ssh/ssh_host_ed25519_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 and that the HostKeyAlgorithms option restricts which of the keys are actually used by sshd(8). It is possible to have multiple host key files. "rsa1" keys are used for version 1 and "dsa", "ecdsa", "ed25519" or "rsa" are used for version 2 of the SSH protocol. It is also possible to specify public host key files instead. In this case operations on the pri- vate key will be delegated to an ssh-agent(1). HostKeyAgent Identifies the UNIX-domain socket used to communicate with an agent that has access to the private host keys. If "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 algorithms that the server offers. The de- fault for this option is: ecdsa-sha2-nistp256-cert-v01@openssh.com, ecdsa-sha2-nistp384-cert-v01@openssh.com, ecdsa-sha2-nistp521-cert-v01@openssh.com, ssh-ed25519-cert-v01@openssh.com, ssh-rsa-cert-v01@openssh.com, ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, ssh-ed25519,ssh-rsa The list of available key types may also be obtained using the -Q option of ssh(1) with an argument of "key". 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 de- fault 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 connec- tion. Accepted values are "af11", "af12", "af13", "af21", "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 argu- ment 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 ses- sions. The default is "lowdelay" for interactive sessions and "throughput" for non-interactive sessions. KbdInteractiveAuthentication Specifies whether to allow keyboard-interactive authentication. The argument to this keyword must be "yes" or "no". The default is to use whatever value ChallengeResponseAuthentication is set to (by default "yes"). 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 al- lows 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 ac- quire 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. Alternately if the specified value begins with a `+' character, then the specified methods will be appended to the default set instead of replacing them. The sup- ported algorithms are: curve25519-sha256@libssh.org diffie-hellman-group1-sha1 diffie-hellman-group14-sha1 diffie-hellman-group-exchange-sha1 diffie-hellman-group-exchange-sha256 ecdh-sha2-nistp256 ecdh-sha2-nistp384 ecdh-sha2-nistp521 The default is: curve25519-sha256@libssh.org, ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, diffie-hellman-group-exchange-sha256, diffie-hellman-group14-sha1 The list of available key exchange algorithms may also be obtained using the -Q option of ssh(1) with an argument of "kex". 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 re- generated. The default is 3600 (seconds). ListenAddress Specifies the local addresses sshd(8) should listen on. The fol- lowing 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 Port options specified. The default is to listen on all local ad- dresses. Multiple ListenAddress options are permitted. LoginGraceTime The server disconnects after this time if the user has not success- fully 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, VER- BOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. The default is INFO. DE- BUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify higher levels of debugging output. Logging with a DEBUG level vio- lates the privacy of users and is not recommended. MACs Specifies the available MAC (message authentication code) algo- rithms. The MAC algorithm is used for data integrity protection. Multiple algorithms must be comma-separated. If the specified value begins with a `+' character, then the specified algorithms will be appended to the default set instead of replacing them. The algorithms that contain "-etm" calculate the MAC after encryp- tion (encrypt-then-mac). These are considered safer and their use recommended. The supported MACs are: hmac-md5 hmac-md5-96 hmac-ripemd160 hmac-sha1 hmac-sha1-96 hmac-sha2-256 hmac-sha2-512 umac-64@openssh.com umac-128@openssh.com hmac-md5-etm@openssh.com hmac-md5-96-etm@openssh.com hmac-ripemd160-etm@openssh.com hmac-sha1-etm@openssh.com hmac-sha1-96-etm@openssh.com hmac-sha2-256-etm@openssh.com hmac-sha2-512-etm@openssh.com umac-64-etm@openssh.com umac-128-etm@openssh.com The default is: umac-64-etm@openssh.com,umac-128-etm@openssh.com, hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, hmac-sha1-etm@openssh.com, umac-64@openssh.com,umac-128@openssh.com, hmac-sha2-256,hmac-sha2-512,hmac-sha1 The list of available MAC algorithms may also be obtained using the -Q option of ssh(1) with an argument of "mac". Match Introduces a conditional block. If all of the criteria on the Match line are satisfied, the keywords on the following lines over- ride those set in the global section of the config file, until ei- ther another Match line or the end of the file. If a keyword ap- pears 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 opera- tors described in the PATTERNS section of ssh_config(5). The patterns in an Address criteria may additionally contain ad- dresses to match in CIDR address/masklen format, e.g. "192.0.2.0/24" or "3ffe:ffff::/32". Note that the mask length pro- vided must be consistent with the address - it is an error to spec- ify 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 AcceptEnv, AllowAgentForwarding, AllowGroups, AllowStreamLocalForwarding, AllowTcpForwarding, AllowUsers, AuthenticationMethods, AuthorizedKeysCommand, AuthorizedKeysCommandUser, AuthorizedKeysFile, AuthorizedPrincipalsCommand, AuthorizedPrincipalsCommandUser, AuthorizedPrincipalsFile, Banner, ChrootDirectory, DenyGroups, DenyUsers, ForceCommand, GatewayPorts, GSSAPIAuthentication, HostbasedAcceptedKeyTypes, HostbasedAuthentication, HostbasedUsesNameFromPacketOnly, IPQoS, KbdInteractiveAuthentication, KerberosAuthentication, MaxAuthTries, MaxSessions, PasswordAuthentication, PermitEmptyPasswords, PermitOpen, PermitRootLogin, PermitTTY, PermitTunnel, PermitUserRC, PubkeyAcceptedKeyTypes, PubkeyAuthentication, RekeyLimit, RevokedKeys, RhostsRSAAuthentication, RSAAuthentication, 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 mul- tiplexing. Setting MaxSessions to 1 will effectively disable ses- sion multiplexing, whereas setting it to 0 will prevent all shell, login and subsystem sessions while still permitting forwarding. The default is 10. MaxStartups Specifies the maximum number of concurrent unauthenticated connec- tions to the SSH daemon. 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) will refuse connection attempts with a probability of "rate/100" (30%) if there are currently "start" (10) unauthenti- cated connections. The probability increases linearly and all con- nection attempts are refused if the number of unauthenticated con- nections reaches "full" (60). PasswordAuthentication Specifies whether password authentication is allowed. See also UsePAM. 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 permit- ted. 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 white- space. An argument of "any" can be used to remove all restrictions and permit any forwarding requests. An argument of "none" can be used to prohibit all forwarding requests. By default all port for- warding requests are permitted. PermitRootLogin Specifies whether root can log in using ssh(1). The argument must be "yes", "prohibit-password", "without-password", "forced-commands-only", or "no". The default is "no". Note that if ChallengeResponseAuthentication is "yes", the root user may be allowed in with its password even if PermitRootLogin is set to "without-password". If this option is set to "prohibit-password" or "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 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 au- thentication 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 argu- ment 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) device must allow access to the user. PermitTTY Specifies whether pty(4) allocation is permitted. The default is "yes". 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. PermitUserRC Specifies whether any ~/.ssh/rc file is executed. The default is "yes". PidFile Specifies the file that contains the process ID of the SSH daemon, or "none" to not write one. 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'. Protocol 1 suffers from a number of crypto- graphic weaknesses and should not be used. It is only offered to support legacy devices. Note that the order of the protocol list does not indicate prefer- ence, because the client selects among multiple protocol versions offered by the server. Specifying "2,1" is identical to "1,2". PubkeyAcceptedKeyTypes Specifies the key types that will be accepted for public key au- thentication as a comma-separated pattern list. Alternately if the specified value begins with a `+' character, then the specified key types will be appended to the default set instead of replacing them. The default for this option is: ecdsa-sha2-nistp256-cert-v01@openssh.com, ecdsa-sha2-nistp384-cert-v01@openssh.com, ecdsa-sha2-nistp521-cert-v01@openssh.com, ssh-ed25519-cert-v01@openssh.com, ssh-rsa-cert-v01@openssh.com, ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, ssh-ed25519,ssh-rsa The -Q option of ssh(1) may be used to list supported key types. PubkeyAuthentication Specifies whether public key authentication is allowed. The de- fault is "yes". RekeyLimit Specifies the maximum amount of data that may be transmitted before the session key is renegotiated, optionally followed a maximum amount of time that may pass before the session key is renegoti- ated. The first argument is specified in bytes and may have a suf- fix of `K', `M', or `G' to indicate Kilobytes, Megabytes, or Giga- bytes, respectively. The default is between `1G' and `4G', depend- ing on the cipher. The optional second value is specified in sec- onds 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 authentica- tion 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). For more in- formation on KRLs, see the KEY REVOCATION LISTS section in ssh-key- gen(1). RhostsRSAAuthentication Specifies whether rhosts or /etc/hosts.equiv authentication to- gether with successful RSA host authentication is allowed. The de- fault 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 default and minimum value is 1024. 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 en- abled, 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) 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). Ar- guments should be a subsystem name and a command (with optional ar- guments) to execute upon subsystem request. The command sftp-server(8) implements the "sftp" file transfer sub- system. 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. SyslogFacility Gives the facility code that is used when logging messages from sshd(8). The possible values are: DAEMON, USER, AUTH, LOCAL0, LO- CAL1, 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". 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 certifi- cates that lack a list of principals will not be permitted for au- thentication using TrustedUserCAKeys. For more details on certifi- cates, see the CERTIFICATES section in ssh-keygen(1). UseBlacklist Specifies whether sshd(8) attempts to send authentication success and failure messages to the blacklistd(8) daemon. The default is "no". UseDNS Specifies whether sshd(8) 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", then only addresses and not host names may be used in ~/.ssh/known_hosts from and sshd_config Match Host directives. 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 speci- fied, 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 addi- tion to PAM account and session module processing for all authenti- cation types. Because PAM challenge-response authentication usually serves an equivalent role to password authentication, you should disable ei- ther PasswordAuthentication or ChallengeResponseAuthentication. If UsePAM is enabled, you will not be able to run sshd(8) as a non- root user. The default is "yes". UsePrivilegeSeparation Specifies whether sshd(8) separates privileges by creating an un- privileged 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 contain- ing any corruption within the unprivileged processes. The argument must be "yes", "no", or "sandbox". If UsePrivilegeSeparation is set to "sandbox" then the pre-authentication unprivileged process is subject to additional restrictions. The default is "sandbox". VersionAddendum Optionally specifies additional text to append to the SSH protocol banner sent by the server upon connection. The default is "FreeBSD-20161230". The value "none" may be used to disable this. X11DisplayOffset Specifies the first display number available for sshd(8)'s X11 for- warding. 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 "yes". 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 authen- tication spoofing and authentication data verification and substi- tution 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 for- warding X11 traffic, as users can always install their own for- warders. 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 configu- ration. X11UseLocalhost may be set to "no" to specify that the forwarding server should be bound to the wildcard address. The ar- gument must be "yes" or "no". The default is "yes". XAuthLocation Specifies the full pathname of the xauth(1) program, or "none" to not use one. The default is /usr/local/bin/xauth. TIME FORMATS 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) FILES /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. SEE ALSO sshd(8) 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. FreeBSD 11.1 February 17, 2016 SSHD_CONFIG(5)
NAME | SYNOPSIS | DESCRIPTION | TIME FORMATS | FILES | SEE ALSO | AUTHORS
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