sshd_config man page

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 interpreted 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 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 available options 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 access, 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 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. HOST criteria may additionally contain addresses to match in CIDR address/masklen format. 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 completed for a user to be granted access. This option must be followed by one or more comma-separated lists of 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, 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 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”, “pam”, or “skey”, 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) verifies that keys that have been used successfully are not reused for subsequent authentications. For example, an AuthenticationMethods of “publickey,publickey” will require successful authentication using 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 “any” 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 following 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 authenticated, %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 authenticate and authorize the user then public key authentication continues using the usual AuthorizedKeysFile files. By default, no AuthorizedKeysCommand 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 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. 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 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 authenticated 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 specified, then certificates offered by the client for authentication 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 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 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 “none”, i.e. 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).

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.
ChallengeResponseAuthentication
Specifies whether challenge-response authentication is allowed (e.g. via PAM or through authentication styles supported in login.conf(5)) The default is “yes”.
ChrootDirectory
Specifies the pathname of a directory to chroot(2) to after authentication. 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 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), 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 may require /dev/log inside the chroot directory on some operating systems (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 (especially those outside the jail). Misconfiguration can lead to unsafe 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-separated. 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

The list of available ciphers may also be obtained using the -Q option 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 default 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, 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. HOST criteria may additionally contain addresses to match in CIDR address/masklen format. 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.

ExposeAuthenticationMethods
When using SSH2, this option controls the exposure of the list of successful authentication methods to PAM during the authentication and to the shell environment via the SSH_USER_AUTH variable. See the description of this variable for more details. Valid options are: “never” (Do not expose successful authentication methods), “pam-only” (Only expose them to PAM during authentication, not afterwards), “pam-and-env” (Expose them to PAM and keep them in the shell environment). The default is “never”.
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) 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”.
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”.
GSSAPICleanupCredentials
Specifies whether to automatically destroy the user's credentials cache on logout. The default is “yes”.
GSSAPIEnablek5users
Specifies whether to look at .k5users file for GSSAPI authentication access control. Further details are described in ksu(1). The default is “no”.
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 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-

The default is “gss-gex-sha1-,gss-group14-sha1-”. This option only applies to protocol version 2 connections using GSSAPI.

HostbasedAcceptedKeyTypes
Specifies the key types that will be accepted for hostbased authentication 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 together with successful public key client host authentication is allowed (host-based authentication). 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, /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 private 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 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 algorithms that the server offers. 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 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 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”, “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 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.
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 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”.
KerberosUseKuserok
Specifies whether to look at .k5login file for user's aliases. 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 supported 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 regenerated. The default is 3600 (seconds).
ListenAddress
Specifies the local addresses sshd(8) should listen on. The following forms may be used:

If port is not specified, sshd will listen on the address and all Port options specified. 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). 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 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 encryption (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 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).

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 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, KerberosUseKuserok, 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 multiplexing. Setting MaxSessions to 1 will effectively disable session 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 connections 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) 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”.
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:

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 argument 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. By default all port forwarding 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 “prohibit-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 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”.

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 cryptographic 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 preference, 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 authentication 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 default 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 renegotiated. The first argument is specified in bytes and may have a suffix of ‘K’, ‘M’, or ‘G’ to indicate Kilobytes, Megabytes, or Gigabytes, respectively. 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). For more information on KRLs, see the KEY REVOCATION LISTS section in ssh-keygen(1).
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 default and minimum value is 1024.
ShowPatchLevel
Specifies whether sshd will display the patch level of the binary in the identification string. The patch level is set at compile-time. The default is “no”.
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) 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.

SyslogFacility
Gives the facility code that is used when logging messages from sshd(8). The possible values are: DAEMON, USER, AUTH, AUTHPRIV, 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”.

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 CERTIFICATES section in ssh-keygen(1).
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” (the default) then only addresses and not host names may be used in ~/.ssh/authorized_keys from and sshd_config Match Host directives.

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 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 “none”.
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, or “none” to not use one. The default is /usr/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 privilege separation.

Referenced By

brlapi_tty(3), gsissh(1), gsissh_config(5), gsisshd(8), gsissh-keygen(1), monkeysphere-authentication(8), sftp-server(8), ssh(1), ssh-add(1), ssh-agent(1), ssh_config(5), sshd(8), ssh-keygen(1), ssh-ldap-helper(8), sss_ssh_authorizedkeys(1), virt-p2v(1).

July 19 2016