gsisshd_config (5) Linux Manual Page

sshd_config – OpenSSH SSH daemon configuration file

Synopsis

/etc/gsissh/sshd_config

Description

sshd(8) reads configuration data from /etc/gsissh/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_config5 for how to configure the client. Note that environment passing is only supported for protocol 2, and that 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-agent1 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 All of the specified user and group tests must succeed, before user is allowed to log in. See PATTERNS in ssh_config5 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.

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 [at] 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 All of the specified user and group tests must succeed, before user is allowed to log in. See PATTERNS in ssh_config5 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. This option is only available for SSH protocol 2 and 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 and not writable by group or others. It will be invoked with a single argument of the username being authenticated, and 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.

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 “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).

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 or though authentication styles supported in login.conf5) 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-server8 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” “aes128-gcm [at] openssh.com” “aes256-gcm [at] openssh.com” “arcfour128” “arcfour256” “arcfour” “blowfish-cbc” “cast128-cbc” and “chacha20-poly1305 [at] openssh.com” The default is:

aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
aes128-gcm [at] openssh.com,aes256-gcm [at] openssh.com,
chacha20-poly1305 [at] openssh.com,
aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
aes256-cbc,arcfour

The list of available ciphers may also be obtained using the –Q option of ssh(1).

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”

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 All of the specified user and group tests must succeed, before user is allowed to log in. See PATTERNS in ssh_config5 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 [at] 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 All of the specified user and group tests must succeed, before user is allowed to log in. See PATTERNS in ssh_config5 for more information on patterns.

DisableUsageStats

This keyword can be followed by "yes" to disable reporting of usage metrics. Or it can be set to "no" to enable reporting of usage metrics. Setting the GLOBUS_USAGE_OPTOUT environment variable to "1" will also disable the reporting of usage metrics. Disabling reporting of usage metrics will cause the UsageStatsTargets setting to be ignored.

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”

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 “yes” Note that this option applies to protocol version 2 only.

GSSAPIDelegateCredentials

Specifies whether delegated credentials are stored in the user’s environment. The default is “yes”

GSSAPIKeyExchange

Specifies whether key exchange based on GSSAPI is allowed. GSSAPI key exchange doesn’t rely on ssh keys to verify host identity. The default is “yes” 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.

GSSAPICredentialsPath

If specified, the delegated GSSAPI credential is stored in the given path, overwriting any existing credentials. Paths can be specified with syntax similar to the AuthorizedKeysFile option (i.e., accepting %h and %u tokens). When using this option, setting ‘GssapiCleanupCredentials no’ is recommended, so logging out of one session doesn’t remove the credentials in use by another session of the same user. Currently only implemented for the GSI mechanism.

GSIAllowLimitedProxy

Specifies whether to accept limited proxy credentials for authentication. The default is “no”

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 “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”

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-group1-sha1-,gss-group14-sha1-” This option only applies to protocol version 2 connections using GSSAPI.

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/gsissh/ssh_host_key for protocol version 1, and /etc/gsissh/ssh_host_dsa_key /etc/gsissh/ssh_host_ecdsa_key /etc/gsissh/ssh_host_ed25519_key and /etc/gsissh/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” “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-agent1.

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.

IgnoreRhosts

Specifies that .rhosts and .shosts files will not be used in RhostsRSAAuthentication or HostbasedAuthentication /etc/hosts.equiv and /etc/gsissh/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. The default is

curve25519-sha256 [at] libssh.org,
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.