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Diffstat (limited to 'sshd_config.5')
| -rw-r--r-- | sshd_config.5 | 1258 |
1 files changed, 788 insertions, 470 deletions
diff --git a/sshd_config.5 b/sshd_config.5 index 6dce0c70..b294efc2 100644 --- a/sshd_config.5 +++ b/sshd_config.5 @@ -33,15 +33,13 @@ .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. .\" -.\" $OpenBSD: sshd_config.5,v 1.194 2015/02/20 23:46:01 djm Exp $ -.Dd $Mdocdate: February 20 2015 $ +.\" $OpenBSD: sshd_config.5,v 1.311 2020/04/17 06:12:41 jmc Exp $ +.Dd $Mdocdate: April 17 2020 $ .Dt SSHD_CONFIG 5 .Os .Sh NAME .Nm sshd_config -.Nd OpenSSH SSH daemon configuration file -.Sh SYNOPSIS -.Nm /etc/ssh/sshd_config +.Nd OpenSSH daemon configuration file .Sh DESCRIPTION .Xr sshd 8 reads configuration data from @@ -50,6 +48,7 @@ reads configuration data from .Fl f on the command line). The file contains keyword-argument pairs, one per line. +For each keyword, the first obtained value will be used. Lines starting with .Ql # and empty lines are interpreted as comments. @@ -67,10 +66,15 @@ the session's .Xr environ 7 . See .Cm SendEnv +and +.Cm SetEnv in .Xr ssh_config 5 for how to configure the client. -Note that environment passing is only supported for protocol 2. +The +.Ev TERM +environment variable is always accepted whenever the client +requests a pseudo-terminal as it is required by the protocol. Variables are specified by name, which may contain the wildcard characters .Ql * and @@ -87,19 +91,18 @@ The default is not to accept any environment variables. Specifies which address family should be used by .Xr sshd 8 . Valid arguments are -.Dq any , -.Dq inet +.Cm any +(the default), +.Cm inet (use IPv4 only), or -.Dq inet6 +.Cm inet6 (use IPv6 only). -The default is -.Dq any . .It Cm AllowAgentForwarding Specifies whether .Xr ssh-agent 1 forwarding is permitted. The default is -.Dq yes . +.Cm 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. @@ -110,54 +113,49 @@ 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: -.Cm DenyUsers , -.Cm AllowUsers , +The allow/deny groups directives are processed in the following order: .Cm DenyGroups , -and finally .Cm AllowGroups . .Pp See PATTERNS in .Xr ssh_config 5 for more information on patterns. -.It Cm AllowTcpForwarding -Specifies whether TCP forwarding is permitted. +.It Cm AllowStreamLocalForwarding +Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted. The available options are -.Dq yes +.Cm yes +(the default) or -.Dq all -to allow TCP forwarding, -.Dq no -to prevent all TCP forwarding, -.Dq local +.Cm all +to allow StreamLocal forwarding, +.Cm no +to prevent all StreamLocal forwarding, +.Cm local to allow local (from the perspective of .Xr ssh 1 ) forwarding only or -.Dq remote +.Cm remote to allow remote forwarding only. -The default is -.Dq yes . -Note that disabling TCP forwarding does not improve security unless +Note that disabling StreamLocal forwarding does not improve security unless users are also denied shell access, as they can always install their own forwarders. -.It Cm AllowStreamLocalForwarding -Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted. +.It Cm AllowTcpForwarding +Specifies whether TCP forwarding is permitted. The available options are -.Dq yes +.Cm yes +(the default) or -.Dq all -to allow StreamLocal forwarding, -.Dq no -to prevent all StreamLocal forwarding, -.Dq local +.Cm all +to allow TCP forwarding, +.Cm no +to prevent all TCP forwarding, +.Cm local to allow local (from the perspective of .Xr ssh 1 ) forwarding only or -.Dq remote +.Cm remote to allow remote forwarding only. -The default is -.Dq yes . -Note that disabling StreamLocal forwarding does not improve security unless +Note that disabling TCP forwarding does not improve security unless users are also denied shell access, as they can always install their own forwarders. .It Cm AllowUsers @@ -170,12 +168,11 @@ 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: +HOST criteria may additionally contain addresses to match in CIDR +address/masklen format. +The allow/deny users directives are processed in the following order: .Cm DenyUsers , -.Cm AllowUsers , -.Cm DenyGroups , -and finally -.Cm AllowGroups . +.Cm AllowUsers . .Pp See PATTERNS in .Xr ssh_config 5 @@ -183,65 +180,84 @@ for more information on patterns. .It Cm 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. -Successful authentication requires completion of every method in at least -one of these lists. +This option must be followed by one or more lists of comma-separated +authentication method names, or by the single string +.Cm 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. .Pp -For example, an argument of -.Dq publickey,password publickey,keyboard-interactive +For example, +.Qq 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 +so for this example it would not be possible to attempt password or keyboard-interactive authentication before public key. .Pp For keyboard interactive authentication it is also possible to restrict authentication to a specific device by appending a colon followed by the device identifier -.Dq bsdauth , -.Dq pam , +.Cm bsdauth or -.Dq skey , +.Cm pam . depending on the server configuration. For example, -.Dq keyboard-interactive:bsdauth +.Qq keyboard-interactive:bsdauth would restrict keyboard interactive authentication to the -.Dq bsdauth +.Cm bsdauth device. .Pp -If the -.Dq publickey -method is listed more than once, +If the publickey method is listed more than once, .Xr sshd 8 verifies that keys that have been used successfully are not reused for subsequent authentications. -For example, an -.Cm AuthenticationMethods -of -.Dq publickey,publickey -will require successful authentication using two different public keys. -.Pp -This option is only available for SSH protocol 2 and will yield a fatal -error if enabled if protocol 1 is also enabled. +For example, +.Qq publickey,publickey +requires successful authentication using two different public keys. +.Pp 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. +.Pp +The available authentication methods are: +.Qq gssapi-with-mic , +.Qq hostbased , +.Qq keyboard-interactive , +.Qq none +(used for access to password-less accounts when +.Cm PermitEmptyPasswords +is enabled), +.Qq password +and +.Qq publickey . .It Cm 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 +The program must be owned by root, not writable by group or others and +specified by an absolute path. +Arguments to +.Cm AuthorizedKeysCommand +accept the tokens described in the +.Sx TOKENS +section. +If no arguments are specified then the username of the target user is used. +.Pp +The program should produce on standard output zero or +more lines of authorized_keys output (see +.Sx AUTHORIZED_KEYS +in .Xr sshd 8 ) . -If a key supplied by AuthorizedKeysCommand does not successfully authenticate -and authorize the user then public key authentication continues using the usual +.Cm AuthorizedKeysCommand +is tried after the usual .Cm AuthorizedKeysFile -files. -By default, no AuthorizedKeysCommand is run. +files and will not be executed if a matching key is found there. +By default, no +.Cm AuthorizedKeysCommand +is run. .It Cm AuthorizedKeysCommandUser -Specifies the user under whose account the AuthorizedKeysCommand is run. +Specifies the user under whose account the +.Cm 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 @@ -252,25 +268,63 @@ is not, then .Xr sshd 8 will refuse to start. .It Cm 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 +Specifies the file that contains the public keys used for user authentication. +The format is described in the AUTHORIZED_KEYS FILE FORMAT section of .Xr sshd 8 . +Arguments to .Cm 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. +accept the tokens described in the +.Sx TOKENS +section. After expansion, .Cm 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 +.Cm none +to skip checking for user keys in files. The default is -.Dq .ssh/authorized_keys .ssh/authorized_keys2 . +.Qq .ssh/authorized_keys .ssh/authorized_keys2 . +.It Cm AuthorizedPrincipalsCommand +Specifies a program to be used to generate the list of allowed +certificate principals as per +.Cm AuthorizedPrincipalsFile . +The program must be owned by root, not writable by group or others and +specified by an absolute path. +Arguments to +.Cm AuthorizedPrincipalsCommand +accept the tokens described in the +.Sx TOKENS +section. +If no arguments are specified then the username of the target user is used. +.Pp +The program should produce on standard output zero or +more lines of +.Cm AuthorizedPrincipalsFile +output. +If either +.Cm AuthorizedPrincipalsCommand +or +.Cm AuthorizedPrincipalsFile +is specified, then certificates offered by the client for authentication +must contain a principal that is listed. +By default, no +.Cm AuthorizedPrincipalsCommand +is run. +.It Cm AuthorizedPrincipalsCommandUser +Specifies the user under whose account the +.Cm 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 +.Cm AuthorizedPrincipalsCommand +is specified but +.Cm AuthorizedPrincipalsCommandUser +is not, then +.Xr sshd 8 +will refuse to start. .It Cm AuthorizedPrincipalsFile Specifies a file that lists principal names that are accepted for certificate authentication. @@ -278,29 +332,28 @@ When using certificates signed by a key listed in .Cm 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 +Names are listed one per line preceded by key options (as described in +.Sx AUTHORIZED_KEYS FILE FORMAT +in .Xr sshd 8 ) . Empty lines and comments starting with .Ql # are ignored. .Pp +Arguments to .Cm 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. +accept the tokens described in the +.Sx TOKENS +section. After expansion, .Cm AuthorizedPrincipalsFile -is taken to be an absolute path or one relative to the user's home -directory. -.Pp +is taken to be an absolute path or one relative to the user's home directory. The default is -.Dq none , +.Cm none , i.e. not to use a principals file \(en in this case, the username of the user must appear in a certificate's principals list for it to be accepted. +.Pp Note that .Cm AuthorizedPrincipalsFile is only used when authentication proceeds using a CA listed in @@ -316,16 +369,26 @@ for details). The contents of the specified file are sent to the remote user before authentication is allowed. If the argument is -.Dq none +.Cm none then no banner is displayed. -This option is only available for protocol version 2. By default, no banner is displayed. +.It Cm CASignatureAlgorithms +Specifies which algorithms are allowed for signing of certificates +by certificate authorities (CAs). +The default is: +.Bd -literal -offset indent +ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, +ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa +.Ed +.Pp +Certificates signed using other algorithms will not be accepted for +public key or host-based authentication. .It Cm ChallengeResponseAuthentication Specifies whether challenge-response authentication is allowed (e.g. via PAM or through authentication styles supported in .Xr login.conf 5 ) The default is -.Dq yes . +.Cm yes . .It Cm ChrootDirectory Specifies the pathname of a directory to .Xr chroot 2 @@ -337,11 +400,11 @@ which are not writable by any other user or group. After the chroot, .Xr sshd 8 changes the working directory to the user's home directory. -.Pp -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. +Arguments to +.Cm ChrootDirectory +accept the tokens described in the +.Sx TOKENS +section. .Pp The .Cm ChrootDirectory @@ -360,10 +423,9 @@ nodes such as and .Xr tty 4 devices. -For file transfer sessions using -.Dq sftp , -no additional configuration of the environment is necessary if the -in-process sftp server is used, +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 .Pa /dev/log inside the chroot directory on some operating systems (see @@ -377,11 +439,26 @@ Misconfiguration can lead to unsafe environments which .Xr sshd 8 cannot detect. .Pp -The default is not to +The default is +.Cm none , +indicating not to .Xr chroot 2 . .It Cm Ciphers -Specifies the ciphers allowed for protocol version 2. +Specifies the ciphers allowed. Multiple ciphers must be comma-separated. +If the specified list begins with a +.Sq + +character, then the specified ciphers will be appended to the default set +instead of replacing them. +If the specified list begins with a +.Sq - +character, then the specified ciphers (including wildcards) will be removed +from the default set instead of replacing them. +If the specified list begins with a +.Sq ^ +character, then the specified ciphers will be placed at the head of the +default set. +.Pp The supported ciphers are: .Pp .Bl -item -compact -offset indent @@ -404,59 +481,45 @@ aes128-gcm@openssh.com .It aes256-gcm@openssh.com .It -arcfour -.It -arcfour128 -.It -arcfour256 -.It -blowfish-cbc -.It -cast128-cbc -.It chacha20-poly1305@openssh.com .El .Pp The default is: .Bd -literal -offset indent +chacha20-poly1305@openssh.com, aes128-ctr,aes192-ctr,aes256-ctr, -aes128-gcm@openssh.com,aes256-gcm@openssh.com, -chacha20-poly1305@openssh.com +aes128-gcm@openssh.com,aes256-gcm@openssh.com .Ed .Pp -The list of available ciphers may also be obtained using the -.Fl Q -option of -.Xr ssh 1 -with an argument of -.Dq cipher . +The list of available ciphers may also be obtained using +.Qq ssh -Q cipher . .It Cm ClientAliveCountMax -Sets the number of client alive messages (see below) which may be -sent without +Sets the number of client alive messages which may be sent without .Xr 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 -.Cm TCPKeepAlive -(below). +.Cm TCPKeepAlive . The client alive messages are sent through the encrypted channel and therefore will not be spoofable. The TCP keepalive option enabled by .Cm TCPKeepAlive is spoofable. The client alive mechanism is valuable when the client or -server depend on knowing when a connection has become inactive. +server depend on knowing when a connection has become unresponsive. .Pp The default value is 3. If .Cm ClientAliveInterval -(see below) is set to 15, and +is set to 15, and .Cm ClientAliveCountMax is left at the default, unresponsive SSH clients will be disconnected after approximately 45 seconds. -This option applies to protocol version 2 only. +Setting a zero +.Cm ClientAliveCountMax +disables connection termination. .It Cm ClientAliveInterval Sets a timeout interval in seconds after which if no data has been received from the client, @@ -465,17 +528,18 @@ 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. .It Cm Compression -Specifies whether compression is allowed, or delayed until +Specifies whether compression is enabled after the user has authenticated successfully. The argument must be -.Dq yes , -.Dq delayed , +.Cm yes , +.Cm delayed +(a legacy synonym for +.Cm yes ) or -.Dq no . +.Cm no . The default is -.Dq delayed . +.Cm yes . .It Cm DenyGroups This keyword can be followed by a list of group name patterns, separated by spaces. @@ -483,11 +547,8 @@ 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: -.Cm DenyUsers , -.Cm AllowUsers , +The allow/deny groups directives are processed in the following order: .Cm DenyGroups , -and finally .Cm AllowGroups . .Pp See PATTERNS in @@ -502,24 +563,37 @@ 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: +HOST criteria may additionally contain addresses to match in CIDR +address/masklen format. +The allow/deny users directives are processed in the following order: .Cm DenyUsers , -.Cm AllowUsers , -.Cm DenyGroups , -and finally -.Cm AllowGroups . +.Cm AllowUsers . .Pp See PATTERNS in .Xr ssh_config 5 for more information on patterns. +.It Cm DisableForwarding +Disables all forwarding features, including X11, +.Xr ssh-agent 1 , +TCP and StreamLocal. +This option overrides all other forwarding-related options and may +simplify restricted configurations. +.It Cm ExposeAuthInfo +Writes a temporary file containing a list of authentication methods and +public credentials (e.g. keys) used to authenticate the user. +The location of the file is exposed to the user session through the +.Ev SSH_USER_AUTH +environment variable. +The default is +.Cm no . .It Cm FingerprintHash Specifies the hash algorithm used when logging key fingerprints. Valid options are: -.Dq md5 +.Cm md5 and -.Dq sha256 . +.Cm sha256 . The default is -.Dq sha256 . +.Cm sha256 . .It Cm ForceCommand Forces the execution of the command specified by .Cm ForceCommand , @@ -535,10 +609,12 @@ The command originally supplied by the client is available in the .Ev SSH_ORIGINAL_COMMAND environment variable. Specifying a command of -.Dq internal-sftp -will force the use of an in-process sftp server that requires no support +.Cm internal-sftp +will force the use of an in-process SFTP server that requires no support files when used with .Cm ChrootDirectory . +The default is +.Cm none . .It Cm GatewayPorts Specifies whether remote hosts are allowed to connect to ports forwarded for the client. @@ -551,45 +627,77 @@ 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 -.Dq no +.Cm no to force remote port forwardings to be available to the local host only, -.Dq yes +.Cm yes to force remote port forwardings to bind to the wildcard address, or -.Dq clientspecified +.Cm clientspecified to allow the client to select the address to which the forwarding is bound. The default is -.Dq no . +.Cm no . .It Cm GSSAPIAuthentication Specifies whether user authentication based on GSSAPI is allowed. The default is -.Dq no . -Note that this option applies to protocol version 2 only. +.Cm no . .It Cm GSSAPICleanupCredentials Specifies whether to automatically destroy the user's credentials cache on logout. The default is -.Dq yes . -Note that this option applies to protocol version 2 only. +.Cm yes . +.It Cm GSSAPIStrictAcceptorCheck +Determines whether to be strict about the identity of the GSSAPI acceptor +a client authenticates against. +If set to +.Cm yes +then the client must authenticate against the host +service on the current hostname. +If set to +.Cm 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 +.Cm yes . .It Cm HostbasedAcceptedKeyTypes Specifies the key types that will be accepted for hostbased authentication -as a comma-separated pattern list. -The default -.Dq * -will allow all key types. -The -.Fl Q -option of -.Xr ssh 1 -may be used to list supported key types. +as a list of comma-separated patterns. +Alternately if the specified list begins with a +.Sq + +character, then the specified key types will be appended to the default set +instead of replacing them. +If the specified list begins with a +.Sq - +character, then the specified key types (including wildcards) will be removed +from the default set instead of replacing them. +If the specified list begins with a +.Sq ^ +character, then the specified key types will be placed at the head of the +default set. +The default for this option is: +.Bd -literal -offset 3n +ecdsa-sha2-nistp256-cert-v01@openssh.com, +ecdsa-sha2-nistp384-cert-v01@openssh.com, +ecdsa-sha2-nistp521-cert-v01@openssh.com, +sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, +ssh-ed25519-cert-v01@openssh.com, +sk-ssh-ed25519-cert-v01@openssh.com, +rsa-sha2-512-cert-v01@openssh.com, +rsa-sha2-256-cert-v01@openssh.com, +ssh-rsa-cert-v01@openssh.com, +ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, +sk-ecdsa-sha2-nistp256@openssh.com, +ssh-ed25519,sk-ssh-ed25519@openssh.com, +rsa-sha2-512,rsa-sha2-256,ssh-rsa +.Ed +.Pp +The list of available key types may also be obtained using +.Qq ssh -Q HostbasedAcceptedKeyTypes . .It Cm 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 -.Cm RhostsRSAAuthentication -and applies to protocol version 2 only. The default is -.Dq no . +.Cm no . .It Cm HostbasedUsesNameFromPacketOnly Specifies whether or not the server will attempt to perform a reverse name lookup when matching the name in the @@ -600,13 +708,13 @@ and files during .Cm HostbasedAuthentication . A setting of -.Dq yes +.Cm yes means that .Xr sshd 8 uses the name supplied by the client rather than attempting to resolve the name from the TCP connection itself. The default is -.Dq no . +.Cm no . .It Cm HostCertificate Specifies a file containing a public host certificate. The certificate's public key must match a private host key already specified @@ -618,27 +726,21 @@ is not to load any certificates. .It Cm HostKey Specifies a file containing a private host key used by SSH. -The default is -.Pa /etc/ssh/ssh_host_key -for protocol version 1, and -.Pa /etc/ssh/ssh_host_dsa_key , +The defaults are .Pa /etc/ssh/ssh_host_ecdsa_key , .Pa /etc/ssh/ssh_host_ed25519_key and -.Pa /etc/ssh/ssh_host_rsa_key -for protocol version 2. +.Pa /etc/ssh/ssh_host_rsa_key . +.Pp Note that .Xr sshd 8 -will refuse to use a file if it is group/world-accessible. +will refuse to use a file if it is group/world-accessible +and that the +.Cm HostKeyAlgorithms +option restricts which of the keys are actually used by +.Xr sshd 8 . +.Pp It is possible to have multiple host key files. -.Dq rsa1 -keys are used for version 1 and -.Dq dsa , -.Dq ecdsa , -.Dq ed25519 -or -.Dq 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 @@ -646,86 +748,137 @@ to an .It Cm HostKeyAgent Identifies the UNIX-domain socket used to communicate with an agent that has access to the private host keys. -If -.Dq SSH_AUTH_SOCK +If the string +.Qq SSH_AUTH_SOCK is specified, the location of the socket will be read from the .Ev SSH_AUTH_SOCK environment variable. +.It Cm HostKeyAlgorithms +Specifies the host key algorithms +that the server offers. +The default for this option is: +.Bd -literal -offset 3n +ecdsa-sha2-nistp256-cert-v01@openssh.com, +ecdsa-sha2-nistp384-cert-v01@openssh.com, +ecdsa-sha2-nistp521-cert-v01@openssh.com, +sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, +ssh-ed25519-cert-v01@openssh.com, +sk-ssh-ed25519-cert-v01@openssh.com, +rsa-sha2-512-cert-v01@openssh.com, +rsa-sha2-256-cert-v01@openssh.com, +ssh-rsa-cert-v01@openssh.com, +ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, +sk-ecdsa-sha2-nistp256@openssh.com, +ssh-ed25519,sk-ssh-ed25519@openssh.com, +rsa-sha2-512,rsa-sha2-256,ssh-rsa +.Ed +.Pp +The list of available key types may also be obtained using +.Qq ssh -Q HostKeyAlgorithms . .It Cm IgnoreRhosts -Specifies that +Specifies whether to ignore per-user .Pa .rhosts and .Pa .shosts -files will not be used in -.Cm RhostsRSAAuthentication -or +files during .Cm HostbasedAuthentication . -.Pp +The system-wide .Pa /etc/hosts.equiv and .Pa /etc/shosts.equiv -are still used. -The default is -.Dq yes . +are still used regardless of this setting. +.Pp +Accepted values are +.Cm yes +(the default) to ignore all per-user files, +.Cm shosts-only +to allow the use of +.Pa .shosts +but to ignore +.Pa .rhosts +or +.Cm no +to allow both +.Pa .shosts +and +.Pa rhosts . .It Cm IgnoreUserKnownHosts Specifies whether .Xr sshd 8 should ignore the user's .Pa ~/.ssh/known_hosts during -.Cm RhostsRSAAuthentication -or -.Cm HostbasedAuthentication . +.Cm HostbasedAuthentication +and use only the system-wide known hosts file +.Pa /etc/ssh/known_hosts . The default is .Dq no . +.It Cm Include +Include the specified configuration file(s). +Multiple pathnames may be specified and each pathname may contain +.Xr glob 7 +wildcards. +Files without absolute paths are assumed to be in +.Pa /etc/ssh . +An +.Cm Include +directive may appear inside a +.Cm Match +block +to perform conditional inclusion. .It Cm IPQoS Specifies the IPv4 type-of-service or DSCP class for the connection. Accepted values are -.Dq af11 , -.Dq af12 , -.Dq af13 , -.Dq af21 , -.Dq af22 , -.Dq af23 , -.Dq af31 , -.Dq af32 , -.Dq af33 , -.Dq af41 , -.Dq af42 , -.Dq af43 , -.Dq cs0 , -.Dq cs1 , -.Dq cs2 , -.Dq cs3 , -.Dq cs4 , -.Dq cs5 , -.Dq cs6 , -.Dq cs7 , -.Dq ef , -.Dq lowdelay , -.Dq throughput , -.Dq reliability , -or a numeric value. +.Cm af11 , +.Cm af12 , +.Cm af13 , +.Cm af21 , +.Cm af22 , +.Cm af23 , +.Cm af31 , +.Cm af32 , +.Cm af33 , +.Cm af41 , +.Cm af42 , +.Cm af43 , +.Cm cs0 , +.Cm cs1 , +.Cm cs2 , +.Cm cs3 , +.Cm cs4 , +.Cm cs5 , +.Cm cs6 , +.Cm cs7 , +.Cm ef , +.Cm le , +.Cm lowdelay , +.Cm throughput , +.Cm reliability , +a numeric value, or +.Cm none +to use the operating system default. This option may take one or two arguments, separated by whitespace. If one argument is specified, it is used as the packet class unconditionally. If two values are specified, the first is automatically selected for interactive sessions and the second for non-interactive sessions. The default is -.Dq lowdelay +.Cm af21 +(Low-Latency Data) for interactive sessions and -.Dq throughput +.Cm cs1 +(Lower Effort) for non-interactive sessions. .It Cm KbdInteractiveAuthentication Specifies whether to allow keyboard-interactive authentication. The argument to this keyword must be -.Dq yes +.Cm yes or -.Dq no . +.Cm no . The default is to use whatever value .Cm ChallengeResponseAuthentication is set to (by default -.Dq yes ) . +.Cm yes ) . .It Cm KerberosAuthentication Specifies whether the password provided by the user for .Cm PasswordAuthentication @@ -733,37 +886,57 @@ 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 -.Dq no . +.Cm no . .It Cm 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 -.Dq no . +.Cm no . .It Cm KerberosOrLocalPasswd If password authentication through Kerberos fails then the password will be validated via any additional local mechanism such as .Pa /etc/passwd . The default is -.Dq yes . +.Cm yes . .It Cm KerberosTicketCleanup Specifies whether to automatically destroy the user's ticket cache file on logout. The default is -.Dq yes . +.Cm yes . .It Cm KexAlgorithms Specifies the available KEX (Key Exchange) algorithms. Multiple algorithms must be comma-separated. +Alternately if the specified list begins with a +.Sq + +character, then the specified methods will be appended to the default set +instead of replacing them. +If the specified list begins with a +.Sq - +character, then the specified methods (including wildcards) will be removed +from the default set instead of replacing them. +If the specified list begins with a +.Sq ^ +character, then the specified methods will be placed at the head of the +default set. The supported algorithms are: .Pp .Bl -item -compact -offset indent .It +curve25519-sha256 +.It curve25519-sha256@libssh.org .It diffie-hellman-group1-sha1 .It diffie-hellman-group14-sha1 .It +diffie-hellman-group14-sha256 +.It +diffie-hellman-group16-sha512 +.It +diffie-hellman-group18-sha512 +.It diffie-hellman-group-exchange-sha1 .It diffie-hellman-group-exchange-sha256 @@ -773,31 +946,21 @@ ecdh-sha2-nistp256 ecdh-sha2-nistp384 .It ecdh-sha2-nistp521 +.It +sntrup4591761x25519-sha512@tinyssh.org .El .Pp The default is: .Bd -literal -offset indent -curve25519-sha256@libssh.org, +curve25519-sha256,curve25519-sha256@libssh.org, ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, diffie-hellman-group-exchange-sha256, -diffie-hellman-group14-sha1 +diffie-hellman-group16-sha512,diffie-hellman-group18-sha512, +diffie-hellman-group14-sha256 .Ed .Pp -The list of available key exchange algorithms may also be obtained using the -.Fl Q -option of -.Xr ssh 1 -with an argument of -.Dq kex . -.It Cm 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). +The list of available key exchange algorithms may also be obtained using +.Qq ssh -Q KexAlgorithms . .It Cm ListenAddress Specifies the local addresses .Xr sshd 8 @@ -808,34 +971,47 @@ The following forms may be used: .It .Cm ListenAddress .Sm off -.Ar host | Ar IPv4_addr | Ar IPv6_addr +.Ar hostname | address .Sm on +.Op Cm rdomain Ar domain .It .Cm ListenAddress .Sm off -.Ar host | Ar IPv4_addr : Ar port +.Ar hostname : port .Sm on +.Op Cm rdomain Ar domain .It .Cm ListenAddress .Sm off -.Oo -.Ar host | Ar IPv6_addr Oc : Ar port +.Ar IPv4_address : port .Sm on +.Op Cm rdomain Ar domain +.It +.Cm ListenAddress +.Sm off +.Oo Ar hostname | address Oc : Ar port +.Sm on +.Op Cm rdomain Ar domain .El .Pp +The optional +.Cm rdomain +qualifier requests +.Xr sshd 8 +listen in an explicit routing domain. If .Ar port is not specified, -sshd will listen on the address and all prior +sshd will listen on the address and all .Cm Port options specified. -The default is to listen on all local addresses. +The default is to listen on all local addresses on the current default +routing domain. Multiple .Cm ListenAddress options are permitted. -Additionally, any -.Cm Port -options must precede this option for non-port qualified addresses. +For more information on routing domains, see +.Xr rdomain 4 . .It Cm LoginGraceTime The server disconnects after this time if the user has not successfully logged in. @@ -852,11 +1028,23 @@ DEBUG2 and DEBUG3 each specify higher levels of debugging output. Logging with a DEBUG level violates the privacy of users and is not recommended. .It Cm MACs Specifies the available MAC (message authentication code) algorithms. -The MAC algorithm is used in protocol version 2 -for data integrity protection. +The MAC algorithm is used for data integrity protection. Multiple algorithms must be comma-separated. +If the specified list begins with a +.Sq + +character, then the specified algorithms will be appended to the default set +instead of replacing them. +If the specified list begins with a +.Sq - +character, then the specified algorithms (including wildcards) will be removed +from the default set instead of replacing them. +If the specified list begins with a +.Sq ^ +character, then the specified algorithms will be placed at the head of the +default set. +.Pp The algorithms that contain -.Dq -etm +.Qq -etm calculate the MAC after encryption (encrypt-then-mac). These are considered safer and their use recommended. The supported MACs are: @@ -867,8 +1055,6 @@ hmac-md5 .It hmac-md5-96 .It -hmac-ripemd160 -.It hmac-sha1 .It hmac-sha1-96 @@ -885,8 +1071,6 @@ hmac-md5-etm@openssh.com .It hmac-md5-96-etm@openssh.com .It -hmac-ripemd160-etm@openssh.com -.It hmac-sha1-etm@openssh.com .It hmac-sha1-96-etm@openssh.com @@ -904,16 +1088,13 @@ The default is: .Bd -literal -offset indent 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-sha2-256,hmac-sha2-512,hmac-sha1 .Ed .Pp -The list of available MAC algorithms may also be obtained using the -.Fl Q -option of -.Xr ssh 1 -with an argument of -.Dq mac . +The list of available MAC algorithms may also be obtained using +.Qq ssh -Q mac . .It Cm Match Introduces a conditional block. If all of the criteria on the @@ -938,28 +1119,30 @@ The available criteria are .Cm Host , .Cm LocalAddress , .Cm LocalPort , +.Cm RDomain , and -.Cm Address . +.Cm Address +(with +.Cm RDomain +representing the +.Xr rdomain 4 +on which the connection was received). +.Pp 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 +.Sx PATTERNS +section of .Xr ssh_config 5 . .Pp The patterns in an .Cm Address criteria may additionally contain addresses to match in CIDR -address/masklen format, e.g.\& -.Dq 192.0.2.0/24 -or -.Dq 3ffe:ffff::/32 . +address/masklen format, +such as 192.0.2.0/24 or 2001:db8::/32. Note that the mask length provided must be consistent with the address - it is an error to specify a mask length that is too long for the address or one with bits set in this host portion of the address. -For example, -.Dq 192.0.2.0/33 -and -.Dq 192.0.2.0/8 -respectively. +For example, 192.0.2.0/33 and 192.0.2.0/8, respectively. .Pp Only a subset of keywords may be used on the lines following a .Cm Match @@ -975,9 +1158,13 @@ Available keywords are .Cm AuthorizedKeysCommand , .Cm AuthorizedKeysCommandUser , .Cm AuthorizedKeysFile , +.Cm AuthorizedPrincipalsCommand , +.Cm AuthorizedPrincipalsCommandUser , .Cm AuthorizedPrincipalsFile , .Cm Banner , .Cm ChrootDirectory , +.Cm ClientAliveCountMax , +.Cm ClientAliveInterval , .Cm DenyGroups , .Cm DenyUsers , .Cm ForceCommand , @@ -986,13 +1173,17 @@ Available keywords are .Cm HostbasedAcceptedKeyTypes , .Cm HostbasedAuthentication , .Cm HostbasedUsesNameFromPacketOnly , +.Cm IgnoreRhosts , +.Cm Include , .Cm IPQoS , .Cm KbdInteractiveAuthentication , .Cm KerberosAuthentication , +.Cm LogLevel , .Cm MaxAuthTries , .Cm MaxSessions , .Cm PasswordAuthentication , .Cm PermitEmptyPasswords , +.Cm PermitListen , .Cm PermitOpen , .Cm PermitRootLogin , .Cm PermitTTY , @@ -1002,15 +1193,15 @@ Available keywords are .Cm PubkeyAuthentication , .Cm RekeyLimit , .Cm RevokedKeys , -.Cm RhostsRSAAuthentication , -.Cm RSAAuthentication , +.Cm RDomain , +.Cm SetEnv , .Cm StreamLocalBindMask , .Cm StreamLocalBindUnlink , .Cm TrustedUserCAKeys , .Cm X11DisplayOffset , .Cm X11Forwarding and -.Cm X11UseLocalHost . +.Cm X11UseLocalhost . .It Cm MaxAuthTries Specifies the maximum number of authentication attempts permitted per connection. @@ -1018,7 +1209,15 @@ Once the number of failures reaches half this value, additional failures are logged. The default is 6. .It Cm MaxSessions -Specifies the maximum number of open sessions permitted per network connection. +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 +.Cm 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. .It Cm MaxStartups Specifies the maximum number of concurrent unauthenticated connections to the @@ -1030,29 +1229,63 @@ The default is 10:30:100. .Pp Alternatively, random early drop can be enabled by specifying the three colon separated values -.Dq start:rate:full -(e.g. "10:30:60"). +start:rate:full (e.g. "10:30:60"). .Xr sshd 8 -will refuse connection attempts with a probability of -.Dq rate/100 -(30%) -if there are currently -.Dq start -(10) -unauthenticated connections. +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 -.Dq full -(60). +are refused if the number of unauthenticated connections reaches full (60). .It Cm PasswordAuthentication Specifies whether password authentication is allowed. The default is -.Dq yes . +.Cm yes . .It Cm PermitEmptyPasswords When password authentication is allowed, it specifies whether the server allows login to accounts with empty password strings. The default is -.Dq no . +.Cm no . +.It Cm PermitListen +Specifies the addresses/ports on which a remote TCP port forwarding may listen. +The listen specification must be one of the following forms: +.Pp +.Bl -item -offset indent -compact +.It +.Cm PermitListen +.Sm off +.Ar port +.Sm on +.It +.Cm PermitListen +.Sm off +.Ar host : port +.Sm on +.El +.Pp +Multiple permissions may be specified by separating them with whitespace. +An argument of +.Cm any +can be used to remove all restrictions and permit any listen requests. +An argument of +.Cm none +can be used to prohibit all listen requests. +The host name may contain wildcards as described in the PATTERNS section in +.Xr ssh_config 5 . +The wildcard +.Sq * +can also be used in place of a port number to allow all ports. +By default all port forwarding listen requests are permitted. +Note that the +.Cm GatewayPorts +option may further restrict which addresses may be listened on. +Note also that +.Xr ssh 1 +will request a listen host of +.Dq localhost +if no listen host was specifically requested, and this name is +treated differently to explicit localhost addresses of +.Dq 127.0.0.1 +and +.Dq ::1 . .It Cm PermitOpen Specifies the destinations to which TCP port forwarding is permitted. The forwarding specification must be one of the following forms: @@ -1077,30 +1310,37 @@ The forwarding specification must be one of the following forms: .Pp Multiple forwards may be specified by separating them with whitespace. An argument of -.Dq any +.Cm any can be used to remove all restrictions and permit any forwarding requests. An argument of -.Dq none +.Cm none can be used to prohibit all forwarding requests. +The wildcard +.Sq * +can be used for host or port to allow all hosts or ports respectively. +Otherwise, no pattern matching or address lookups are performed on supplied +names. By default all port forwarding requests are permitted. .It Cm PermitRootLogin Specifies whether root can log in using .Xr ssh 1 . The argument must be -.Dq yes , -.Dq without-password , -.Dq forced-commands-only , +.Cm yes , +.Cm prohibit-password , +.Cm forced-commands-only , or -.Dq no . +.Cm no . The default is -.Dq yes . +.Cm prohibit-password . .Pp If this option is set to -.Dq without-password , -password authentication is disabled for root. +.Cm prohibit-password +(or its deprecated alias, +.Cm without-password ) , +password and keyboard-interactive authentication are disabled for root. .Pp If this option is set to -.Dq forced-commands-only , +.Cm forced-commands-only , root login with public key authentication will be allowed, but only if the .Ar command @@ -1110,37 +1350,37 @@ normally not allowed). All other authentication methods are disabled for root. .Pp If this option is set to -.Dq no , +.Cm no , root is not allowed to log in. +.It Cm PermitTTY +Specifies whether +.Xr pty 4 +allocation is permitted. +The default is +.Cm yes . .It Cm PermitTunnel Specifies whether .Xr tun 4 device forwarding is allowed. The argument must be -.Dq yes , -.Dq point-to-point +.Cm yes , +.Cm point-to-point (layer 3), -.Dq ethernet +.Cm ethernet (layer 2), or -.Dq no . +.Cm no . Specifying -.Dq yes +.Cm yes permits both -.Dq point-to-point +.Cm point-to-point and -.Dq ethernet . +.Cm ethernet . The default is -.Dq no . +.Cm no . .Pp Independent of this setting, the permissions of the selected .Xr tun 4 device must allow access to the user. -.It Cm PermitTTY -Specifies whether -.Xr pty 4 -allocation is permitted. -The default is -.Dq yes . .It Cm PermitUserEnvironment Specifies whether .Pa ~/.ssh/environment @@ -1150,8 +1390,14 @@ options in .Pa ~/.ssh/authorized_keys are processed by .Xr sshd 8 . +Valid options are +.Cm yes , +.Cm no +or a pattern-list specifying which environment variable names to accept +(for example +.Qq LANG,LC_* ) . The default is -.Dq no . +.Cm no . Enabling environment processing may enable users to bypass access restrictions in some configurations using mechanisms such as .Ev LD_PRELOAD . @@ -1160,10 +1406,12 @@ Specifies whether any .Pa ~/.ssh/rc file is executed. The default is -.Dq yes . +.Cm yes . .It Cm PidFile Specifies the file that contains the process ID of the -SSH daemon. +SSH daemon, or +.Cm none +to not write one. The default is .Pa /var/run/sshd.pid . .It Cm Port @@ -1180,7 +1428,7 @@ Specifies whether should print the date and time of the last user login when a user logs in interactively. The default is -.Dq yes . +.Cm yes . .It Cm PrintMotd Specifies whether .Xr sshd 8 @@ -1191,41 +1439,69 @@ when a user logs in interactively. .Pa /etc/profile , or equivalent.) The default is -.Dq yes . -.It Cm Protocol -Specifies the protocol versions -.Xr sshd 8 -supports. -The possible values are -.Sq 1 -and -.Sq 2 . -Multiple versions must be comma-separated. -The default is -.Sq 2 . -Note that the order of the protocol list does not indicate preference, -because the client selects among multiple protocol versions offered -by the server. -Specifying -.Dq 2,1 -is identical to -.Dq 1,2 . +.Cm yes . .It Cm PubkeyAcceptedKeyTypes Specifies the key types that will be accepted for public key authentication -as a comma-separated pattern list. -The default -.Dq * -will allow all key types. +as a list of comma-separated patterns. +Alternately if the specified list begins with a +.Sq + +character, then the specified key types will be appended to the default set +instead of replacing them. +If the specified list begins with a +.Sq - +character, then the specified key types (including wildcards) will be removed +from the default set instead of replacing them. +If the specified list begins with a +.Sq ^ +character, then the specified key types will be placed at the head of the +default set. +The default for this option is: +.Bd -literal -offset 3n +ecdsa-sha2-nistp256-cert-v01@openssh.com, +ecdsa-sha2-nistp384-cert-v01@openssh.com, +ecdsa-sha2-nistp521-cert-v01@openssh.com, +sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, +ssh-ed25519-cert-v01@openssh.com, +sk-ssh-ed25519-cert-v01@openssh.com, +rsa-sha2-512-cert-v01@openssh.com, +rsa-sha2-256-cert-v01@openssh.com, +ssh-rsa-cert-v01@openssh.com, +ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, +sk-ecdsa-sha2-nistp256@openssh.com, +ssh-ed25519,sk-ssh-ed25519@openssh.com, +rsa-sha2-512,rsa-sha2-256,ssh-rsa +.Ed +.Pp +The list of available key types may also be obtained using +.Qq ssh -Q PubkeyAcceptedKeyTypes . +.It Cm PubkeyAuthOptions +Sets one or more public key authentication options. +Two option keywords are currently supported: +.Cm none +(the default; indicating no additional options are enabled) +and +.Cm touch-required . +.Pp The -.Fl Q -option of -.Xr ssh 1 -may be used to list supported key types. +.Cm touch-required +option causes public key authentication using a FIDO authenticator algorithm +(i.e.\& +.Cm ecdsa-sk +or +.Cm ed25519-sk ) +to always require the signature to attest that a physically present user +explicitly confirmed the authentication (usually by touching the authenticator). +By default, +.Xr sshd 8 +requires user presence unless overridden with an authorized_keys option. +The +.Cm touch-required +flag disables this override. +This option has no effect for other, non-authenticator public key types. .It Cm PubkeyAuthentication Specifies whether public key authentication is allowed. The default is -.Dq yes . -Note that this option applies to protocol version 2 only. +.Cm yes . .It Cm RekeyLimit Specifies the maximum amount of data that may be transmitted before the session key is renegotiated, optionally followed a maximum amount of @@ -1248,12 +1524,13 @@ section. The default value for .Cm RekeyLimit is -.Dq default none , +.Cm 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. -This option applies to protocol version 2 only. .It Cm RevokedKeys -Specifies revoked public keys. +Specifies revoked public keys file, or +.Cm 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. @@ -1262,20 +1539,34 @@ an OpenSSH Key Revocation List (KRL) as generated by .Xr ssh-keygen 1 . For more information on KRLs, see the KEY REVOCATION LISTS section in .Xr ssh-keygen 1 . -.It Cm RhostsRSAAuthentication -Specifies whether rhosts or /etc/hosts.equiv authentication together -with successful RSA host authentication is allowed. -The default is -.Dq no . -This option applies to protocol version 1 only. -.It Cm RSAAuthentication -Specifies whether pure RSA authentication is allowed. -The default is -.Dq yes . -This option applies to protocol version 1 only. -.It Cm ServerKeyBits -Defines the number of bits in the ephemeral protocol version 1 server key. -The minimum value is 512, and the default is 1024. +.It Cm RDomain +Specifies an explicit routing domain that is applied after authentication +has completed. +The user session, as well and any forwarded or listening IP sockets, +will be bound to this +.Xr rdomain 4 . +If the routing domain is set to +.Cm \&%D , +then the domain in which the incoming connection was received will be applied. +.It Cm SecurityKeyProvider +Specifies a path to a library that will be used when loading +FIDO authenticator-hosted keys, overriding the default of using +the built-in USB HID support. +.It Cm SetEnv +Specifies one or more environment variables to set in child sessions started +by +.Xr sshd 8 +as +.Dq NAME=VALUE . +The environment value may be quoted (e.g. if it contains whitespace +characters). +Environment variables set by +.Cm SetEnv +override the default environment and any variables specified by the user +via +.Cm AcceptEnv +or +.Cm PermitUserEnvironment . .It Cm StreamLocalBindMask Sets the octal file creation mode mask .Pq umask @@ -1298,11 +1589,11 @@ 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. .Pp The argument must be -.Dq yes +.Cm yes or -.Dq no . +.Cm no . The default is -.Dq no . +.Cm no . .It Cm StrictModes Specifies whether .Xr sshd 8 @@ -1311,7 +1602,7 @@ 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 -.Dq yes . +.Cm yes . Note that this does not apply to .Cm ChrootDirectory , whose permissions and ownership are checked unconditionally. @@ -1321,22 +1612,17 @@ Arguments should be a subsystem name and a command (with optional arguments) to execute upon subsystem request. .Pp The command -.Xr sftp-server 8 -implements the -.Dq sftp -file transfer subsystem. +.Cm sftp-server +implements the SFTP file transfer subsystem. .Pp Alternately the name -.Dq internal-sftp -implements an in-process -.Dq sftp -server. +.Cm internal-sftp +implements an in-process SFTP server. This may simplify configurations using .Cm ChrootDirectory to force a different filesystem root on clients. .Pp By default no subsystems are defined. -Note that this option applies to protocol version 2 only. .It Cm SyslogFacility Gives the facility code that is used when logging messages from .Xr sshd 8 . @@ -1353,20 +1639,22 @@ 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 -.Dq ghost +.Qq ghost users and consuming server resources. .Pp The default is -.Dq yes +.Cm 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. .Pp To disable TCP keepalive messages, the value should be set to -.Dq no . +.Cm no . .It Cm TrustedUserCAKeys Specifies a file containing public keys of certificate authorities that are -trusted to sign user certificates for authentication. +trusted to sign user certificates for authentication, or +.Cm none +to not use one. Keys are listed one per line; empty lines and comments starting with .Ql # are allowed. @@ -1381,34 +1669,24 @@ For more details on certificates, see the CERTIFICATES section in .It Cm UseDNS Specifies whether .Xr sshd 8 -should look up the remote host name and check that +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. -The default is -.Dq no . -.It Cm UseLogin -Specifies whether -.Xr login 1 -is used for interactive login sessions. -The default is -.Dq no . -Note that -.Xr login 1 -is never used for remote command execution. -Note also, that if this is enabled, -.Cm X11Forwarding -will be disabled because -.Xr login 1 -does not know how to handle -.Xr xauth 1 -cookies. -If -.Cm UsePrivilegeSeparation -is specified, it will be disabled after authentication. +.Pp +If this option is set to +.Cm no +(the default) then only addresses and not host names may be used in +.Pa ~/.ssh/authorized_keys +.Cm from +and +.Nm +.Cm Match +.Cm Host +directives. .It Cm UsePAM Enables the Pluggable Authentication Module interface. If set to -.Dq yes +.Cm yes this will enable PAM authentication using .Cm ChallengeResponseAuthentication and @@ -1428,29 +1706,12 @@ is enabled, you will not be able to run .Xr sshd 8 as a non-root user. The default is -.Dq no . -.It Cm UsePrivilegeSeparation -Specifies whether -.Xr sshd 8 -separates privileges by creating an unprivileged child process -to deal with incoming network traffic. -After successful authentication, another process will be created that has -the privilege of the authenticated user. -The goal of privilege separation is to prevent privilege -escalation by containing any corruption within the unprivileged processes. -The default is -.Dq yes . -If -.Cm UsePrivilegeSeparation -is set to -.Dq sandbox -then the pre-authentication unprivileged process is subject to additional -restrictions. +.Cm no . .It Cm VersionAddendum Optionally specifies additional text to append to the SSH protocol banner sent by the server upon connection. The default is -.Dq none . +.Cm none . .It Cm X11DisplayOffset Specifies the first display number available for .Xr sshd 8 Ns 's @@ -1460,18 +1721,18 @@ The default is 10. .It Cm X11Forwarding Specifies whether X11 forwarding is permitted. The argument must be -.Dq yes +.Cm yes or -.Dq no . +.Cm no . The default is -.Dq no . +.Cm no . .Pp When X11 forwarding is enabled, there may be additional exposure to the server and to client displays if the .Xr sshd 8 proxy display is configured to listen on the wildcard address (see -.Cm X11UseLocalhost -below), though this is not the default. +.Cm X11UseLocalhost ) , +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 @@ -1483,14 +1744,11 @@ in 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 -.Dq no +.Cm no setting. .Pp 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 -.Cm UseLogin -is enabled. .It Cm X11UseLocalhost Specifies whether .Xr sshd 8 @@ -1501,25 +1759,27 @@ sshd binds the forwarding server to the loopback address and sets the hostname part of the .Ev DISPLAY environment variable to -.Dq localhost . +.Cm localhost . This prevents remote hosts from connecting to the proxy display. However, some older X11 clients may not function with this configuration. .Cm X11UseLocalhost may be set to -.Dq no +.Cm no to specify that the forwarding server should be bound to the wildcard address. The argument must be -.Dq yes +.Cm yes or -.Dq no . +.Cm no . The default is -.Dq yes . +.Cm yes . .It Cm XAuthLocation Specifies the full pathname of the .Xr xauth 1 -program. +program, or +.Cm none +to not use one. The default is .Pa /usr/X11R6/bin/xauth . .El @@ -1564,6 +1824,57 @@ Time format examples: .It 1h30m 1 hour 30 minutes (90 minutes) .El +.Sh TOKENS +Arguments to some keywords can make use of tokens, +which are expanded at runtime: +.Pp +.Bl -tag -width XXXX -offset indent -compact +.It %% +A literal +.Sq % . +.It \&%D +The routing domain in which the incoming connection was received. +.It %F +The fingerprint of the CA key. +.It %f +The fingerprint of the key or certificate. +.It %h +The home directory of the user. +.It %i +The key ID in the certificate. +.It %K +The base64-encoded CA key. +.It %k +The base64-encoded key or certificate for authentication. +.It %s +The serial number of the certificate. +.It \&%T +The type of the CA key. +.It %t +The key or certificate type. +.It \&%U +The numeric user ID of the target user. +.It %u +The username. +.El +.Pp +.Cm AuthorizedKeysCommand +accepts the tokens %%, %f, %h, %k, %t, %U, and %u. +.Pp +.Cm AuthorizedKeysFile +accepts the tokens %%, %h, %U, and %u. +.Pp +.Cm AuthorizedPrincipalsCommand +accepts the tokens %%, %F, %f, %h, %i, %K, %k, %s, %T, %t, %U, and %u. +.Pp +.Cm AuthorizedPrincipalsFile +accepts the tokens %%, %h, %U, and %u. +.Pp +.Cm ChrootDirectory +accepts the tokens %%, %h, %U, and %u. +.Pp +.Cm RoutingDomain +accepts the token %D. .Sh FILES .Bl -tag -width Ds .It Pa /etc/ssh/sshd_config @@ -1573,15 +1884,22 @@ This file should be writable by root only, but it is recommended (though not necessary) that it be world-readable. .El .Sh SEE ALSO +.Xr sftp-server 8 , .Xr sshd 8 .Sh AUTHORS +.An -nosplit 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 +ssh 1.2.12 release by +.An Tatu Ylonen . +.An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos , +.An Theo de Raadt +and +.An 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. +.An Markus Friedl +contributed the support for SSH protocol versions 1.5 and 2.0. +.An Niels Provos +and +.An Markus Friedl +contributed support for privilege separation. |