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diff --git a/sshd_config.5 b/sshd_config.5 new file mode 100644 index 0000000..7313a7f --- /dev/null +++ b/sshd_config.5 @@ -0,0 +1,2082 @@ +.\" +.\" Author: Tatu Ylonen <ylo@cs.hut.fi> +.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland +.\" All rights reserved +.\" +.\" As far as I am concerned, the code I have written for this software +.\" can be used freely for any purpose. Any derived versions of this +.\" software must be clearly marked as such, and if the derived work is +.\" incompatible with the protocol description in the RFC file, it must be +.\" called by a name other than "ssh" or "Secure Shell". +.\" +.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. +.\" Copyright (c) 1999 Aaron Campbell. All rights reserved. +.\" Copyright (c) 1999 Theo de Raadt. All rights reserved. +.\" +.\" Redistribution and use in source and binary forms, with or without +.\" modification, are permitted provided that the following conditions +.\" are met: +.\" 1. Redistributions of source code must retain the above copyright +.\" notice, this list of conditions and the following disclaimer. +.\" 2. Redistributions in binary form must reproduce the above copyright +.\" notice, this list of conditions and the following disclaimer in the +.\" documentation and/or other materials provided with the distribution. +.\" +.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR +.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES +.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. +.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, +.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT +.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, +.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY +.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT +.\" (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 1.347 2023/01/18 06:55:32 jmc Exp $ +.Dd $Mdocdate: January 18 2023 $ +.Dt SSHD_CONFIG 5 +.Os +.Sh NAME +.Nm sshd_config +.Nd OpenSSH daemon configuration file +.Sh DESCRIPTION +.Xr sshd 8 +reads configuration data from +.Pa /etc/ssh/sshd_config +(or the file specified with +.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. +Arguments may optionally be enclosed in double quotes +.Pq \&" +in order to represent arguments containing spaces. +.Pp +The possible +keywords and their meanings are as follows (note that +keywords are case-insensitive and arguments are case-sensitive): +.Bl -tag -width Ds +.It Cm AcceptEnv +Specifies what environment variables sent by the client will be copied into +the session's +.Xr environ 7 . +See +.Cm SendEnv +and +.Cm SetEnv +in +.Xr ssh_config 5 +for how to configure the client. +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 +.Ql \&? . +Multiple environment variables may be separated by whitespace or spread +across multiple +.Cm 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. +.It Cm AddressFamily +Specifies which address family should be used by +.Xr sshd 8 . +Valid arguments are +.Cm any +(the default), +.Cm inet +(use IPv4 only), or +.Cm inet6 +(use IPv6 only). +.It Cm AllowAgentForwarding +Specifies whether +.Xr ssh-agent 1 +forwarding is permitted. +The default is +.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. +.It Cm 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 groups directives are processed in the following order: +.Cm DenyGroups , +.Cm AllowGroups . +.Pp +See PATTERNS in +.Xr ssh_config 5 +for more information on patterns. +.It Cm AllowStreamLocalForwarding +Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted. +The available options are +.Cm yes +(the default) +or +.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 +.Cm remote +to allow remote forwarding only. +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 AllowTcpForwarding +Specifies whether TCP forwarding is permitted. +The available options are +.Cm yes +(the default) +or +.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 +.Cm remote +to allow remote forwarding only. +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 +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 users directives are processed in the following order: +.Cm DenyUsers , +.Cm AllowUsers . +.Pp +See PATTERNS in +.Xr ssh_config 5 +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 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, +.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 +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 +.Cm bsdauth +or +.Cm pam . +depending on the server configuration. +For example, +.Qq keyboard-interactive:bsdauth +would restrict keyboard interactive authentication to the +.Cm bsdauth +device. +.Pp +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, +.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. +.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, 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 ) . +.Cm AuthorizedKeysCommand +is tried after the usual +.Cm AuthorizedKeysFile +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 +.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 +.Cm AuthorizedKeysCommand +is specified but +.Cm AuthorizedKeysCommandUser +is not, then +.Xr sshd 8 +will refuse to start. +.It Cm AuthorizedKeysFile +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 +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 +.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. +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 +.Sx AUTHORIZED_KEYS FILE FORMAT +in +.Xr sshd 8 ) . +Empty lines and comments starting with +.Ql # +are ignored. +.Pp +Arguments to +.Cm AuthorizedPrincipalsFile +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. +The default is +.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 +.Cm TrustedUserCAKeys +and is not consulted for certification authorities trusted via +.Pa ~/.ssh/authorized_keys , +though the +.Cm principals= +key option offers a similar facility (see +.Xr sshd 8 +for details). +.It Cm Banner +The contents of the specified file are sent to the remote user before +authentication is allowed. +If the argument is +.Cm none +then no banner is displayed. +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 +ssh-ed25519,ecdsa-sha2-nistp256, +ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, +sk-ssh-ed25519@openssh.com, +sk-ecdsa-sha2-nistp256@openssh.com, +rsa-sha2-512,rsa-sha2-256 +.Ed +.Pp +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. +.Pp +Certificates signed using other algorithms will not be accepted for +public key or host-based authentication. +.It Cm ChannelTimeout +Specifies whether and how quickly +.Xr sshd 8 +should close inactive channels. +Timeouts are specified as one or more +.Dq type=interval +pairs separated by whitespace, where the +.Dq type +must be a channel type name (as described in the table below), optionally +containing wildcard characters. +.Pp +The timeout value +.Dq interval +is specified in seconds or may use any of the units documented in the +.Sx TIME FORMATS +section. +For example, +.Dq session:*=5m +would cause all sessions to terminate after five minutes of inactivity. +Specifying a zero value disables the inactivity timeout. +.Pp +The available channel types include: +.Bl -tag -width Ds +.It Cm agent-connection +Open connections to +.Xr ssh-agent 1 . +.It Cm direct-tcpip , Cm direct-streamlocal@openssh.com +Open TCP or Unix socket (respectively) connections that have +been established from a +.Xr ssh 1 +local forwarding, i.e.\& +.Cm LocalForward +or +.Cm DynamicForward . +.It Cm forwarded-tcpip , Cm forwarded-streamlocal@openssh.com +Open TCP or Unix socket (respectively) connections that have been +established to a +.Xr sshd 8 +listening on behalf of a +.Xr ssh 1 +remote forwarding, i.e.\& +.Cm RemoteForward . +.It Cm session:command +Command execution sessions. +.It Cm session:shell +Interactive shell sessions. +.It Cm session:subsystem:... +Subsystem sessions, e.g. for +.Xr sftp 1 , +which could be identified as +.Cm session:subsystem:sftp . +.It Cm x11-connection +Open X11 forwarding sessions. +.El +.Pp +Note that in all the above cases, terminating an inactive session does not +guarantee to remove all resources associated with the session, e.g. shell +processes or X11 clients relating to the session may continue to execute. +.Pp +Moreover, terminating an inactive channel or session does not necessarily +close the SSH connection, nor does it prevent a client from +requesting another channel of the same type. +In particular, expiring an inactive forwarding session does not prevent +another identical forwarding from being subsequently created. +See also +.Cm UnusedConnectionTimeout , +which may be used in conjunction with this option. +.Pp +The default is not to expire channels of any type for inactivity. +.It Cm ChrootDirectory +Specifies the pathname of a directory to +.Xr chroot 2 +to after authentication. +At session startup +.Xr 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, +.Xr sshd 8 +changes the working directory to the user's home directory. +Arguments to +.Cm ChrootDirectory +accept the tokens described in the +.Sx TOKENS +section. +.Pp +The +.Cm 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 +.Xr sh 1 , +and basic +.Pa /dev +nodes such as +.Xr null 4 , +.Xr zero 4 , +.Xr stdin 4 , +.Xr stdout 4 , +.Xr stderr 4 , +and +.Xr 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 +.Pa /dev/log +inside the chroot directory on some operating systems (see +.Xr sftp-server 8 +for details). +.Pp +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 +.Xr sshd 8 +cannot detect. +.Pp +The default is +.Cm none , +indicating not to +.Xr chroot 2 . +.It Cm Ciphers +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 +.It +3des-cbc +.It +aes128-cbc +.It +aes192-cbc +.It +aes256-cbc +.It +aes128-ctr +.It +aes192-ctr +.It +aes256-ctr +.It +aes128-gcm@openssh.com +.It +aes256-gcm@openssh.com +.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 +.Ed +.Pp +The list of available ciphers may also be obtained using +.Qq ssh -Q cipher . +.It Cm ClientAliveCountMax +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 . +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 unresponsive. +.Pp +The default value is 3. +If +.Cm ClientAliveInterval +is set to 15, and +.Cm ClientAliveCountMax +is left at the default, unresponsive SSH clients +will be disconnected after approximately 45 seconds. +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, +.Xr 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. +.It Cm Compression +Specifies whether compression is enabled after +the user has authenticated successfully. +The argument must be +.Cm yes , +.Cm delayed +(a legacy synonym for +.Cm yes ) +or +.Cm no . +The default is +.Cm yes . +.It Cm 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 groups directives are processed in the following order: +.Cm DenyGroups , +.Cm AllowGroups . +.Pp +See PATTERNS in +.Xr ssh_config 5 +for more information on patterns. +.It Cm 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 users directives are processed in the following order: +.Cm DenyUsers , +.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: +.Cm md5 +and +.Cm sha256 . +The default is +.Cm sha256 . +.It Cm ForceCommand +Forces the execution of the command specified by +.Cm ForceCommand , +ignoring any command supplied by the client and +.Pa ~/.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 +.Cm Match +block. +The command originally supplied by the client is available in the +.Ev SSH_ORIGINAL_COMMAND +environment variable. +Specifying a command of +.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. +By default, +.Xr sshd 8 +binds remote port forwardings to the loopback address. +This prevents other remote hosts from connecting to forwarded ports. +.Cm 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 +.Cm no +to force remote port forwardings to be available to the local host only, +.Cm yes +to force remote port forwardings to bind to the wildcard address, or +.Cm clientspecified +to allow the client to select the address to which the forwarding is bound. +The default is +.Cm no . +.It Cm GSSAPIAuthentication +Specifies whether user authentication based on GSSAPI is allowed. +The default is +.Cm no . +.It Cm GSSAPICleanupCredentials +Specifies whether to automatically destroy the user's credentials cache +on logout. +The default is +.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 HostbasedAcceptedAlgorithms +Specifies the signature algorithms that will be accepted for hostbased +authentication as a list of comma-separated patterns. +Alternately if the specified list begins with a +.Sq + +character, then the specified signature algorithms will be appended to +the default set instead of replacing them. +If the specified list begins with a +.Sq - +character, then the specified signature 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 signature algorithms will be placed at +the head of the default set. +The default for this option is: +.Bd -literal -offset 3n +ssh-ed25519-cert-v01@openssh.com, +ecdsa-sha2-nistp256-cert-v01@openssh.com, +ecdsa-sha2-nistp384-cert-v01@openssh.com, +ecdsa-sha2-nistp521-cert-v01@openssh.com, +sk-ssh-ed25519-cert-v01@openssh.com, +sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, +rsa-sha2-512-cert-v01@openssh.com, +rsa-sha2-256-cert-v01@openssh.com, +ssh-ed25519, +ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, +sk-ssh-ed25519@openssh.com, +sk-ecdsa-sha2-nistp256@openssh.com, +rsa-sha2-512,rsa-sha2-256 +.Ed +.Pp +The list of available signature algorithms may also be obtained using +.Qq ssh -Q HostbasedAcceptedAlgorithms . +This was formerly named 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). +The default is +.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 +.Pa ~/.shosts , +.Pa ~/.rhosts , +and +.Pa /etc/hosts.equiv +files during +.Cm HostbasedAuthentication . +A setting of +.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 +.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 +by +.Cm HostKey . +The default behaviour of +.Xr sshd 8 +is not to load any certificates. +.It Cm HostKey +Specifies a file containing a private host key +used by SSH. +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 . +.Pp +Note that +.Xr sshd 8 +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. +It is also possible to specify public host key files instead. +In this case operations on the private key will be delegated +to an +.Xr ssh-agent 1 . +.It Cm HostKeyAgent +Identifies the UNIX-domain socket used to communicate +with an agent that has access to the private host keys. +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 signature algorithms +that the server offers. +The default for this option is: +.Bd -literal -offset 3n +ssh-ed25519-cert-v01@openssh.com, +ecdsa-sha2-nistp256-cert-v01@openssh.com, +ecdsa-sha2-nistp384-cert-v01@openssh.com, +ecdsa-sha2-nistp521-cert-v01@openssh.com, +sk-ssh-ed25519-cert-v01@openssh.com, +sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, +rsa-sha2-512-cert-v01@openssh.com, +rsa-sha2-256-cert-v01@openssh.com, +ssh-ed25519, +ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, +sk-ssh-ed25519@openssh.com, +sk-ecdsa-sha2-nistp256@openssh.com, +rsa-sha2-512,rsa-sha2-256 +.Ed +.Pp +The list of available signature algorithms may also be obtained using +.Qq ssh -Q HostKeyAlgorithms . +.It Cm IgnoreRhosts +Specifies whether to ignore per-user +.Pa .rhosts +and +.Pa .shosts +files during +.Cm HostbasedAuthentication . +The system-wide +.Pa /etc/hosts.equiv +and +.Pa /etc/shosts.equiv +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 HostbasedAuthentication +and use only the system-wide known hosts file +.Pa /etc/ssh/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 that will be expanded and processed in lexical order. +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 +.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 +.Cm af21 +(Low-Latency Data) +for interactive sessions and +.Cm cs1 +(Lower Effort) +for non-interactive sessions. +.It Cm KbdInteractiveAuthentication +Specifies whether to allow keyboard-interactive authentication. +All authentication styles from +.Xr login.conf 5 +are supported. +The default is +.Cm yes . +The argument to this keyword must be +.Cm yes +or +.Cm no . +.Cm ChallengeResponseAuthentication +is a deprecated alias for this. +.It Cm KerberosAuthentication +Specifies whether the password provided by the user for +.Cm 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 +.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 +.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 +.Cm yes . +.It Cm KerberosTicketCleanup +Specifies whether to automatically destroy the user's ticket cache +file on logout. +The default is +.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 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. +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 +.It +ecdh-sha2-nistp256 +.It +ecdh-sha2-nistp384 +.It +ecdh-sha2-nistp521 +.It +sntrup761x25519-sha512@openssh.com +.El +.Pp +The default is: +.Bd -literal -offset indent +sntrup761x25519-sha512@openssh.com, +curve25519-sha256,curve25519-sha256@libssh.org, +ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, +diffie-hellman-group-exchange-sha256, +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 +.Qq ssh -Q KexAlgorithms . +.It Cm ListenAddress +Specifies the local addresses +.Xr sshd 8 +should listen on. +The following forms may be used: +.Pp +.Bl -item -offset indent -compact +.It +.Cm ListenAddress +.Sm off +.Ar hostname | address +.Sm on +.Op Cm rdomain Ar domain +.It +.Cm ListenAddress +.Sm off +.Ar hostname : port +.Sm on +.Op Cm rdomain Ar domain +.It +.Cm ListenAddress +.Sm off +.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 +.Cm Port +options specified. +The default is to listen on all local addresses on the current default +routing domain. +Multiple +.Cm ListenAddress +options are permitted. +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. +If the value is 0, there is no time limit. +The default is 120 seconds. +.It Cm LogLevel +Gives the verbosity level that is used when logging messages from +.Xr 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. +.It Cm LogVerbose +Specify one or more overrides to LogLevel. +An override consists of a pattern lists that matches the source file, function +and line number to force detailed logging for. +For example, an override pattern of: +.Bd -literal -offset indent +kex.c:*:1000,*:kex_exchange_identification():*,packet.c:* +.Ed +.Pp +would enable detailed logging for line 1000 of +.Pa kex.c , +everything in the +.Fn kex_exchange_identification +function, and all code in the +.Pa packet.c +file. +This option is intended for debugging and no overrides are enabled by default. +.It Cm 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 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 +.Qq -etm +calculate the MAC after encryption (encrypt-then-mac). +These are considered safer and their use recommended. +The supported MACs are: +.Pp +.Bl -item -compact -offset indent +.It +hmac-md5 +.It +hmac-md5-96 +.It +hmac-sha1 +.It +hmac-sha1-96 +.It +hmac-sha2-256 +.It +hmac-sha2-512 +.It +umac-64@openssh.com +.It +umac-128@openssh.com +.It +hmac-md5-etm@openssh.com +.It +hmac-md5-96-etm@openssh.com +.It +hmac-sha1-etm@openssh.com +.It +hmac-sha1-96-etm@openssh.com +.It +hmac-sha2-256-etm@openssh.com +.It +hmac-sha2-512-etm@openssh.com +.It +umac-64-etm@openssh.com +.It +umac-128-etm@openssh.com +.El +.Pp +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-sha1 +.Ed +.Pp +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 +.Cm Match +line are satisfied, the keywords on the following lines override those +set in the global section of the config file, until either another +.Cm Match +line or the end of the file. +If a keyword appears in multiple +.Cm Match +blocks that are satisfied, only the first instance of the keyword is +applied. +.Pp +The arguments to +.Cm Match +are one or more criteria-pattern pairs or the single token +.Cm All +which matches all criteria. +The available criteria are +.Cm User , +.Cm Group , +.Cm Host , +.Cm LocalAddress , +.Cm LocalPort , +.Cm RDomain , +and +.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 +.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, +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, 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 +keyword. +Available keywords are +.Cm AcceptEnv , +.Cm AllowAgentForwarding , +.Cm AllowGroups , +.Cm AllowStreamLocalForwarding , +.Cm AllowTcpForwarding , +.Cm AllowUsers , +.Cm AuthenticationMethods , +.Cm AuthorizedKeysCommand , +.Cm AuthorizedKeysCommandUser , +.Cm AuthorizedKeysFile , +.Cm AuthorizedPrincipalsCommand , +.Cm AuthorizedPrincipalsCommandUser , +.Cm AuthorizedPrincipalsFile , +.Cm Banner , +.Cm CASignatureAlgorithms , +.Cm ChannelTimeout , +.Cm ChrootDirectory , +.Cm ClientAliveCountMax , +.Cm ClientAliveInterval , +.Cm DenyGroups , +.Cm DenyUsers , +.Cm DisableForwarding , +.Cm ExposeAuthInfo , +.Cm ForceCommand , +.Cm GatewayPorts , +.Cm GSSAPIAuthentication , +.Cm HostbasedAcceptedAlgorithms , +.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 , +.Cm PermitTunnel , +.Cm PermitUserRC , +.Cm PubkeyAcceptedAlgorithms , +.Cm PubkeyAuthentication , +.Cm PubkeyAuthOptions , +.Cm RekeyLimit , +.Cm RevokedKeys , +.Cm RDomain , +.Cm SetEnv , +.Cm StreamLocalBindMask , +.Cm StreamLocalBindUnlink , +.Cm TrustedUserCAKeys , +.Cm UnusedConnectionTimeout , +.Cm X11DisplayOffset , +.Cm X11Forwarding +and +.Cm X11UseLocalhost . +.It Cm 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. +.It Cm 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 +.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 +SSH daemon. +Additional connections will be dropped until authentication succeeds or the +.Cm LoginGraceTime +expires for a connection. +The default is 10:30:100. +.Pp +Alternatively, random early drop can be enabled by specifying +the three colon separated values +start:rate:full (e.g. "10:30:60"). +.Xr 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). +.It Cm ModuliFile +Specifies the +.Xr moduli 5 +file that contains the Diffie-Hellman groups used for the +.Dq diffie-hellman-group-exchange-sha1 +and +.Dq diffie-hellman-group-exchange-sha256 +key exchange methods. +The default is +.Pa /etc/moduli . +.It Cm PasswordAuthentication +Specifies whether password authentication is allowed. +The default is +.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 +.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: +.Pp +.Bl -item -offset indent -compact +.It +.Cm PermitOpen +.Sm off +.Ar host : port +.Sm on +.It +.Cm PermitOpen +.Sm off +.Ar IPv4_addr : port +.Sm on +.It +.Cm PermitOpen +.Sm off +.Ar \&[ IPv6_addr \&] : port +.Sm on +.El +.Pp +Multiple forwards may be specified by separating them with whitespace. +An argument of +.Cm any +can be used to remove all restrictions and permit any forwarding requests. +An argument of +.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 +.Cm yes , +.Cm prohibit-password , +.Cm forced-commands-only , +or +.Cm no . +The default is +.Cm prohibit-password . +.Pp +If this option is set to +.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 +.Cm forced-commands-only , +root login with public key authentication will be allowed, +but only if the +.Ar 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. +.Pp +If this option is set to +.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 +.Cm yes , +.Cm point-to-point +(layer 3), +.Cm ethernet +(layer 2), or +.Cm no . +Specifying +.Cm yes +permits both +.Cm point-to-point +and +.Cm ethernet . +The default is +.Cm no . +.Pp +Independent of this setting, the permissions of the selected +.Xr tun 4 +device must allow access to the user. +.It Cm PermitUserEnvironment +Specifies whether +.Pa ~/.ssh/environment +and +.Cm environment= +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 +.Cm no . +Enabling environment processing may enable users to bypass access +restrictions in some configurations using mechanisms such as +.Ev LD_PRELOAD . +.It Cm PermitUserRC +Specifies whether any +.Pa ~/.ssh/rc +file is executed. +The default is +.Cm yes . +.It Cm PerSourceMaxStartups +Specifies the number of unauthenticated connections allowed from a +given source address, or +.Dq none +if there is no limit. +This limit is applied in addition to +.Cm MaxStartups , +whichever is lower. +The default is +.Cm none . +.It Cm PerSourceNetBlockSize +Specifies the number of bits of source address that are grouped together +for the purposes of applying PerSourceMaxStartups limits. +Values for IPv4 and optionally IPv6 may be specified, separated by a colon. +The default is +.Cm 32:128 , +which means each address is considered individually. +.It Cm PidFile +Specifies the file that contains the process ID of the +SSH daemon, or +.Cm none +to not write one. +The default is +.Pa /var/run/sshd.pid . +.It Cm Port +Specifies the port number that +.Xr sshd 8 +listens on. +The default is 22. +Multiple options of this type are permitted. +See also +.Cm ListenAddress . +.It Cm PrintLastLog +Specifies whether +.Xr sshd 8 +should print the date and time of the last user login when a user logs +in interactively. +The default is +.Cm yes . +.It Cm PrintMotd +Specifies whether +.Xr sshd 8 +should print +.Pa /etc/motd +when a user logs in interactively. +(On some systems it is also printed by the shell, +.Pa /etc/profile , +or equivalent.) +The default is +.Cm yes . +.It Cm PubkeyAcceptedAlgorithms +Specifies the signature algorithms that will be accepted for public key +authentication as a list of comma-separated patterns. +Alternately 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. +The default for this option is: +.Bd -literal -offset 3n +ssh-ed25519-cert-v01@openssh.com, +ecdsa-sha2-nistp256-cert-v01@openssh.com, +ecdsa-sha2-nistp384-cert-v01@openssh.com, +ecdsa-sha2-nistp521-cert-v01@openssh.com, +sk-ssh-ed25519-cert-v01@openssh.com, +sk-ecdsa-sha2-nistp256-cert-v01@openssh.com, +rsa-sha2-512-cert-v01@openssh.com, +rsa-sha2-256-cert-v01@openssh.com, +ssh-ed25519, +ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, +sk-ssh-ed25519@openssh.com, +sk-ecdsa-sha2-nistp256@openssh.com, +rsa-sha2-512,rsa-sha2-256 +.Ed +.Pp +The list of available signature algorithms may also be obtained using +.Qq ssh -Q PubkeyAcceptedAlgorithms . +.It Cm PubkeyAuthOptions +Sets one or more public key authentication options. +The supported keywords are: +.Cm none +(the default; indicating no additional options are enabled), +.Cm touch-required +and +.Cm verify-required . +.Pp +The +.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. +.Pp +The +.Cm verify-required +option requires a FIDO key signature attest that the user was verified, +e.g. via a PIN. +.Pp +Neither the +.Cm touch-required +or +.Cm verify-required +options have any effect for other, non-FIDO, public key types. +.It Cm PubkeyAuthentication +Specifies whether public key authentication is allowed. +The default is +.Cm yes . +.It Cm RekeyLimit +Specifies the maximum amount of data that may be transmitted or received +before the session key is renegotiated, optionally followed by 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 +.Sq K , +.Sq M , +or +.Sq G +to indicate Kilobytes, Megabytes, or Gigabytes, respectively. +The default is between +.Sq 1G +and +.Sq 4G , +depending on the cipher. +The optional second value is specified in seconds and may use any of the +units documented in the +.Sx TIME FORMATS +section. +The default value for +.Cm RekeyLimit +is +.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. +.It Cm RequiredRSASize +Specifies the minimum RSA key size (in bits) that +.Xr sshd 8 +will accept. +User and host-based authentication keys smaller than this limit will be +refused. +The default is +.Cm 1024 +bits. +Note that this limit may only be raised from the default. +.It Cm RevokedKeys +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. +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 +.Xr ssh-keygen 1 . +For more information on KRLs, see the KEY REVOCATION LISTS section in +.Xr ssh-keygen 1 . +.It Cm RDomain +Specifies an explicit routing domain that is applied after authentication +has completed. +The user session, as well as 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 +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. +.Pp +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. +.It Cm 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 +.Cm StreamLocalBindUnlink +is not enabled, +.Nm 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. +.Pp +The argument must be +.Cm yes +or +.Cm no . +The default is +.Cm no . +.It Cm StrictModes +Specifies whether +.Xr 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 +.Cm yes . +Note that this does not apply to +.Cm ChrootDirectory , +whose permissions and ownership are checked unconditionally. +.It Cm 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. +.Pp +The command +.Cm sftp-server +implements the SFTP file transfer subsystem. +.Pp +Alternately the name +.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. +.It Cm SyslogFacility +Gives the facility code that is used when logging messages from +.Xr sshd 8 . +The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, +LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. +The default is AUTH. +.It Cm 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 +.Qq ghost +users and consuming server resources. +.Pp +The default is +.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 +.Cm no . +.It Cm TrustedUserCAKeys +Specifies a file containing public keys of certificate authorities that are +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. +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 +.Cm TrustedUserCAKeys . +For more details on certificates, see the CERTIFICATES section in +.Xr ssh-keygen 1 . +.It Cm UnusedConnectionTimeout +Specifies whether and how quickly +.Xr sshd 8 +should close client connections with no open channels. +Open channels include active shell, command execution or subsystem +sessions, connected network, socket, agent or X11 forwardings. +Forwarding listeners, such as those from the +.Xr ssh 1 +.Fl R +flag, are not considered as open channels and do not prevent the timeout. +The timeout value +is specified in seconds or may use any of the units documented in the +.Sx TIME FORMATS +section. +.Pp +Note that this timeout starts when the client connection completes +user authentication but before the client has an opportunity to open any +channels. +Caution should be used when using short timeout values, as they may not +provide sufficient time for the client to request and open its channels +before terminating the connection. +.Pp +The default +.Cm none +is to never expire connections for having no open channels. +This option may be useful in conjunction with +.Cm ChannelTimeout . +.It Cm UseDNS +Specifies whether +.Xr 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. +.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 +.Cm yes +this will enable PAM authentication using +.Cm KbdInteractiveAuthentication +and +.Cm PasswordAuthentication +in addition to PAM account and session module processing for all +authentication types. +.Pp +Because PAM keyboard-interactive authentication usually serves an equivalent +role to password authentication, you should disable either +.Cm PasswordAuthentication +or +.Cm KbdInteractiveAuthentication . +.Pp +If +.Cm UsePAM +is enabled, you will not be able to run +.Xr sshd 8 +as a non-root user. +The default is +.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 +.Cm none . +.It Cm X11DisplayOffset +Specifies the first display number available for +.Xr sshd 8 Ns 's +X11 forwarding. +This prevents sshd from interfering with real X11 servers. +The default is 10. +.It Cm X11Forwarding +Specifies whether X11 forwarding is permitted. +The argument must be +.Cm yes +or +.Cm no . +The default is +.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 ) , +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 +.Cm ForwardX11 +in +.Xr 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 +.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. +.It Cm X11UseLocalhost +Specifies whether +.Xr 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 +.Ev DISPLAY +environment variable to +.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 +.Cm no +to specify that the forwarding server should be bound to the wildcard +address. +The argument must be +.Cm yes +or +.Cm no . +The default is +.Cm yes . +.It Cm XAuthLocation +Specifies the full pathname of the +.Xr xauth 1 +program, or +.Cm none +to not use one. +The default is +.Pa /usr/X11R6/bin/xauth . +.El +.Sh TIME FORMATS +.Xr sshd 8 +command-line arguments and configuration file options that specify time +may be expressed using a sequence of the form: +.Sm off +.Ar time Op Ar qualifier , +.Sm on +where +.Ar time +is a positive integer value and +.Ar qualifier +is one of the following: +.Pp +.Bl -tag -width Ds -compact -offset indent +.It Aq Cm none +seconds +.It Cm s | Cm S +seconds +.It Cm m | Cm M +minutes +.It Cm h | Cm H +hours +.It Cm d | Cm D +days +.It Cm w | Cm W +weeks +.El +.Pp +Each member of the sequence is added together to calculate +the total time value. +.Pp +Time format examples: +.Pp +.Bl -tag -width Ds -compact -offset indent +.It 600 +600 seconds (10 minutes) +.It 10m +10 minutes +.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 +Contains configuration data for +.Xr sshd 8 . +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 +.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. +.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. |