From 588df31ab61e8733b3abeb0d7dc81fc07e5384e7 Mon Sep 17 00:00:00 2001 From: mouring Date: Fri, 21 Jun 2002 00:59:05 +0000 Subject: [PATCH] - stevesk@cvs.openbsd.org 2002/06/20 19:56:07 [ssh.1 sshd.8] move configuration file options from ssh.1/sshd.8 to ssh_config.5/sshd_config.5; ok deraadt@ millert@ --- ChangeLog | 4 + ssh.1 | 569 +--------------------------------------- ssh_config.5 | 621 +++++++++++++++++++++++++++++++++++++++++++ sshd.8 | 641 +-------------------------------------------- sshd_config.5 | 711 ++++++++++++++++++++++++++++++++++++++++++++++++++ 5 files changed, 1355 insertions(+), 1191 deletions(-) create mode 100644 ssh_config.5 create mode 100644 sshd_config.5 diff --git a/ChangeLog b/ChangeLog index 454827e7..982a1abf 100644 --- a/ChangeLog +++ b/ChangeLog @@ -35,6 +35,10 @@ make the monitor sync the transfer ssh1 session key; transfer keycontext only for RC4 (this is still depends on EVP implementation details and is broken). + - stevesk@cvs.openbsd.org 2002/06/20 19:56:07 + [ssh.1 sshd.8] + move configuration file options from ssh.1/sshd.8 to + ssh_config.5/sshd_config.5; ok deraadt@ millert@ - (bal) Cygwin special handling of empty passwords wrong. Patch by vinschen@redhat.com diff --git a/ssh.1 b/ssh.1 index 4bd0f074..8ada41f2 100644 --- a/ssh.1 +++ b/ssh.1 @@ -34,7 +34,7 @@ .\" (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: ssh.1,v 1.157 2002/06/19 00:27:55 deraadt Exp $ +.\" $OpenBSD: ssh.1,v 1.158 2002/06/20 19:56:07 stevesk Exp $ .Dd September 25, 1999 .Dt SSH 1 .Os @@ -384,7 +384,7 @@ this mechanism is to prevent man-in-the-middle attacks which could otherwise be used to circumvent the encryption. The .Cm StrictHostKeyChecking -option (see below) can be used to prevent logins to machines whose +option can be used to prevent logins to machines whose host key is not known or has changed. .Pp The options are as follows: @@ -568,13 +568,13 @@ and the .Dq level can be controlled by the .Cm CompressionLevel -option (see below). +option. Compression is desirable on modem lines and other slow connections, but will only slow down things on fast networks. The default value can be set on a host-by-host basis in the configuration files; see the .Cm Compression -option below. +option. .It Fl F Ar configfile Specifies an alternative per-user configuration file. If a configuration file is given on the command line, @@ -649,547 +649,10 @@ to use IPv6 addresses only. .El .Sh CONFIGURATION FILES .Nm -obtains configuration data from the following sources in -the following order: -command line options, user's configuration file -.Pq Pa $HOME/.ssh/config , -and system-wide configuration file -.Pq Pa /etc/ssh/ssh_config . -For each parameter, the first obtained value -will be used. -The configuration files contain sections bracketed by -.Dq Host -specifications, and that section is only applied for hosts that -match one of the patterns given in the specification. -The matched host name is the one given on the command line. -.Pp -Since the first obtained value for each parameter is used, more -host-specific declarations should be given near the beginning of the -file, and general defaults at the end. -.Pp -The configuration file has the following format: -.Pp -Empty lines and lines starting with -.Ql # -are comments. -.Pp -Otherwise a line is of the format -.Dq keyword arguments . -Configuration options may be separated by whitespace or -optional whitespace and exactly one -.Ql = ; -the latter format is useful to avoid the need to quote whitespace -when specifying configuration options using the -.Nm ssh , -.Nm scp -and -.Nm sftp -.Fl o -option. -.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 Host -Restricts the following declarations (up to the next -.Cm Host -keyword) to be only for those hosts that match one of the patterns -given after the keyword. -.Ql \&* -and -.Ql ? -can be used as wildcards in the -patterns. -A single -.Ql \&* -as a pattern can be used to provide global -defaults for all hosts. -The host is the -.Ar hostname -argument given on the command line (i.e., the name is not converted to -a canonicalized host name before matching). -.It Cm AFSTokenPassing -Specifies whether to pass AFS tokens to remote host. -The argument to this keyword must be -.Dq yes -or -.Dq no . -This option applies to protocol version 1 only. -.It Cm BatchMode -If set to -.Dq yes , -passphrase/password querying will be disabled. -This option is useful in scripts and other batch jobs where no user -is present to supply the password. -The argument must be -.Dq yes -or -.Dq no . -The default is -.Dq no . -.It Cm BindAddress -Specify the interface to transmit from on machines with multiple -interfaces or aliased addresses. -Note that this option does not work if -.Cm UsePrivilegedPort -is set to -.Dq yes . -.It Cm ChallengeResponseAuthentication -Specifies whether to use challenge response authentication. -The argument to this keyword must be -.Dq yes -or -.Dq no . -The default is -.Dq yes . -.It Cm CheckHostIP -If this flag is set to -.Dq yes , -ssh will additionally check the host IP address in the -.Pa known_hosts -file. -This allows ssh to detect if a host key changed due to DNS spoofing. -If the option is set to -.Dq no , -the check will not be executed. -The default is -.Dq yes . -.It Cm Cipher -Specifies the cipher to use for encrypting the session -in protocol version 1. -Currently, -.Dq blowfish , -.Dq 3des , -and -.Dq des -are supported. -.Ar des -is only supported in the -.Nm -client for interoperability with legacy protocol 1 implementations -that do not support the -.Ar 3des -cipher. Its use is strongly discouraged due to cryptographic -weaknesses. -The default is -.Dq 3des . -.It Cm Ciphers -Specifies the ciphers allowed for protocol version 2 -in order of preference. -Multiple ciphers must be comma-separated. -The default is -.Pp -.Bd -literal - ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour, - aes192-cbc,aes256-cbc'' -.Ed -.It Cm ClearAllForwardings -Specifies that all local, remote and dynamic port forwardings -specified in the configuration files or on the command line be -cleared. This option is primarily useful when used from the -.Nm -command line to clear port forwardings set in -configuration files, and is automatically set by -.Xr scp 1 -and -.Xr sftp 1 . -The argument must be -.Dq yes -or -.Dq no . -The default is -.Dq no . -.It Cm Compression -Specifies whether to use compression. -The argument must be -.Dq yes -or -.Dq no . -The default is -.Dq no . -.It Cm CompressionLevel -Specifies the compression level to use if compression is enabled. -The argument must be an integer from 1 (fast) to 9 (slow, best). -The default level is 6, which is good for most applications. -The meaning of the values is the same as in -.Xr gzip 1 . -Note that this option applies to protocol version 1 only. -.It Cm ConnectionAttempts -Specifies the number of tries (one per second) to make before exiting. -The argument must be an integer. -This may be useful in scripts if the connection sometimes fails. -The default is 1. -.It Cm DynamicForward -Specifies that a TCP/IP port on the local machine be forwarded -over the secure channel, and the application -protocol is then used to determine where to connect to from the -remote machine. The argument must be a port number. -Currently the SOCKS4 protocol is supported, and -.Nm -will act as a SOCKS4 server. -Multiple forwardings may be specified, and -additional forwardings can be given on the command line. Only -the superuser can forward privileged ports. -.It Cm EscapeChar -Sets the escape character (default: -.Ql ~ ) . -The escape character can also -be set on the command line. -The argument should be a single character, -.Ql ^ -followed by a letter, or -.Dq none -to disable the escape -character entirely (making the connection transparent for binary -data). -.It Cm ForwardAgent -Specifies whether the connection to the authentication agent (if any) -will be forwarded to the remote machine. -The argument must be -.Dq yes -or -.Dq no . -The default is -.Dq no . -.It Cm ForwardX11 -Specifies whether X11 connections will be automatically redirected -over the secure channel and -.Ev DISPLAY -set. -The argument must be -.Dq yes -or -.Dq no . -The default is -.Dq no . -.It Cm GatewayPorts -Specifies whether remote hosts are allowed to connect to local -forwarded ports. -By default, -.Nm -binds local port forwardings to the loopback address. This -prevents other remote hosts from connecting to forwarded ports. -.Cm GatewayPorts -can be used to specify that -.Nm -should bind local port forwardings to the wildcard address, -thus allowing remote hosts to connect to forwarded ports. -The argument must be -.Dq yes -or -.Dq no . -The default is -.Dq no . -.It Cm GlobalKnownHostsFile -Specifies a file to use for the global -host key database instead of -.Pa /etc/ssh/ssh_known_hosts . -.It Cm HostbasedAuthentication -Specifies whether to try rhosts based authentication with public key -authentication. -The argument must be -.Dq yes -or -.Dq no . -The default is -.Dq no . -This option applies to protocol version 2 only and -is similar to -.Cm RhostsRSAAuthentication . -.It Cm HostKeyAlgorithms -Specifies the protocol version 2 host key algorithms -that the client wants to use in order of preference. -The default for this option is: -.Dq ssh-rsa,ssh-dss . -.It Cm HostKeyAlias -Specifies an alias that should be used instead of the -real host name when looking up or saving the host key -in the host key database files. -This option is useful for tunneling ssh connections -or for multiple servers running on a single host. -.It Cm HostName -Specifies the real host name to log into. -This can be used to specify nicknames or abbreviations for hosts. -Default is the name given on the command line. -Numeric IP addresses are also permitted (both on the command line and in -.Cm HostName -specifications). -.It Cm IdentityFile -Specifies a file from which the user's RSA or DSA authentication identity -is read. The default is -.Pa $HOME/.ssh/identity -for protocol version 1, and -.Pa $HOME/.ssh/id_rsa -and -.Pa $HOME/.ssh/id_dsa -for protocol version 2. -Additionally, any identities represented by the authentication agent -will be used for authentication. -The file name may use the tilde -syntax to refer to a user's home directory. -It is possible to have -multiple identity files specified in configuration files; all these -identities will be tried in sequence. -.It Cm KeepAlive -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. -.Pp -The default is -.Dq yes -(to send keepalives), and the client will notice -if the network goes down or the remote host dies. -This is important in scripts, and many users want it too. -.Pp -To disable keepalives, the value should be set to -.Dq no . -.It Cm KerberosAuthentication -Specifies whether Kerberos authentication will be used. -The argument to this keyword must be -.Dq yes -or -.Dq no . -.It Cm KerberosTgtPassing -Specifies whether a Kerberos TGT will be forwarded to the server. -This will only work if the Kerberos server is actually an AFS kaserver. -The argument to this keyword must be -.Dq yes -or -.Dq no . -.It Cm LocalForward -Specifies that a TCP/IP port on the local machine be forwarded over -the secure channel to the specified host and port from the remote machine. -The first argument must be a port number, and the second must be -.Ar host:port . -IPv6 addresses can be specified with an alternative syntax: -.Ar host/port . -Multiple forwardings may be specified, and additional -forwardings can be given on the command line. -Only the superuser can forward privileged ports. -.It Cm LogLevel -Gives the verbosity level that is used when logging messages from -.Nm ssh . -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 verbose output. -.It Cm MACs -Specifies the MAC (message authentication code) algorithms -in order of preference. -The MAC algorithm is used in protocol version 2 -for data integrity protection. -Multiple algorithms must be comma-separated. -The default is -.Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 . -.It Cm NoHostAuthenticationForLocalhost -This option can be used if the home directory is shared across machines. -In this case localhost will refer to a different machine on each of -the machines and the user will get many warnings about changed host keys. -However, this option disables host authentication for localhost. -The argument to this keyword must be -.Dq yes -or -.Dq no . -The default is to check the host key for localhost. -.It Cm NumberOfPasswordPrompts -Specifies the number of password prompts before giving up. -The argument to this keyword must be an integer. -Default is 3. -.It Cm PasswordAuthentication -Specifies whether to use password authentication. -The argument to this keyword must be -.Dq yes -or -.Dq no . -The default is -.Dq yes . -.It Cm Port -Specifies the port number to connect on the remote host. -Default is 22. -.It Cm PreferredAuthentications -Specifies the order in which the client should try protocol 2 -authentication methods. This allows a client to prefer one method (e.g. -.Cm keyboard-interactive ) -over another method (e.g. -.Cm password ) -The default for this option is: -.Dq hostbased,publickey,keyboard-interactive,password . -.It Cm Protocol -Specifies the protocol versions -.Nm -should support in order of preference. -The possible values are -.Dq 1 -and -.Dq 2 . -Multiple versions must be comma-separated. -The default is -.Dq 2,1 . -This means that -.Nm -tries version 2 and falls back to version 1 -if version 2 is not available. -.It Cm ProxyCommand -Specifies the command to use to connect to the server. -The command -string extends to the end of the line, and is executed with -.Pa /bin/sh . -In the command string, -.Ql %h -will be substituted by the host name to -connect and -.Ql %p -by the port. -The command can be basically anything, -and should read from its standard input and write to its standard output. -It should eventually connect an -.Xr sshd 8 -server running on some machine, or execute -.Ic sshd -i -somewhere. -Host key management will be done using the -HostName of the host being connected (defaulting to the name typed by -the user). -Note that -.Cm CheckHostIP -is not available for connects with a proxy command. -.Pp -.It Cm PubkeyAuthentication -Specifies whether to try public key authentication. -The argument to this keyword must be -.Dq yes -or -.Dq no . -The default is -.Dq yes . -This option applies to protocol version 2 only. -.It Cm RemoteForward -Specifies that a TCP/IP port on the remote machine be forwarded over -the secure channel to the specified host and port from the local machine. -The first argument must be a port number, and the second must be -.Ar host:port . -IPv6 addresses can be specified with an alternative syntax: -.Ar host/port . -Multiple forwardings may be specified, and additional -forwardings can be given on the command line. -Only the superuser can forward privileged ports. -.It Cm RhostsAuthentication -Specifies whether to try rhosts based authentication. -Note that this -declaration only affects the client side and has no effect whatsoever -on security. -Most servers do not permit RhostsAuthentication because it -is not secure (see -.Cm RhostsRSAAuthentication ) . -The argument to this keyword must be -.Dq yes -or -.Dq no . -The default is -.Dq no . -This option applies to protocol version 1 only. -.It Cm RhostsRSAAuthentication -Specifies whether to try rhosts based authentication with RSA host -authentication. -The argument must be -.Dq yes -or -.Dq no . -The default is -.Dq no . -This option applies to protocol version 1 only and requires -.Nm -to be setuid root. -.It Cm RSAAuthentication -Specifies whether to try RSA authentication. -The argument to this keyword must be -.Dq yes -or -.Dq no . -RSA authentication will only be -attempted if the identity file exists, or an authentication agent is -running. -The default is -.Dq yes . -Note that this option applies to protocol version 1 only. -.It Cm SmartcardDevice -Specifies which smartcard device to use. The argument to this keyword is -the device -.Nm -should use to communicate with a smartcard used for storing the user's -private RSA key. By default, no device is specified and smartcard support -is not activated. -.It Cm StrictHostKeyChecking -If this flag is set to -.Dq yes , -.Nm -will never automatically add host keys to the -.Pa $HOME/.ssh/known_hosts -file, and refuses to connect to hosts whose host key has changed. -This provides maximum protection against trojan horse attacks, -however, can be annoying when the -.Pa /etc/ssh/ssh_known_hosts -file is poorly maintained, or connections to new hosts are -frequently made. -This option forces the user to manually -add all new hosts. -If this flag is set to -.Dq no , -.Nm -will automatically add new host keys to the -user known hosts files. -If this flag is set to -.Dq ask , -new host keys -will be added to the user known host files only after the user -has confirmed that is what they really want to do, and -.Nm -will refuse to connect to hosts whose host key has changed. -The host keys of -known hosts will be verified automatically in all cases. -The argument must be -.Dq yes , -.Dq no -or -.Dq ask . -The default is -.Dq ask . -.It Cm UsePrivilegedPort -Specifies whether to use a privileged port for outgoing connections. -The argument must be -.Dq yes -or -.Dq no . -The default is -.Dq no . -Note that this option must be set to -.Dq yes -if -.Cm RhostsAuthentication -and -.Cm RhostsRSAAuthentication -authentications are needed with older servers. -.It Cm User -Specifies the user to log in as. -This can be useful when a different user name is used on different machines. -This saves the trouble of -having to remember to give the user name on the command line. -.It Cm UserKnownHostsFile -Specifies a file to use for the user -host key database instead of -.Pa $HOME/.ssh/known_hosts . -.It Cm XAuthLocation -Specifies the location of the -.Xr xauth 1 -program. -The default is -.Pa /usr/X11R6/bin/xauth . -.El +may additionally obtain configuration data from +a per-user configuration file and a system-wide configuration file. +The file format and configuration options are described in +.Xr ssh_config 5 . .Sh ENVIRONMENT .Nm will normally set the following environment variables: @@ -1324,13 +787,8 @@ never used automatically and are not necessary; they are only provided for the convenience of the user. .It Pa $HOME/.ssh/config This is the per-user configuration file. -The format of this file is described above. -This file is used by the -.Nm -client. -This file does not usually contain any sensitive information, -but the recommended permissions are read/write for the user, and not -accessible by others. +The file format and configuration options are described in +.Xr ssh_config 5 . .It Pa $HOME/.ssh/authorized_keys Lists the public keys (RSA/DSA) that can be used for logging in as this user. The format of this file is described in the @@ -1365,10 +823,8 @@ checking the key, because someone with access to the name servers would then be able to fool host authentication. .It Pa /etc/ssh/ssh_config Systemwide configuration file. -This file provides defaults for those -values that are not specified in the user's configuration file, and -for those users who do not have a configuration file. -This file must be world-readable. +The file format and configuration options are described in +.Xr ssh_config 5 . .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key These three files contain the private parts of the host keys and are used for @@ -1496,6 +952,7 @@ protocol versions 1.5 and 2.0. .Xr ssh-agent 1 , .Xr ssh-keygen 1 , .Xr telnet 1 , +.Xr ssh_config 4 , .Xr ssh-keysign 8, .Xr sshd 8 .Rs diff --git a/ssh_config.5 b/ssh_config.5 new file mode 100644 index 00000000..53cb0fe9 --- /dev/null +++ b/ssh_config.5 @@ -0,0 +1,621 @@ +.\" -*- nroff -*- +.\" +.\" Author: Tatu Ylonen +.\" Copyright (c) 1995 Tatu Ylonen , 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: ssh_config.5,v 1.1 2002/06/20 19:56:07 stevesk Exp $ +.Dd September 25, 1999 +.Dt SSH_CONFIG 5 +.Os +.Sh NAME +.Nm ssh_config +.Nd OpenSSH SSH client configuration files +.Sh SYNOPSIS +.Bl -tag -width Ds -compact +.It Pa $HOME/.ssh/config +.It Pa /etc/ssh/ssh_config +.El +.Sh DESCRIPTION +.Nm ssh +obtains configuration data from the following sources in +the following order: +command line options, user's configuration file +.Pq Pa $HOME/.ssh/config , +and system-wide configuration file +.Pq Pa /etc/ssh/ssh_config . +.Pp +For each parameter, the first obtained value +will be used. +The configuration files contain sections bracketed by +.Dq Host +specifications, and that section is only applied for hosts that +match one of the patterns given in the specification. +The matched host name is the one given on the command line. +.Pp +Since the first obtained value for each parameter is used, more +host-specific declarations should be given near the beginning of the +file, and general defaults at the end. +.Pp +The configuration file has the following format: +.Pp +Empty lines and lines starting with +.Ql # +are comments. +.Pp +Otherwise a line is of the format +.Dq keyword arguments . +Configuration options may be separated by whitespace or +optional whitespace and exactly one +.Ql = ; +the latter format is useful to avoid the need to quote whitespace +when specifying configuration options using the +.Nm ssh , +.Nm scp +and +.Nm sftp +.Fl o +option. +.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 Host +Restricts the following declarations (up to the next +.Cm Host +keyword) to be only for those hosts that match one of the patterns +given after the keyword. +.Ql \&* +and +.Ql ? +can be used as wildcards in the +patterns. +A single +.Ql \&* +as a pattern can be used to provide global +defaults for all hosts. +The host is the +.Ar hostname +argument given on the command line (i.e., the name is not converted to +a canonicalized host name before matching). +.It Cm AFSTokenPassing +Specifies whether to pass AFS tokens to remote host. +The argument to this keyword must be +.Dq yes +or +.Dq no . +This option applies to protocol version 1 only. +.It Cm BatchMode +If set to +.Dq yes , +passphrase/password querying will be disabled. +This option is useful in scripts and other batch jobs where no user +is present to supply the password. +The argument must be +.Dq yes +or +.Dq no . +The default is +.Dq no . +.It Cm BindAddress +Specify the interface to transmit from on machines with multiple +interfaces or aliased addresses. +Note that this option does not work if +.Cm UsePrivilegedPort +is set to +.Dq yes . +.It Cm ChallengeResponseAuthentication +Specifies whether to use challenge response authentication. +The argument to this keyword must be +.Dq yes +or +.Dq no . +The default is +.Dq yes . +.It Cm CheckHostIP +If this flag is set to +.Dq yes , +ssh will additionally check the host IP address in the +.Pa known_hosts +file. +This allows ssh to detect if a host key changed due to DNS spoofing. +If the option is set to +.Dq no , +the check will not be executed. +The default is +.Dq yes . +.It Cm Cipher +Specifies the cipher to use for encrypting the session +in protocol version 1. +Currently, +.Dq blowfish , +.Dq 3des , +and +.Dq des +are supported. +.Ar des +is only supported in the +.Nm ssh +client for interoperability with legacy protocol 1 implementations +that do not support the +.Ar 3des +cipher. Its use is strongly discouraged due to cryptographic +weaknesses. +The default is +.Dq 3des . +.It Cm Ciphers +Specifies the ciphers allowed for protocol version 2 +in order of preference. +Multiple ciphers must be comma-separated. +The default is +.Pp +.Bd -literal + ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour, + aes192-cbc,aes256-cbc'' +.Ed +.It Cm ClearAllForwardings +Specifies that all local, remote and dynamic port forwardings +specified in the configuration files or on the command line be +cleared. This option is primarily useful when used from the +.Nm ssh +command line to clear port forwardings set in +configuration files, and is automatically set by +.Xr scp 1 +and +.Xr sftp 1 . +The argument must be +.Dq yes +or +.Dq no . +The default is +.Dq no . +.It Cm Compression +Specifies whether to use compression. +The argument must be +.Dq yes +or +.Dq no . +The default is +.Dq no . +.It Cm CompressionLevel +Specifies the compression level to use if compression is enabled. +The argument must be an integer from 1 (fast) to 9 (slow, best). +The default level is 6, which is good for most applications. +The meaning of the values is the same as in +.Xr gzip 1 . +Note that this option applies to protocol version 1 only. +.It Cm ConnectionAttempts +Specifies the number of tries (one per second) to make before exiting. +The argument must be an integer. +This may be useful in scripts if the connection sometimes fails. +The default is 1. +.It Cm DynamicForward +Specifies that a TCP/IP port on the local machine be forwarded +over the secure channel, and the application +protocol is then used to determine where to connect to from the +remote machine. The argument must be a port number. +Currently the SOCKS4 protocol is supported, and +.Nm ssh +will act as a SOCKS4 server. +Multiple forwardings may be specified, and +additional forwardings can be given on the command line. Only +the superuser can forward privileged ports. +.It Cm EscapeChar +Sets the escape character (default: +.Ql ~ ) . +The escape character can also +be set on the command line. +The argument should be a single character, +.Ql ^ +followed by a letter, or +.Dq none +to disable the escape +character entirely (making the connection transparent for binary +data). +.It Cm ForwardAgent +Specifies whether the connection to the authentication agent (if any) +will be forwarded to the remote machine. +The argument must be +.Dq yes +or +.Dq no . +The default is +.Dq no . +.It Cm ForwardX11 +Specifies whether X11 connections will be automatically redirected +over the secure channel and +.Ev DISPLAY +set. +The argument must be +.Dq yes +or +.Dq no . +The default is +.Dq no . +.It Cm GatewayPorts +Specifies whether remote hosts are allowed to connect to local +forwarded ports. +By default, +.Nm ssh +binds local port forwardings to the loopback address. This +prevents other remote hosts from connecting to forwarded ports. +.Cm GatewayPorts +can be used to specify that +.Nm ssh +should bind local port forwardings to the wildcard address, +thus allowing remote hosts to connect to forwarded ports. +The argument must be +.Dq yes +or +.Dq no . +The default is +.Dq no . +.It Cm GlobalKnownHostsFile +Specifies a file to use for the global +host key database instead of +.Pa /etc/ssh/ssh_known_hosts . +.It Cm HostbasedAuthentication +Specifies whether to try rhosts based authentication with public key +authentication. +The argument must be +.Dq yes +or +.Dq no . +The default is +.Dq no . +This option applies to protocol version 2 only and +is similar to +.Cm RhostsRSAAuthentication . +.It Cm HostKeyAlgorithms +Specifies the protocol version 2 host key algorithms +that the client wants to use in order of preference. +The default for this option is: +.Dq ssh-rsa,ssh-dss . +.It Cm HostKeyAlias +Specifies an alias that should be used instead of the +real host name when looking up or saving the host key +in the host key database files. +This option is useful for tunneling ssh connections +or for multiple servers running on a single host. +.It Cm HostName +Specifies the real host name to log into. +This can be used to specify nicknames or abbreviations for hosts. +Default is the name given on the command line. +Numeric IP addresses are also permitted (both on the command line and in +.Cm HostName +specifications). +.It Cm IdentityFile +Specifies a file from which the user's RSA or DSA authentication identity +is read. The default is +.Pa $HOME/.ssh/identity +for protocol version 1, and +.Pa $HOME/.ssh/id_rsa +and +.Pa $HOME/.ssh/id_dsa +for protocol version 2. +Additionally, any identities represented by the authentication agent +will be used for authentication. +The file name may use the tilde +syntax to refer to a user's home directory. +It is possible to have +multiple identity files specified in configuration files; all these +identities will be tried in sequence. +.It Cm KeepAlive +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. +.Pp +The default is +.Dq yes +(to send keepalives), and the client will notice +if the network goes down or the remote host dies. +This is important in scripts, and many users want it too. +.Pp +To disable keepalives, the value should be set to +.Dq no . +.It Cm KerberosAuthentication +Specifies whether Kerberos authentication will be used. +The argument to this keyword must be +.Dq yes +or +.Dq no . +.It Cm KerberosTgtPassing +Specifies whether a Kerberos TGT will be forwarded to the server. +This will only work if the Kerberos server is actually an AFS kaserver. +The argument to this keyword must be +.Dq yes +or +.Dq no . +.It Cm LocalForward +Specifies that a TCP/IP port on the local machine be forwarded over +the secure channel to the specified host and port from the remote machine. +The first argument must be a port number, and the second must be +.Ar host:port . +IPv6 addresses can be specified with an alternative syntax: +.Ar host/port . +Multiple forwardings may be specified, and additional +forwardings can be given on the command line. +Only the superuser can forward privileged ports. +.It Cm LogLevel +Gives the verbosity level that is used when logging messages from +.Nm ssh . +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 verbose output. +.It Cm MACs +Specifies the MAC (message authentication code) algorithms +in order of preference. +The MAC algorithm is used in protocol version 2 +for data integrity protection. +Multiple algorithms must be comma-separated. +The default is +.Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 . +.It Cm NoHostAuthenticationForLocalhost +This option can be used if the home directory is shared across machines. +In this case localhost will refer to a different machine on each of +the machines and the user will get many warnings about changed host keys. +However, this option disables host authentication for localhost. +The argument to this keyword must be +.Dq yes +or +.Dq no . +The default is to check the host key for localhost. +.It Cm NumberOfPasswordPrompts +Specifies the number of password prompts before giving up. +The argument to this keyword must be an integer. +Default is 3. +.It Cm PasswordAuthentication +Specifies whether to use password authentication. +The argument to this keyword must be +.Dq yes +or +.Dq no . +The default is +.Dq yes . +.It Cm Port +Specifies the port number to connect on the remote host. +Default is 22. +.It Cm PreferredAuthentications +Specifies the order in which the client should try protocol 2 +authentication methods. This allows a client to prefer one method (e.g. +.Cm keyboard-interactive ) +over another method (e.g. +.Cm password ) +The default for this option is: +.Dq hostbased,publickey,keyboard-interactive,password . +.It Cm Protocol +Specifies the protocol versions +.Nm ssh +should support in order of preference. +The possible values are +.Dq 1 +and +.Dq 2 . +Multiple versions must be comma-separated. +The default is +.Dq 2,1 . +This means that +.Nm ssh +tries version 2 and falls back to version 1 +if version 2 is not available. +.It Cm ProxyCommand +Specifies the command to use to connect to the server. +The command +string extends to the end of the line, and is executed with +.Pa /bin/sh . +In the command string, +.Ql %h +will be substituted by the host name to +connect and +.Ql %p +by the port. +The command can be basically anything, +and should read from its standard input and write to its standard output. +It should eventually connect an +.Xr sshd 8 +server running on some machine, or execute +.Ic sshd -i +somewhere. +Host key management will be done using the +HostName of the host being connected (defaulting to the name typed by +the user). +Note that +.Cm CheckHostIP +is not available for connects with a proxy command. +.Pp +.It Cm PubkeyAuthentication +Specifies whether to try public key authentication. +The argument to this keyword must be +.Dq yes +or +.Dq no . +The default is +.Dq yes . +This option applies to protocol version 2 only. +.It Cm RemoteForward +Specifies that a TCP/IP port on the remote machine be forwarded over +the secure channel to the specified host and port from the local machine. +The first argument must be a port number, and the second must be +.Ar host:port . +IPv6 addresses can be specified with an alternative syntax: +.Ar host/port . +Multiple forwardings may be specified, and additional +forwardings can be given on the command line. +Only the superuser can forward privileged ports. +.It Cm RhostsAuthentication +Specifies whether to try rhosts based authentication. +Note that this +declaration only affects the client side and has no effect whatsoever +on security. +Most servers do not permit RhostsAuthentication because it +is not secure (see +.Cm RhostsRSAAuthentication ) . +The argument to this keyword must be +.Dq yes +or +.Dq no . +The default is +.Dq no . +This option applies to protocol version 1 only. +.It Cm RhostsRSAAuthentication +Specifies whether to try rhosts based authentication with RSA host +authentication. +The argument must be +.Dq yes +or +.Dq no . +The default is +.Dq no . +This option applies to protocol version 1 only and requires +.Nm ssh +to be setuid root. +.It Cm RSAAuthentication +Specifies whether to try RSA authentication. +The argument to this keyword must be +.Dq yes +or +.Dq no . +RSA authentication will only be +attempted if the identity file exists, or an authentication agent is +running. +The default is +.Dq yes . +Note that this option applies to protocol version 1 only. +.It Cm SmartcardDevice +Specifies which smartcard device to use. The argument to this keyword is +the device +.Nm ssh +should use to communicate with a smartcard used for storing the user's +private RSA key. By default, no device is specified and smartcard support +is not activated. +.It Cm StrictHostKeyChecking +If this flag is set to +.Dq yes , +.Nm ssh +will never automatically add host keys to the +.Pa $HOME/.ssh/known_hosts +file, and refuses to connect to hosts whose host key has changed. +This provides maximum protection against trojan horse attacks, +however, can be annoying when the +.Pa /etc/ssh/ssh_known_hosts +file is poorly maintained, or connections to new hosts are +frequently made. +This option forces the user to manually +add all new hosts. +If this flag is set to +.Dq no , +.Nm ssh +will automatically add new host keys to the +user known hosts files. +If this flag is set to +.Dq ask , +new host keys +will be added to the user known host files only after the user +has confirmed that is what they really want to do, and +.Nm ssh +will refuse to connect to hosts whose host key has changed. +The host keys of +known hosts will be verified automatically in all cases. +The argument must be +.Dq yes , +.Dq no +or +.Dq ask . +The default is +.Dq ask . +.It Cm UsePrivilegedPort +Specifies whether to use a privileged port for outgoing connections. +The argument must be +.Dq yes +or +.Dq no . +The default is +.Dq no . +Note that this option must be set to +.Dq yes +if +.Cm RhostsAuthentication +and +.Cm RhostsRSAAuthentication +authentications are needed with older servers. +.It Cm User +Specifies the user to log in as. +This can be useful when a different user name is used on different machines. +This saves the trouble of +having to remember to give the user name on the command line. +.It Cm UserKnownHostsFile +Specifies a file to use for the user +host key database instead of +.Pa $HOME/.ssh/known_hosts . +.It Cm XAuthLocation +Specifies the location of the +.Xr xauth 1 +program. +The default is +.Pa /usr/X11R6/bin/xauth . +.El +.Sh FILES +.Bl -tag -width Ds +.It Pa $HOME/.ssh/config +This is the per-user configuration file. +The format of this file is described above. +This file is used by the +.Nm ssh +client. +This file does not usually contain any sensitive information, +but the recommended permissions are read/write for the user, and not +accessible by others. +.It Pa /etc/ssh/ssh_config +Systemwide configuration file. +This file provides defaults for those +values that are not specified in the user's configuration file, and +for those users who do not have a configuration file. +This file must be world-readable. +.El +.Sh AUTHORS +OpenSSH is a derivative of the original and free +ssh 1.2.12 release by Tatu Ylonen. +Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, +Theo de Raadt and Dug Song +removed many bugs, re-added newer features and +created OpenSSH. +Markus Friedl contributed the support for SSH +protocol versions 1.5 and 2.0. +.Sh SEE ALSO +.Xr ssh 1 diff --git a/sshd.8 b/sshd.8 index aae973c5..22f81432 100644 --- a/sshd.8 +++ b/sshd.8 @@ -34,7 +34,7 @@ .\" (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.8,v 1.183 2002/05/29 03:06:30 stevesk Exp $ +.\" $OpenBSD: sshd.8,v 1.184 2002/06/20 19:56:07 stevesk Exp $ .Dd September 25, 1999 .Dt SSHD 8 .Os @@ -320,638 +320,8 @@ reads configuration data from (or the file specified with .Fl f on the command line). -The file contains keyword-argument pairs, one per line. -Lines starting with -.Ql # -and empty lines are interpreted as comments. -.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 AFSTokenPassing -Specifies whether an AFS token may be forwarded to the server. -Default is -.Dq no . -.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. -.Ql \&* -and -.Ql ? -can be used as -wildcards in the patterns. -Only group names are valid; a numerical group ID is not recognized. -By default, login is allowed for all groups. -.Pp -.It Cm AllowTcpForwarding -Specifies whether TCP forwarding is permitted. -The default is -.Dq yes . -Note that disabling TCP forwarding does not improve security unless -users are also denied shell access, as they can always install their -own forwarders. -.Pp -.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 users names that -match one of the patterns. -.Ql \&* -and -.Ql ? -can be used as -wildcards in 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. -.Pp -.It Cm AuthorizedKeysFile -Specifies the file that contains the public keys that can be used -for user authentication. -.Cm AuthorizedKeysFile -may contain tokens of the form %T which are substituted during connection -set-up. The following tokens are defined: %% is replaced by a literal '%', -%h is replaced by the home directory of the user being authenticated and -%u is replaced by the username of that user. -After expansion, -.Cm AuthorizedKeysFile -is taken to be an absolute path or one relative to the user's home -directory. -The default is -.Dq .ssh/authorized_keys . -.It Cm Banner -In some jurisdictions, sending a warning message before authentication -may be relevant for getting legal protection. -The contents of the specified file are sent to the remote user before -authentication is allowed. -This option is only available for protocol version 2. -By default, no banner is displayed. -.Pp -.It Cm ChallengeResponseAuthentication -Specifies whether challenge response authentication is allowed. -All authentication styles from -.Xr login.conf 5 -are supported. -The default is -.Dq yes . -.It Cm Ciphers -Specifies the ciphers allowed for protocol version 2. -Multiple ciphers must be comma-separated. -The default is -.Pp -.Bd -literal - ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour, - aes192-cbc,aes256-cbc'' -.Ed -.It Cm ClientAliveInterval -Sets a timeout interval in seconds after which if no data has been received -from the client, -.Nm -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 ClientAliveCountMax -Sets the number of client alive messages (see above) which may be -sent without -.Nm -receiving any messages back from the client. If this threshold is -reached while client alive messages are being sent, -.Nm -will disconnect the client, terminating the session. It is important -to note that the use of client alive messages is very different from -.Cm KeepAlive -(below). The client alive messages are sent through the -encrypted channel and therefore will not be spoofable. The TCP keepalive -option enabled by -.Cm KeepAlive -is spoofable. The client alive mechanism is valuable when the client or -server depend on knowing when a connection has become inactive. -.Pp -The default value is 3. If -.Cm ClientAliveInterval -(above) is set to 15, and -.Cm ClientAliveCountMax -is left at the default, unresponsive ssh clients -will be disconnected after approximately 45 seconds. -.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. -.Ql \&* -and -.Ql ? -can be used as -wildcards in the patterns. -Only group names are valid; a numerical group ID is not recognized. -By default, login is allowed for all groups. -.Pp -.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. -.Ql \&* -and -.Ql ? -can be used as wildcards in 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. -.It Cm GatewayPorts -Specifies whether remote hosts are allowed to connect to ports -forwarded for the client. -By default, -.Nm -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 -.Nm -should bind remote port forwardings to the wildcard address, -thus allowing remote hosts to connect to forwarded ports. -The argument must be -.Dq yes -or -.Dq no . -The default is -.Dq no . -.It Cm HostbasedAuthentication -Specifies whether rhosts or /etc/hosts.equiv authentication together -with successful public key client host authentication is allowed -(hostbased authentication). -This option is similar to -.Cm RhostsRSAAuthentication -and applies to protocol version 2 only. -The default is -.Dq no . -.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_rsa_key -and -.Pa /etc/ssh/ssh_host_dsa_key -for protocol version 2. -Note that -.Nm -will refuse to use a file if it is group/world-accessible. -It is possible to have multiple host key files. -.Dq rsa1 -keys are used for version 1 and -.Dq dsa -or -.Dq rsa -are used for version 2 of the SSH protocol. -.It Cm IgnoreRhosts -Specifies that -.Pa .rhosts -and -.Pa .shosts -files will not be used in -.Cm RhostsAuthentication , -.Cm RhostsRSAAuthentication -or -.Cm HostbasedAuthentication . -.Pp -.Pa /etc/hosts.equiv -and -.Pa /etc/shosts.equiv -are still used. -The default is -.Dq yes . -.It Cm IgnoreUserKnownHosts -Specifies whether -.Nm -should ignore the user's -.Pa $HOME/.ssh/known_hosts -during -.Cm RhostsRSAAuthentication -or -.Cm HostbasedAuthentication . -The default is -.Dq no . -.It Cm KeepAlive -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 keepalives are not sent, -sessions may hang indefinitely on the server, leaving -.Dq ghost -users and consuming server resources. -.Pp -The default is -.Dq yes -(to send keepalives), and the server will notice -if the network goes down or the client host crashes. -This avoids infinitely hanging sessions. -.Pp -To disable keepalives, the value should be set to -.Dq no . -.It Cm KerberosAuthentication -Specifies whether Kerberos authentication is allowed. -This can be in the form of a Kerberos ticket, or if -.Cm PasswordAuthentication -is yes, the password provided by the user 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. -Default is -.Dq no . -.It Cm KerberosOrLocalPasswd -If set then if password authentication through Kerberos fails then -the password will be validated via any additional local mechanism -such as -.Pa /etc/passwd . -Default is -.Dq yes . -.It Cm KerberosTgtPassing -Specifies whether a Kerberos TGT may be forwarded to the server. -Default is -.Dq no , -as this only works when the Kerberos KDC is actually an AFS kaserver. -.It Cm KerberosTicketCleanup -Specifies whether to automatically destroy the user's ticket cache -file on logout. -Default is -.Dq yes . -.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). -.It Cm ListenAddress -Specifies the local addresses -.Nm -should listen on. -The following forms may be used: -.Pp -.Bl -item -offset indent -compact -.It -.Cm ListenAddress -.Sm off -.Ar host No | Ar IPv4_addr No | Ar IPv6_addr -.Sm on -.It -.Cm ListenAddress -.Sm off -.Ar host No | Ar IPv4_addr No : Ar port -.Sm on -.It -.Cm ListenAddress -.Sm off -.Oo -.Ar host No | Ar IPv6_addr Oc : Ar port -.Sm on -.El -.Pp -If -.Ar port -is not specified, -.Nm -will listen on the address and all prior -.Cm Port -options specified. The default is to listen on all local -addresses. Multiple -.Cm ListenAddress -options are permitted. Additionally, any -.Cm Port -options must precede this option for non port qualified addresses. -.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 600 (seconds). -.It Cm LogLevel -Gives the verbosity level that is used when logging messages from -.Nm sshd . -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 MACs -Specifies the available MAC (message authentication code) algorithms. -The MAC algorithm is used in protocol version 2 -for data integrity protection. -Multiple algorithms must be comma-separated. -The default is -.Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 . -.It Cm MaxStartups -Specifies the maximum number of concurrent unauthenticated connections to the -.Nm -daemon. -Additional connections will be dropped until authentication succeeds or the -.Cm LoginGraceTime -expires for a connection. -The default is 10. -.Pp -Alternatively, random early drop can be enabled by specifying -the three colon separated values -.Dq start:rate:full -(e.g., "10:30:60"). -.Nm -will refuse connection attempts with a probability of -.Dq rate/100 -(30%) -if there are currently -.Dq start -(10) -unauthenticated connections. -The probability increases linearly and all connection attempts -are refused if the number of unauthenticated connections reaches -.Dq full -(60). -.It Cm PAMAuthenticationViaKbdInt -Specifies whether PAM challenge response authentication is allowed. This -allows the use of most PAM challenge response authentication modules, but -it will allow password authentication regardless of whether -.Cm PasswordAuthentication -is disabled. -The default is -.Dq no . -.It Cm PasswordAuthentication -Specifies whether password authentication is allowed. -The default is -.Dq 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 . -.It Cm PermitRootLogin -Specifies whether root can login using -.Xr ssh 1 . -The argument must be -.Dq yes , -.Dq without-password , -.Dq forced-commands-only -or -.Dq no . -The default is -.Dq yes . -.Pp -If this option is set to -.Dq without-password -password authentication is disabled for root. -.Pp -If this option is set to -.Dq 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 -.Dq no -root is not allowed to login. -.It Cm PidFile -Specifies the file that contains the process identifier of the -.Nm -daemon. -The default is -.Pa /var/run/sshd.pid . -.It Cm Port -Specifies the port number that -.Nm -listens on. -The default is 22. -Multiple options of this type are permitted. -See also -.Cm ListenAddress . -.It Cm PrintLastLog -Specifies whether -.Nm -should print the date and time when the user last logged in. -The default is -.Dq yes . -.It Cm PrintMotd -Specifies whether -.Nm -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 -.Dq yes . -.It Cm Protocol -Specifies the protocol versions -.Nm -should support. -The possible values are -.Dq 1 -and -.Dq 2 . -Multiple versions must be comma-separated. -The default is -.Dq 2,1 . -.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. -.It Cm RhostsAuthentication -Specifies whether authentication using rhosts or /etc/hosts.equiv -files is sufficient. -Normally, this method should not be permitted because it is insecure. -.Cm RhostsRSAAuthentication -should be used -instead, because it performs RSA-based host authentication in addition -to normal rhosts or /etc/hosts.equiv authentication. -The default is -.Dq no . -This option applies to protocol version 1 only. -.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 768. -.It Cm StrictModes -Specifies whether -.Nm -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 -.Dq yes . -.It Cm Subsystem -Configures an external subsystem (e.g., file transfer daemon). -Arguments should be a subsystem name and a command to execute upon subsystem -request. -The command -.Xr sftp-server 8 -implements the -.Dq sftp -file transfer subsystem. -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 -.Nm sshd . -The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, -LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. -The default is AUTH. -.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. -.It Cm UsePrivilegeSeparation -Specifies whether -.Nm -separated 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 . -.It Cm VerifyReverseMapping -Specifies whether -.Nm -should try to verify the remote host name and 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 X11DisplayOffset -Specifies the first display number available for -.Nm sshd Ns 's -X11 forwarding. -This prevents -.Nm -from interfering with real X11 servers. -The default is 10. -.It Cm X11Forwarding -Specifies whether X11 forwarding is permitted. -The default is -.Dq no . -Note that disabling X11 forwarding does not improve security in any -way, as users can always install their own forwarders. -X11 forwarding is automatically disabled if -.Cm UseLogin -is enabled. -.It Cm X11UseLocalhost -Specifies whether -.Nm -should bind the X11 forwarding server to the loopback address or to -the wildcard address. By default, -.Nm -binds the forwarding server to the loopback address and sets the -hostname part of the -.Ev DISPLAY -environment variable to -.Dq localhost . -This prevents remote hosts from connecting to the fake display. -However, some older X11 clients may not function with this -configuration. -.Cm X11UseLocalhost -may be set to -.Dq no -to specify that the forwarding server should be bound to the wildcard -address. -The argument must be -.Dq yes -or -.Dq no . -The default is -.Dq yes . -.It Cm XAuthLocation -Specifies the location of the -.Xr xauth 1 -program. -The default is -.Pa /usr/X11R6/bin/xauth . -.El -.Ss Time Formats -.Pp -.Nm -command-line arguments and configuration file options that specify time -may be expressed using a sequence of the form: -.Sm off -.Ar time Oo Ar qualifier Oc , -.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 Cm -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 +The file format and configuration options are described in +.Xr sshd_config 5 . .Sh LOGIN PROCESS When a user successfully logs in, .Nm @@ -1187,8 +557,8 @@ cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....= .It Pa /etc/ssh/sshd_config Contains configuration data for .Nm sshd . -This file should be writable by root only, but it is recommended -(though not necessary) that it be world-readable. +The file format and configuration options are described in +.Xr sshd_config 5 . .It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key These three files contain the private parts of the host keys. These files should only be owned by root, readable only by root, and not @@ -1389,6 +759,7 @@ for privilege separation. .Xr ssh-keygen 1 , .Xr login.conf 5 , .Xr moduli 5 , +.Xr sshd_config 5 , .Xr sftp-server 8 .Rs .%A T. Ylonen diff --git a/sshd_config.5 b/sshd_config.5 new file mode 100644 index 00000000..1aecd485 --- /dev/null +++ b/sshd_config.5 @@ -0,0 +1,711 @@ +.\" -*- nroff -*- +.\" +.\" Author: Tatu Ylonen +.\" Copyright (c) 1995 Tatu Ylonen , 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 1.3 2002/06/20 23:37:12 markus Exp $ +.Dd September 25, 1999 +.Dt SSHD_CONFIG 5 +.Os +.Sh NAME +.Nm sshd_config +.Nd OpenSSH SSH daemon configuration file +.Sh SYNOPSIS +.Bl -tag -width Ds -compact +.It Pa /etc/ssh/sshd_config +.El +.Sh DESCRIPTION +.Nm sshd +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. +Lines starting with +.Ql # +and empty lines are interpreted as comments. +.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 AFSTokenPassing +Specifies whether an AFS token may be forwarded to the server. +Default is +.Dq no . +.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. +.Ql \&* +and +.Ql ? +can be used as +wildcards in the patterns. +Only group names are valid; a numerical group ID is not recognized. +By default, login is allowed for all groups. +.Pp +.It Cm AllowTcpForwarding +Specifies whether TCP forwarding is permitted. +The default is +.Dq yes . +Note that disabling TCP forwarding does not improve security unless +users are also denied shell access, as they can always install their +own forwarders. +.Pp +.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 users names that +match one of the patterns. +.Ql \&* +and +.Ql ? +can be used as +wildcards in 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. +.Pp +.It Cm AuthorizedKeysFile +Specifies the file that contains the public keys that can be used +for user authentication. +.Cm AuthorizedKeysFile +may contain tokens of the form %T which are substituted during connection +set-up. The following tokens are defined: %% is replaced by a literal '%', +%h is replaced by the home directory of the user being authenticated and +%u is replaced by the username of that user. +After expansion, +.Cm AuthorizedKeysFile +is taken to be an absolute path or one relative to the user's home +directory. +The default is +.Dq .ssh/authorized_keys . +.It Cm Banner +In some jurisdictions, sending a warning message before authentication +may be relevant for getting legal protection. +The contents of the specified file are sent to the remote user before +authentication is allowed. +This option is only available for protocol version 2. +By default, no banner is displayed. +.Pp +.It Cm ChallengeResponseAuthentication +Specifies whether challenge response authentication is allowed. +All authentication styles from +.Xr login.conf 5 +are supported. +The default is +.Dq yes . +.It Cm Ciphers +Specifies the ciphers allowed for protocol version 2. +Multiple ciphers must be comma-separated. +The default is +.Pp +.Bd -literal + ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour, + aes192-cbc,aes256-cbc'' +.Ed +.It Cm ClientAliveInterval +Sets a timeout interval in seconds after which if no data has been received +from the client, +.Nm sshd +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 ClientAliveCountMax +Sets the number of client alive messages (see above) which may be +sent without +.Nm sshd +receiving any messages back from the client. If this threshold is +reached while client alive messages are being sent, +.Nm 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 KeepAlive +(below). The client alive messages are sent through the +encrypted channel and therefore will not be spoofable. The TCP keepalive +option enabled by +.Cm KeepAlive +is spoofable. The client alive mechanism is valuable when the client or +server depend on knowing when a connection has become inactive. +.Pp +The default value is 3. If +.Cm ClientAliveInterval +(above) is set to 15, and +.Cm ClientAliveCountMax +is left at the default, unresponsive ssh clients +will be disconnected after approximately 45 seconds. +.It Cm Compression +Specifies whether compression is allowed. +The argument must be +.Dq yes +or +.Dq no . +The default is +.Dq 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. +.Ql \&* +and +.Ql ? +can be used as +wildcards in the patterns. +Only group names are valid; a numerical group ID is not recognized. +By default, login is allowed for all groups. +.Pp +.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. +.Ql \&* +and +.Ql ? +can be used as wildcards in 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. +.It Cm GatewayPorts +Specifies whether remote hosts are allowed to connect to ports +forwarded for the client. +By default, +.Nm sshd +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 +.Nm sshd +should bind remote port forwardings to the wildcard address, +thus allowing remote hosts to connect to forwarded ports. +The argument must be +.Dq yes +or +.Dq no . +The default is +.Dq no . +.It Cm HostbasedAuthentication +Specifies whether rhosts or /etc/hosts.equiv authentication together +with successful public key client host authentication is allowed +(hostbased authentication). +This option is similar to +.Cm RhostsRSAAuthentication +and applies to protocol version 2 only. +The default is +.Dq no . +.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_rsa_key +and +.Pa /etc/ssh/ssh_host_dsa_key +for protocol version 2. +Note that +.Nm sshd +will refuse to use a file if it is group/world-accessible. +It is possible to have multiple host key files. +.Dq rsa1 +keys are used for version 1 and +.Dq dsa +or +.Dq rsa +are used for version 2 of the SSH protocol. +.It Cm IgnoreRhosts +Specifies that +.Pa .rhosts +and +.Pa .shosts +files will not be used in +.Cm RhostsAuthentication , +.Cm RhostsRSAAuthentication +or +.Cm HostbasedAuthentication . +.Pp +.Pa /etc/hosts.equiv +and +.Pa /etc/shosts.equiv +are still used. +The default is +.Dq yes . +.It Cm IgnoreUserKnownHosts +Specifies whether +.Nm sshd +should ignore the user's +.Pa $HOME/.ssh/known_hosts +during +.Cm RhostsRSAAuthentication +or +.Cm HostbasedAuthentication . +The default is +.Dq no . +.It Cm KeepAlive +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 keepalives are not sent, +sessions may hang indefinitely on the server, leaving +.Dq ghost +users and consuming server resources. +.Pp +The default is +.Dq yes +(to send keepalives), and the server will notice +if the network goes down or the client host crashes. +This avoids infinitely hanging sessions. +.Pp +To disable keepalives, the value should be set to +.Dq no . +.It Cm KerberosAuthentication +Specifies whether Kerberos authentication is allowed. +This can be in the form of a Kerberos ticket, or if +.It Cm PAMAuthenticationViaKbdInt +Specifies whether PAM challenge response authentication is allowed. This +allows the use of most PAM challenge response authentication modules, but +it will allow password authentication regardless of whether +.Cm PasswordAuthentication +is yes, the password provided by the user 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. +Default is +.Dq no . +.It Cm KerberosOrLocalPasswd +If set then if password authentication through Kerberos fails then +the password will be validated via any additional local mechanism +such as +.Pa /etc/passwd . +Default is +.Dq yes . +.It Cm KerberosTgtPassing +Specifies whether a Kerberos TGT may be forwarded to the server. +Default is +.Dq no , +as this only works when the Kerberos KDC is actually an AFS kaserver. +.It Cm KerberosTicketCleanup +Specifies whether to automatically destroy the user's ticket cache +file on logout. +Default is +.Dq yes . +.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). +.It Cm ListenAddress +Specifies the local addresses +.Nm sshd +should listen on. +The following forms may be used: +.Pp +.Bl -item -offset indent -compact +.It +.Cm ListenAddress +.Sm off +.Ar host No | Ar IPv4_addr No | Ar IPv6_addr +.Sm on +.It +.Cm ListenAddress +.Sm off +.Ar host No | Ar IPv4_addr No : Ar port +.Sm on +.It +.Cm ListenAddress +.Sm off +.Oo +.Ar host No | Ar IPv6_addr Oc : Ar port +.Sm on +.El +.Pp +If +.Ar port +is not specified, +.Nm sshd +will listen on the address and all prior +.Cm Port +options specified. The default is to listen on all local +addresses. Multiple +.Cm ListenAddress +options are permitted. Additionally, any +.Cm Port +options must precede this option for non port qualified addresses. +.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 600 (seconds). +.It Cm LogLevel +Gives the verbosity level that is used when logging messages from +.Nm sshd . +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 MACs +Specifies the available MAC (message authentication code) algorithms. +The MAC algorithm is used in protocol version 2 +for data integrity protection. +Multiple algorithms must be comma-separated. +The default is +.Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 . +.It Cm MaxStartups +Specifies the maximum number of concurrent unauthenticated connections to the +.Nm sshd +daemon. +Additional connections will be dropped until authentication succeeds or the +.Cm LoginGraceTime +expires for a connection. +The default is 10. +.Pp +Alternatively, random early drop can be enabled by specifying +the three colon separated values +.Dq start:rate:full +(e.g., "10:30:60"). +.Nm sshd +will refuse connection attempts with a probability of +.Dq rate/100 +(30%) +if there are currently +.Dq start +(10) +unauthenticated connections. +The probability increases linearly and all connection attempts +are refused if the number of unauthenticated connections reaches +.Dq full +(60). +.It Cm PasswordAuthentication +Specifies whether password authentication is allowed. +The default is +.Dq 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 . +.It Cm PermitRootLogin +Specifies whether root can login using +.Xr ssh 1 . +The argument must be +.Dq yes , +.Dq without-password , +.Dq forced-commands-only +or +.Dq no . +The default is +.Dq yes . +.Pp +If this option is set to +.Dq without-password +password authentication is disabled for root. +.Pp +If this option is set to +.Dq 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 +.Dq no +root is not allowed to login. +.It Cm PidFile +Specifies the file that contains the process identifier of the +.Nm sshd +daemon. +The default is +.Pa /var/run/sshd.pid . +.It Cm Port +Specifies the port number that +.Nm sshd +listens on. +The default is 22. +Multiple options of this type are permitted. +See also +.Cm ListenAddress . +.It Cm PrintLastLog +Specifies whether +.Nm sshd +should print the date and time when the user last logged in. +The default is +.Dq yes . +.It Cm PrintMotd +Specifies whether +.Nm sshd +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 +.Dq yes . +.It Cm Protocol +Specifies the protocol versions +.Nm sshd +should support. +The possible values are +.Dq 1 +and +.Dq 2 . +Multiple versions must be comma-separated. +The default is +.Dq 2,1 . +.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. +.It Cm RhostsAuthentication +Specifies whether authentication using rhosts or /etc/hosts.equiv +files is sufficient. +Normally, this method should not be permitted because it is insecure. +.Cm RhostsRSAAuthentication +should be used +instead, because it performs RSA-based host authentication in addition +to normal rhosts or /etc/hosts.equiv authentication. +The default is +.Dq no . +This option applies to protocol version 1 only. +.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 768. +.It Cm StrictModes +Specifies whether +.Nm sshd +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 +.Dq yes . +.It Cm Subsystem +Configures an external subsystem (e.g., file transfer daemon). +Arguments should be a subsystem name and a command to execute upon subsystem +request. +The command +.Xr sftp-server 8 +implements the +.Dq sftp +file transfer subsystem. +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 +.Nm sshd . +The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, +LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. +The default is AUTH. +.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. +.It Cm UsePrivilegeSeparation +Specifies whether +.Nm sshd +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 . +.It Cm VerifyReverseMapping +Specifies whether +.Nm sshd +should try to verify the remote host name and 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 X11DisplayOffset +Specifies the first display number available for +.Nm sshd Ns 's +X11 forwarding. +This prevents +.Nm sshd +from interfering with real X11 servers. +The default is 10. +.It Cm X11Forwarding +Specifies whether X11 forwarding is permitted. +The default is +.Dq no . +Note that disabling X11 forwarding does not improve security in any +way, as users can always install their own forwarders. +X11 forwarding is automatically disabled if +.Cm UseLogin +is enabled. +.It Cm X11UseLocalhost +Specifies whether +.Nm sshd +should bind the X11 forwarding server to the loopback address or to +the wildcard address. By default, +.Nm sshd +binds the forwarding server to the loopback address and sets the +hostname part of the +.Ev DISPLAY +environment variable to +.Dq localhost . +This prevents remote hosts from connecting to the fake display. +However, some older X11 clients may not function with this +configuration. +.Cm X11UseLocalhost +may be set to +.Dq no +to specify that the forwarding server should be bound to the wildcard +address. +The argument must be +.Dq yes +or +.Dq no . +The default is +.Dq yes . +.It Cm XAuthLocation +Specifies the location of the +.Xr xauth 1 +program. +The default is +.Pa /usr/X11R6/bin/xauth . +.El +.Ss Time Formats +.Pp +.Nm sshd +command-line arguments and configuration file options that specify time +may be expressed using a sequence of the form: +.Sm off +.Ar time Oo Ar qualifier Oc , +.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 Cm +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 FILES +.Bl -tag -width Ds +.It Pa /etc/ssh/sshd_config +Contains configuration data for +.Nm sshd . +This file should be writable by root only, but it is recommended +(though not necessary) that it be world-readable. +.El +.Sh AUTHORS +OpenSSH is a derivative of the original and free +ssh 1.2.12 release by Tatu Ylonen. +Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, +Theo de Raadt and Dug Song +removed many bugs, re-added newer features and +created OpenSSH. +Markus Friedl contributed the support for SSH +protocol versions 1.5 and 2.0. +Niels Provos and Markus Friedl contributed support +for privilege separation. +.Sh SEE ALSO +.Xr sshd 8 -- 2.45.2