From aae4dbd4c058d3b1fe1eb5c4e6ddf35827271377 Mon Sep 17 00:00:00 2001 From: "jmc@openbsd.org" Date: Fri, 7 Oct 2016 14:41:52 +0000 Subject: [PATCH] upstream commit tidy up the formatting in this file. more specifically, replace .Dq, which looks appalling, with .Cm, where appropriate; Upstream-ID: ff8e90aa0343d9bb56f40a535e148607973cc738 --- sshd_config.5 | 467 +++++++++++++++++++++++--------------------------- 1 file changed, 212 insertions(+), 255 deletions(-) diff --git a/sshd_config.5 b/sshd_config.5 index b5d361e1d..82a3ad14a 100644 --- a/sshd_config.5 +++ b/sshd_config.5 @@ -33,8 +33,8 @@ .\" (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 1.236 2016/09/28 20:32:42 djm Exp $ -.Dd $Mdocdate: September 28 2016 $ +.\" $OpenBSD: sshd_config.5,v 1.237 2016/10/07 14:41:52 jmc Exp $ +.Dd $Mdocdate: October 7 2016 $ .Dt SSHD_CONFIG 5 .Os .Sh NAME @@ -90,19 +90,18 @@ The default is not to accept any environment variables. Specifies which address family should be used by .Xr sshd 8 . Valid arguments are -.Dq any , -.Dq inet +.Cm any +(the default), +.Cm inet (use IPv4 only), or -.Dq inet6 +.Cm inet6 (use IPv6 only). -The default is -.Dq any . .It Cm AllowAgentForwarding Specifies whether .Xr ssh-agent 1 forwarding is permitted. The default is -.Dq yes . +.Cm yes . Note that disabling agent forwarding does not improve security unless users are also denied shell access, as they can always install their own forwarders. @@ -126,40 +125,38 @@ for more information on patterns. .It Cm AllowStreamLocalForwarding Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted. The available options are -.Dq yes +.Cm yes +(the default) or -.Dq all +.Cm all to allow StreamLocal forwarding, -.Dq no +.Cm no to prevent all StreamLocal forwarding, -.Dq local +.Cm local to allow local (from the perspective of .Xr ssh 1 ) forwarding only or -.Dq remote +.Cm remote to allow remote forwarding only. -The default is -.Dq yes . Note that disabling StreamLocal forwarding does not improve security unless 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 -.Dq yes +.Cm yes +(the default) or -.Dq all +.Cm all to allow TCP forwarding, -.Dq no +.Cm no to prevent all TCP forwarding, -.Dq local +.Cm local to allow local (from the perspective of .Xr ssh 1 ) forwarding only or -.Dq remote +.Cm remote to allow remote forwarding only. -The default is -.Dq yes . Note that disabling TCP forwarding does not improve security unless users are also denied shell access, as they can always install their own forwarders. @@ -190,52 +187,44 @@ Specifies the authentication methods that must be successfully completed for a user to be granted access. This option must be followed by one or more comma-separated lists of authentication method names, or by the single string -.Dq any +.Cm any to indicate the default behaviour of accepting any single authentication method. -if the default is overridden, then successful authentication requires +If the default is overridden, then successful authentication requires completion of every method in at least one of these lists. .Pp -For example, an argument of -.Dq publickey,password publickey,keyboard-interactive +For example, +.Qq publickey,password publickey,keyboard-interactive would require the user to complete public key authentication, followed by either password or keyboard interactive authentication. Only methods that are next in one or more lists are offered at each stage, -so for this example, it would not be possible to attempt password or +so for this example it would not be possible to attempt password or keyboard-interactive authentication before public key. .Pp For keyboard interactive authentication it is also possible to restrict authentication to a specific device by appending a colon followed by the device identifier -.Dq bsdauth , -.Dq pam , +.Cm bsdauth , +.Cm pam , or -.Dq skey , +.Cm skey , depending on the server configuration. For example, -.Dq keyboard-interactive:bsdauth +.Qq keyboard-interactive:bsdauth would restrict keyboard interactive authentication to the -.Dq bsdauth +.Cm bsdauth device. .Pp -If the -.Dq publickey -method is listed more than once, +If the publickey method is listed more than once, .Xr sshd 8 verifies that keys that have been used successfully are not reused for subsequent authentications. -For example, an -.Cm AuthenticationMethods -of -.Dq publickey,publickey -will require successful authentication using two different public keys. +For example, +.Qq publickey,publickey +requires successful authentication using two different public keys. .Pp Note that each authentication method listed should also be explicitly enabled in the configuration. -The default -.Dq any -is not to require multiple authentication; successful completion -of a single authentication method is sufficient. .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 @@ -258,9 +247,13 @@ does not successfully authenticate and authorize the user then public key authentication continues using the usual .Cm AuthorizedKeysFile files. -By default, no AuthorizedKeysCommand is run. +By default, no +.Cm AuthorizedKeysCommand +is run. .It Cm AuthorizedKeysCommandUser -Specifies the user under whose account the AuthorizedKeysCommand is run. +Specifies the user under whose account the +.Cm AuthorizedKeysCommand +is run. It is recommended to use a dedicated user that has no other role on the host than running authorized keys commands. If @@ -287,10 +280,10 @@ 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 -.Dq none +.Cm none to skip checking for user keys in files. The default is -.Dq .ssh/authorized_keys .ssh/authorized_keys2 . +.Qq .ssh/authorized_keys .ssh/authorized_keys2 . .It Cm AuthorizedPrincipalsCommand Specifies a program to be used to generate the list of allowed certificate principals as per @@ -354,7 +347,7 @@ After expansion, .Cm AuthorizedPrincipalsFile is taken to be an absolute path or one relative to the user's home directory. The default is -.Dq none , +.Cm none , i.e. not to use a principals file \(en in this case, the username of the user must appear in a certificate's principals list for it to be accepted. @@ -374,7 +367,7 @@ for details). The contents of the specified file are sent to the remote user before authentication is allowed. If the argument is -.Dq none +.Cm none then no banner is displayed. By default, no banner is displayed. .It Cm ChallengeResponseAuthentication @@ -382,7 +375,7 @@ Specifies whether challenge-response authentication is allowed (e.g. via PAM or through authentication styles supported in .Xr login.conf 5 ) The default is -.Dq yes . +.Cm yes . .It Cm ChrootDirectory Specifies the pathname of a directory to .Xr chroot 2 @@ -417,10 +410,9 @@ nodes such as and .Xr tty 4 devices. -For file transfer sessions using -.Dq sftp , -no additional configuration of the environment is necessary if the -in-process sftp server is used, +For file transfer sessions using SFTP +no additional configuration of the environment is necessary if the in-process +sftp-server is used, though sessions which use logging may require .Pa /dev/log inside the chroot directory on some operating systems (see @@ -435,7 +427,7 @@ Misconfiguration can lead to unsafe environments which cannot detect. .Pp The default is -.Dq none , +.Cm none , indicating not to .Xr chroot 2 . .It Cm Ciphers @@ -488,23 +480,17 @@ 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 the -.Fl Q -option of -.Xr ssh 1 -with an argument of -.Dq cipher . +The list of available ciphers may also be obtained using +.Qq ssh -Q cipher . .It Cm ClientAliveCountMax -Sets the number of client alive messages (see below) which may be -sent without +Sets the number of client alive messages which may be sent without .Xr sshd 8 receiving any messages back from the client. If this threshold is reached while client alive messages are being sent, sshd will disconnect the client, terminating the session. It is important to note that the use of client alive messages is very different from -.Cm TCPKeepAlive -(below). +.Cm TCPKeepAlive . The client alive messages are sent through the encrypted channel and therefore will not be spoofable. The TCP keepalive option enabled by @@ -516,7 +502,7 @@ server depend on knowing when a connection has become inactive. The default value is 3. If .Cm ClientAliveInterval -(see below) is set to 15, and +is set to 15, and .Cm ClientAliveCountMax is left at the default, unresponsive SSH clients will be disconnected after approximately 45 seconds. @@ -532,14 +518,14 @@ is 0, indicating that these messages will not be sent to the client. Specifies whether compression is enabled after the user has authenticated successfully. The argument must be -.Dq yes , -.Dq delayed +.Cm yes , +.Cm delayed (a legacy synonym for -.Dq yes ) +.Cm yes ) or -.Dq no . +.Cm no . The default is -.Dq yes . +.Cm yes . .It Cm DenyGroups This keyword can be followed by a list of group name patterns, separated by spaces. @@ -581,11 +567,11 @@ for more information on patterns. .It Cm FingerprintHash Specifies the hash algorithm used when logging key fingerprints. Valid options are: -.Dq md5 +.Cm md5 and -.Dq sha256 . +.Cm sha256 . The default is -.Dq sha256 . +.Cm sha256 . .It Cm ForceCommand Forces the execution of the command specified by .Cm ForceCommand , @@ -601,12 +587,12 @@ The command originally supplied by the client is available in the .Ev SSH_ORIGINAL_COMMAND environment variable. Specifying a command of -.Dq internal-sftp -will force the use of an in-process sftp server that requires no support +.Cm internal-sftp +will force the use of an in-process SFTP server that requires no support files when used with .Cm ChrootDirectory . The default is -.Dq none . +.Cm none . .It Cm GatewayPorts Specifies whether remote hosts are allowed to connect to ports forwarded for the client. @@ -619,38 +605,37 @@ can be used to specify that sshd should allow remote port forwardings to bind to non-loopback addresses, thus allowing other hosts to connect. The argument may be -.Dq no +.Cm no to force remote port forwardings to be available to the local host only, -.Dq yes +.Cm yes to force remote port forwardings to bind to the wildcard address, or -.Dq clientspecified +.Cm clientspecified to allow the client to select the address to which the forwarding is bound. The default is -.Dq no . +.Cm no . .It Cm GSSAPIAuthentication Specifies whether user authentication based on GSSAPI is allowed. The default is -.Dq no . +.Cm no . .It Cm GSSAPICleanupCredentials Specifies whether to automatically destroy the user's credentials cache on logout. The default is -.Dq yes . +.Cm yes . .It Cm GSSAPIStrictAcceptorCheck Determines whether to be strict about the identity of the GSSAPI acceptor a client authenticates against. If set to -.Dq yes -then the client must authenticate against the -.Pa host +.Cm yes +then the client must authenticate against the host service on the current hostname. If set to -.Dq no +.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 -.Dq yes . +.Cm yes . .It Cm HostbasedAcceptedKeyTypes Specifies the key types that will be accepted for hostbased authentication as a comma-separated pattern list. @@ -669,17 +654,14 @@ ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, ssh-ed25519,ssh-rsa .Ed .Pp -The -.Fl Q -option of -.Xr ssh 1 -may be used to list supported key types. +The list of available key types may also be obtained using +.Qq ssh -Q key . .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 -.Dq no . +.Cm no . .It Cm HostbasedUsesNameFromPacketOnly Specifies whether or not the server will attempt to perform a reverse name lookup when matching the name in the @@ -690,13 +672,13 @@ and files during .Cm HostbasedAuthentication . A setting of -.Dq yes +.Cm yes means that .Xr sshd 8 uses the name supplied by the client rather than attempting to resolve the name from the TCP connection itself. The default is -.Dq no . +.Cm no . .It Cm HostCertificate Specifies a file containing a public host certificate. The certificate's public key must match a private host key already specified @@ -732,7 +714,7 @@ to an Identifies the UNIX-domain socket used to communicate with an agent that has access to the private host keys. If the string -.Dq SSH_AUTH_SOCK +.Qq SSH_AUTH_SOCK is specified, the location of the socket will be read from the .Ev SSH_AUTH_SOCK environment variable. @@ -750,12 +732,8 @@ ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, ssh-ed25519,ssh-rsa .Ed .Pp -The list of available key types may also be obtained using the -.Fl Q -option of -.Xr ssh 1 -with an argument of -.Dq key . +The list of available key types may also be obtained using +.Qq ssh -Q key . .It Cm IgnoreRhosts Specifies that .Pa .rhosts @@ -769,7 +747,7 @@ and .Pa /etc/shosts.equiv are still used. The default is -.Dq yes . +.Cm yes . .It Cm IgnoreUserKnownHosts Specifies whether .Xr sshd 8 @@ -778,55 +756,55 @@ should ignore the user's during .Cm HostbasedAuthentication . The default is -.Dq no . +.Cm no . .It Cm IPQoS Specifies the IPv4 type-of-service or DSCP class for the connection. Accepted values are -.Dq af11 , -.Dq af12 , -.Dq af13 , -.Dq af21 , -.Dq af22 , -.Dq af23 , -.Dq af31 , -.Dq af32 , -.Dq af33 , -.Dq af41 , -.Dq af42 , -.Dq af43 , -.Dq cs0 , -.Dq cs1 , -.Dq cs2 , -.Dq cs3 , -.Dq cs4 , -.Dq cs5 , -.Dq cs6 , -.Dq cs7 , -.Dq ef , -.Dq lowdelay , -.Dq throughput , -.Dq reliability , +.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 lowdelay , +.Cm throughput , +.Cm reliability , or a numeric value. This option may take one or two arguments, separated by whitespace. If one argument is specified, it is used as the packet class unconditionally. If two values are specified, the first is automatically selected for interactive sessions and the second for non-interactive sessions. The default is -.Dq lowdelay +.Cm lowdelay for interactive sessions and -.Dq throughput +.Cm throughput for non-interactive sessions. .It Cm KbdInteractiveAuthentication Specifies whether to allow keyboard-interactive authentication. The argument to this keyword must be -.Dq yes +.Cm yes or -.Dq no . +.Cm no . The default is to use whatever value .Cm ChallengeResponseAuthentication is set to (by default -.Dq yes ) . +.Cm yes ) . .It Cm KerberosAuthentication Specifies whether the password provided by the user for .Cm PasswordAuthentication @@ -834,24 +812,24 @@ will be validated through the Kerberos KDC. To use this option, the server needs a Kerberos servtab which allows the verification of the KDC's identity. The default is -.Dq no . +.Cm no . .It Cm KerberosGetAFSToken If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire an AFS token before accessing the user's home directory. The default is -.Dq no . +.Cm no . .It Cm KerberosOrLocalPasswd If password authentication through Kerberos fails then the password will be validated via any additional local mechanism such as .Pa /etc/passwd . The default is -.Dq yes . +.Cm yes . .It Cm KerberosTicketCleanup Specifies whether to automatically destroy the user's ticket cache file on logout. The default is -.Dq yes . +.Cm yes . .It Cm KexAlgorithms Specifies the available KEX (Key Exchange) algorithms. Multiple algorithms must be comma-separated. @@ -890,12 +868,8 @@ diffie-hellman-group-exchange-sha256, diffie-hellman-group14-sha1 .Ed .Pp -The list of available key exchange algorithms may also be obtained using the -.Fl Q -option of -.Xr ssh 1 -with an argument of -.Dq kex . +The list of available key exchange algorithms may also be obtained using +.Qq ssh -Q kex . .It Cm ListenAddress Specifies the local addresses .Xr sshd 8 @@ -955,7 +929,7 @@ character, then the specified algorithms will be appended to the default set instead of replacing them. .Pp The algorithms that contain -.Dq -etm +.Qq -etm calculate the MAC after encryption (encrypt-then-mac). These are considered safer and their use recommended. The supported MACs are: @@ -1008,12 +982,8 @@ 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 the -.Fl Q -option of -.Xr ssh 1 -with an argument of -.Dq mac . +The list of available MAC algorithms may also be obtained using +.Qq ssh -Q mac . .It Cm Match Introduces a conditional block. If all of the criteria on the @@ -1042,24 +1012,19 @@ and .Cm Address . The match patterns may consist of single entries or comma-separated lists and may use the wildcard and negation operators described in the -PATTERNS section of +.Sx PATTERNS +section of .Xr ssh_config 5 . .Pp The patterns in an .Cm Address criteria may additionally contain addresses to match in CIDR -address/masklen format, e.g.\& -.Dq 192.0.2.0/24 -or -.Dq 2001:db8::/32 . +address/masklen format, +such as 192.0.2.0/24 or 2001:db8::/32. Note that the mask length provided must be consistent with the address - it is an error to specify a mask length that is too long for the address or one with bits set in this host portion of the address. -For example, -.Dq 192.0.2.0/33 -and -.Dq 192.0.2.0/8 -respectively. +For example, 192.0.2.0/33 and 192.0.2.0/8, respectively. .Pp Only a subset of keywords may be used on the lines following a .Cm Match @@ -1138,29 +1103,21 @@ The default is 10:30:100. .Pp Alternatively, random early drop can be enabled by specifying the three colon separated values -.Dq start:rate:full -(e.g. "10:30:60"). +start:rate:full (e.g. "10:30:60"). .Xr sshd 8 -will refuse connection attempts with a probability of -.Dq rate/100 -(30%) -if there are currently -.Dq start -(10) -unauthenticated connections. +will refuse connection attempts with a probability of rate/100 (30%) +if there are currently start (10) unauthenticated connections. The probability increases linearly and all connection attempts -are refused if the number of unauthenticated connections reaches -.Dq full -(60). +are refused if the number of unauthenticated connections reaches full (60). .It Cm PasswordAuthentication Specifies whether password authentication is allowed. The default is -.Dq yes . +.Cm yes . .It Cm PermitEmptyPasswords When password authentication is allowed, it specifies whether the server allows login to accounts with empty password strings. The default is -.Dq no . +.Cm no . .It Cm PermitOpen Specifies the destinations to which TCP port forwarding is permitted. The forwarding specification must be one of the following forms: @@ -1185,36 +1142,36 @@ The forwarding specification must be one of the following forms: .Pp Multiple forwards may be specified by separating them with whitespace. An argument of -.Dq any +.Cm any can be used to remove all restrictions and permit any forwarding requests. An argument of -.Dq none +.Cm none can be used to prohibit all forwarding requests. The wildcard -.Dq * +.Sq * can be used for host or port to allow all hosts or ports, respectively. By default all port forwarding requests are permitted. .It Cm PermitRootLogin Specifies whether root can log in using .Xr ssh 1 . The argument must be -.Dq yes , -.Dq prohibit-password , -.Dq without-password , -.Dq forced-commands-only , +.Cm yes , +.Cm prohibit-password , +.Cm without-password , +.Cm forced-commands-only , or -.Dq no . +.Cm no . The default is -.Dq prohibit-password . +.Cm prohibit-password . .Pp If this option is set to -.Dq prohibit-password +.Cm prohibit-password or -.Dq without-password , +.Cm without-password , password and keyboard-interactive authentication are disabled for root. .Pp If this option is set to -.Dq forced-commands-only , +.Cm forced-commands-only , root login with public key authentication will be allowed, but only if the .Ar command @@ -1224,33 +1181,33 @@ normally not allowed). All other authentication methods are disabled for root. .Pp If this option is set to -.Dq no , +.Cm no , root is not allowed to log in. .It Cm PermitTTY Specifies whether .Xr pty 4 allocation is permitted. The default is -.Dq yes . +.Cm yes . .It Cm PermitTunnel Specifies whether .Xr tun 4 device forwarding is allowed. The argument must be -.Dq yes , -.Dq point-to-point +.Cm yes , +.Cm point-to-point (layer 3), -.Dq ethernet +.Cm ethernet (layer 2), or -.Dq no . +.Cm no . Specifying -.Dq yes +.Cm yes permits both -.Dq point-to-point +.Cm point-to-point and -.Dq ethernet . +.Cm ethernet . The default is -.Dq no . +.Cm no . .Pp Independent of this setting, the permissions of the selected .Xr tun 4 @@ -1265,7 +1222,7 @@ options in are processed by .Xr sshd 8 . The default is -.Dq no . +.Cm no . Enabling environment processing may enable users to bypass access restrictions in some configurations using mechanisms such as .Ev LD_PRELOAD . @@ -1274,11 +1231,11 @@ Specifies whether any .Pa ~/.ssh/rc file is executed. The default is -.Dq yes . +.Cm yes . .It Cm PidFile Specifies the file that contains the process ID of the SSH daemon, or -.Dq none +.Cm none to not write one. The default is .Pa /var/run/sshd.pid . @@ -1296,7 +1253,7 @@ Specifies whether should print the date and time of the last user login when a user logs in interactively. The default is -.Dq yes . +.Cm yes . .It Cm PrintMotd Specifies whether .Xr sshd 8 @@ -1307,7 +1264,7 @@ when a user logs in interactively. .Pa /etc/profile , or equivalent.) The default is -.Dq yes . +.Cm yes . .It Cm PubkeyAcceptedKeyTypes Specifies the key types that will be accepted for public key authentication as a comma-separated pattern list. @@ -1326,15 +1283,12 @@ ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521, ssh-ed25519,ssh-rsa .Ed .Pp -The -.Fl Q -option of -.Xr ssh 1 -may be used to list supported key types. +The list of available key types may also be obtained using +.Qq ssh -Q key . .It Cm PubkeyAuthentication Specifies whether public key authentication is allowed. The default is -.Dq yes . +.Cm yes . .It Cm RekeyLimit Specifies the maximum amount of data that may be transmitted before the session key is renegotiated, optionally followed a maximum amount of @@ -1357,12 +1311,12 @@ section. The default value for .Cm RekeyLimit is -.Dq default none , +.Cm default none , which means that rekeying is performed after the cipher's default amount of data has been sent or received and no time based rekeying is done. .It Cm RevokedKeys Specifies revoked public keys file, or -.Dq none +.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 @@ -1394,11 +1348,11 @@ will be unable to forward the port to the Unix-domain socket file. This option is only used for port forwarding to a Unix-domain socket file. .Pp The argument must be -.Dq yes +.Cm yes or -.Dq no . +.Cm no . The default is -.Dq no . +.Cm no . .It Cm StrictModes Specifies whether .Xr sshd 8 @@ -1407,7 +1361,7 @@ user's files and home directory before accepting login. This is normally desirable because novices sometimes accidentally leave their directory or files world-writable. The default is -.Dq yes . +.Cm yes . Note that this does not apply to .Cm ChrootDirectory , whose permissions and ownership are checked unconditionally. @@ -1417,16 +1371,12 @@ Arguments should be a subsystem name and a command (with optional arguments) to execute upon subsystem request. .Pp The command -.Xr sftp-server 8 -implements the -.Dq sftp -file transfer subsystem. +.Cm sftp-server +implements the SFTP file transfer subsystem. .Pp Alternately the name -.Dq internal-sftp -implements an in-process -.Dq sftp -server. +.Cm internal-sftp +implements an in-process SFTP server. This may simplify configurations using .Cm ChrootDirectory to force a different filesystem root on clients. @@ -1448,21 +1398,21 @@ connections will die if the route is down temporarily, and some people find it annoying. On the other hand, if TCP keepalives are not sent, sessions may hang indefinitely on the server, leaving -.Dq ghost +.Qq ghost users and consuming server resources. .Pp The default is -.Dq yes +.Cm yes (to send TCP keepalive messages), and the server will notice if the network goes down or the client host crashes. This avoids infinitely hanging sessions. .Pp To disable TCP keepalive messages, the value should be set to -.Dq no . +.Cm no . .It Cm TrustedUserCAKeys Specifies a file containing public keys of certificate authorities that are trusted to sign user certificates for authentication, or -.Dq none +.Cm none to not use one. Keys are listed one per line; empty lines and comments starting with .Ql # @@ -1483,7 +1433,7 @@ the resolved host name for the remote IP address maps back to the very same IP address. .Pp If this option is set to -.Dq no +.Cm no (the default) then only addresses and not host names may be used in .Pa ~/.ssh/authorized_keys .Cm from @@ -1495,7 +1445,7 @@ directives. .It Cm UsePAM Enables the Pluggable Authentication Module interface. If set to -.Dq yes +.Cm yes this will enable PAM authentication using .Cm ChallengeResponseAuthentication and @@ -1515,7 +1465,7 @@ is enabled, you will not be able to run .Xr sshd 8 as a non-root user. The default is -.Dq no . +.Cm no . .It Cm UsePrivilegeSeparation Specifies whether .Xr sshd 8 @@ -1526,23 +1476,23 @@ the privilege of the authenticated user. The goal of privilege separation is to prevent privilege escalation by containing any corruption within the unprivileged processes. The argument must be -.Dq yes , -.Dq no , +.Cm yes , +.Cm no , or -.Dq sandbox . +.Cm sandbox . If .Cm UsePrivilegeSeparation is set to -.Dq sandbox +.Cm sandbox then the pre-authentication unprivileged process is subject to additional restrictions. The default is -.Dq sandbox . +.Cm sandbox . .It Cm VersionAddendum Optionally specifies additional text to append to the SSH protocol banner sent by the server upon connection. The default is -.Dq none . +.Cm none . .It Cm X11DisplayOffset Specifies the first display number available for .Xr sshd 8 Ns 's @@ -1552,18 +1502,18 @@ The default is 10. .It Cm X11Forwarding Specifies whether X11 forwarding is permitted. The argument must be -.Dq yes +.Cm yes or -.Dq no . +.Cm no . The default is -.Dq no . +.Cm no . .Pp When X11 forwarding is enabled, there may be additional exposure to the server and to client displays if the .Xr sshd 8 proxy display is configured to listen on the wildcard address (see -.Cm X11UseLocalhost -below), though this is not the default. +.Cm X11UseLocalhost ) , +though this is not the default. Additionally, the authentication spoofing and authentication data verification and substitution occur on the client side. The security risk of using X11 forwarding is that the client's X11 @@ -1575,7 +1525,7 @@ in A system administrator may have a stance in which they want to protect clients that may expose themselves to attack by unwittingly requesting X11 forwarding, which can warrant a -.Dq no +.Cm no setting. .Pp Note that disabling X11 forwarding does not prevent users from @@ -1590,26 +1540,26 @@ sshd binds the forwarding server to the loopback address and sets the hostname part of the .Ev DISPLAY environment variable to -.Dq localhost . +.Cm localhost . This prevents remote hosts from connecting to the proxy display. However, some older X11 clients may not function with this configuration. .Cm X11UseLocalhost may be set to -.Dq no +.Cm no to specify that the forwarding server should be bound to the wildcard address. The argument must be -.Dq yes +.Cm yes or -.Dq no . +.Cm no . The default is -.Dq yes . +.Cm yes . .It Cm XAuthLocation Specifies the full pathname of the .Xr xauth 1 program, or -.Dq none +.Cm none to not use one. The default is .Pa /usr/X11R6/bin/xauth . @@ -1708,15 +1658,22 @@ This file should be writable by root only, but it is recommended (though not necessary) that it be world-readable. .El .Sh SEE ALSO +.Xr sftp-server 8 , .Xr sshd 8 .Sh AUTHORS +.An -nosplit OpenSSH is a derivative of the original and free -ssh 1.2.12 release by Tatu Ylonen. -Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, -Theo de Raadt and Dug Song +ssh 1.2.12 release by +.An Tatu Ylonen . +.An Aaron Campbell , Bob Beck , Markus Friedl , Niels Provos , +.An Theo de Raadt +and +.An Dug Song removed many bugs, re-added newer features and created OpenSSH. -Markus Friedl contributed the support for SSH -protocol versions 1.5 and 2.0. -Niels Provos and Markus Friedl contributed support -for privilege separation. +.An Markus Friedl +contributed the support for SSH protocol versions 1.5 and 2.0. +.An Niels Provos +and +.An Markus Friedl +contributed support for privilege separation.