Differences between version 3 and predecessor to the previous major change of sshd(8).
Other diffs: Previous Revision, Previous Author, or view the Annotated Edit History
| Newer page: | version 3 | Last edited on Tuesday, May 27, 2003 1:02:58 pm | by JohnMcPherson | Revert |
| Older page: | version 1 | Last edited on Monday, June 3, 2002 11:58:53 pm | by perry | Revert |
@@ -2,520 +2,285 @@
__NAME__
sshd - OpenSSH SSH daemon
+
__SYNOPSIS__
-sshd [[-deiqtD46] [[-b bits] [[-f config_file] [[-g
-
-
+sshd [[-deiqtD46] [[-b bits] [[-f config_file] [[-g login_grace_time] [[-h host_key_file] [[-k key_gen_time] [[-p port] [[-u len]
+
__DESCRIPTION__
-sshd (SSH Daemon) is the daemon program for ssh(1).
-
-
-
-
+sshd (SSH Daemon) is the daemon program for ssh(1). Together these programs replace rlogin and rsh, and provide secure encrypted communications between two untrusted hosts over an insecure network. The programs are intended to be as easy to install and use as possible.
-sshd is the daemon that listens for connections from clients. It is normally started at boot from /etc/rc. It forks a new daemon for each incoming connection. The forked daemons handle key exchange, encryption, authentication, command execution, and data exchange. This implementation of sshd supports both SSH protocol version 1 and 2 simulta- neously. sshd works as follows.
+sshd is the daemon that listens for connections from clients. It is
normally started at boot from /etc/rc. It forks a new daemon for each incoming connection. The forked daemons handle key exchange, encryption, authentication, command execution, and data exchange. This implementation of sshd supports both SSH protocol version 1 and 2 simulta- neously. sshd works as follows.
__SSH protocol version 1__
-Each host has a host-specific RSA key (normally 1024 bits)
-
-
-
-
+Each host has a host-specific RSA key (normally 1024 bits) used to identify the host. Additionally, when the daemon starts, it generates a server RSA key (normally 768 bits). This key is normally regenerated every hour if it has been used, and is never stored on disk.
-Whenever a client connects the daemon responds with its pub-
-lic
host and server keys. The client compares the RSA host
-
-
-
-
-
-
-
-
-
-algorithm to use from those offered by the
-
+Whenever a client connects the daemon responds with its public
host and server keys. The client compares the RSA host key against its own database to verify that it has not changed. The client then generates a 256 bit random number. It encrypts this random number using both the host key and the server key, and sends the encrypted number to the server. Both sides then use this random number as a session key which is used to encrypt all further communications in the session. The rest of the session is encrypted using a conventional cipher, currently Blowfish or 3DES, with 3DES being used by default. The client selects the encryption
+algorithm to use from those offered by the server.
-Next, the server and the client enter an authentication dia-
-log
. The client tries to authenticate itself using
-
-
-
-
+Next, the server and the client enter an authentication dialog
. The client tries to authenticate itself using .rhosts authentication, .rhosts authentication combined with RSA host authentication, RSA challenge-response authentication, or password based authentication.
-Rhosts authentication is normally disabled because it is
-
-
-
-
-rlogin(1) and rsh(1) into the
-
+Rhosts authentication is normally disabled because it is fundamentally insecure, but can be enabled in the server configuration file if desired. System security is not improved unless rshd(8), rlogind(8), and rexecd(8) are dis- abled (thus completely disabling
+rlogin(1) and rsh(1) into the machine).
__SSH protocol version 2__
-Version 2 works similarly: Each host has a host-specific key
-
-
-
-
-
+Version 2 works similarly: Each host has a host-specific key (RSA or DSA) used to identify the host. However, when the daemon starts, it does not generate a server key. Forward security is provided through a Diffie-Hellman key agreement. This key agreement results in a shared session key.
-The rest of the session is encrypted using a symmetric
-Arc-
-four
, 192 bit AES, or 256 bit AES. The client selects the
-
-
-
-
+The rest of the session is encrypted using a symmetric cipher, currently 128 bit AES, Blowfish, 3DES, CAST128, Arcfour
, 192 bit AES, or 256 bit AES. The client selects the encryption algorithm to use from those offered by the server. Additionally, session integrity is provided through a cryptographic message authentication code (hmac-sha1 or hmac-md5).
-Protocol version 2 provides a public key based user (Pub-
-keyAuthentication
) or client host (HostbasedAuthentication)
-
-
+Protocol version 2 provides a public key based user (!PubkeyAuthentication
) or client host (!
HostbasedAuthentication) authentication method, conventional password authentication and challenge response based methods.
__Command execution and data forwarding__
-If the client successfully authenticates itself, a dialog
-
-for-
-warding
X11 connections, forwarding TCP/IP connections, or
-
-
+If the client successfully authenticates itself, a dialog for preparing the session is entered. At this time the client may request things like allocating a pseudo-tty, forwarding
X11 connections, forwarding TCP/IP connections, or forwarding the authentication agent connection over the secure channel.
-Finally, the client either requests a shell or execution of
-
-
-
-
+Finally, the client either requests a shell or execution of a command. The sides then enter session mode. In this mode, either side may send data at any time, and such data is forwarded to/from the shell or command on the server side, and the user terminal in the client side.
-When the user program terminates and all forwarded X11 and
-
-
+When the user program terminates and all forwarded X11 and other connections have been closed, the server sends command exit status to the client, and both sides exit.
-sshd can be configured using command-line options or a con-
-figuration
file. Command-line options override values spec-
-ified
in the configuration file.
+sshd can be configured using command-line options or a configuration
file. Command-line options override values specified
in the configuration file.
-sshd rereads its configuration file when it receives a
-
-
-
+sshd rereads its configuration file when it receives a hangup signal, SIGHUP, by executing itself with the name it was started as, i.e., /usr/sbin/sshd.
The options are as follows:
-
-b bits
+
-b bits
Specifies the number of bits in the ephemeral proto-col version 1 server key (default 768).-dDebug mode. The server sends verbose debug output tothe system log, and does not put itself in the back-ground. The server also will not fork and will onlyprocess one connection. This option is only intendedfor debugging for the server. Multiple -d optionsincrease the debugging level. Maximum is 3.-eWhen this option is specified, sshd will send theoutput to the standard error instead of the systemlog.-f configuration_fileSpecifies the name of the configuration file. Thedefault is /etc/ssh/sshd_config. sshd refuses to start if there is no configuration file.
--g login_grace_time
-Gives the grace time for clients to authenticate themselves
-
-
-
+
-g login_grace_time
+Gives the grace time for clients to authenticate themselves (default 600 seconds). If the client fails to authenticate the user within this many sec- onds, the server disconnects and exits. A value of zero indicates no limit.
--h host_key_file
-Specifies the file from which the host key is read (default
-
-
-
-
-algo- rithms
.
+
-h host_key_file
+Specifies the file from which the host key is read (default /etc/ssh/ssh_host_key). This option must be given if sshd is not run as root (as the normal host file is normally not readable by anyone but root). It is possible to have multiple host key files for the different protocol versions and host key algorithms
.
--i
-Specifies that sshd is being run from inetd. sshd is
-gen- erate
-
-
-
-
-
+
-i
+Specifies that sshd is being run from inetd. sshd is normally not run from inetd because it needs to generate
the server key before it can respond to the client, and this may take tens of seconds. Clients would have to wait too long if the key was regener- ated every time. However, with small key sizes (e.g., 512) using sshd from inetd may be feasible.
--k key_gen_time
-Specifies how often the ephemeral protocol version 1 server
-
-
-
-
-communica- tions
even if the machine is cracked
-physi- cally
seized. A value of zero indicates that
-
+
-k key_gen_time
+Specifies how often the ephemeral protocol version 1 server key is regenerated (default 3600 seconds, or one hour). The motivation for regenerating the key fairly often is that the key is not stored anywhere, and after about an hour, it becomes impossible to recover the key for decrypting intercepted communications
even if the machine is cracked into or physically
seized. A value of zero indicates that the key will never be regenerated.
--p port
-Specifies the port on which the server listens for
-
+
-p port
+Specifies the port on which the server listens for connections (default 22).
--q
-Quiet mode. Only fatal errors are sent to the system log.
-
-
-
+
-q
+Quiet mode. Only fatal errors are sent to the system log. Normally the beginning, authentication, and termination of each connection is logged. If a sec- ond -q is given then nothing is sent to the system log.
--t
-Test mode. Only check the validity of the configura- tion
-
-
-
+
-t
+Test mode. Only check the validity of the configuration
file and sanity of the keys. This is useful for updating sshd reliably as configuration options may change.
--u len
-This option is used to specify the size of the field in the
-
-
-
-
-Specify- ing
-u0 indicates that only
-
-utmp file. -u0 is also be used to prevent sshd from
-
-
-
-
-
-
+
-u len
+This option is used to specify the size of the field in the utmp structure that holds the remote host name. If the resolved host name is longer than len, the dotted decimal value will be used instead. This allows hosts with very long host names that overflow this field to still be uniquely identified. Specifying
-u0 indicates that only dotted decimal addresses should be put into the
+utmp file. -u0 is also be used to prevent sshd from making DNS requests unless the authentication mechanism or configuration requires it. Authentication mechanisms that may require DNS include !
RhostsAuthentication, !
RhostsRSAAuthentication, !
HostbasedAuthentication and using a from="pattern-list" option in a key file. Configuration options that require DNS include using a USER@HOST pattern in AllowUsers or DenyUsers.
--D
-When this option is specified sshd will not detach and does
-moni- toring
of
-
+
-D
+When this option is specified sshd will not detach and does not become a daemon. This allows easy monitoring
of sshd.
--4
+
-4
Forces sshd to use IPv4 addresses only.
--6
+
-6
Forces sshd to use IPv6 addresses only.
__CONFIGURATION FILE__
-sshd reads configuration data from
-
-
-start- ing
with # and
-
+sshd reads configuration data from /etc/ssh/sshd_config (or the file specified with -f on the command line). The file contains keyword-argument pairs, one per line. Lines starting
with # and empty lines are interpreted as comments.
-The possible keywords and their meanings are as follows
-
-
+The possible keywords and their meanings are as follows (note that keywords are case-insensitive and arguments are case-sensitive):
AFSTokenPassing
-Specifies whether an AFS token may be forwarded tothe server. Default is ``yes''.AllowGroupsThis keyword can be followed by a list of groupnames, separated by spaces. If specified, login isallowed only for users whose primary group or supple-mentary group list matches one of the patterns. and? can be used as wildcards in the patterns. Onlygroup names are valid; a numerical group ID is notrecognized. By default login is allowed regardlessof the group list.AllowTcpForwardingSpecifies whether TCP forwarding is permitted. Thedefault is ``yes''. Note that disabling TCP forward-ing does not improve security unless users are alsodenied shell access, as they can always install theirown forwarders.AllowUsersThis keyword can be followed by a list of user names,separated by spaces. If specified, login is allowedonly for users names that match one of the patterns.and ? can be used as wildcards in the patterns.Only user names are valid; a numerical user ID is notrecognized. By default login is allowed regardlessof the user name. If the pattern takes the formUSER@HOST then USER and HOST are separately checked,restricting logins to particular users from particu-lar hosts.AuthorizedKeysFileSpecifies the file that contains the public keys thatcan be used for user authentication.AuthorizedKeysFile may contain tokens of the form %Twhich are substituted during connection set-up. Thefollowing tokens are defined: %% is replaced by aliteral '%', %h is replaced by the home directory ofthe user being authenticated and %u is replaced bythe username of that user. After expansion,AuthorizedKeysFile is taken to be an absolute path orone relative to the user's home directory. Thedefault is ``.ssh/authorized_keys''BannerIn some jurisdictions, sending a warning messagebefore authentication may be relevant for gettinglegal protection. The contents of the specified fileare sent to the remote user before authentication isallowed. This option is only available for protocolversion 2.ChallengeResponseAuthenticationSpecifies whether challenge response authenticationis allowed. All authentication styles fromlogin.conf(5) are supported. The default is ``yes''.
+Specifies whether an AFS token may be forwarded tothe server. Default is ``yes''.!
AllowGroupsThis keyword can be followed by a list of groupnames, separated by spaces. If specified, login isallowed only for users whose primary group or supple-mentary group list matches one of the patterns. and? can be used as wildcards in the patterns. Onlygroup names are valid; a numerical group ID is notrecognized. By default login is allowed regardlessof the group list.!
AllowTcpForwardingSpecifies whether TCP forwarding is permitted. Thedefault is ``yes''. Note that disabling TCP forward-ing does not improve security unless users are alsodenied shell access, as they can always install theirown forwarders.!
AllowUsersThis keyword can be followed by a list of user names,separated by spaces. If specified, login is allowedonly for users names that match one of the patterns.and ? can be used as wildcards in the patterns.Only user names are valid; a numerical user ID is notrecognized. By default login is allowed regardlessof the user name. If the pattern takes the formUSER@HOST then USER and HOST are separately checked,restricting logins to particular users from particu-lar hosts.!
AuthorizedKeysFileSpecifies the file that contains the public keys thatcan be used for user authentication.!
AuthorizedKeysFile may contain tokens of the form %Twhich are substituted during connection set-up. Thefollowing tokens are defined: %% is replaced by aliteral '%', %h is replaced by the home directory ofthe user being authenticated and %u is replaced bythe username of that user. After expansion,!
AuthorizedKeysFile is taken to be an absolute path orone relative to the user's home directory. Thedefault is ``.ssh/authorized_keys''!
BannerIn some jurisdictions, sending a warning messagebefore authentication may be relevant for gettinglegal protection. The contents of the specified fileare sent to the remote user before authentication isallowed. This option is only available for protocolversion 2.!
ChallengeResponseAuthenticationSpecifies whether challenge response authenticationis allowed. All authentication styles fromlogin.conf(5) are supported. The default is ``yes''.
-Ciphers
-Specifies the ciphers allowed for protocol version 2.
-
-
+
Ciphers
+Specifies the ciphers allowed for protocol version 2. Multiple ciphers must be comma-separated. The default is ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour.''
-ClientAliveInterval
-Sets a timeout interval in seconds after which if no data
-
-
-
-
-
+ !
ClientAliveInterval
+Sets a timeout interval in seconds after which if no data has been received from the client, sshd will send a message through the encrypted channel to request a response from the client. The default is , indicating that these messages will not be sent to the client. This option applies to protocol version 2 only.
-ClientAliveCountMax
-Sets the number of client alive messages (see above) which
-
-
-
-
-mes- sages
is very different from
-
-
-
-
-
-
+ !
ClientAliveCountMax
+Sets the number of client alive messages (see above) which may be sent without sshd 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 !
KeepAlive (below). The client alive messages are sent through the encrypted channel and therefore will not be spoofable. The TCP keepalive option enabled by !
KeepAlive is spoofable. The client alive mechanism is valuable when the client or server depend on knowing when a connection has become inactive.
-The default value is 3. If ClientAliveInterval (above) is
-
-dis- connected
after
-
+The default value is 3. If !
ClientAliveInterval (above) is set to 15, and !
ClientAliveCountMax is left at the default, unresponsive ssh clients will be disconnected
after approximately 45 seconds.
-DenyGroups
-This keyword can be followed by a number of group names,
-
-
-
-
-
-
+!
DenyGroups
+This keyword can be followed by a number of group names, separated by spaces. Users whose primary group or supplementary group list matches one of the patterns aren't allowed to log in. and
? 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 regardless of the group list.
-DenyUsers
-This keyword can be followed by a number of user names,
-
-
-
-recog- nized
. By default login is allowed regardless
-
+!
DenyUsers
+This keyword can be followed by a number of user names, separated by spaces. Login is disallowed for user names that match one of the patterns. and ? 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 regardless of the user name.
-GatewayPorts
-Specifies whether remote hosts are allowed to connect to
-
-
-
-
-
-
-
-
+!
GatewayPorts
+Specifies whether remote hosts are allowed to connect to ports forwarded for the client. By default, sshd binds remote port forwardings to the loopback addresss. This prevents other remote hosts from con- necting to forwarded ports. !
GatewayPorts can be used to specify that sshd should bind remote port forward- ings to the wildcard address, thus allowing remote hosts to connect to forwarded ports. The argument must be ``yes'' or ``no''. The default is ``no''.
-HostbasedAuthentication
-Specifies whether rhosts or /etc/hosts.equiv authen-
-tication
together with successful public key client host
-
-
-ver- sion
2 only. The default is
-
+!
HostbasedAuthentication
+Specifies whether rhosts or /etc/hosts.equiv authentication
together with successful public key client host authentication is allowed (hostbased authentica- tion). This option is similar to RhostsRSAAuthentication and applies to protocol version
2 only. The default is ``no''.
-HostKey
-Specifies the file containing the private host keys (default
-
-
-
-
-
-
+!
HostKey
+Specifies the file containing the private host keys (default /etc/ssh/ssh_host_key) used by SSH protocol versions 1 and 2. Note that sshd will refuse to use a file if it is group/world-accessible. It is possi- ble to have multiple host key files. ``rsa1'' keys are used for version 1 and ``dsa'' or ``rsa'' are used for version 2 of the SSH protocol.
-IgnoreRhosts
-Specifies that .rhosts and .shosts files
-
-
-
+!
IgnoreRhosts
+Specifies that .rhosts and .shosts files will not be used in !
RhostsAuthentication, RhostsRSAAuthentication or !
HostbasedAuthentication.
+/etc/hosts.equiv and /etc/ssh/shosts.equiv are still used. The default is ``yes''.
-/etc/hosts.equiv and /etc/ssh/shosts.equiv
-are still used. The default is ``yes''.
+!IgnoreUserKnownHosts
+Specifies whether sshd should ignore the user's $HOME/.ssh/known_hosts during RhostsRSAAuthentication or !HostbasedAuthentication. The default is ``no''.
-IgnoreUserKnownHosts
-Specifies whether sshd should ignore the user's
-$HOME/.ssh/known_hosts during
-RhostsRSAAuthentication or HostbasedAuthentication. The
-default is ``no''.
+!KeepAlive
+Specifies whether the system should send 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 ``ghost'' users and consuming server resources.
-KeepAlive
-Specifies whether the system should send 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 temporar- ily, and some people find it annoying. On
-the other hand, if keepalives are not sent, sessions may
-hang indefinitely on the server, leaving ``ghost'' users and
-consuming server resources.
+The default is ``yes'' (to send keepalives), and the server will notice if the network goes down or the client host reboots. This avoids infinitely hanging sessions.
-The default is ``yes'' (to send keepalives), and the server
-will notice if the network goes down or the client host
-reboots. This avoids infinitely hanging
-sessions.
+To disable keepalives, the value should be set to ``no'' in both the server and the client configuration files.
-To disable keepalives, the value should be set to ``no'' in
-both the server and the client configura- tion
-files.
+!KerberosAuthentication
+Specifies whether Kerberos authentication is allowed. This can be in the form of a Kerberos ticket, or if !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 ``yes''.
-KerberosAuthentication
-Specifies whether Kerberos authentication is allowed. This
-can be in the form of a Kerberos ticket, or if
-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
-``yes''.
-
-
-If set then if password authentication through Ker- beros
-
-
-
+!
KerberosOrLocalPasswd
+If set then if password authentication through Kerberos
fails then the password will be validated via any additional local mechanism such as /etc/passwd. Default is ``yes''.
-KerberosTgtPassing
-Specifies whether a Kerberos TGT may be forwarded to the
-
-
+!
KerberosTgtPassing
+Specifies whether a Kerberos TGT may be forwarded to the server. Default is ``no'', as this only works when the Kerberos KDC is actually an AFS kaserver.
-KerberosTicketCleanup
-Specifies whether to automatically destroy the user's ticket
-
+!
KerberosTicketCleanup
+Specifies whether to automatically destroy the user's ticket cache file on logout. Default is ``yes''.
-KeyRegenerationInterval
-In protocol version 1, the ephemeral server key is
-
-
-
-
-
-
+!
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 break- ing into the machine and stealing the keys. The key is never stored anywhere. If the value is , the key is never regenerated. The default is 3600 (seconds).
-ListenAddress
-Specifies the local addresses sshd should listen on. The
-
+!
ListenAddress
+Specifies the local addresses sshd should listen on. The following forms may be used:
-ListenAddress host|IPv4_addr|IPv6_addr
-ListenAddress host|IPv4_addr:port
-ListenAddress [[
-
+ !
ListenAddress host|IPv4_addr|IPv6_addr
+ !
ListenAddress host|IPv4_addr:port
+ !
ListenAddress [[ host|IPv6_addr]:port
-If port is not specified, sshd will listen on the address
-
-Multi- ple
ListenAddress
-
-
-
+If port is not specified, sshd will listen on the address and all prior Port options specified. The default is to listen on all local addresses. Multiple !
ListenAddress options are permitted. Addition- ally, any Port options must precede this option for non port qualified addresses.
-LoginGraceTime
-The server disconnects after this time if the user has not
-
-sec- onds
).
+!
LoginGraceTime
+The server disconnects after this time if the user has not successfully logged in. If the value is , there is no time limit. The default is 600 (seconds
).
-LogLevel
-Gives the verbosity level that is used when logging messages
-
-
-pri- vacy
of users and is not
-
+!
LogLevel
+Gives the verbosity level that is used when logging messages from sshd. The possible values are: QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG. The default is INFO. Logging with level DEBUG violates the privacy
of users and is not recommended.
MACs
-Specifies the available MAC (message authentication code)
-
-Mul- tiple
algorithms must
-
-
+Specifies the available MAC (message authentication code) algorithms. The MAC algorithm is used in pro- tocol version 2 for data integrity protection. Multiple
algorithms must be comma-separated. The default is ``hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96''.
-MaxStartups
-Specifies the maximum number of concurrent unauthen- ticated
-
-suc- ceeds
or the
-
-
+!
MaxStartups
+Specifies the maximum number of concurrent unauthenticated
connections to the sshd daemon. Additional connections will be dropped until authentication succeeds
or the !
LoginGraceTime expires for a connection. The default is 10.
-Alternatively, random early drop can be enabled by
-
-
+Alternatively, random early drop can be enabled by specifying the three colon separated values ``start:rate:full'' (e.g.,
PAMAuthenticationViaKbdInt
-Specifies whether PAM challenge response authentica- tion
is
-
-
-
-
+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 !
PasswordAuthentication is disabled. The default is ``no''.
-PasswordAuthentication
-Specifies whether password authentication is allowed. The
-
+!
PasswordAuthentication
+Specifies whether password authentication is allowed. The default is ``yes''.
-PermitEmptyPasswords
-When password authentication is allowed, it specifies
-
-
+!
PermitEmptyPasswords
+When password authentication is allowed, it specifies whether the server allows login to accounts with empty password strings. The default is ``no''.
-PermitRootLogin
-Specifies whether root can login using ssh(1). The
-
-
-
+!
PermitRootLogin
+Specifies whether root can login using ssh(1). The argument must be ``yes'', ``without-password'', ``forced-commands-only'' or ``no''. The default is ``yes''.
-If this option is set to ``without-password'' pass- word
-
+If this option is set to ``without-password'' password
authentication is disabled for root.
-If this option is set to ``forced-commands-only'' root login
-
-
-
-
-
+If this option is set to ``forced-commands-only'' root login with public key authentication will be allowed, but only if the 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.
If this option is set to ``no'' root is not allowed to
login.
-PidFile
+!
PidFile
Specifies the file that contains the process identi- fier of
the sshd daemon. The default is
/var/run/sshd.pid.
Port
Specifies the port number that sshd listens on. The default
is 22. Multiple options of this type are permitted. See also
-ListenAddress.
+!
ListenAddress.
-PrintLastLog
+!
PrintLastLog
Specifies whether sshd should print the date and time when
the user last logged in. The default is
``yes''.
-PrintMotd
+!
PrintMotd
Specifies whether sshd should print /etc/motd when
a user logs in interactively. (On some systems it is also
printed by the shell, /etc/profile, or equiva-
lent.) The default is ``yes''.
@@ -526,22 +291,22 @@
possible values are ``1'' and ``2''. Multiple versions must
be comma-separated. The default is ``2,1''.
-PubkeyAuthentication
+!
PubkeyAuthentication
Specifies whether public key authentication is allowed. The
default is ``yes''. Note that this option applies to
protocol version 2 only.
-ReverseMappingCheck
+!
ReverseMappingCheck
Specifies whether 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 ``no''.
-RhostsAuthentication
+!
RhostsAuthentication
Specifies whether authentication using rhosts or
/etc/hosts.equiv files is sufficient. Normally, this method
should not be permitted because it is inse- cure.
RhostsRSAAuthentication should be used instead, because it
@@ -563,15 +328,15 @@
default is ``yes''. This option applies to pro- tocol
version 1 only.
-ServerKeyBits
+!
ServerKeyBits
Defines the number of bits in the ephemeral protocol version
1 server key. The minimum value is 512, and the default is
768.
-StrictModes
+!
StrictModes
Specifies whether 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.
@@ -587,16 +352,16 @@
Note that this option applies to protocol version 2
only.
-SyslogFacility
+!
SyslogFacility
Gives the facility code that is used when logging messages
from sshd. The possible values are: DAEMON, USER, AUTH,
LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6,
LOCAL7. The default is AUTH.
-UseLogin
+!
UseLogin
Specifies whether login(1) is used for interactive
login sessions. The default is ``no''. Note that
login(1) is never used for remote command
execution. Note also, that if this is enabled, X11Forwarding
@@ -614,9 +379,9 @@
Specifies whether X11 forwarding is permitted. The default
is ``no''. Note that disabling X11 forward- ing does not
improve security in any way, as users can always install
their own forwarders. X11 for- warding is automatically
-disabled if UseLogin is enabled.
+disabled if !
UseLogin is enabled.
XAuthLocation
Specifies the location of the xauth(1) program. The
@@ -707,9 +472,9 @@
$HOME/.ssh/authorized_keys is the default file that
lists the public keys that are permitted for RSA
authentication in protocol version 1 and for public key
authentication (Pub- keyAuthentication) in protocol version
-2. AuthorizedKeysFile may be used to specify an alternative
+2. !
AuthorizedKeysFile may be used to specify an alternative
file.
Each line of the file contains one key (empty lines and
@@ -849,177 +614,108 @@
contents can be copied to known hosts files. These files are
created using ssh-keygen(1).
-/etc/ssh/moduli
-Contains Diffie-Hellman groups used for the
+
/etc/ssh/moduli
+Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
-/var/run/sshd.pid
-Contains the process ID of the sshd listening for
-connections (if there are several daemons running
-concurrently for different ports, this contains the pid of
-the one started last). The content of this file is not
-sensitive; it can be world-readable.
+ /var/run/sshd.pid
+Contains the process ID of the sshd listening for connections (if there are several daemons running concurrently for different ports, this contains the pid of the one started last). The content of this file is not sensitive; it can be world-readable.
-$HOME/.ssh/authorized_keys
-Lists the public keys (RSA or DSA) that can be used to log
-into the user's account. This file must be readable by root
-(which may on some machines imply it being world-readable if
-the user's home directory resides on an NFS volume). It is
-recommended that it not be accessible by others. The format
-of this file is described above. Users will place the
-contents of their identity.pub, id_dsa.pub
-and/or id_rsa.pub files into this file, as
-described in ssh-keygen(1).
+ $HOME/.ssh/authorized_keys
+Lists the public keys (RSA or DSA) that can be used to log into the user's account. This file must be readable by root (which may on some machines imply it being world-readable if the user's home directory resides on an NFS volume). It is recommended that it not be accessible by others. The format of this file is described above. Users will place the contents of their identity.pub, id_dsa.pub and/or id_rsa.pub files into this file, as described in ssh-keygen(1).
-/etc/ssh/ssh_known_hosts and
-$HOME/.ssh/known_hosts
-These files are consulted when using rhosts with RSA host
-authentication or protocol version 2 hostbased
-authentication to check the public key of the host. The key
-must be listed in one of these files to be accepted. The
-client uses the same files to verify that it is connecting
-to the correct remote host. These files should be writable
-only by root/the owner. /etc/ssh/ssh_known_hosts
-should be world-readable, and
-$HOME/.ssh/known_hosts can but need not be
-world-readable.
+ /etc/ssh/ssh_known_hosts and
+ $HOME/.ssh/known_hosts
+These files are consulted when using rhosts with RSA host authentication or protocol version 2 hostbased authentication to check the public key of the host. The key must be listed in one of these files to be accepted. The client uses the same files to verify that it is connecting to the correct remote host. These files should be writable only by root/the owner. /etc/ssh/ssh_known_hosts should be world-readable, and $HOME/.ssh/known_hosts can but need not be world-readable.
-/etc/nologin
-If this file exists, sshd refuses to let anyone except root
-log in. The contents of the file are displayed to anyone
-trying to log in, and non-root connections are refused. The
-file should be world-readable.
+ /etc/nologin
+If this file exists, sshd refuses to let anyone except root log in. The contents of the file are displayed to anyone trying to log in, and non-root connections are refused. The file should be world-readable.
-/etc/hosts.allow, /etc/hosts.deny
-Access controls that should be enforced by tcp-wrap- pers
-are defined here. Further details are described in
-hosts_access(5).
+ /etc/hosts.allow, /etc/hosts.deny
+Access controls that should be enforced by tcp-wrappers are defined here. Further details are described in hosts_access(5).
-$HOME/.rhosts
-This file contains host-username pairs, separated by a
-space, one per line. The given user on the corre- sponding
-host is permitted to log in without pass- word. The same
-file is used by rlogind and rshd. The file must be writable
-only by the user; it is recommended that it not be
-accessible by others.
+ $HOME/.rhosts
+This file contains host-username pairs, separated by a space, one per line. The given user on the corre- sponding host is permitted to log in without pass- word. The same file is used by rlogind and rshd. The file must be writable only by the user; it is recommended that it not be accessible by others.
-If is also possible to use netgroups in the file. Either
-host or user name may be of the form +@group- name to
-specify all hosts or all users in the group.
+If is also possible to use netgroups in the file. Either host or user name may be of the form +@group- name to specify all hosts or all users in the group.
-$HOME/.shosts
-For ssh, this file is exactly the same as for
-.rhosts. However, this file is not used by rlogin
-and rshd, so using this permits access using SSH
-only.
+ $HOME/.shosts
+For ssh, this file is exactly the same as for .rhosts. However, this file is not used by rlogin and rshd, so using this permits access using SSH only.
-/etc/hosts.equiv
-This file is used during .rhosts authentication. In
-the simplest form, this file contains host names, one per
-line. Users on those hosts are permitted to log in without a
-password, provided they have the same user name on both
-machines. The host name may also be followed by a user name;
-such users are permitted to log in as ''any'' user on
-this machine (except root). Additionally, the syntax
-``+@group'' can be used to specify netgroups. Negated
-entries start with -.
+ /etc/hosts.equiv
+This file is used during .rhosts authentication. In the simplest form, this file contains host names, one per line. Users on those hosts are permitted to log in without a password, provided they have the same user name on both machines. The host name may also be followed by a user name; such users are permitted to log in as ''any'' user on this machine (except root). Additionally, the syntax ``+@group'' can be used to specify netgroups. Negated entries start with -.
-If the client host/user is successfully matched in this
-file, login is automatically permitted provided the client
-and server user names are the same. Addi- tionally,
-successful RSA host authentication is nor- mally required.
-This file must be writable only by root; it is recommended
-that it be world-readable.
+If the client host/user is successfully matched in this file, login is automatically permitted provided the client and server user names are the same. Additionally, successful RSA host authentication is normally required. This file must be writable only by root; it is recommended that it be world-readable.
-__Warning: It is almost never a good idea to use user names
-in__ hosts.equiv. Beware that it really means
-that the named user(s) can log in as ''anybody'', which
-includes bin, daemon, adm, and other accounts that own
-critical binaries and directories. Using a user name
-practically grants the user root access. The only valid use
-for user names that I can think of is in negative
-entries.
+__Warning: It is almost never a good idea to use user names in__ hosts.equiv. Beware that it really means that the named user(s) can log in as ''anybody'', which includes bin, daemon, adm, and other accounts that own critical binaries and directories. Using a user name practically grants the user root access. The only valid use for user names that I can think of is in negative entries.
-Note that this warning also applies to
-rsh/rlogin.
+Note that this warning also applies to rsh/rlogin.
-/etc/ssh/shosts.equiv
-This is processed exactly as /etc/hosts.equiv. How-
-ever, this file may be useful in environments that want to
-run both rsh/rlogin and ssh.
+ /etc/ssh/shosts.equiv
+This is processed exactly as /etc/hosts.equiv. However, this file may be useful in environments that want to run both rsh/rlogin and ssh.
-$HOME/.ssh/environment
-This file is read into the environment at login (if it
-exists). It can only contain empty lines, comment lines
-(that start with #), and assignment lines of the
-form name=value. The file should be writable only by the
-user; it need not be readable by anyone else.
+ $HOME/.ssh/environment
+This file is read into the environment at login (if it exists). It can only contain empty lines, comment lines (that start with #), and assignment lines of the form name=value. The file should be writable only by the user; it need not be readable by anyone else.
-$HOME/.ssh/rc
-If this file exists, it is run with /bin/sh after reading
-the environment files but before starting the user's shell
-or command. If X11 spoofing is in use, this will receive the
-DISPLAY in environment). This must call
-xauth(1) in that case.
+ $HOME/.ssh/rc
+If this file exists, it is run with /bin/sh after reading the environment files but before starting the user's shell or command. It must not produce any output on stdout; stderr must be used instead. If X11 forwarding is in use, it will receive the "proto cookie" pair in its standard input (and DISPLAY in its environment). The script must call xauth(1) because sshd will not run xauth automatically to add X11 cookies.
-The primary purpose of this file is to run any ini-
-tialization routines which may be needed before the user's
-home directory becomes accessible; AFS is a particular
-example of such an environment.
-This
file will probably contain some initialization code
-followed by something similar
to:
+The primary purpose of this
file is
to run any initialization routines which may be needed before the user's home directory becomes accessible; AFS is a particular example of such an environment.
-
if read proto cookie; then
-echo add $DISPLAY $proto $cookie |
xauth -q -
+This file will probably contain some initialization code followed by something similar to:
+
+
+
if read proto cookie && [ -n "$DISPLAY" ]
; then
+ if [ `
echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
+ # X11UseLocalhost=yes
+ xauth
add unix:`echo
$DISPLAY |
+ cut -c11-`
$proto $cookie
+ else
+ # X11UseLocalhost=no
+
xauth add $DISPLAY $proto $cookie
+ fi
fi
+
If this file does not exist, /etc/ssh/sshrc is run, and if that does not exist either, xauth is used to store the cookie.
+
+This file should be writable only by the user, and need not be readable by anyone else.
-This file should be writable only by the user, and need not
-be readable by anyone else.
/etc/ssh/sshrc
-
-
-
-
-
-
+Like $HOME/.ssh/rc. This can be used to specify machine-specific login-time initializations globally. This file should be writable only by root, and should be world-readable.
__AUTHORS__
-OpenSSH is a derivative of the original and free ssh 1.2.12
-
-
-
-ver-
-sions
1.5 and 2..
+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..
+
__SEE ALSO__
+scp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), login.conf(5), moduli(5), sftp-server(8)
+
+T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne, and S. Lehtinen, "SSH Protocol Architecture'', draft-ietf-secsh-architecture-09.txt, July 2001, work in progress material.
+
+M. Friedl, N. Provos, and W. A. Simpson, "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol", draft-ietf-secsh-dh-group-exchange-01.txt, April 2001, work in progress material.
-scp(1), sftp(1), ssh(1),
-ssh-add(1), ssh-agent(1),
-ssh-keygen(1), login.conf(5),
-moduli(5), sftp-server(8)
- T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne, and S. Lehtinen,'' SSH Protocol Architecture'', draft-ietf-secsh-architecture-09.txt, July 2001, work inprogress material. M. Friedl, N. Provos, and W. A. Simpson,'' Diffie-Hellman Group Exchange for the SSH Transport LayerProtocol'', draft-ietf-secsh-dh-group-exchange-01.txt, April 2001, work in progress material.
BSD
September 25, 1999 1
+BSD September 25, 1999 1
----
