Content-type: text/html Manpage of SSHD

SSHD

Section: SSH (8)
Updated: November 8, 1995
Index

NAME

sshd - secure shell daemon

SYNOPSIS

sshd [-b bits] [-d ] [-f config_file] [-g login_grace_time] [-h host_key_file] [-i ] [-k key_gen_time] [-p port] [-q ] [-V version]

DESCRIPTION

Sshd (Secure Shell Daemon) is the daemon program for ssh. Together these programs replace rlogin and rsh programs, 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.local or equivalent. It forks a new daemon for each incoming connection. The forked daemons handle key exchange, encryption, authentication, command execution, and data exchange.

Sshd works as follows. 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, the daemon sends its host and server public keys to the client. The client compares the 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 start to 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, IDEA, DES, 3DES, ARCFOUR, and TSS (a fast home-grown algorithm) are supported. IDEA is 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 dialog. The client tries to authenticate itself using .rhosts authentication, .rhosts authentication combined with RSA host authentication, RSA challenge-response authentication, TIS channenge response authentication, or password based authentication.

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), rexecd(8), and rexd (8) are disabled (thus completely disabling rlogin(1) and rsh(1) into that machine).

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 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 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 configuration file. Command-line options override values specified in the configuration file.

Sshd rereads its configuration file if it is sent the hangup signal, SIGHUP.

OPTIONS

-b bits: Specifies the number of bits in the server key (default 768).
-d: Debug mode. The server sends verbose debug output to the system log, and does not put itself in the background. The server also will not fork and will only process one connection. This option is only intended for debugging for the server.
-f configuration_file: Specifies the name of the configuration file. The default is /etc/ssh/sshd_config.
-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 seconds, 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 /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).
-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 regenerated 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 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 connections (default 22).
-q: Quiet mode. Nothing is sent to the system log. Normally the beginning, authentication, and termination of each connection is logged.
-V: SSH version 2 compatibility mode. Server assumes that SSH version 2 daemon has already read the version number string from the client and this option gives the version string read from the client.

CONFIGURATION FILE

Sshd reads configuration data from /etc/ssh/sshd_config (or the file specified with -f on the command line). The file contains keyword-value pairs, one per line. Lines starting with '#' and empty lines are interpreted as comments.

The following keywords are possible. Keywords are case insensitive.

AllowGroups

This keyword can be followed by any number of group name patterns, separated by spaces. If specified, login is allowed only if users primary group name matches one of the patterns. '*' and '?' can be used as wildcards in the patterns. By default, logins as all users are allowed.

Note that the all other login authentication steps must still be sucessfully completed. AllowGroups and DenyGroups are additional restrictions.

AllowHosts

This keyword can be followed by any number of host name patterns, separated by spaces. If specified, login is allowed only from hosts whose name matches one of the patterns. '*' and '?' can be used as wildcards in the patterns. Normal name servers are used to map the client's host into a canonical host name. If the name cannot be mapped, its IP-address is used as the host name. By default all hosts are allowed to connect.

Note that sshd can also be configured to use tcp_wrappers using the --with-libwrap compile-time configuration option.

AccountExpireWarningDays

Specifies when to start print warning messages that the account is going to expire. The value is number of days before the account expiration. The default value is 14 days, and if set to 0 the warning messages are disabled.

AllowSHosts

This keyword can be followed by any number of host name patterns, separated by spaces. If specified, .shosts (and .rhosts and /etc/hosts.equiv) entries are only honoured for hosts whose name matches one of the patterns. servers are used to map the client's host into a canonical host name. If the name cannot be mapped, its IP-address is used as the host name. By default all hosts are allowed to connect.

AllowTcpForwarding

Specifies whether tcp forwarding is permitted. The default is "yes". Note that disabling tcp forwarding does not improve security in any way, as users can always install their own forwarders.

AllowUsers

This keyword can be followed by any number of user name patterns or user@host patterns, separated by spaces. Host name may be either the dns name or the ip address. If specified, login is allowed only as users whose name matches one of the patterns. '*' and '?' can be used as wildcards in the patterns. By default, logins as all users are allowed.

Note that the all other login authentication steps must still be sucessfully completed. AllowUsers and DenyUsers are additional restrictions.

CheckMail

Specifies whether sshd should print information whether you have new mail or not when a user logs in interactively. (On some systems it is also printed by the shell, /etc/profile, or equivalent.) The default is "yes".

DenyGroups

This keyword can be followed by any number of group name patterns, separated by spaces. If specified, login is disallowed if users primary group name name matches any of the patterns.

DenyHosts

This keyword can be followed by any number of host name patterns, separated by spaces. If specified, login is disallowed from the hosts whose name matches any of the patterns.

DenySHosts

This keyword can be followed by any number of host name patterns, separated by spaces. If specified, .shosts (and .rhosts and /etc/hosts.equiv) entries whose name matches any of the patterns are ignored.

DenyUsers

This keyword can be followed by any number of user name patterns or user@host patterns, separated by spaces. Host name may be either the dns name or the ip address. If specified, login is disallowed as users whose name matches any of the patterns.

FascistLogging

Specifies whether to use verbose logging. Verbose logging violates the privacy of users and is not recommended. The argument must be "yes" or "no" (without the quotes). The default is "no".

ForcedEmptyPasswdChange

Specifies whether to force password change if the password is empty (first login). . The argument must be "yes" or "no" (without the quotes). The default is "no".

ForcedPasswdChange

Specifies whether to force password change if the password is expired. The argument must be "yes" or "no" (without the quotes). The default is "yes".

HostKey

Specifies the file containing the private host key (default /etc/ssh/ssh_host_key).

IdleTimeout time

Sets idle timeout limit to time in seconds (s or nothing after number), in minutes (m), in hours (h), in days (d), or in weeks (w). If the connection have been idle (all channels) for that long time the child process is killed with SIGHUP, and connection is closed down.

IgnoreRhosts

Specifies that rhosts and shosts files will not be used in authentication. /etc/hosts.equiv and /etc/ssh/shosts.equiv are still used. The default is "no".

IgnoreRootRhosts

Specifies that rhosts and shosts files will not be used in authentication for root. The default is the value of IgnoreRhosts.

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 send, 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.

To disable keepalives, the value should be set to "no" in both the server and the client configuration files.

KerberosAuthentication

Specifies whether Kerberos V5 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 or DCE Security Server. Default is yes.

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 or SecurID. Default is no.

KerberosTgtPassing

Specifies whether a Kerberos V5 TGT may be forwarded to the server. Default is yes.

KeyRegenerationInterval

The 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).

ListenAddress

Specifies the ip address of the interface where the sshd server socket is bind.

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).

PasswordAuthentication

Specifies whether password authentication is allowed. The default is "yes".

PasswordExpireWarningDays

Specifies when to start print warning messages that the password is going to expire. The value is number of days before the password expiration. The default value is 14 days, and if set to 0 the warning messages are disabled.

PermitEmptyPasswords

When password authentication is allowed, it specifies whether the server allows login to accounts with empty password strings. The default is "yes".

PermitRootLogin

Specifies whether the root can log in using ssh. May be set to "yes", "nopwd", or "no". The default is "yes", allowing root logins through any of the authentication types allowed for other users. The "nopwd" value disables password-authenticated root logins. The "no" value disables root logins through any of the authentication methods. ("nopwd" and "no" are equivalent unless you have a .rhosts, .shosts, or .ssh/authorized_keys file in the root home directory.)

Root login with RSA authentication when the "command" option has been specified will be allowed regardless of the value of this setting (which may be useful for taking remote backups even if root login is normally not allowed).

PidFile

Specifies the location of the file containing the process ID of the master sshd daemon (default: /etc/sshd.pid or /var/run/sshd.pid, depending on the system).

Port

Specifies the port number that sshd listens on. The default is 22.

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 equivalent.) The default is "yes".

QuietMode

Specifies whether the system runs in quiet mode. In quiet mode, nothing is logged in the system log, except fatal errors. The default is "no".

RandomSeed

Specifies the file containing the random seed for the server; this file is created automatically and updated regularly. The default is /etc/ssh/ssh_random_seed.

RhostsAuthentication

Specifies whether authentication using rhosts or /etc/hosts.equiv files is sufficient. Normally, this method should not be permitted because it is insecure. 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 "no".

RhostsRSAAuthentication

Specifies whether rhosts or /etc/hosts.equiv authentication together with successful RSA host authentication is allowed. The default is "yes".

RSAAuthentication

Specifies whether pure RSA authentication is allowed. The default is "yes".

ServerKeyBits

Defines the number of bits in the server key. The minimum value is 512, and the default is 768.

SilentDeny

Specifies wheter denied (or not allowed) connections are denied silently (just close the connection, no logging etc) or are they closed cleanly (send error message and log connection attempt).

StrictModes

Specifies whether ssh should check file modes and ownership of the user's home directory and rhosts files before accepting login. This is normally desirable because novices sometimes accidentally leave their directory or files world-writable. The default is "yes".

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 DAEMON.

TISAuthentication

Specifies wether authentication through TIS authsrv (8) is allowed. The default is "no".

Umask

Sets default umask for sshd and its childs. Remember to add 0 in front of the number to make it octal. Default is to not set umask at all.

X11Forwarding

Specifies whether X11 forwarding is permitted. The default is "yes". Note that disabling X11 forwarding does not improve security in any way, as users can always install their own forwarders.

X11DisplayOffset

Specifies the first display number available for sshd's X11 forwarding. This prevents sshd from interfering with real X11 servers.

XAuthLocation

Specifies the default path to xauth program.

LOGIN PROCESS

When a user successfully logs in, sshd does the following:

1.

If the login is on a tty, and no command has been specified, prints last login time and /etc/motd (unless prevented in the configuration file or by $HOME/.hushlogin; see the FILES section).

2.

If the login is on a tty, records login time.

3.

Checks /etc/nologin; if it exists, prints contents and quits (unless root).

4.

Changes to run with normal user privileges.

5.

Sets up basic environment.

6.

Reads /etc/environment if it exists.

7.

Reads $HOME/.ssh/environment if it exists.

8.

Changes to user's home directory.

9.

If $HOME/.ssh/rc exists, runs it (with the user's shell); else if /etc/ssh/sshrc exists, runs it (with /bin/sh); otherwise runs xauth. The "rc" files are given the X11 authentication protocol and cookie in standard input.

10.

Runs user's shell or command.

AUTHORIZED_KEYS FILE FORMAT

The $HOME/.ssh/authorized_keys file lists the RSA keys that are permitted for RSA authentication. Each line of the file contains one key (empty lines and lines starting with a '#' are ignored as comments). Each line consists of the following fields, separated by spaces: options, bits, exponent, modulus, comment. The options field is optional; its presence is determined by whether the line starts with a number or not (the option field never starts with a number). The bits, exponent, modulus and comment fields give the RSA key; the comment field is not used for anything (but may be convenient for the user to identify the key).

Note that lines in this file are usually several hundred bytes long (because of the size of the RSA key modulus). You don't want to type them in; instead, copy the identity.pub file and edit it.

The options (if present) consists of comma-separated option specifications. No spaces are permitted, except within double quotes. Option names are case insensitive. The following option specifications are supported:

: from="pattern-list"
Specifies that in addition to RSA authentication, the canonical name of the remote host must be present in the comma-separated list of patterns ('*' and '?' serve as wildcards). The list may also contain patterns negated by prefixing them with '!'; if the canonical host name matches a negated pattern, the key is not accepted. The purpose of this option is to optionally increase security: RSA authentication by itself does not trust the network or name servers or anything (but the key); however, if somebody somehow steals the key, the key permits an intruder to log in from anywhere in the world. This additional option makes using a stolen key more difficult (name servers and/or routers would have to be compromised in addition to just the key).
: command="command"
Specifies that the command is executed whenever this key is used for authentication. The command supplied by the user (if any) is ignored. The command is run on a pty if the connection requests a pty; otherwise it is run without a tty. A quote may be included in the command by quoting it with a backslash. This option might be useful to restrict certain RSA keys to perform just a specific operation. An example might be a key that permits remote backups but nothing else. Notice that the client may specify TCP/IP and/or X11 forwardings unless they are explicitly prohibited.
: environment="NAME=value"
Specifies that the string is to be added to the environment when logging in using this key. Environment variables set this way override other default environment values. Multiple options of this type are permitted.
: idle-timeout=time
Sets idle timeout limit to time in seconds (s or nothing after number), in minutes (m), in hours (h), in days (d), or in weeks (w). If the connection have been idle (all channels) for that long time the child process is killed with SIGHUP, and connection is closed down.
: no-port-forwarding
Forbids TCP/IP forwarding when this key is used for authentication. Any port forward requests by the client will return an error. This might be used e.g. in connection with the command option.
: no-X11-forwarding
Forbids X11 forwarding when this key is used for authentication. Any X11 forward requests by the client will return an error.
: no-agent-forwarding
Forbids authentication agent forwarding when this key is used for authentication.
: no-pty
Prevents tty allocation (a request to allocate a pty will fail).

Examples

1024 33 12121...312314325 ylo@foo.bar

from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23...2334 ylo@niksula

command="dump /home",no-pty,no-port-forwarding 1024 33 23...2323 backup.hut.fi

SSH WITH TCP WRAPPERS

When sshd is compiled with tcp wrappers libraries, then the host.allow/deny files also controls who can connect to ports forwarded by sshd.

The program names in the hosts.allow/deny files are sshdfwd-<portname>, sshdfwd-<portnumber>, and sshdfwd-X11 for forwarded ports the ssh client or server is listening.

If the port has name defined then you must use it.

SSH_KNOWN_HOSTS FILE FORMAT

The /etc/ssh/ssh_known_hosts and $HOME/.ssh/known_hosts files contain host public keys for all known hosts. The global file should be prepared by the admistrator (optional), and the per-user file is maintained automatically: whenever the user connects an unknown host its key is added to the per-user file. The recommended way to create /etc/ssh/ssh_known_hosts is to use the make-ssh-known-hosts command.

Each line in these files contains the following fields: hostnames, bits, exponent, modulus, comment. The fields are separated by spaces.

Hostnames is a comma-separated list of patterns ('*' and '?' act as wildcards); each pattern in turn is matched against the canonical host name (when authenticating a client) or against the user-supplied name (when authenticating a server). A pattern may also be preceded by '!' to indicate negation: if the host name matches a negated pattern, it is not accepted (by that line) even if it matched another pattern on the line.

Bits, exponent, and modulus are taken directly from the host key; they can be obtained e.g. from /etc/ssh/ssh_host_key.pub. The optional comment field continues to the end of the line, and is not used.

Lines starting with '#' and empty lines are ignored as comments.

When performing host authentication, authentication is accepted if any matching line has the proper key. It is thus permissible (but not recommended) to have several lines or different host keys for the same names. This will inevitably happen when short forms of host names from different domains are put in the file. It is possible that the files contain conflicting information; authentication is accepted if valid information can be found from either file.

Note that the lines in these files are typically hundreds of characters long, and you definitely don't want to type in the host keys by hand. Rather, generate them by a script (see make-ssh-known-hosts(1)) or by taking /etc/ssh/ssh_host_key.pub and adding the host names at the front.

Examples

closenet,closenet.hut.fi,...,130.233.208.41 1024 37 159...93 closenet.hut.fi

FILES

/etc/ssh/sshd_config

Contains configuration data for sshd. This file should be writable by root only, but it is recommended (though not necessary) that it be world-readable.

/etc/ssh/ssh_host_key

Contains the private part of the host key. This file is normally created automatically by "make install", but can also be created manually using ssh-keygen(1). This file should only be owned by root, readable only by root, and not accessible to others.

/etc/ssh/ssh_host_key.pub

Contains the public part of the host key. This file is normally created automatically by "make install", but can also be created manually. This file should be world-readable but writable only by root. Its contents should match the private part. This file is not really used for anything; it is only provided for the convenience of the user so its contents can be copied to known hosts files.

/etc/ssh/ssh_random_seed

This file contains a seed for the random number generator. This file should only be accessible by root.

/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 contents of this file are not sensitive; it can be world-readable.

$HOME/.ssh/authorized_keys

Lists the RSA keys 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.

/etc/ssh/ssh_known_hosts and $HOME/.ssh/known_hosts

These files are consulted when using rhosts with RSA host 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 the remote host is the one we intended to connect.) 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.

$HOME/.rhosts

This file contains host-username pairs, separated by a space, one per line. The given user on the corresponding host is permitted to log in without password. The same file is used by rlogind and rshd. Ssh differs from rlogind and rshd in that it requires RSA host authentication in addition to validating the host name retrieved from domain name servers (unless compiled with the --with-rhosts configuration option). The file must be writable only by the user; it is recommended that it not be accessible by others.

It is also possible to use netgroups in the file. Either host or user name may be of the form +@groupname 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.

/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. 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. Note that this warning also applies to rsh/rlogin.

/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.

/etc/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. This file is processed in all environments (normal rsh/rlogin only process it on AIX and potentially some other systems). The file should be writable only by root, and should be world-readable.

$HOME/.ssh/environment

This file is read into the environment after /etc/environment. It has the same format. 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 the user's shell after reading the environment files but before starting the user's shell or command. If X11 spoofing is in use, this will receive the "proto cookie" pair in standard input (and DISPLAY in environment). This must call xauth in that case.

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.

This file will probably contain some initialization code followed by something similar to: "if read proto cookie; then echo add $DISPLAY $proto $cookie | xauth -q -; 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.

/etc/ssh/sshrc

Like $HOME/.ssh/rc, but run with /bin/sh. 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.

/etc/ssh/sshd_tis.map

Establishes a mapping between a local username and its corresponding name in the TIS database. Each line contains the local name followed by a ":" followed by the corresponding name. If the file does not exist or the user is not found, the corresponding name in the TIS database is supposed to be the same.

INSTALLATION

Sshd is normally run as root. If it is not run as root, it can only log in as the user it is running as, and password authentication may not work if the system uses shadow passwords. An alternative host key file must also be used.

Sshd is normally started from /etc/rc.local or equivalent at system boot.

Considerable work has been put to making sshd secure. However, if you find a security problem, please report it immediately to <ssh-bugs@cs.hut.fi>.