Install Now
sshd — OpenSSH daemon
Contents
Authentication
The OpenSSH SSH daemon supports SSH protocol 2 only. Each host has a host-specific key, used to identify
the host. Whenever a client connects, the daemon responds with its public host key. The client compares
the host key against its own database to verify that it has not changed. Forward secrecy 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 cipher. 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 (MAC).
Finally, the server and the client enter an authentication dialog. The client tries to authenticate
itself using host-based authentication, public key authentication, challenge-response authentication, or
password authentication.
Regardless of the authentication type, the account is checked to ensure that it is accessible. An
account is not accessible if it is locked, listed in DenyUsers or its group is listed in DenyGroups .
The definition of a locked account is system dependent. Some platforms have their own account database
(eg AIX) and some modify the passwd field ( ‘*LK*’ on Solaris and UnixWare, ‘*’ on HP-UX, containing
‘Nologin’ on Tru64, a leading ‘*LOCKED*’ on FreeBSD and a leading ‘!’ on most Linuxes). If there is a
requirement to disable password authentication for the account while allowing still public-key, then the
passwd field should be set to something other than these values (eg ‘NP’ or ‘*NP*’ ).
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 connections, or forwarding the authentication agent connection over the secure channel.
After this, the client either requests an interactive shell or execution of a non-interactive command,
which sshd will execute via the user's shell using its -c option. 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.
Description
sshd (OpenSSH Daemon) is the daemon program for ssh(1). It provides secure encrypted communications between two untrusted hosts over an insecure network. sshd listens for connections from clients. It is normally started at boot from /etc/init.d/ssh. It forks a new daemon for each incoming connection. The forked daemons handle key exchange, encryption, authentication, command execution, and data exchange. sshd can be configured using command-line options or a configuration file (by default sshd_config(5)); command-line options override values specified in the configuration file. sshd rereads its configuration file when it receives a hangup signal, SIGHUP, by executing itself with the name and options it was started with, e.g. /usr/sbin/sshd. The options are as follows: -4 Forces sshd to use IPv4 addresses only. -6 Forces sshd to use IPv6 addresses only. -Cconnection_spec Specify the connection parameters to use for the -T extended test mode. If provided, any Match directives in the configuration file that would apply are applied before the configuration is written to standard output. The connection parameters are supplied as keyword=value pairs and may be supplied in any order, either with multiple -C options or as a comma-separated list. The keywords are “addr”, “user”, “host”, “laddr”, “lport”, and “rdomain” and correspond to source address, user, resolved source host name, local address, local port number and routing domain respectively. Additionally the “invalid-user” flag (which does not take a value argument) may be specified to simulate a connection from an unrecognised username. -chost_certificate_file Specifies a path to a certificate file to identify sshd during key exchange. The certificate file must match a host key file specified using the -h option or the HostKey configuration directive. -D When this option is specified, sshd will not detach and does not become a daemon. This allows easy monitoring of sshd. -d Debug mode. The server sends verbose debug output to standard error, and does not put itself in the background. The server also will not fork(2) and will only process one connection. This option is only intended for debugging for the server. Multiple -d options increase the debugging level. Maximum is 3. -Elog_file Append debug logs to log_file instead of the system log. -e Write debug logs to standard error instead of the system log. -fconfig_file Specifies the name of the configuration file. The default is /etc/ssh/sshd_config. sshd refuses to start if there is no configuration file. -G Parse and print configuration file. Check the validity of the configuration file, output the effective configuration to stdout and then exit. Optionally, Match rules may be applied by specifying the connection parameters using one or more -C options. -glogin_grace_time Gives the grace time for clients to authenticate themselves (default 120 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. -hhost_key_file Specifies a file from which a host key is read. This option must be given if sshd is not run as root (as the normal host key files are normally not readable by anyone but root). The default is /etc/ssh/ssh_host_ecdsa_key, /etc/ssh/ssh_host_ed25519_key and /etc/ssh/ssh_host_rsa_key. It is possible to have multiple host key files for the different host key algorithms. -i Specifies that sshd is being run from inetd(8). -ooption Can be used to give options in the format used in the configuration file. This is useful for specifying options for which there is no separate command-line flag. For full details of the options, and their values, see sshd_config(5). -pport Specifies the port on which the server listens for connections (default 22). Multiple port options are permitted. Ports specified in the configuration file with the Port option are ignored when a command-line port is specified. Ports specified using the ListenAddress option override command-line ports. -q Quiet mode. Nothing is sent to the system log. Normally the beginning, authentication, and termination of each connection is logged. -T Extended test mode. Check the validity of the configuration file, output the effective configuration to stdout and then exit. Optionally, Match rules may be applied by specifying the connection parameters using one or more -C options. This is similar to the -G flag, but it includes the additional testing performed by the -t flag. -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. -ulen 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 may 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 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. -V Display the version number and exit.
Files
~/.hushlogin This file is used to suppress printing the last login time and /etc/motd, if PrintLastLog and PrintMotd, respectively, are enabled. It does not suppress printing of the banner specified by Banner. ~/.rhosts This file is used for host-based authentication (see ssh(1) for more information). On some machines this file may need to be world-readable if the user's home directory is on an NFS partition, because sshd reads it as root. Additionally, this file must be owned by the user, and must not have write permissions for anyone else. The recommended permission for most machines is read/write for the user, and not accessible by others. ~/.shosts This file is used in exactly the same way as .rhosts, but allows host-based authentication without permitting login with rlogin/rsh. ~/.ssh/ This directory is the default location for all user-specific configuration and authentication information. There is no general requirement to keep the entire contents of this directory secret, but the recommended permissions are read/write/execute for the user, and not accessible by others. ~/.ssh/authorized_keys Lists the public keys (ECDSA, Ed25519, RSA) that can be used for logging in as this user. The format of this file is described above. The content of the file is not highly sensitive, but the recommended permissions are read/write for the user, and not accessible by others. If this file, the ~/.ssh directory, or the user's home directory are writable by other users, then the file could be modified or replaced by unauthorized users. In this case, sshd will not allow it to be used unless the StrictModes option has been set to “no”. ~/.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. Environment processing is disabled by default and is controlled via the PermitUserEnvironment option. ~/.ssh/known_hosts Contains a list of host keys for all hosts the user has logged into that are not already in the systemwide list of known host keys. The format of this file is described above. This file should be writable only by root/the owner and can, but need not be, world-readable. ~/.ssh/rc Contains initialization routines to be run before the user's home directory becomes accessible. This file should be writable only by the user, and need not be readable by anyone else. /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). /etc/hosts.equiv This file is for host-based authentication (see ssh(1)). It should only be writable by root. /etc/ssh/moduli Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange" key exchange method. The file format is described in moduli(5). If no usable groups are found in this file then fixed internal groups will be used. /etc/motd See motd(5). /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/ssh/shosts.equiv This file is used in exactly the same way as hosts.equiv, but allows host-based authentication without permitting login with rlogin/rsh. /etc/ssh/ssh_host_ecdsa_key/etc/ssh/ssh_host_ed25519_key/etc/ssh/ssh_host_rsa_key These files contain the private parts of the host keys. These files should only be owned by root, readable only by root, and not accessible to others. Note that sshd does not start if these files are group/world-accessible. /etc/ssh/ssh_host_ecdsa_key.pub/etc/ssh/ssh_host_ed25519_key.pub/etc/ssh/ssh_host_rsa_key.pub These files contain the public parts of the host keys. These files should be world-readable but writable only by root. Their contents should match the respective private parts. These files are not really used for anything; they are provided for the convenience of the user so their contents can be copied to known hosts files. These files are created using ssh-keygen(1). /etc/ssh/ssh_known_hosts Systemwide list of known host keys. This file should be prepared by the system administrator to contain the public host keys of all machines in the organization. The format of this file is described above. This file should be writable only by root/the owner and should be world- readable. /etc/ssh/sshd_config Contains configuration data for sshd. The file format and configuration options are described in sshd_config(5). /etc/ssh/sshrc Similar to ~/.ssh/rc, it can be used to specify machine-specific login-time initializations globally. This file should be writable only by root, and should be world-readable. /run/sshdchroot(2) directory used by sshd during privilege separation in the pre-authentication phase. The directory should not contain any files and must be owned by root and not group or world- writable. /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 process ID of the one started last). The content of this file is not sensitive; it can be world-readable.
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 ~/.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 the file ~/.ssh/environment, if it exists, and users are allowed to change their
environment. See the PermitUserEnvironment option in sshd_config(5).
7. Changes to user's home directory.
8. If ~/.ssh/rc exists and the sshd_config(5) PermitUserRC option is set, runs it; else if
/etc/ssh/sshrc exists, runs it; otherwise runs xauth(1). The “rc” files are given the X11
authentication protocol and cookie in standard input. See “SSHRC”, below.
9. Runs user's shell or command. All commands are run under the user's login shell as specified
in the system password database.
Name
sshd — OpenSSH daemon
See Also
scp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1), chroot(2), hosts_access(5), moduli(5), sshd_config(5), inetd(8), sftp-server(8)
Ssh_Known_Hosts File Format
The /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts files contain host public keys for all known hosts.
The global file should be prepared by the administrator (optional), and the per-user file is maintained
automatically: whenever the user connects to an unknown host, its key is added to the per-user file.
Each line in these files contains the following fields: marker (optional), hostnames, keytype,
base64-encoded key, comment. The fields are separated by spaces.
The marker is optional, but if it is present then it must be one of “@cert-authority”, to indicate that
the line contains a certification authority (CA) key, or “@revoked”, to indicate that the key contained
on the line is revoked and must not ever be accepted. Only one marker should be used on a key line.
Hostnames is a comma-separated list of patterns (‘*’ and ‘?’ act as wildcards); each pattern in turn is
matched against the host name. When sshd is authenticating a client, such as when using
HostbasedAuthentication, this will be the canonical client host name. When ssh(1) is authenticating a
server, this will be the host name given by the user, the value of the ssh(1) HostkeyAlias if it was
specified, or the canonical server hostname if the ssh(1) CanonicalizeHostname option was used.
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. A hostname or address
may optionally be enclosed within ‘[’ and ‘]’ brackets then followed by ‘:’ and a non-standard port
number.
Alternately, hostnames may be stored in a hashed form which hides host names and addresses should the
file's contents be disclosed. Hashed hostnames start with a ‘|’ character. Only one hashed hostname may
appear on a single line and none of the above negation or wildcard operators may be applied.
The keytype and base64-encoded key are taken directly from the host key; they can be obtained, for
example, from /etc/ssh/ssh_host_rsa_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;
either one that matches exactly or, if the server has presented a certificate for authentication, the key
of the certification authority that signed the certificate. For a key to be trusted as a certification
authority, it must use the “@cert-authority” marker described above.
The known hosts file also provides a facility to mark keys as revoked, for example when it is known that
the associated private key has been stolen. Revoked keys are specified by including the “@revoked”
marker at the beginning of the key line, and are never accepted for authentication or as certification
authorities, but instead will produce a warning from ssh(1) when they are encountered.
It is 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, ssh-keyscan(1) or by taking,
for example, /etc/ssh/ssh_host_rsa_key.pub and adding the host names at the front. ssh-keygen(1) also
offers some basic automated editing for ~/.ssh/known_hosts including removing hosts matching a host name
and converting all host names to their hashed representations.
An example ssh_known_hosts file:
# Comments allowed at start of line
cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
# A hashed hostname
|1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
AAAA1234.....=
# A revoked key
@revoked * ssh-rsa AAAAB5W...
# A CA key, accepted for any host in *.mydomain.com or *.mydomain.org
@cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...
Sshrc
If the file ~/.ssh/rc exists, sh(1) runs it 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 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 && [ -n "$DISPLAY" ]; then
if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
# X11UseLocalhost=yes
echo add unix:`echo $DISPLAY |
cut -c11-` $proto $cookie
else
# X11UseLocalhost=no
echo add $DISPLAY $proto $cookie
fi | 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
add the cookie.
Synopsis
sshd [-46DdeGiqTtV] [-Cconnection_spec] [-chost_certificate_file] [-Elog_file] [-fconfig_file]
[-glogin_grace_time] [-hhost_key_file] [-ooption] [-pport] [-ulen]
PNG Icons
Emojis