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🔖 NAME

sshd — OpenSSH daemon

🚀 Quick Reference

Use CaseCommandDescription
🧪 Test config validitysshd -tCheck configuration file and key sanity
📋 Extended config testsshd -T -C user=... -C addr=...Output effective config with Match rules applied
🔍 Debug modesshd -dRun in foreground with verbose debug output (add -ddd for max)
🕐 Set login grace timesshd -g 60Set authentication timeout to 60 seconds
🔑 Specify host keysshd -h /path/to/host_keyUse a custom host key file
🔌 Listen on alternate portsshd -p 2222Listen on port 2222 instead of default 22
📄 Log to filesshd -E /var/log/sshd.logAppend debug logs to a specific file
🛑 No daemon modesshd -DRun in foreground (does not detach)
🔒 Forced-command keycommand="dump /home" ssh-rsa ... in authorized_keysRestrict key to run only one command
🌐 From-address restrictionfrom="192.0.2.*" ssh-rsa ... in authorized_keysOnly allow key from specific IP patterns
📡 Tunnel forwardingtunnel="0",command="..." ssh-rsa ... in authorized_keysForce a tunnel device and run a command

📋 SYNOPSIS

sshd [-46DdeiqTt] [-C connection_spec] [-c host_certificate_file]
     [-E log_file] [-f config_file] [-g login_grace_time]
     [-h host_key_file] [-o option] [-p port] [-u len]

📝 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:

🔐 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 (e.g. 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 (e.g. '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 or 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.

🚪 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).
  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 PermitUserEnvironment in sshd_config(5)).
  7. 🔹 Changes to user's home directory.
  8. 🔹 If ~/.ssh/rc exists and PermitUserRC is set, runs it; else if /etc/ssh/sshrc exists, runs it; otherwise runs xauth(1).
  9. 🔹 Runs user's shell or command. All commands are run under the user's login shell as specified in the system password database.

📄 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.

🔑 AUTHORIZED_KEYS FILE FORMAT

AuthorizedKeysFile specifies the files containing public keys for public key authentication; if this option is not specified, the default is ~/.ssh/authorized_keys and ~/.ssh/authorized_keys2. Each line of the file contains one key (empty lines and lines starting with a '#' are ignored as comments). Public keys consist of the following space-separated fields: options, keytype, base64-encoded key, comment. The options field is optional. The supported key types are:

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 can be several hundred bytes long (because of the size of the public key encoding) up to a limit of 8 kilobytes, which permits RSA keys up to 16 kilobits. You don't want to type them in; instead, copy the id_dsa.pub, id_ecdsa.pub, id_ecdsa_sk.pub, id_ed25519.pub, id_ed25519_sk.pub, or the id_rsa.pub file and edit it.

sshd enforces a minimum RSA key modulus size of 1024 bits.

The options (if present) consist of comma-separated option specifications. No spaces are permitted, except within double quotes. The following option specifications are supported (note that option keywords are case-insensitive):

An example authorized_keys file:

# Comments are allowed at start of line. Blank lines are allowed.
# Plain key, no restrictions
ssh-rsa ...
# Forced command, disable PTY and all forwarding
restrict,command="dump /home" ssh-rsa ...
# Restriction of ssh -L forwarding destinations
permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-rsa ...
# Restriction of ssh -R forwarding listeners
permitlisten="localhost:8080",permitlisten="[::1]:22000" ssh-rsa ...
# Configuration for tunnel forwarding
tunnel="0",command="sh /etc/netstart tun0" ssh-rsa ...
# Override of restriction to allow PTY allocation
restrict,pty,command="nethack" ssh-rsa ...
# Allow FIDO key without requiring touch
no-touch-required sk-ecdsa-sha2-nistp256@openssh.com ...
# Require user-verification (e.g. PIN or biometric) for FIDO key
verify-required sk-ecdsa-sha2-nistp256@openssh.com ...
# Trust CA key, allow touch-less FIDO if requested in certificate
cert-authority,no-touch-required,principals="user_a" ssh-rsa ...

🏠 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
closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net
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...

📁 FILES

👀 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)

✍️ AUTHORS

OpenSSH is a derivative of the original and free ssh 1.2.12 release by Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo de Raadt and Dug Song removed many bugs, re-added newer features and created OpenSSH. Markus Friedl contributed the support for SSH protocol versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support for privilege separation.

sshd(8) □✕
🔖 NAME 🚀 Quick Reference 📋 SYNOPSIS 📝 DESCRIPTION 🔐 AUTHENTICATION 🚪 LOGIN PROCESS 📄 SSHRC 🔑 AUTHORIZED_KEYS FILE FORMAT 🏠 SSH_KNOWN_HOSTS FILE FORMAT 📁 FILES 👀 SEE ALSO ✍️ AUTHORS

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