Linux/Unix Command: sshd

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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 simultaneously.

SSH Protocol Version 1

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 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 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 fundamentally insecure, but can be enabled in the server configuration file if desired. System security is not improved unless rshdrlogind and rexecd are disabled (thus completely disabling rlogin and rsh into the machine).

SSH Protocol Version 2

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 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 (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 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 on 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 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

Specifies the number of bits in the ephemeral protocol version 1 server key (default 768).


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 work 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.


When this option is specified, sshd will send the output to the standard error instead of the system log.

-f configuration_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 login_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.

-h host_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_key for protocol version 1, and /etc/ssh/ssh_host_rsa_key and /etc/ssh/ssh_host_dsa_key for protocol version 2. It is possible to have multiple host key files for the different protocol versions and host key algorithms.


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 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.

-o option

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.

-p port

Specifies the port on which the server listens for connections (default 22). Multiple port options are permitted. Ports specified in the configuration file are ignored when a command-line port is specified.


Quiet mode. Nothing is sent to the system log. Normally the beginning, authentication, and termination of each connection are logged.


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 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 RhostsAuthenticationRhostsRSAAuthentication 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.


When this option is specified sshd will not detach and does not become a daemon. This allows easy monitoring of sshd.


Forces sshd to use IPv4 addresses only.


Forces sshd to use IPv6 addresses only.

Configuration File

sshd reads configuration data from /etc/ssh/sshd_config (or the file specified with -f on the command line). The file format and configuration options are described in sshd_config5.

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 Sx 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 $HOME/.ssh/environment if it exists and users are allowed to change their environment. See the PermitUserEnvironment option in sshd_config5.
  7. Changes to user's home directory.
  8. If $HOME/.ssh/rc exists, runs it; else if /etc/ssh/sshrc exists, runs it; otherwise runs xauth. The ``rc'' files are given the X11 authentication protocol and cookie in standard input.
  9. Runs user's shell or command.

Authorized_Keys File Format

$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 (PubkeyAuthentication) in protocol version 2. AuthorizedKeysFile may be used to specify an alternative file.

Each line of the file contains one key (empty lines and lines starting with a '#' are ignored as comments). Each RSA public key consists of the following fields, separated by spaces: options, bits, exponent, modulus, comment. Each protocol version 2 public key consists of: options, keytype, base64 encoded key, comment. The options field is optional; its presence is determined by whether the line starts with a number or not (the options field never starts with a number). The bits, exponent, modulus and comment fields give the RSA key for protocol version 1; the comment field is not used for anything (but may be convenient for the user to identify the key). For protocol version 2 the keytype is "ssh-dss'' or "ssh-rsa.''

Lines in this file are usually several hundred bytes long (because of the size of the public key encoding). You don't want to type them in; instead, copy the or the file and edit it.

sshd enforces a minimum RSA key modulus size for protocol 1 and protocol 2 keys of 768 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):


Specifies that in addition to public key 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: public key 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).


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 client requests a pty; otherwise, it is run without a tty. If an 8-bit clean channel is required, one must not request a pty or should specify no-pty A quote may be included in the command by quoting it with a backslash. This option might be useful to restrict certain public keys to perform just a specific operation. An example might be a key that permits remote backups but nothing else. Note that the client may specify TCP/IP and/or X11 forwarding unless they are explicitly prohibited. Note that this option applies to shell, command or subsystem execution.


Specifies that the string is to be added to the environment when logging in using this key. Environment variables set this way will override other default environment values. Multiple options of this type are permitted. Environment processing is disabled by default and is controlled via the PermitUserEnvironment option. This option is automatically disabled if UseLogin is enabled.


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.


Forbids X11 forwarding when this key is used for authentication. Any X11 forward requests by the client will return an error.


Forbids authentication agent forwarding when this key is used for authentication.


Prevents tty allocation (a request to allocate a pty will fail).


Limit local "ssh -L" port forwarding such that it may only connect to the specified host and port. IPv6 addresses can be specified with an alternative syntax: host/port Multiple permitopen options may be applied separated by commas. No pattern matching is performed on the specified hostnames, they must be literal domains or addresses.


1024 33 12121...312314325

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

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

permitopen="",permitopen="" 1024 33 23...2323

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 administrator (optional), and the per-user file is maintained automatically: whenever the user connects from an unknown host its key is added to the per-user file.

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

Hostnames are 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 RSA host key; they can be obtained, e.g., from /etc/ssh/ 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.

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 or by taking /etc/ssh/ and adding the host names at the front.


closenet,..., 1024 37 159...93, ssh-rsa AAAA1234.....=

See Also

scp(1), sftp(1), ssh(1), ssh-add1, ssh-agent1, ssh-keygen1, login.conf5, moduli(5), sshd_config5, sftp-server8

T. Ylonen T. Kivinen M. Saarinen T. Rinne S. Lehtinen "SSH Protocol Architecture" draft-ietf-secsh-architecture-12.txt January 2002 work in progress material

M. Friedl N. Provos W. A. Simpson "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol" draft-ietf-secsh-dh-group-exchange-02.txt January 2002 work in progress material

Use the man command (% man) to see how a command is used on your particular computer.