sshd -words [-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 ]
sshd (OpenSSH 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.
sshd listens for connections from clients. It is normally started at boot from /etc/init.d/ssh (or /etc/init/ssh.conf on systems using the Upstart init daemon). 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_config5); 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:
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 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-md5, hmac-sha1, umac-64, umac-128, hmac-ripemd160, hmac-sha2-256 or hmac-sha2-512).
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 dependant. 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 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.
When a user successfully logs in, sshd does the following:
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.
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 keytype is ``ecdsa-sha2-nistp256'' ``ecdsa-sha2-nistp384'' ``ecdsa-sha2-nistp521'' ``ssh-ed25519'' ``ssh-dss'' or ``ssh-rsa'' 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 DSA keys up to 8 kilobits and RSA keys up to 16 kilobits. You don't want to type them in; instead, copy the id_dsa.pub id_ecdsa.pub id_ed25519.pub or the id_rsa.pub file and edit it.
sshd enforces a minimum RSA key modulus size 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):
Certificates may encode access restrictions similar to these key options. If both certificate restrictions and key options are present, the most restrictive union of the two is applied.
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 and/or X11 forwarding unless they are explicitly prohibited, e.g. using the restrict key option.
The command originally supplied by the client is available in the SSH_ORIGINAL_COMMAND environment variable. Note that this option applies to shell, command or subsystem execution. Also note that this command may be superseded by a sshd_config5 ForceCommand directive.
If a command is specified and a forced-command is embedded in a certificate used for authentication, then the certificate will be accepted only if the two commands are identical.
In addition to the wildcard matching that may be applied to hostnames or addresses, a from stanza may match IP addresses using CIDR address/masklen notation.
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).
An example authorized_keys file:
# Comments allowed at start of line ssh-rsa AAAAB3Nza...LiPk== email@example.com from="*.sales.example.net,!pc.sales.example.net" ssh-rsa AAAAB2...19Q== firstname.lastname@example.org command="dump /home",no-pty,no-port-forwarding ssh-dss AAAAC3...51R== example.net permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss AAAAB5...21S== tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...== email@example.com restrict,command="uptime" ssh-rsa AAAA1C8...32Tv== firstname.lastname@example.org restrict,pty,command="nethack" ssh-rsa AAAA1f8...IrrC5== email@example.com
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 from an unknown host, its key is added to the per-user file.
Each line in these files contains the following fields: markers (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 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. 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-keyscan1 or by taking, for example, /etc/ssh/ssh_host_rsa_key.pub and adding the host names at the front. ssh-keygen1 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...
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''
scp(1), sftp(1), ssh(1), ssh-add1, ssh-agent1, ssh-keygen1, ssh-keyscan1, chroot(2), hosts_access5, moduli(5), sshd_config5, inetd(8), sftp-server8
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.