The sshd (Secure Shell 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 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 versions 1 and 2 simultaneously. Because of security weaknesses in the v1 protocol, sites should run only v2, if possible. In the default configuration, only protocol v2 is enabled for the server. To enable v1 and v2 simultaneously, see the instructions in sshd_config(4).
Support for v1 is provided to help sites with existing ssh v1 clients and servers to transition to v2. v1 might not be supported in a future release.
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 rshd(1M), rlogind(1M), rexecd(1M), and rexd(1M) are disabled (thus completely disabling rlogin(1) and rsh(1) into the machine).
Version 2 works similarly to version 1: Each host has a host-specific DSA/RSA key. 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 Blowfish, 3DES, or 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 authentication method (PubKeyAuthentication) and conventional password authentication.
If the client successfully authenticates itself, a dialog for preparing the session is entered. At this time the client can 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 the configuration files /etc/ssh/ssh_config and ~/.ssh/config, both described in ssh_config(4). Command-line options override values specified in the configuration file.
sshd rereads its configuration file when it receives a hangup signal, SIGHUP.
The sshd daemon uses TCP Wrappers to restrict access to hosts. It uses the service name of sshd for hosts_access(). For more information on TCP Wrappers see tcpd(1M) and hosts_access(3) man pages, which are part of the SUNWsfman package (they are not SunOS man pages). TCP wrappers binaries, including libwrap, are in SUNWtcpd, a required package for SUNWsshdu, the package containing sshd.
The options for sshd are as follows:
Specifies the number of bits in the server key (the default is 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 fork 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.
Specifies the name of the configuration file. The default is /etc/ssh/sshd_config. sshd refuses to start if there is no configuration file.
Gives the grace time for clients to authenticate themselves (the default is 300 seconds). If the client fails to authenticate the user within this number of seconds, the server disconnects and exits. A value of zero indicates no limit.
Specifies the file from which the host key is read (the default is /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).
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 (for example, 512) using sshd from inetd may be reasonable.
Specifies how often the server key is regenerated (the default is 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.
Specifies the port on which the server listens for connections (the default is 22).
Quiet mode. Nothing is sent to the system log. Normally the beginning, authentication, and termination of each connection is logged.
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.
Forces sshd to use IPv4 addresses only.
Forces sshd to use IPv6 addresses only.
The $HOME/.ssh/authorized_keys file lists the public keys that are permitted for RSA authentication. Each line of the file contains one key (empty lines and lines starting with a hash mark [#] 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).
Lines in this file are usually several hundred bytes long (because of the size of the RSA key modulus). You will find it very inconvenient to type them in; instead, copy the identity.pub file and edit it.
Permissions of this file must be set so that it is not world or group writable. See the StrickModes option of sshd_config(4).
The options (if present) consist of comma-separated option specifications. No spaces are permitted, except within double quotes. The following option specifications are supported:
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 can 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).
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 can be included in the command by quoting it with a backslash. This option might be useful to restrict certain RSA keys to perform only a specific operation. An example might be a key that permits remote backups but nothing else. The client might specify TCP/IP and/or X11 forwarding unless they are explicitly prohibited.
Specifies that the string NAME=value 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.
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, for example, 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).
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 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 can 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, 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 a hash mark (#) 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.
The lines in these files are typically hundreds of characters long.You should definitely not type in the host keys by hand. Rather, generate them by a script or by taking /etc/ssh/ssh_host_rsa_key.pub and adding the host names at the front.
1024 33 12121...312314325 firstname.lastname@example.org 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
The following are examples of ssh_known_hosts file entries.
closenet,closenet.hut.fi,...,22.214.171.124 1024 37 159...93 closenet.hut.fi
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.
Contains the private part of the host key. This file should only be owned by root, readable only by root, and not accessible to others. sshd does not start if this file is group/world-accessible.
Contains the public part of the host key. This file should be world-readable but writable only by root. Its contents should match the private part. This file is not used for encryption; it is provided only for the convenience of the user so its contents can be copied to known hosts files. These two files are created using ssh-keygen(1).
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 content of this file is not sensitive; it can be world-readable. You can use the PidFile keyword in sshd_config to specify a file other than /var/run/sshd.pid. See sshd_config(4).
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 it intended to connect. These files should be writable only by root or the owner. /etc/ssh/ssh_known_hosts should be world-readable, and $HOME/.ssh/known_hosts can but need not be world-readable.
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.
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. 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.
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.
This file is used during .rhosts authentication. In its simplest form, this file contains host names, one per line. Users on these hosts are permitted to log in without a password, provided they have the same user name on both machines. The host name can 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 a hyphen (-).
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. For practical purposes, using a user name grants the user root access. Probably the only valid use for user names is in negative entries. This warning also applies to rsh/rlogin.
This file is processed exactly as /etc/hosts.equiv. However, this file might be useful in environments that want to run both rsh/rlogin and ssh.
This file is read into the environment at login (if it exists). It can contain only 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.
If this file exists, it is run with /bin/sh 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(1) in that case.
The primary purpose of $HOME/.ssh/rc is to run any initialization routines that might 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, xauth is used to store the cookie. $HOME/.ssh/rc should be writable only by the user, and need not be readable by anyone else.
Similar to $HOME/.ssh/rc. 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.
See attributes(5) for descriptions of the following attributes:
To view license terms, attribution, and copyright for OpenSSH, the default path is /var/sadm/pkg/SUNWsshdr/install/copyright. If the Solaris operating environment has been installed anywhere other than the default, modify the given path to access the file at the installed location.
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, added newer features and created Open SSH. Markus Friedl contributed the support for SSH protocol versions 1.4 and 2.0.