In most cases, the client-side characteristics of a Secure Shell session are governed by the system-wide configuration file, /etc/ssh/ssh_config, which is set up by the administrator. The settings in the system-wide configuration file can be overridden by the user's configuration in $HOME/.ssh/config. In addition, the user can override both configuration files on the command line.
The command line options are client requests and are permitted or denied on the server side by the /etc/ssh/sshd_config file (see ssh_config(4)). The configuration file keywords and command options are introduced in the following sections and are described in detail in the ssh(1), scp(1), sftp(1), and ssh_config(4) man pages. Note that in the two user configuration files, the Host keyword indicates a host or wildcard expression to which all following keywords up to the next Host keyword apply.
If it is useful to have different Secure Shell characteristics for different local hosts, the administrator can define separate sets of parameters in the /etc/ssh/ssh_config file to be applied according to host or regular expression. This task is done by grouping entries in the file by Host keyword. If the Host keyword is not used, the entries in the client configuration file apply to whichever local host a user is working on.
The Protocol keyword sets the Secure Shell protocol version to v1 or v2. You can specify both versions separated by a comma. The first version is tried and upon failure, the second version is used.
The keyword Cipher specifies the v1 encryption algorithm, which might be blowfish or 3des. The keyword Ciphers specifies an order of preference for the v2 encryption algorithms: 3des-cbc, blowfish-cbc, and aes128–cbc. The commands ssh and scp have a -c option for specifying the encryption algorithm on the command line.
The known host files (/etc/ssh/ssh_known_hosts and $HOME/.ssh/known_hosts) contain the public keys for all hosts with which the client can communicate by using Secure Shell. The GlobalKnownHostsFile keyword specifies an alternate file instead of /etc/ssh/ssh_known_hosts. The UserKnownHostsFile keyword specifies an alternate to $HOME/.ssh/known_hosts.
The StrictHostKeyChecking keyword requires new hosts to be added manually to the known hosts file, and refuses any host whose public key has changed or whose public key is not in the known hosts file. The keyword CheckHostIP enables the IP address for hosts in the known host files to be checked, in case a key has been changed due to DNS spoofing.
The LocalForward keyword specifies a local TCP/IP port to be forwarded over a secure channel to a specified port on a remote host. The GatewayPorts keyword enables remote hosts to connect to local forwarded ports.
The command ssh enables port forwarding through these options:
-L, which specifies the local port to be forwarded to the specified port on the remote host
The NumberOfPasswordPrompts keyword specifies how many times the user is prompted for a password before Secure Shell quits. The ConnectionAttempts keyword specifies how many tries (at one try per second) are made before Secure Shell either quits or falls back to rsh if the FallBackToRsh keyword is set.
The Compression keyword enables compression of transmitted data. The CompressionLevel keyword sets a level of 1 to 9, trading off between speed and amount of compression.
User specifies an alternate user name. Hostname specifies an alternate name for a remote host. ProxyCommand specifies an alternate command name for starting Secure Shell. Any command that can connect to your proxy server can be used. The command should read from its standard input and write to its standard output.