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System Administration Guide: Security Services

Configuring the Solaris Secure Shell

The characteristics of a Solaris Secure Shell session are controlled by configuration files, which can be overridden to a certain degree by options on the command line.

Solaris Secure Shell Client Configuration

In most cases, the client-side characteristics of a Solaris 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.

Host-Specific Parameters

If it is useful to have different Solaris 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.

Client-Side Authentication Parameters

The authentication method is determined by setting one of the following keywords to “yes”:

The keyword UseRsh specifies that the rlogin and rsh commands be used, probably due to no Secure Shell support.

The Protocol keyword sets the Solaris 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 IdentityFile keyword specifies an alternate file name to hold the user's private key.

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.

Known Host File Parameters

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 Solaris 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.

Client-Side X11 Forwarding and Port Forwarding Parameters

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:

The ForwardX11 keyword redirects X11 connections to the remote host with the DISPLAY environment variable set. The XAuthLocation keyword specifies the location of the xauth program.

Client-Side Connection and Other Parameters

The NumberOfPasswordPrompts keyword specifies how many times the user is prompted for a password before Solaris Secure Shell quits. The ConnectionAttempts keyword specifies how many tries (at one try per second) are made before Solaris 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 Solaris 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.

Batchmode disables password prompts, which is useful for scripts and other batch jobs.

KeepAlive enables messages to indicate network problems due to host crashes. LogLevel sets the verbosity level for ssh messages.

EscapeChar defines a single character that is used as a prefix for displaying special characters as plain text.

Solaris Secure Shell Server Configuration

The server-side characteristics of a Solaris Secure Shell session are governed by the /etc/ssh/sshd_config file, which is set up by the administrator.

Server-Side Authentication Parameters

Permitted authentication methods are indicated by theses keywords:

HostKey and HostDSAKey identify files that hold host public keys when the default file name is not used. KeyRegenerationInterval defines how often the server key is regenerated.

Protocol specifies the version. Ciphers specifies the encryption algorithms for v2. ServerKeyBits defines the number of bits in the server's key.

Ports and Forwarding Parameters

AllowTCPForwarding specifies whether TCP forwarding is permitted.

GatewayPorts allows remote hosts to connect to ports forwarded for the client. Port specifies the port number that sshd listens on. ListenAddress designates a specific local address that sshd listens to. If there is no ListenAddress specification, sshd listens to all addresses by default.

X11Forwarding allows X11 forwarding. X11DisplayOffset specifies the first display number that is available for forwarding. This keyword prevents sshd from interfering with real X11 servers. XAuthLocation specifies the location of the xauth program.

Session Control Parameters

KeepAlive displays messages regarding broken connections and host crashes. LogLevel sets the verbosity level of messages from sshd. SyslogFacility provides a facility code for messages that are logged from sshd.

Server Connection and Other Parameters

The AllowGroups, AllowUsers, DenyGroups, and DenyUsers keywords control which users can or cannot use ssh.

The LoginGraceTime, MaxStartups, PermitRootLogin, and PermitEmptyPasswords keywords set controls on users who are logging in. StrictModes causes sshd to check file modes and ownership of the user's files and home directory before login. UseLogin specifies whether login is used for interactive login sessions. Turning this keyword on should not be necessary and is not recommended for the Solaris environment.

Subsystem configures a file transfer daemon for using sftp.