Linux Command Directory


Linux in a Nutshell

This directory of Linux commands is from Linux in a Nutshell, 5th Edition.

Click on any of the 687 commands below to get a description and list of available options. All links in the command summaries point to the online version of the book on Safari Bookshelf.

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sshd

sshd [options]

TCP/IP command. Server for the ssh program, providing a secure remote-login and remote-execution facility equivalent to rlogin and rsh. Normally started at boot, sshd listens for incoming connections, forking a new daemon when one is detected. The forked daemon handles authentication, command execution, and encryption. Most implementations of sshd support both SSH protocols 1 and 2. The following options are those used by OpenSSH, OpenBSD's Secure Shell implementation.

Options

-4

Use only IPv4 addresses.

-6

Use only IPv6 addresses.

-b bits

Use the specified number of bits in the server key. Default is 768.

-d

Run sshd in the foreground and send verbose debug information to the system log. Process only one connection. Use the specified number of bits in the server key. This option may be specified from one to three times. Each additional -d increases the level of information sent to the system log.

-D

Do not detach from the foreground process.

-e

Send output to standard error instead of the system log.

-f file

Read configuration information from file instead of the default configuration file /etc/ssh/sshd_config.

-g seconds

Set the grace time a client has to authenticate itself before the server disconnects and exits. The default is 600 seconds. A value of 0 means there is no limit.

-h keyfile

Read the host's cryptographic key from the specified keyfile instead of from the default file /etc/ssh/ssh_host_key for SSH protocol 1, and the default files /etc/ssh/ssh_host_rsa_key and /etc/ssh/ssh_host_dsa_key for SSH protocol 2. The -h option may be given more than once to specify multiple keyfiles.

-i

Use when running sshd from inetd.

-k seconds

Set how often the Version 1 server key should be regenerated. Default value is 3600 seconds. If set to 0 seconds, the key will never be regenerated.

-o setting

Pass a configuration file setting as an option.

-p port

Listen for connections on port. The default is 22. More than one -p option may be specified. This option overrides ports specified in a configuration file.

-q

Send no messages to the system log.

-t

Test configuration files and keys, then exit.

-u namelength

Specify the length of the remote hostname field in the UTMP structure as specifed in utmp.h. A namelength of 0 will cause sshd to write dotted decimal values instead of hostnames to the utmp file and prevent DNS requests unless required by the authentication mechanism.


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