Config File - LINUX

SSH looks for the user config file in ~/.ssh/config. A sample might look like:

ForwardX11 yes
Protocol 2,1

Using ForwardX11 yes is the same as specifying -X on the command line. The Protocol line tells SSH to try SSH2 first and then fall back to SSH1. If you want to use only SSH2, delete the ,1.

The config file can include sections that take effect only for certain remote hosts by using the Host option. Another useful config file option is User, which specifies the remote user name. If you often log in to a machine with ssh -l remoteuser remotehost or ssh remoteuser@remotehost, you can shorten this by placing the following lines in your config file:

Host remotehost
ForwardX11 yes
User remoteuser

Host *
ForwardX11 no

Now, you can type ssh remotehost to log on as user remoteuser with the ForwardX11 option turned on. Otherwise, ForwardX11 is turned off, as recommended above. The asterisk matches all hosts, including hosts already matched in a Host section, but only the first matching option is used. Put specific Host sections before generic sections in your config file.

A system-wide SSH config file, /etc/ssh/ssh_config, also is available. SSH obtains configuration data in the following order: command-line options, user's configuration file and system-wide configuration file. All of the options can be explored by browsing man ssh_config.

Speeding Things Up: Compression and Ciphers

SSH can use gzip compression on any connection. The default compression level is equivalent to approximately 4× compression for text. Compression is a great idea if you are forwarding X sessions on a dial-up or slow network. Turn on compression with ssh -C or put Compression yes in your config file.

Another speed tweak involves changing your encryption cipher. The default cipher on many older systems is triple DES (3DES), which is slower than Blowfish and AES. New versions of OpenSSH default to Blowfish. You can change the cipher to blowfish with ssh -c blowfish.

Cipher changes to your config file depend on whether you are connecting with SSH1 or SSH2. For SSH1, use Cipher blowfish; for SSH2, use:

Ciphers blowfish-cbc,aes128-cbc,3des-cbc,cast128-cbc,arcfour,aes192-cbc,aes256-cbc