[Contents] [Index]

About This Document>>     Introduction to SSH Secure Shell >>     Configuring SSH Secure Shell >>         Basic Configuration>>             Default Locations of Secure Shell Files             Generating the Host Key             Ciphers and MACs             Compression             Configuring Root Logins             Restricting User Logins         Subconfigurations >>         Configuring SSH Secure Shell for TCP Wrappers Support>>         Configuring SSH2 for SSH1 Compatibility         Forwarding>>     Authentication >>     Log Messages >>     Using SSH Secure Shell >>     Tool Syntax>>     Technical Specifications >>

Compression

SSH Secure Shell uses GNU ZLIB (LZ77) for compression. The "zlib" compression is described in RFC 1950 and in RFC 1951.

By default, compression is disabled. Compression can be enabled in the ssh2_config file:

        Compression     yes

Alternatively, compression can be enabled on the command line:

$ ssh2 +C username@remote

The client can request a compression level with a number after +C (from +C1 to +C9). In this case, the compression level is between the levels requested by the client and offered by the server. For example, if the server offers level 6 (the default) and the client asks for level 1, level 2 is used.

Compression is worth using if your connection is slow (for example a modem connection). The efficiency of the compression depends on the type of the file, and varies widely. It is close to 0% for already compressed files like zip and often 50% or even more for text files.