ServerTokens in apache2 debian

ServerTokens will display description of the generic OS-type of the server as well as information about compiled-in modules.

By setting this to `prod` will avoid displaying all module information under server-generated documents (error messages, mod_proxy, ftp directory listings, mod_info output). The default for this directive is 'Full'

In debian you can set this directive in /etc/apache2/conf.d/security file

Also set ServerSignature Off to diable Signature

This will helps to hide your Apache server information from others

Look at apache online document for more information

http://httpd.apache.org/docs/2.2/mod/core.html