NASL is a scripting language designed for the Nessus security scanner. Its
aim is to allow anyone to write a test for a given security hole in a few
minutes, to allow people to share their tests without having to worry
about their operating system, and to garantee everyone that a NASL script
can not do anything nasty except performing a given security test against
a given target. Thus, NASL allows you to easily forge IP packets, or to
send regular packets. It provides you some convenient functions that will
make the test of web and ftp server more easy to write. NASL
garantees you that a NASL script : 

       will not send any packet to a host other than the target host 
       will not execute any commands on your local system 

NASL is not a powerful scripting language. Its purpose is to make scripts
that are security tests. So, do not expect to write a third generation web
server in this language, nor a file conversion utility. Use perl, python
or whatever scripting language to do this.

WWW: http://www.nessus.org/doc/nasl.html