question about firewalling

[David Hajoglou]

What sort of applications does linux have that either make up, or aid a
firewall.  I have seen some stuff on ipfdw and ip_masq, but are these
firewall components, or just methods of passing packets from point to
point?  Is there any gpl software that is intended for just a firewall
application?  Any remarks would be great!

Thank You
David