Thx Antony But: >Circuit level filtering means packet filtering - what netfilter does - in >other words you filter packets based on where they've come from and where >they're going to (IP addresses), and on *assumptions* about what the TCP/UDP >port numbers mean, rather than based on anything that's actually inside the body of the packets (data). I read it's sth more than packet filtering, and it work on session layer. Working on session layer is a little hard for me to underestand. I'm looking for some example. I guess ESTABLISHED state option of iptables make it work on such level, but I'm not sure. >Application level filtering means proxies - software which can understand >protocols like http, smtp, pop3, ftp, irc.... and look at the data and >commands which are being transferred between machines, then base the >filtering decisions on that (as well as IP addresses and hostnames). >Gateway simply refers to a machine which is in the path between your network >and the outside world - can mean anything from a simple router with no >filtering capabilities to a multi-protocol proxy server with intrusion detection. Thx but, I meant "Circuit Level gateway" not a simple gateway I mean IP(or network level). >Netfilter (iptables) is a stateful packet filter, and therefore operates at >layers 3/4 of the OSI model - the network layers. It does not meaningfully >operate at layer 7 - the application layer. But I think matching RELATED state of ftp data connection means working on layer 7. >If you want realistic application layer filtering on a linux system you need >proxy applications like sendmail/exim/apache/squid/frox. Netfilter won't do >it for you. So u mean there's no such matching module or action in Netfilter. P.S. Below is what I read about Circuit Level (Filtering and Gateway). But they are alittle ambiguous! Regards __Radien__ http://www.pc-help.org/www.nwinternet.com/pchelp/lockdown/claims/firewalls.htm http://csrc.nist.gov/publications/nistpubs/800-10/node53.html&e=7317 http://www.pcstats.com/articleview.cfm?articleid=1450&page=5 http://www.firewall-software.com/firewall_faqs/types_of_firewall.html
