Snort mailing list archives
Re: fragroute vs. snort: the tempest in a teacup
From: Darren Reed <avalon () coombs anu edu au>
Date: Fri, 19 Apr 2002 08:10:54 +1000 (Australia/ACT)
In some mail from Dug Song, sie said:
Most firewalls these days (especially Linux and OpenBSD ones) actually do reassembly inbound.this isn't quite true. most stateful inspection firewalls do "virtual reassembly" for IP fragments, and a few do basic window tracking for TCP connections, but will still allow most fragroute-style attacks through (e.g. duplicate overwriting TCP segments with older TCP timestamp options for PAWS elimination, short TTLs, etc.).
Well then IDS software needs to be smarter. IMHO it makes little sense for an IDS to be *behind* a firewall as it's going to miss out on lots of useful data points. Maybe this means telling your IDS software how big your network is so it can make intelligent decisions about how far a packet will go based on its TTL.
This was an interesting point discovered recently when it was realized that the snort defragger was actually never getting touched at all in some installations.IP fragmentation is rare to begin with [5], so i wouldn't chalk this up to firewall magic - especially when all major firewalls still pass fragments in their default configuration, and ONLY OpenBSD pf and Linux netfilter can actually be configured to reassemble. even fewer track TCP windows, options, etc...
IP Fragmentation is rare across the WAN, maybe, but anyone who's used NFSv2 knows how common it is on the LAN. There are good reasons NOT to do reassembly and I imagine those that do not do so because they understand this better than the desire to simply add yet another feature which some consider "cool". Mind you, if you don't configure OpenBSD pf to reassemble packets then you cannot make pf drop them, either. Darren _______________________________________________ Snort-users mailing list Snort-users () lists sourceforge net Go to this URL to change user options or unsubscribe: https://lists.sourceforge.net/lists/listinfo/snort-users Snort-users list archive: http://www.geocrawler.com/redir-sf.php3?list=snort-users
Current thread:
- fragroute vs. snort: the tempest in a teacup Dragos Ruiu (Apr 17)
- Re: fragroute vs. snort: the tempest in a teacup Dug Song (Apr 18)
- Re: fragroute vs. snort: the tempest in a teacup Darren Reed (Apr 18)
- Re: fragroute vs. snort: the tempest in a teacup Ron DuFresne (Apr 19)
- RE: fragroute vs. snort: the tempest in a teacup Enno Rey (Apr 19)
- Re: fragroute vs. snort: the tempest in a teacup Marco Thorbruegge (Apr 19)
- Re: fragroute vs. snort: the tempest in a teacup Crist J. Clark (Apr 20)
- Re: fragroute vs. snort: the tempest in a teacup Francis Cianfrocca (Apr 18)
- Re: Re: fragroute vs. snort: the tempest in a teacup Jason Haar (Apr 18)
- Re: fragroute vs. snort: the tempest in a teacup Darren Reed (Apr 18)
- <Possible follow-ups>
- Re: fragroute vs. snort: the tempest in a teacup Brad Powell (Apr 19)
- Re: fragroute vs. snort: the tempest in a teacup Steven M. Bellovin (Apr 19)
- RE: fragroute vs. snort: the tempest in a teacup Craig, Scott (Apr 25)
- RE: fragroute vs. snort: the tempest in a teacup Ron DuFresne (Apr 25)
- Re: fragroute vs. snort: the tempest in a teacup Dug Song (Apr 18)