Firewall Wizards mailing list archives
Re: TTL, works with Cisco ACL's to :)
From: Lance Spitzner <lance () spitzner net>
Date: Thu, 9 Nov 2000 23:23:51 -0600 (CST)
On Thu, 9 Nov 2000, Alex Goldney wrote:
I know a lot of sites don't do good egress filtering, and I guess that is the point that needs to be hammered home.....
Actually many sites do have egress filtering. However, the filtering device is filtering outbound traffic generated by the internal network. What many sites are NOT doing is egress filtering of traffic generated by the filtering device itself. The filtering device is trusted, so it is allowed to generate and send any traffic it wants to. That is why I belive the use of TTL within port scans can be effective against many filtering devices.
On Thu, 9 Nov 2000, Alex Goldney wrote:OK, so you aren't blocking any ICMP packets with access-lists. That should avoid the problem, no? Of course, it can be considered a bit unfriendly to block the lot. PATH MTU discovery stuff should be allowed at least in general. I guess that opens up the possiblility for the same type of attack if the MTU for one of your routers links is less than the MTU of the incoming internet link. This case should be pretty rare though.Keep in mind, many Firewalls/Screening Routers do not block ICMP error messages. Those that do block ICMP error messages block them inbound from the untrusted networks, such as the Internet, or block them inbound from internal networks. However, most rulebases/ACLs do NOT block ICMP error messages generated by the filtering device itself. Keep in mind, this is a generalization based on my experience. lance
-- Lance Spitzner http://www.enteract.com/~lspitz _______________________________________________ firewall-wizards mailing list firewall-wizards () nfr com http://www.nfr.com/mailman/listinfo/firewall-wizards
Current thread:
- TTL, works with Cisco ACL's to :) Lance Spitzner (Nov 08)
- <Possible follow-ups>
- Re: TTL, works with Cisco ACL's to :) Alex Goldney (Nov 09)
- Re: TTL, works with Cisco ACL's to :) Lance Spitzner (Nov 09)
- Re: TTL, works with Cisco ACL's to :) Alex Goldney (Nov 09)
- Re: TTL, works with Cisco ACL's to :) Lance Spitzner (Nov 09)
- Re: TTL, works with Cisco ACL's to :) Alex Goldney (Nov 10)
- Re: TTL, works with Cisco ACL's to :) Lance Spitzner (Nov 11)
- RE: TTL, works with Cisco ACL's to :) Ofir Arkin (Nov 12)
- Re: TTL, works with Cisco ACL's to :) Lance Spitzner (Nov 11)