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From: Lyndon Nerenberg (lyndon_at_orthanc.ab.ca)
Date: Sun Oct 20 2002 - 14:03:25 CDT
>Think of ECN; should older stacks simply reject a packet with Syn+0x42
>because they don't know what 0x42 is?
>If I've understood correctly, you were suggesting to drop "bad" packets.
>I agree; only let established traffic through your firewall, and only
>let packets with Syn or Syn+Ack set and with Fin and Rst unset establish
>state in the firewall. Ignore the rest of the flags.
>Of course, if anyone finds this un-interoperable, please chime in!
Before people get too paranoid about accepting packets I recommend
they read RFC 3360: Inappropriate TCP Resets Considered Harmful.
TCP uses the RST (Reset) bit in the TCP header to reset a TCP
connection. Resets are appropriately sent in response to a
connection request to a nonexistent connection, for example. The TCP
receiver of the reset aborts the TCP connection, and notifies the
application [RFC793, RFC1122, Ste94].
Unfortunately, a number of firewalls and load-balancers in the
current Internet send a reset in response to a TCP SYN packet that
use flags from the Reserved field in the TCP header. Section 3 below
discusses the specific example of firewalls that send resets in
response to TCP SYN packets from ECN-capable hosts.
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