Paul Starzetz <paul@starzetz.de> writes: > * Linux 2.4.19 > > The examination of the source code of the TCP engine reveals that a > TCP connection can be opened by any combination of the TCP flags > having the SYN bit set and the ACK bit reset. For example we can open > a TCP connection by sending an obviously bogus SYN,RST packet: > > 14:25:43.888897 192.168.1.184.12345 > 192.168.1.111.9999: SR > 420:420(0) win 512 (DF) [tos 0x18] > 14:25:43.889143 192.168.1.111.9999 > 192.168.1.184.12345: S > 2168208394:2168208394(0) ack 421 win 5840 <mss 1460> (DF) As a result of this bug, it's quite complicated (if not impossible in some configurations) to properly filter connection attempts to Linux hosts on Cisco IOS routers. If your access list is a whitelist with a "permit tcp any any established" statement somewhere, it's very likely that you can bypass the filter just by setting the RST in the initial SYN packet, as described above. The router will forward the packet, and the Linux host will happily initiate the three-way handshake. "established" in Cisco parlance does not mean "SYN unset", but "ACK or RST set". This means that the impact for non-Linux hosts (which do not react to SYN-RST packets according to Paul's survey) is less severe if your filters run IOS. -- Florian Weimer Weimer@CERT.Uni-Stuttgart.DE University of Stuttgart http://CERT.Uni-Stuttgart.DE/people/fw/ RUS-CERT fax +49-711-685-5898
