Snort_inline and out of order packets

In Snort_inline’s stream4 modifications, one of the changes is that out of order TCP packets are treated differently from unmodified stream4. This can cause some new alerts to appear and some unexpected behaviour. So I’ll try to explain what happens here.

First of all let me explain quickly what out of order packets are. To put it simple, TCP packets are send out by the source host in a specific order but can arrive in a different order at the destination. Packetloss, link saturation, routing issues are among many things that can cause this. A Snort_inline specific issue is that when Snort_inline can’t keep up with the packets it needs to process, it will drop packets which causes packetloss. These packets will then have to be resent by the sending host.

Out of order packets become a problem when dealing with stream reassembly. Stream reassembly basically is putting all data from the packets in the right order to get the original data as it was sent. We can’t do stream reassembly if we don’t have all packets. Unmodified stream4 basically ignores gaps in the stream. Designed for passive listening for traffic, it has to deal with packetloss differently than Snort_inline.

Next, some definitions of this functionality in Snort_inline. Out-of-order packets: The number of packets that we have in queue that are out of order for a stream. This means they have a higher sequence number than the next in-sequence packet we are expecting. Out-of-order bytes: The number of bytes of the combined data of the out-of-order packets in the stream. Sequence number hole: A gap between two packets, that can be closed by one or more missing packets.

To prevent Snort_inline from using to much memory on bad connections or when an attacker sends lots of out of order packets, Snort_inline can enforce limits to protect itself. Snort_inline can even force a stream to be completely in-order by dropping all packets that are out of order. Sadly, this has a bad effect on the performance of the connections, so you can set certain limits that balance between performance and protection.

When Snort_inline hits these limits, it will (optionally) fire alerts that look like this:

(spp_stream4) TCP out-of-order packets limit reached for stream
(spp_stream4) TCP out-of-order bytes limit reached for stream
(spp_stream4) TCP sequence number holes limit reached for stream

You can disable the alerts by adding the following option to the preprocessor stream4 line: disable_ooo_alerts. The limits themselves can be adjusted by using the following options: max_seq_holes 2, max_ooo_pkts 25, max_ooo_bytes 7000. These are the values I currently use on my home gateway. I got the idea of implementing these limits from this paper by Vern Paxson. However, it seems to me that his suggestion that at max one sequence hole per stream (even per host) was a bit optimistic. Maybe DSL has more packetloss than the university links he studied.

By default Snort_inline uses the settings that were chosen a bit randomly, so they may not fit your usage. Like with the wscaling, please let me know in a comment what values you use!

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