[Bro] TCP normalization and reassembly decision

[Christian Kreibich]

Hi Shuai,

On 11/13/2017 02:30 PM, Shuai Hao wrote:
> In /src/analyzer/protocol/tcp/tcp.cc, I find a comment "we could be fooled
> by an inconsistent SYN retransmission. Where is a normalizer". So I assume
> Bro doesn't come with a TCP normalizer. What is the consideration for such
> decision? It will be not necessary, or it will be implemented in future?

A TCP normalizer, in the sense referred to here, is a middlebox that 
removes ambiguities in the traffic by actually modifying the packet flow 
and payloads in-path, to simplify the job of subsequent network 
monitors. So in order to implement this Bro would need to support 
in-path deployment, which isn't a priority for us.

There's old (entirely unsupported) code for such a normalizer available 
here, if you'd like to experiment:

http://icir.org/christian/downloads/norm-0.2.0.tar.gz

There are also commercial products in this space that support varying 
extents of traffic normalization.

Best,
-C.