[Snort-users] Snort+flexresp

Sonika Malhotra sonikam at ...4044...
Tue Mar 12 22:06:06 EST 2002

    Thanks for this detail explanation.Does this indicate that in every( WAN )
case where the delay is more ,the possibility of RST packet, generated by snort,
reaching the other end increases? That is if the no. of hops are more, it is more
likely that the snort terminates the connection.But in any case it is not


Roelof JT Jonkman wrote:

> Sonika,
> This is a somwhat common problem. I'll try my best to explain this somewhat.
> (Marty and other have explained this well in the past)
> Whenever you see the alert gets generated, snort has to fabricate two
> packets with the RST flag set, one for the server, and one for the client.
> The crucial piece is that the sequence number matches that of the connection.
> If the sequence number is off, it simply gets discarded. It obviously takes
> some time to fabricate these packets. In the mean time the server is also
> working on a response to the client. The gotcha is when you do this
> on a LAN, the delays are so low, that the server is likely to get back to
> the client before snort/flexresp is able to generate the RST packets, and
> the connection will have advanced beyond the sequence number that the
> RST packets have, and swat, they get ignored. However on a WAN connection
> where the delays are more than ~2ms, the RST packets will still have
> a sequence number that matches the current sequence number of the connection,
> and hence will convince both ends that the connection has ended.
> This is sort of a terse background behind flexible response, and why
> it works in some (WAN) cases, and not in others (LAN) This is
> by no means complete, rather it reflects upon my understanding
> of it.
> Hope this helps.
> Roel Jonkman
> Security Engineer
> http://www.SiliconDefense.com
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