This rule will match on the STREAM_3WHS_ACK_DATA_INJECT, that is
set if we're:
- in IPS mode
- get a data packet from the server
- that matches the exact SEQ/ACK expectations for the 3whs
The action of the rule is set to drop as the stream engine will drop.
So the rule action is actually not needed, but for consistency it
is drop.
Reported in bug 1238 is an issue where stream reassembly can be
disrupted.
A packet that was in-window, but otherwise unexpected set the
window to a really low value, causing the next *expected* packet
to be considered out of window. This lead to missing data in the
stream reassembly.
The packet was unexpected in various ways:
- it would ack unseen traffic
- it's sequence number would not match the expected next_seq
- set a really low window, while not being a proper window update
Detection however, it greatly hampered by the fact that in case of
packet loss, quite similar packets come in. Alerting in this case
is unwanted. Ignoring/skipping packets in this case as well.
The logic used in this patch is as follows. If:
- the packet is not a window update AND
- packet seq > next_seq AND
- packet acq > next_seq (packet acks unseen data) AND
- packet shrinks window more than it's own data size
THEN set event and skip the packet in the stream engine.
So in case of a segment with no data, any window shrinking is rejected.
Bug #1238.
Until now, when processing the TCP 3 way handshake (3whs), retransmissions
of SYN/ACKs are silently accepted, unless they are different somehow. If
the SEQ or ACK values are different they are considered wrong and events
are set. The stream events rules will match on this.
In some cases, this is wrong. If the client missed the SYN/ACK, the server
may send a different one with a different SEQ. This commit deals with this.
As it is impossible to predict which one the client will accept, each is
added to a list. Then on receiving the final ACK from the 3whs, the list
is checked and the state is updated according to the queued SYN/ACK.
Set event on overlapping data segments that have different data.
Add stream-events option stream-event:reassembly_overlap_different_data and
add an example rule.
Issue 603.
Victor Julien
Eric Leblond