Hartmut Brandt wrote:
Rui Paulo wrote:
On 15 Nov 2008, at 20:08, Hartmut Brandt wrote:
Hi,
in tcp_syncache.c:syncache_expand() there is a test that the
acknowledgement number and the sequence number of an incoming ACK
segment are in the expected range. If they are not, syncache_expand()
returns 0 and tcp_input drops the segment and sets a reset. So far so
good. But syncache_expand() also deletes the syncache entry, and so
destroys the connection. I cannot see why it does it. It seems to me
that such a wrong segment should be interpreted as to be from another
connection and as such the segment should be ignored (but a reset
sent). When the correct ACK comes, the connection could still be
established. As it is now, the establishment of incoming connections
can seriously be disturbed by someone sending fake ACK packets.
The same test (for the ack number, not for the sequence number) is
also further down in tcp_input.c:tcp_do_segment() (just after the
header prediction stuff) and here the handling is correct: the goto
dropwithreset just sends a reset and drops the segment but leaves the
connection in the SYN-RECEIVED state. This test is probably never
reached now, because of syncache_expand(), though.
Correct.
Maybe I fail to see something obvious, though...
Well, if the RST is sent, why should we keep the syncache entry?
Because this effectively destroys the connection in SYN-RECEIVED which
is wrong according to RFC793. On page 69 the handling of incoming
segments for connections in SYN-RECEIVED is described: first you check
the sequence number and, if it is wrong, you send an RST (unless the RST
bit is set in the incoming segment), but otherwise ignore the segment.
>
A segment with a bad sequence number in SYN-RECEIVED is either forged or
from an old connection. In both cases you don't want to destroy the
embryonic connection, because the correct ACK from the correct peer may
still arrive.
I see your problem. Syncookies mitigate this problem (if not disabled) as
the correct ACK will pass that test later even if the syncache entry went
away before (which can also happen due to a generally high SYN load).
RFC793 wants us to do the following:
Page 69: Send back a challenge ACK with the correct parameters to help
to re-synchronize the connection when
!(RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND).
Page 72: Send back a RST when !(SND.UNA =< SEG.ACK =< SND.NXT).
At the moment we send the RST and delete the syncache entry for both cases.
However we should send the ACK in the former, the RST in the latter, and
and keep the syncache entry in either case.
Fixing this requires some re-shuffling of the syncache_expand(). I'll
post a version later today.
--
Andre
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