TCP and syncache question

Hartmut Brandt hartmut.brandt at
Sun Nov 16 05:02:26 PST 2008

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.
>> 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.


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