Hi.
>> > I think sendig FIN is required when closing SYN_RECV connections
>> > even if they are on a backlog-queue(sk->tp_pinfo.af_tcp.syn_wait_queue),
>> > but Linux releases SYN_RECV connections without anything.
>> > Or it would not be bad idea to send reset (RST) to the peer
>> > than doing nothing.
>>
>> RST is not good when the SYN-RECV ACK has not been received yet -- you
>> risk killing innocent connections because only after the three way handshake
>> you're fully synchronized. FIN is not good neither. For Established sockets
>> we already do the right thing.
>>
>> -Andi
No risk there.
It is easy to specify the peer to send RST/FIN and it doesn't matter
that the peer is SYN-SENT or ESTABLISHED.
The information of the peer -- inetaddress,port,seq ... -- is contained
in the open_request.
If a SYN-SENT socket receives RST, status of the socket turns into CLOSE
and make an ECONNREFUSED error.
If a ESTABLISHED socket receives RST, status of the socket turns into CLOSE
and make an ECONNRESET error.
RFC916 says:
RFC916> 5.2.3. SYN-RECEIVED
RFC916>
RFC916> Arrived at from the LISTEN and SYN-SENT states in response to
RFC916> an arriving SYN packet.
RFC916>
RFC916> Departures
RFC916>
RFC916> - A CLOSE request is made by the user. Create a packet with
RFC916> FIN set. Send it and go to the FIN-WAIT state.
And here is a state diagram of tcp-connection.
We can see how SYN-RECV connection should work.
+---------+ ---------\ active OPEN
| CLOSED | \ -----------
+---------+<---------\ \ create TCB
| ^ \ \ snd SYN
passive OPEN | | CLOSE \ \
------------ | | ---------- \ \
create TCB | | delete TCB \ \
V | \ \
+---------+ CLOSE | \
| LISTEN | ---------- | |
+---------+ delete TCB | |
rcv SYN | | SEND | |
----------- | | ------- | V
+---------+ snd SYN,ACK / \ snd SYN +---------+
| |<----------------- ------------------>| |
| SYN | rcv SYN | SYN |
| RCVD |<-----------------------------------------------| SENT |
| | snd ACK | |
| |------------------ -------------------| |
+---------+ rcv ACK of SYN \ / rcv SYN,ACK +---------+
| -------------- | | -----------
| x | | snd ACK
| V V
| CLOSE +---------+
| ------- | ESTAB |
| snd FIN +---------+
| CLOSE | | rcv FIN
V ------- | | -------
+---------+ snd FIN / \ snd ACK +---------+
| FIN |<----------------- ------------------>| CLOSE |
| WAIT-1 |------------------ | WAIT |
+---------+ rcv FIN \ +---------+
| rcv ACK of FIN ------- | CLOSE |
| -------------- snd ACK | ------- |
V x V snd FIN V
+---------+ +---------+ +---------+
|FINWAIT-2| | CLOSING | | LAST-ACK|
+---------+ +---------+ +---------+
| rcv ACK of FIN | rcv ACK of FIN |
| rcv FIN -------------- | Timeout=2MSL -------------- |
| ------- x V ------------ x V
\ snd ACK +---------+delete TCB +---------+
------------------------>|TIME WAIT|------------------>| CLOSED |
+---------+ +---------+
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