mirror of
https://github.com/sheumann/hush.git
synced 2024-08-14 11:28:58 +00:00
94 lines
3.2 KiB
Plaintext
94 lines
3.2 KiB
Plaintext
Some less-widely known details of TCP connections.
|
|
|
|
Properly closing the connection.
|
|
|
|
After this code sequence:
|
|
|
|
sock = socket(AF_INET, SOCK_STREAM, 0);
|
|
connect(sock, &remote, sizeof(remote));
|
|
write(sock, buffer, 1000000);
|
|
|
|
a large block of data is only buffered by kernel, it can't be sent all at once.
|
|
What will happen if we close the socket?
|
|
|
|
"A host MAY implement a 'half-duplex' TCP close sequence, so that
|
|
an application that has called close() cannot continue to read
|
|
data from the connection. If such a host issues a close() call
|
|
while received data is still pending in TCP, or if new data is
|
|
received after close() is called, its TCP SHOULD send a RST
|
|
to show that data was lost."
|
|
|
|
IOW: if we just close(sock) now, kernel can reset the TCP connection
|
|
(send RST packet).
|
|
|
|
This is problematic for two reasons: it discards some not-yet sent
|
|
data, and it may be reported as error, not EOF, on peer's side.
|
|
|
|
What can be done about it?
|
|
|
|
Solution #1: block until sending is done:
|
|
|
|
/* When enabled, a close(2) or shutdown(2) will not return until
|
|
* all queued messages for the socket have been successfully sent
|
|
* or the linger timeout has been reached.
|
|
*/
|
|
struct linger {
|
|
int l_onoff; /* linger active */
|
|
int l_linger; /* how many seconds to linger for */
|
|
} linger;
|
|
linger.l_onoff = 1;
|
|
linger.l_linger = SOME_NUM;
|
|
setsockopt(sock, SOL_SOCKET, SO_LINGER, &linger, sizeof(linger));
|
|
close(sock);
|
|
|
|
Solution #2: tell kernel that you are done sending.
|
|
This makes kernel send FIN after all data is written:
|
|
|
|
shutdown(sock, SHUT_WR);
|
|
close(sock);
|
|
|
|
However, experiments on Linux 3.9.4 show that kernel can return from
|
|
shutdown() and from close() before all data is sent,
|
|
and if peer sends any data to us after this, kernel still responds with
|
|
RST before all our data is sent.
|
|
|
|
In practice the protocol in use often does not allow peer to send
|
|
such data to us, in which case this solution is acceptable.
|
|
|
|
Solution #3: if you know that peer is going to close its end after it sees
|
|
our FIN (as EOF), it might be a good idea to perform a read after shutdown().
|
|
When read finishes with 0-sized result, we conclude that peer received all
|
|
the data, saw EOF, and closed its end.
|
|
|
|
However, this incurs small performance penalty (we run for a longer time)
|
|
and requires safeguards (nonblocking reads, timeouts etc) against
|
|
malicious peers which don't close the connection.
|
|
|
|
Solutions #1 and #2 can be combined:
|
|
|
|
/* ...set up struct linger... then: */
|
|
setsockopt(sock, SOL_SOCKET, SO_LINGER, &linger, sizeof(linger));
|
|
shutdown(sock, SHUT_WR);
|
|
/* At this point, kernel sent FIN packet, not RST, to the peer, */
|
|
/* even if there is buffered read data from the peer. */
|
|
close(sock);
|
|
|
|
Defeating Nagle.
|
|
|
|
Method #1: manually control whether partial sends are allowed:
|
|
|
|
This prevents partially filled packets being sent:
|
|
|
|
int state = 1;
|
|
setsockopt(fd, IPPROTO_TCP, TCP_CORK, &state, sizeof(state));
|
|
|
|
and this forces last, partially filled packet (if any) to be sent:
|
|
|
|
int state = 0;
|
|
setsockopt(fd, IPPROTO_TCP, TCP_CORK, &state, sizeof(state));
|
|
|
|
Method #2: make any write to immediately send data, even if it's partial:
|
|
|
|
int state = 1;
|
|
setsockopt(fd, IPPROTO_TCP, TCP_NODELAY, &state, sizeof(state));
|