c - listen() ignores the backlog argument?

Question

I have the following problem:

I have sockfd = socket(AF_INET, SOCK_STREAM, 0)

After I set up and bind the socket (let's say with sockfd.sin_port = htons(666)), I immediately do:

listen(sockfd, 3);

sleep(50); // for test purposes

I'm sleeping for 50 seconds to test the backlog argument, which seems to be ignored because I can establish a connection* more than 3 times on port 666.

*: What I mean is that I get a syn/ack for each Nth SYN (n>3) sent from the client and placed in the listen queue, instead of being dropped. What could be wrong? I've read the man pages of listen(2) and tcp(7) and found:

The behavior of the backlog argument on TCP sockets changed with Linux 2.2. Now it specifies the queue length for completely established sockets waiting to be accepted, instead of the number of incomplete connection requests. The maximum length of the queue for incomplete sockets can be set using /proc/sys/net/ipv4/tcp_max_syn_backlog. When syncookies are enabled there is no logical maximum length and this setting is ignored. See tcp(7) for more information.

, but even with sysctl -w sys.net.ipv4.tcp_max_syn_backlog=2 and sysctl -w net.ipv4.tcp_syncookies=0, I still get the same results! I must be missing something or completely missunderstand listen()'s backlog purpose.

Answer 1

The backlog argument to listen() is only advisory.

POSIX says:

The backlog argument provides a hint to the implementation which the implementation shall use to limit the number of outstanding connections in the socket's listen queue.

Current versions of the Linux kernel round it up to the next highest power of two, with a minimum of 16. The revelant code is in reqsk_queue_alloc().