sockaddr.sin_port = 1337 doesn't match the "real" opened port

Question

I am trying to make a very simple server that accept a connection.

int sock, serv;

struct sockaddr_in in_sock;

serv = socket(AF_INET, SOCK_STREAM, 0);

in_sock.sin_addr.s_addr = 0;

in_sock.sin_port = 1337;

in_sock.sin_family = AF_INET;

bind(serv, (struct sockaddr *)&in_sock, sizeof(in_sock)); 

listen(serv, 0);

client = accept(serv, 0, 0);

However when trying to connect to 127.0.0.1:1337, I get a connection refused message :

(UNKNOWN) [127.0.0.1] 1337 (?) : Connection refused

However a simple

netstat -tcpan

shows me that a port is indeed opened :

tcp 0 0 0.0.0.0:14597 0.0.0.0:* LISTEN

If I set sin_port with much higher ports it seems to work properly though.

What am I missing here ? Why isn't the 1337 port opened ? It seems to be free too.

Answer 1

The port number field in struct sockaddr_in is stored in network byte order. This means that you must use htons() when storing a value to it:

in_sock.sin_port = htons(1337);

Otherwise, the port number will be left byte-swapped. Which is exactly what has happened here:

 1337 = 0x0539
14597 = 0x3905

sockaddr.sin_port = 1337 doesn't match the "real" opened port

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