binding a port number to a TCP socket (outgoin) to send packets

Question

I know that it is not easy to bind a port number to TCP socket that you would use to send data (because systems usually bind a random port to sockets).

But I read on one article that by using some low level networking methods you can bind a port number to a TCP socket then use it to send data ?

Does anyone has an idea about how that could be done ?

I am using c programming language

similar: http://stackoverflow.com/questions/204169/how-to-create-a-tcp-socket-connect-using-c-to-a-predefined-port — , Jun 24 '11 at 12:59
The reasons for wanting to do this at all are usually spurious. — user207421, Jun 25 '11 at 00:21
I want to do a port prediction, (because my NAT box don't use port preservation) — Ayoub M., Jun 29 '11 at 20:14

score 11 · Accepted Answer · answered Jun 24 '11 at 13:00

11

Bind it before connecting.

s = socket(AF_INET, SOCK_STREAM, 0);
/* ... */

memset(&client_addr, 0, sizeof(client_addr));
client_addr.sin_family = AF_INET;
client_addr.sin_port = htons(22222);

if (bind(s, (struct sockaddr *) &client_addr, sizeof(client_addr)) < 0) {
    perror("bind");
    exit(1);
}

connect(s, (struct sockaddr *) &server_addr, sizeof(server_addr));

answered Jun 24 '11 at 13:00

cnicutar

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then, will I able to listen through same port(in this case 22222) also? I think NO. but please confirm. – Harshit Gupta Apr 26 '14 at 05:04

score 6 · Answer 2 · answered Jun 24 '11 at 13:00

6

Actually it is quite easy. Simply use the bind function as you would for a server.

answered Jun 24 '11 at 13:00

Dark Falcon

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then, will I able to listen through same port also? I think NO. but please confirm. – Harshit Gupta Apr 26 '14 at 05:06

score 6 · Answer 3 · answered Jun 24 '11 at 13:01

6

You can simply call bind() before connect() in a client in the same way that you would call bind() before listen() in a server. There is nothing more complicated to it than that.

answered Jun 24 '11 at 13:01

caf

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then, will I able to listen through same port(in this case 22222) also? I think NO. but please confirm. – Harshit Gupta Apr 26 '14 at 05:05

binding a port number to a TCP socket (outgoin) to send packets

3 Answers3

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