Question

I’m a little confused about the difference between the definitions of protocols on Linux when using socket(). I am attempting to listen for connections over TCP using socket(PF_INET, SOCK_STREAM, proto), where proto is (in my mind) disputed, or at least seems odd.

From <netinet/in.h>:

...
IPPROTO_IP = 0,    /* Dummy protocol for TCP.  */
...
IPPROTO_TCP = 6,       /* Transmission Control Protocol.  */
...

Agreed with by /etc/protocols:

ip      0       IP              # internet protocol, pseudo protocol number
hopopt  0       HOPOPT          # hop-by-hop options for ipv6
...
tcp     6       TCP             # transmission control protocol
...

I learned from an online tutorial, and also from the man page tcp(7) that you initialise a TCP socket using

tcp_socket = socket(AF_INET, SOCK_STREAM, 0);

which works absolutely fine, and certainly is a TCP socket. One thing about using the above arguments to initialise a socket is that the code

struct timeval timeout = {1, 0};
setsockopt(tcp_socket, 0, SO_RCVTIMEO, &timeout, sizeof(timeout); // 1s timeout
// Exactly the same for SO_SNDTIMEO here

works absolutely fine, but not after replacing all protocol arguments (including in socket()) with IPPROTO_TCP, as opposed to IPPROTO_IP which they have, as above.

So after experimenting with the difference, I’ve needed to ask a few searching questions:

1. Why, when I replace all protocol arguments with IPPROTO_TCP, do I get error 92 (“Protocol not available”) when setting timeouts, when protocol 0 is apparently just a ‘dummy’ TCP?
2. Why does socket() require the information of whether it should be a stream, datagram or raw socket when that information is (always?) implicitly known from the protocol, and vice versa? (i.e. TCP is a stream protocol, UDP is a datagram protocol, …)
3. What could be meant by “dummy TCP”?
4. What is hopopt, and why does it have the same protocol number as ‘ip’?

Many thanks.

Answer 1

Giving 0 as protocol to socket just means that you want to use the default protocol for the family/socktype pair. In this case that is TCP, and thus you get the same result as with IPPROTO_TCP.

Your error is in the setsockopt call. The correct one would be

setsockopt(tcp_socket, SOL_SOCKET, SO_RCVTIMEO, &timeout, sizeof(timeout)); // 1s timeout

the 0 there is not for protocol, but for option level. IPPROTO_TCP is another option level, but you can’t combine that with SO_RCVTIMEO. It can only be used together with SOL_SOCKET.
The ones you use with IPPROTO_TCP are the ones listed in tcp(7), e.g. TCP_NODELAY.