IPPROTO_IP vs IPPROTO_TCP/IPPROTO_UDP
ip, sockets
Solution
Documentation for `socket()` on Linux is split between various manpages including `ip(7)` that specifies that you have to use `0` or `IPPROTO_UDP` for UDP and `0` or `IPPROTO_TCP` for TCP. When you use `0`, which happens to be the value of `IPPROTO_IP`, UDP is used for `SOCK_DGRAM` and TCP is used for `SOCK_STREAM`.
In my opinion the clean way to create a UDP or a TCP IPv4 socket object is as follows:
int sock_udp = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
int sock_tcp = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
The reason is that it is generally better to be explicit than implicit. In this specific case using `0` or worse `IPPROTO_IP` for the third argument doesn't gain you anything.
Also imagine using a protocol that can do both streams and datagrams like sctp. By always specifying both socktype and protocol you are safe from any ambiguity.
Problem
I'm having some trouble finding documentation on what the distinction between these settings for the third argument to `socket` is. I know about TCP and UDP and their differences and also that IP is one layer up (down?) on the stack... My UDP code seems to work the same whether I set it to `IPPROTO_IP` or `IPPROTO_UDP`.