Using getaddrinfo to query a host, I get a number of results:
struct addrinfo hints;
hints.ai_flags = 0
hints.ai_family = AF_INET;
hints.ai_socktype = SOCK_STREAM;
hints.ai_protocol = 0
hints.ai_addrlen = 0;
hints.ai_addr = NULL;
hints.ai_canonname= NULL;
hints.ai_next = NULL;
struct addrinfo *res = NULL;
getaddrinfo("google.com", "http", &hints, &res);
res gets filled in with a list of 11 possible addresses I could use to connect to google.com with the given parameters:
1 family=2, addr.port=80, addr.address=74.125.226.98, protocol=6, socktype=1, flags=0
2 family=2, addr.port=80, addr.address=74.125.226.104, protocol=6, socktype=1, flags=0
3 family=2, addr.port=80, addr.address=74.125.226.100, protocol=6, socktype=1, flags=0
4 family=2, addr.port=80, addr.address=74.125.226.99, protocol=6, socktype=1, flags=0
5 family=2, addr.port=80, addr.address=74.125.226.96, protocol=6, socktype=1, flags=0
6 family=2, addr.port=80, addr.address=74.125.226.102, protocol=6, socktype=1, flags=0
7 family=2, addr.port=80, addr.address=74.125.226.110, protocol=6, socktype=1, flags=0
8 family=2, addr.port=80, addr.address=74.125.226.97, protocol=6, socktype=1, flags=0
9 family=2, addr.port=80, addr.address=74.125.226.105, protocol=6, socktype=1, flags=0
10 family=2, addr.port=80, addr.address=74.125.226.103, protocol=6, socktype=1, flags=0
11 family=2, addr.port=80, addr.address=74.125.226.101, protocol=6, socktype=1, flags=0
My question is, which do I use? Should I just always take the first one, or pick one at random? Or should I try each one in order until connect() succeeds?
There is not an officially defined order, but unless you have a good reason not to: try them in order, skipping any that don't match your requirements (eg, wrong protocol). Usually DNS servers will round-robin or somehow permute the results. (You can probably see that if your run your test multiple times in a row or running nslookup or dig a few times on google.com.)