Usage of getaddrinfo() with AI_PASSIVE

Question

The getaddrinfo() function not only allows for client programs to efficiently find the correct data for creating a socket to a given host, it also allows for se

Answer 1

Your getaddrinfo is returning the wrong result for some reason. It's supposed to return the IPv6 socket first. The only thing I can think of is if your OS detects that your system has a low prio IPv6 (6to4 or Teredo) and avoids them, IMO wrongly so in that case. Edit: Just noticed my own computer does the same thing, I use 6to4.

However, you can either listen to both of them, or use AF_INET6 instead of AF_UNSPEC. Then you can do setsockopt to disable IPV6_V6ONLY.

getaddrinfo does the reasonable thing here and returns all applicable results (though in the wrong order, as I mentioned). Both one and two listen sockets are valid approaches, depending on your application.

Answer 2

JFTR: It seems now that the program given in the manpage is wrong.

There are two possible approaches for listening to both IP types:

1. Create only a IPv6 socket and switch off the v6 only flag:

   from socket import *
   s = socket(AF_INET6, SOCK_STREAM)
   s.setsockopt(IPPROTO_IPV6, IPV6_V6ONLY, 0)
   s.bind(...)
   

   resp.

   from socket import *
   ai = getaddrinfo(None, ..., AF_INET6, SOCK_STREAM, 0, AI_PASSIVE)[0]
   s = socket(ai[0], ai[1], ai[2])
   s.setsockopt(IPPROTO_IPV6, IPV6_V6ONLY, 0)
   s.bind(ai[4])
   

   Pros:

   • simpler to handle

   Cons:

   • doesn't work under XP (AFAIK) - there are two different protocol stacks

2. work with two sockets and switch on the v6only flag:

   from socket import *
   aii = getaddrinfo(None, ..., AF_UNSPEC, SOCK_STREAM, 0, AI_PASSIVE)
   sl = []
   for ai in aii:
       s = socket(ai[0], ai[1], ai[2])
       if ai[0] == AF_INET6: s.setsockopt(IPPROTO_IPV6, IPV6_V6ONLY, 1)
       s.bind(ai[4])
       sl.append(s)
   

   and handle all sockets in sl in accepting loop (use select() or nonblocking IO to do so)

   Pros:

   • uses a (nearly) protocol independent handling with getaddrinfo()
   • works under XP as well

   Cons:

   • complicated to handle