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POUsage of getaddrinfo() with AI_PASSIVE
 

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    <p>The <code>getaddrinfo()</code> 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 servers to bind to the correct socket - in theory.</p> <p>I just <a href="https://stackoverflow.com/questions/8111044/c-sockets-recv-syscall-returning-1/8111252#comment9950077_8111252">learned about that</a> and started to play around with it via Python:</p> <pre><code>from socket import * for i in getaddrinfo(None, 22, AF_UNSPEC, SOCK_STREAM, IPPROTO_IP, AI_PASSIVE): i </code></pre> <p>yields</p> <pre><code>(2, 1, 6, '', ('0.0.0.0', 22)) (10, 1, 6, '', ('::', 22, 0, 0)) </code></pre> <p>what makes me wonder about if there is something wrong.</p> <p>What exactly am I supposed to do with these answers? Should I</p> <ul> <li>make a <code>listen()</code>ing socket of all of these answers, or should I</li> <li>just pick the first one which really works? </li> </ul> <p>The example in the <a href="http://linux.die.net/man/3/getaddrinfo" rel="nofollow noreferrer">manpage</a> suggests me to only take the first one and be happy with it if it is error-free, but then I only get a connection via IPv4 n my example.</p> <p>But if I try all of them, I have to worry with 2 server sockets, which is unnecessary due to the fact that IPv6 server sockets also listen to IPv4 if certain conditions are met (OS, socket flags etc.).</p> <p>Where am I thinking wrong?</p> <hr> <p>EDIT: Obviously, I'm not thinking wrong, but my PC does the wrong thing. I use the default <code>/etc/gai.conf</code> shipped with OpenSUSE. It would be nice if anyone could point me towards the right direction.</p> <p>EDIT 2: In the given case, <code>strace</code> gives the following calls made internally after reading <code>/etc/gai.conf</code> (now with port 54321, as I thought that using port 22 might have some bad influence, which was not the case):</p> <pre><code>socket(PF_INET6, SOCK_DGRAM, IPPROTO_IP) = 3 connect(3, {sa_family=AF_INET6, sin6_port=htons(54321), inet_pton(AF_INET6, "::", &amp;sin6_addr), sin6_flowinfo=0, sin6_scope_id=0}, 28) = 0 getsockname(3, {sa_family=AF_INET6, sin6_port=htons(38289), inet_pton(AF_INET6, "::1", &amp;sin6_addr), sin6_flowinfo=0, sin6_scope_id=0}, [28]) = 0 connect(3, {sa_family=AF_UNSPEC, sa_data="\0\0\0\0\0\0\0\0\0\0\0\0\0\0"}, 16) = 0 connect(3, {sa_family=AF_INET, sin_port=htons(54321), sin_addr=inet_addr("0.0.0.0")}, 16) = 0 getsockname(3, {sa_family=AF_INET6, sin6_port=htons(60866), inet_pton(AF_INET6, "::ffff:127.0.0.1", &amp;sin6_addr), sin6_flowinfo=0, sin6_scope_id=0}, [28]) = 0 close(3) = 0 </code></pre> <p>Obviously, the decision is intended to take place according to the results of the <code>getsockname()</code> calls...</p> <p>BTW: <a href="https://bugs.launchpad.net/ubuntu/+source/eglibc/+bug/673708" rel="nofollow noreferrer">https://bugs.launchpad.net/ubuntu/+source/eglibc/+bug/673708</a> and the other bug reports mentionned there confirm my observations. Several people there claim that the new behaviour is correct, so I'm obviously stuck to using <code>AF_INET6</code>... :-(</p>
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    1. COThe easiest thing is just to pass INADDR_ANY as the IP address in your call to bind(), and then you'll have a socket that is listening on all interfaces and you don't have to worry about getaddrinfo() or multiple listening sockets.
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      1. USJeremy Friesner
    2. COBut with `INADDR_ANY` I am limited to IPv4. I would like to cover IPv6 and IPv4.
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    3. COThat's incorrect.... for IPv6 there is the constant in6addr_any that works the same. (Really both constants are just more readable names for zero; specifying a zero address to bind to gives you wildcarding behavior) Example code here: http://publib.boulder.ibm.com/infocenter/iseries/v5r3/index.jsp?topic=%2Frzab6%2Frzab6xacceptboth.htm
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      1. POUsage of getaddrinfo() with AI_PASSIVE
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