Inconsistent behaviour of getaddrinfo() with AI_ADDRCONFIG

[Joachim Haga]

If the AI_ADDRCONFIG flag is passed to getaddrinfo() with AF_INET or AF_INET6 address families, addresses in /etc/hosts are not resolved properly.

The following test case illustrates the problem. Its output depends on the configured networks, but if only "lo" is configured, the output is:
  Requested family: 2 [AF_INET]
  Error -2: Name or service not known
  Requested family: 10 [AF_INET6]
  Error -2: Name or service not known
  Requested family: 0 [AF_UNSPEC]
  Returned family: 10 [AF_INET6]
  ...
  Returned family: 2 [AF_INET]
  ...

This is inconsistent and wrong (explicit requests for INET or INET6 should both succeed, given the contents of /etc/hosts). /etc/hosts includes the lines:
  127.0.0.1 localhost
  ::1 localhost ip6-localhost ip6-loopback

I see a failure from this behaviour when mounting an NFS share from localhost when the network is down. It might be argued that AI_ADDRCONFIG is meaningless with explicit address families, in which case this is an NFS bug? But the current behaviour is surprising.

--------------------------------------
cat <<EOF >gai.c
#include <netdb.h>
#include <string.h>
#include <stdio.h>

void query(int family)
{
  struct addrinfo *ai;
  struct addrinfo hint = {
        .ai_family = family,
        .ai_flags = AI_ADDRCONFIG,
        };

  int ret = getaddrinfo("localhost", NULL, &hint, &ai);
  printf("Requested family: %d\n", hint.ai_family);
  if (ret == 0)
    while (ai) {
      printf("Returned family: %d\n", ai->ai_family);
      ai = ai->ai_next;
    }
  else
    printf("Error %d: %s\n", ret, gai_strerror(ret));
}

int
main()
{
  query(AF_INET);
  query(AF_INET6);
  query(AF_UNSPEC);
}
EOF
gcc -o gai gai.c
./gai

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Pavel Šimerda (pavlix-a)]

glibc bug report:

http://sourceware.org/bugzilla/show_bug.cgi?id=14966

Fedora bug report:

https://bugzilla.redhat.com/show_bug.cgi?id=887577

More information on the topic:

https://fedoraproject.org/wiki/Networking/NameResolution/ADDRCONFIG