glibc: getaddrinfo only returns ipv4 address for localhost even if ipv6 is enabled

[nhooyr]

Issue description

When localhost is mapped to both 127.0.0.1 and ::1 in /etc/hosts, getaddrinfo(3) still only returns 127.0.0.1. It should be returning both with ipv6 first.

Steps to reproduce

Run the following program as ./a.out localhost

#include <arpa/inet.h>
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char* argv[])
{
    if (argc < 2)
    exit(1);

    struct addrinfo* hints = calloc(1, sizeof(struct addrinfo));
    hints->ai_socktype = SOCK_STREAM;

    struct addrinfo* res;
    int ecode = getaddrinfo(argv[1], NULL, hints, &res) != 0;
    if (ecode != 0) {
    printf("error: %s\n", gai_strerror(ecode));
    return ecode;
    }

    while (res) {
    void* ptr;
    switch (res->ai_family) {
    case AF_INET:
        ptr = &((struct sockaddr_in*)res->ai_addr)->sin_addr;
        break;
    case AF_INET6:
        ptr = &((struct sockaddr_in6*)res->ai_addr)->sin6_addr;
        break;
    }
    char addr[INET6_ADDRSTRLEN];
    inet_ntop(res->ai_family, ptr, addr, sizeof(addr));
    printf("IPv%d address: %s\n", res->ai_family == PF_INET6 ? 6 : 4,
        addr);
    res = res->ai_next;
    }

    return 0;
}

On gentoo, OSX and arch the program returns something like:

IPv6 address: ::1
IPv4 address: 127.0.0.1

Whereas on nixos, it returns:

IPv4 address: 127.0.0.1

Technical details

• System: (NixOS: nixos-version, Ubuntu/Fedora: lsb_release -a, ...) 17.03.git.792d277 (Gorilla)
• Nix version: (run nix-env --version) nix-env (Nix) 1.11.4
• Nixpkgs version: (run nix-instantiate --eval '<nixpkgs>' -A lib.nixpkgsVersion) 17.03.git.792d277

[Mic92]

There is also no /etc/gai.conf in nixos.

[vcunat]

On Ubuntu 14.04.4 LTS, compiling your code with nix's gcc:

$ ./tests/localhost localhost
IPv6 address: ::1
IPv4 address: 127.0.0.1

The machine has those entries in /etc/hosts file.
(I compiled with gcc from the latest unstable channel, as your nixpkgs version seems to be unreachable from official branches.)

[abbradar]

FWIW neither adding /etc/gai.conf from Arch or stopping nscd (a shot in the dark) fixed this.

[bennofs]

The same issue can be demonstrated with getent ahosts localhost. Note that it works in a docker archlinux container on NixOS (using --net host), which means that the network stack itself is not the problem since the container shares the network stack with the host.

[bennofs]

Found the issue: The difference is in the file /etc/host.conf. On archlinux, this file contains the following line:

multi on

NixOS doesn't use a non-default /etc/host.conf, so multi is set to its default value, which is off.

From man host.conf:

multi  Valid values are on and off.  If set to on, the resolver library will return all valid addresses for a
       host that appears in the /etc/hosts file, instead of only the first.  This is off by  default,  as  it
       may cause a substantial performance loss at sites with large hosts files.

[nhooyr]

@ben0x539 Should be a easy fix then right?

[cstrahan]

Following other popular distros is probably the way to go, perhaps with a config option to opt out.