(Copied here from a post by me to debian-devel.)
Executive summary: The getaddrinfo() returns different values
depending on the OS and on nsswitch.conf settings, making it
very difficult to use getaddrinfo() return values to deciding how
to handle an error.
Here are the results of further experiments with getaddrinfo().
I am using the attached x.c program. It tries to look up the
valid domain name 'www.google.com' and an invalid name
four times:
* once with empty /etc/resolv.conf (in which case the resolver tries
127.0.0.1:53)
* once with /etc/resolv.conf pointing to a working nameserver on my LAN
* once with empty /etc/resolv.conf (again)
* once with /etc/resolv.conf pointing to an IP address where there is no
working nameserver
OS is Debian 7.0.
================================
# ./a.out
Making resolv.conf empty
Results of looking up www.google.com: status = -2, errno = 2
Results of looking up a bogus name: status = -2, errno = 2
Writing nameserver option to resolv.conf
Results of looking up www.google.com: status = 0, errno = 101
Results of looking up a bogus name: status = -2, errno = 2
Making resolv.conf empty
Results of looking up www.google.com: status = -2, errno = 2
Results of looking up a bogus name: status = -2, errno = 2
Writing incorrect nameserver option to resolv.conf
Results of looking up www.google.com: status = -2, errno = 2
Results of looking up a bogus name: status = -2, errno = 2
================================
As I saw before, status is always -2 (EAI_NONAME). The manpage
doesn't say that errno is significant in that case. (It is significant
when status is -11.)
Helmut got different results. Is the difference between my machine
and Helmut's machine attributable to some diff in nsswitch.conf,
perhaps? I have:
hosts: files mdns4_minimal [NOTFOUND=return] dns mdns4
I tested next with
hosts: dns
and got different results.
================================
Making resolv.conf empty
Results of looking up www.google.com: status = -2, errno = 111
Results of looking up a bogus name: status = -2, errno = 111
Writing nameserver option to resolv.conf
Results of looking up www.google.com: status = 0, errno = 101
Results of looking up a bogus name: status = -2, errno = 101
Making resolv.conf empty
Results of looking up www.google.com: status = -2, errno = 111
Results of looking up a bogus name: status = -2, errno = 111
Writing incorrect nameserver option to resolv.conf
Results of looking up www.google.com: status = -2, errno = 111
Results of looking up a bogus name: status = -2, errno = 111
================================
Now errno for empty or incorrect resolv.conf is 111 (ECONNREFUSED ).
And with correct resolv.conf and bogus domain name errno is 101
(ENETUNREACH). That doesn't make too much sense but as I
said I don't think we are supposed to pay attention to errno if
status is -2.
Next I ran the program on Ubuntu 13.04 with "hosts: dns".
================================
Making resolv.conf empty
Results of looking up www.google.com: status = -11, errno = 111
Results of looking up a bogus name: status = -11, errno = 111
Writing nameserver option to resolv.conf
Results of looking up www.google.com: status = 0, errno = 101
Results of looking up a bogus name: status = -2, errno = 101
Making resolv.conf empty
Results of looking up www.google.com: status = -11, errno = 111
Results of looking up a bogus name: status = -11, errno = 111
Writing incorrect nameserver option to resolv.conf
Results of looking up www.google.com: status = -11, errno = 110
Results of looking up a bogus name: status = -11, errno = 110
================================
This is different in two ways. First the status is -11 (EAI_SYSTEM)
instead of -2 (EAI_NONAME) when no nameserver can be reached.
Second, there is now a difference between the empty-resolv.conf
case and the resolv.conf-with-bogus-address case. In the latter
case errno is 110 (ETIMEDOUT) instead of 111 (ECONNREFUSED).
This is better.
Debian 7.0 has libc6 version 2.13-37.
Ubuntu 13.04 has libc6 version 2.17-0ubuntu5.
What's behind all this? [google, google...] I see that in
November 2012 a change was made upstream
http://sourceware.org/git/?p=glibc.git;a=commitdiff;h=cfde9b463d63092ff0908d4c2748ace648e2ead8
in response to a complaint that the wrong status was returned
in the case of an internal error (out-of-fds).
http://sourceware.org/bugzilla/show_bug.cgi?id=14719
This is possibly the reason that I get status -11 instead of
-2 on Ubuntu.
But the result of the change was that getaddrinfo() returned
EAI_SYSTEM in too many cases.
http://sourceware.org/bugzilla/show_bug.cgi?id=15339
This has recently (May 2013) been fixed
http://sourceware.org/bugzilla/show_bug.cgi?id=15635
http://sourceware.org/git/?p=glibc.git;a=commitdiff;h=3d04f5db20c8f0d1ba3881b5f5373586a18cf188
such that (IIUC) now getaddrinfo() should return different
status/errno combinations for internal errors, network errors
and name resolution failures. I haven't tested the upstream
code yet.
I don't get the impression that the handling of return values
by the various eglibc layers has been well thought out and
documented; the developers seem to be making changes ad-hoc.
In any case, because of all these differences and changes we
won't have a good, stable getaddrinfo() interface to program
against for a while. In the meantime a program that needs to
distinguish between different causes for a name resolution
failure will have to do more than just check the status and
errno from getaddinfo().
** Bug watch added: Sourceware.org Bugzilla #14719
http://sourceware.org/bugzilla/show_bug.cgi?id=14719
** Bug watch added: Sourceware.org Bugzilla #15635
http://sourceware.org/bugzilla/show_bug.cgi?id=15635
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https://bugs.launchpad.net/bugs/1154599
Title:
getaddrinfo() returns -11 (EAI_SYSTEM) instead of -2
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