[Bug network/14308] New: getaddrinfo DNS referral response returns host not found when A and AAAA questions are sent and one response is a referral

["law at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=14308

             Bug #: 14308
           Summary: getaddrinfo DNS referral response returns host not
                    found when A and AAAA questions are sent and one
                    response is a referral
           Product: glibc
           Version: 2.15
            Status: NEW
          Severity: normal
          Priority: P2
         Component: network
        AssignedTo: unassigned@sourceware.org
        ReportedBy: law@redhat.com
    Classification: Unclassified


Created attachment 6494
  --> http://sourceware.org/bugzilla/attachment.cgi?id=6494
Wireshark Packet Capture

[ This is a cut-n-paste from a bug report in Red Hat's bugzilla database.
Refiling it here on Gino's behalf as this is really the right place. ]


I'm seeing a very edge-case issue with Linux hosts complaining about "host not
found" name resolution errors when using the getaddrinfo() API call as provided
by glibc. When getaddrinfo() is called on hosts with the IPv6 module loaded, it
sends a parallel query for A and AAAA records. If one of the responses is a
referral response and the other is not (e.g. A is not-referral but AAAA is
referral), the function will terminate and return value -2, "No address
associated with hostname" error.

Version-Release number of selected component (if applicable):

Any Linux host running glibc 2.9 or later
Affected distributions:
- Fedora 10-16, rawhide
- EL 6.0 and later

How reproducible:
100% under specific conditions:
- Linux running glibc 2.9 or later with IPv6 enabled
- Citrix Netscaler as a DNS server

Steps to Reproduce:
1. Install Fedora 16 (e.g. Live ISO)
2. Setup and install any DNS server (e.g. ISC Bind) and configure it to be an
authoritative name server for a zone (e.g. example.com)
3. Setup and install a Citrix Netscaler load-balancer (e.g. VPX appliance) and
configure a DNS proxy VIP
4. Enable DNS caching on the Citrix Netscaler load-balancer (set dns parameter
-cache YES)
5. Configure the Linux host to use the DNS VIP as its resolver
6. Disable nscd on the Linux host to bypass name caching (alternatively, use
nscd -i hosts in between calls)
7. Run a program that uses getaddrinfo() (e.g. curl, wget, telnet, etc) to
resolve a hostname

Actual results:
curl -v hostname.example.com
* getaddrinfo(3) failed for hostname.example.com:80
* Couldn't resolve host 'hostname.example.com
* Closing connection #0
curl: (6) Couldn't resolve host 'hostname.example.com'

Expected results:
Connection should be fine

Additional Info:
A buggy DNS server is exposing an edge-case bug in glibc

Related Bugs:
Citrix SR 60783234

[reply] [-]
Private
Comment 1 Gino LV. Ledesma 2012-05-22 21:14:19 EDT

Created attachment 586223 [details]
Wireshark Packet Capture

[reply] [-]
Private
Comment 2 Gino LV. Ledesma 2012-05-22 21:14:34 EDT

I took a packet capture of the DNS request-response scenario (dns.pcap) and
found the following

Case 1: Response is served fresh (not from cache)
Frame 01    Client:    A?    active-mrepo.me.com
Frame 02    Client:    AAAA?    active-mrepo.me.com
Frame 03    VIP:    A    1/1/1 active-mrepo.me.com. 17.172.194.16
Frame 04    VIP:    AAAA    0/1/0 hostmaster... 

Case 2: Response is served from cache
Frame 05    Client:    A?    active-mrepo.me.com
Frame 06    Client: AAAA?    active-mrepo.me.com
Frame 07    VIP:    A    1/0/0 active-mrepo.me.com. 17.172.194.16
Frame 08    VIP:    AAAA    0/0/0 hostmaster... 

# Try 2 (this is done automatically by glibc)
Frame 09    Client:    A?    active-mrepo.me.com
Frame 10    Client:    AAAA?    active-mrepo.me.com
Frame 11    VIP:    A    1/0/0 active-mrepo.me.com. 17.172.194.16
Frame 12    VIP:    AAAA    0/0/0 hostmaster... 

As best as I can tell, there are two things happening here:
1) This DNS server is serving a DNS response that falls under NODATA type 3
(RFC 2308 section 2.2)
2) glibc's (mis-?) interpretation of a referral response and short-circuiting
its logic

glibc interprets DNS responses as "referral" if the following conditions are
met (see glibc 2.15, resolv/res_send.c lines 1301-1303):
a) rcode == NOERROR
b) ancount == 0
c) aa == 0
d) ra == 0
e) arcount == 0

I see that this change was introduced in glibc 2.9 and is still present in
2.15.

In the above situation when two responses come in (A response = authoritative,
AAAA = referral), send_dg() immediately returns 0, causing __libc_res_nsend to
try the next nameserver and repeat the query, ignoring any valid responses that
may have come in.

--

Here is a debug call-trace of glibc with RES_DEBUG enabled:

looking up: active-mrepo
;; res_setoptions(" timeout:600 debug
", "conf")..
;;    debug
dots=0, statp->ndots=1, trailing_dot=0, name=active-mrepo
;; res_nquerydomain(active-mrepo, me.com, 1, 62321)
;; res_query(active-mrepo.me.com, 1, 62321)
;; res_nmkquery(QUERY, active-mrepo.me.com, IN, A)
;; res_nmkquery(QUERY, active-mrepo.me.com, IN, AAAA)
;; res_send()
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 49477
;; flags: rd; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0
;;    active-mrepo.me.com, type = A, class = IN
;; Querying server (# 1) address = 17.230.128.24
referred query:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 8813
;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0
;;    active-mrepo.me.com, type = AAAA, class = IN
;; got answer:
;; ns_initparse: Message too long
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 8813
;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0
;;    active-mrepo.me.com, type = AAAA, class = IN

The call trace won't show the A response because the AAAA referral response
returns 0 immediately.

Adjusting the next_ns: goto label to be moved above the if(buf2 != null) under
the SERVAIL/NOTIMP/REFUSED code checks seems like a work-around but is most
likely incorrect (due to the subsequent goto wait call).

--

On a side note, I can only make this happen with the Citrix Netscaler acting as
a DNS cache. Other caching resolvers (bind, dnsmasq, dnscache, pdns-recursor,
etc) do not expose this behavior in glibc because they will either have the
"aa" or "ra" bits set to 1. The Netscaler seems to be unique in serving the
following combination of flags for type=AAAA:

rcode == NOERROR
ancount == 0
ra == 0
arcount == 0
nscount=0

I've filed a bug with Citrix for this problem: Citrix SR 60783234

--


Possibly related bugs:
https://bugzilla.redhat.com/show_bug.cgi?id=459756
https://bugs.launchpad.net/ubuntu/+source/apt/+bug/326718

--


Work-arounds:
1. Disable Netscaler caching (set dns param -cache NO)
2. Switch service type from DNS/DNS_TCP to UDP/TCP
3. Make Netscaler authoritative for all zones that its DNS proxying for (add ns
soaRecord ...)
4. Disable IPv6 on client-side

Some notes / tidbits:
1. This behavior does NOT affect older glibc hosts (e.g. EL 5.x), presumably
because gethostbyname3_r does two separate calls for A/AAAA
2. The order of the response (whether A or AAAA comes first) doesn't seem to
matter


--




I just installed Fedora 17 i386 on a VM to test this. Problem still persists
there (glibc-2.15-37.fc17.i686).

I've also confirmed that the problem exists with upstream, stock glibc 2.15.

-- 
Configure bugmail: http://sourceware.org/bugzilla/userprefs.cgi?tab=email
------- You are receiving this mail because: -------
You are on the CC list for the bug.