[Bug network/14307] New: getaddrinfo() triggers duplicate DNS requests when answer contains CNAME

[Bug network/14307] New: getaddrinfo() triggers duplicate DNS requests when answer contains CNAME

[jvb127 at gmail dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=14307

             Bug #: 14307
           Summary: getaddrinfo() triggers duplicate DNS requests when
                    answer contains CNAME
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: network
        AssignedTo: unassigned@sourceware.org
        ReportedBy: jvb127@gmail.com
    Classification: Unclassified


Created attachment 6493
  --> http://sourceware.org/bugzilla/attachment.cgi?id=6493
Packet capture illustrating duplicate (redundant) DNS requests

I noticed spurious DNS requests on the network, and traced them down to
getaddrinfo() calls (e.g. using wget). This occurs on a Fedora 16 machine with
latest glibc, as well as a CentOS machine with older versions

To reproduce, perform the following in 2 consoles:

$tcpdump -v -n -i eth0 udp port 53 -w getaddr.pcap

$wget -4 http://www.gmail.com/

Result:
reading from file ./getaddr.pcap, link-type EN10MB (Ethernet)
17:59:38.263471 IP 192.168.1.9.54553 > 8.8.8.8.domain: 32628+ A? www.gmail.com.
(31)
17:59:38.313021 IP 8.8.8.8.domain > 192.168.1.9.54553: 32628 4/0/0 CNAME
mail.google.com., CNAME googlemail.l.google.com., A 74.125.226.22, A
74.125.226.21 (116)
17:59:38.313311 IP 192.168.1.9.57920 > 8.8.8.8.domain: 44145+ A? www.gmail.com.
(31)
17:59:38.367738 IP 8.8.8.8.domain > 192.168.1.9.57920: 44145 4/0/0 CNAME
mail.google.com., CNAME googlemail.l.google.com., A 74.125.226.22, A
74.125.226.21 (116)
17:59:38.476300 IP 192.168.1.9.50993 > 8.8.8.8.domain: 25319+ A?
mail.google.com. (33)
17:59:38.561056 IP 8.8.8.8.domain > 192.168.1.9.50993: 25319 3/0/0 CNAME
googlemail.l.google.com., A 74.125.226.21, A 74.125.226.22 (92)
17:59:38.561306 IP 192.168.1.9.54508 > 8.8.8.8.domain: 35020+ A?
mail.google.com. (33)
17:59:38.610485 IP 8.8.8.8.domain > 192.168.1.9.54508: 35020 3/0/0 CNAME
googlemail.l.google.com., A 74.125.226.21, A 74.125.226.22 (92)
17:59:38.716233 IP 192.168.1.9.59783 > 8.8.8.8.domain: 48556+ A?
accounts.google.com. (37)
17:59:38.765100 IP 8.8.8.8.domain > 192.168.1.9.59783: 48556 2/0/0 CNAME
accounts.l.google.com., A 209.85.225.84 (78)
17:59:38.765348 IP 192.168.1.9.41685 > 8.8.8.8.domain: 24558+ A?
accounts.google.com. (37)
17:59:38.819577 IP 8.8.8.8.domain > 192.168.1.9.41685: 24558 2/0/0 CNAME
accounts.l.google.com., A 209.85.225.84 (78)

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[Bug network/14307] getaddrinfo() triggers duplicate DNS requests when answer contains CNAME

[jvb127 at gmail dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=14307

--- Comment #1 from Jeroen van Bemmel <jvb127 at gmail dot com> 2012-06-29 06:04:59 UTC ---
Created attachment 6495
  --> http://sourceware.org/bugzilla/attachment.cgi?id=6495
Test program that triggers the bug (when compiled in x86_64 mode)

The attached test program triggers the bug when compiled as a 64-bit executable

Use: gcc -O -g dnstest.c

When compiled as 32-bit (-m32), it works as expected

When setting hints.ai_flags = AI_CANONNAME; both an A and an AAAA query are
sent, even though hints.ai_family = AF_INET;

It looks like a struct member alignment problem, causing hints to be
misinterpreted

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[Bug network/14307] getaddrinfo() sends superfluous DNS requests in x86_64 programs when answer contains CNAME

[jvb127 at gmail dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=14307

Jeroen van Bemmel <jvb127 at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Target|                            |x86_64
            Version|unspecified                 |2.14
            Summary|getaddrinfo() triggers      |getaddrinfo() sends
                   |duplicate DNS requests when |superfluous DNS requests in
                   |answer contains CNAME       |x86_64 programs when answer
                   |                            |contains CNAME

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[Bug network/14307] getaddrinfo() sends superfluous DNS requests in x86_64 programs when answer contains CNAME

[siddhesh at redhat dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=14307

Siddhesh Poyarekar <siddhesh at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |siddhesh at redhat dot com

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[Bug network/14307] getaddrinfo() sends superfluous DNS requests in x86_64 programs when answer contains CNAME

[siddhesh at redhat dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=14307

Siddhesh Poyarekar <siddhesh at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at sourceware    |siddhesh at redhat dot com
                   |dot org                     |

--- Comment #2 from Siddhesh Poyarekar <siddhesh at redhat dot com> 2012-06-29 12:30:36 UTC ---
Reproducible on HEAD.

The size of the response is what seems to be causing the second query -- 6 or
more seems to be the trigger. My initial guess was that the first lookup led to
an ERANGE internally, resulting in the second query, but that should have
happened for 32-bit as well. So this might well be something else. This has
nothing to do with whether there is a CNAME in the response.

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[Bug network/14307] getaddrinfo() sends superfluous DNS requests in x86_64 programs for certain hostnames

[jvb127 at gmail dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=14307

Jeroen van Bemmel <jvb127 at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|getaddrinfo() sends         |getaddrinfo() sends
                   |superfluous DNS requests in |superfluous DNS requests in
                   |x86_64 programs when answer |x86_64 programs for certain
                   |contains CNAME              |hostnames

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[Bug network/14307] getaddrinfo() sends superfluous DNS requests in x86_64 programs for certain hostnames

[jvb127 at gmail dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=14307

--- Comment #3 from Jeroen van Bemmel <jvb127 at gmail dot com> 2012-06-30 02:23:01 UTC ---
(In reply to comment #2)
> Reproducible on HEAD.
> 
> The size of the response is what seems to be causing the second query -- 6 or
> more seems to be the trigger. My initial guess was that the first lookup led to
> an ERANGE internally, resulting in the second query, but that should have
> happened for 32-bit as well. So this might well be something else. This has
> nothing to do with whether there is a CNAME in the response.

You are correct that __gethostbyname2_r in sysdeps/posix/getaddrinfo.c returns
ERANGE the first time when using a 512-byte buffer, in 64-bit mode. It is then
retried with a 1024-byte buffer, and the call succeeds

However, the same call in 32-bit mode also uses a 512-byte buffer, and does not
return ERANGE.

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[Bug network/14307] getaddrinfo() sends superfluous DNS requests in x86_64 programs for certain hostnames

[jvb127 at gmail dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=14307

--- Comment #4 from Jeroen van Bemmel <jvb127 at gmail dot com> 2012-06-30 03:16:07 UTC ---
in function getanswer_r (resolv/nss_dns/dns-host.c) the temporary buffer is
used for the following struct:

struct host_data
  {
    char *aliases[MAX_NR_ALIASES];
    unsigned char host_addr[16];        /* IPv4 or IPv6 */
    char *h_addr_ptrs[0];
  } *host_data

sizeof( struct host_data ) == 400 for x86_64, and 208 for x86 32-bit.

One could argue that the code works as designed, but I believe it is desirable
if glibc exhibits the same externally observable behaviour for both x86 and
x86_64 versions.

512 - 400 leaves little room for common DNS query responses like
"www.google.com".

One could simply allocate a larger temporary buffer for x86_64 ( e.g. 512 *
sizeof(void*) / 4 ), but perhaps a better fix is to not use the temporary
buffer for the host_data struct at all (and e.g. use alloca instead, or simply
place it on the stack - although the code does explicitly align it in case the
buffer is unaligned, which it isn't btw)

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[Bug network/14307] getaddrinfo() sends superfluous DNS requests in x86_64 programs for certain hostnames

[jvb127 at gmail dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=14307

--- Comment #5 from Jeroen van Bemmel <jvb127 at gmail dot com> 2012-06-30 05:35:43 UTC ---
Created attachment 6498
  --> http://sourceware.org/bugzilla/attachment.cgi?id=6498
Proposed patch for /sysdeps/posix/getaddrinfo.c

The current code initially allocates a buffer of 512 bytes for the query
response, and repeatedly increases the buffer size (*2) until the call
succeeds.

Out of this 512 bytes, sizeof(host_name_struct) is used for alias pointers. The
size of this struct is 208 bytes on x86 and 400 bytes on x86_64, creating a
difference in network behaviour for these targets. Furthermore, 512-400 leaves
only 112 bytes for the answer, which is too small for common DNS lookups such
as "www.google.com"

The 512 value was probably based on the maximum response size limit imposed by
DNS. This patch adds the size of the host_name_struct, such that 512 bytes
remain for the DNS response, for both the x86 and x86_64 platforms.

Note that there should be a cleaner way for linking the size of the struct to
the code in resolv/nss_dns/dns-host.c; the constant of 48 for MAX_NR_ALIASES
might be changed in the future.

Also, I noticed that there is no limit imposed on the size of the temp buffer.
Not sure what the effect would be of receiving a UDP jumbogram over IPv6 in
response to a DNS query?

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[Bug network/14307] getaddrinfo() sends superfluous DNS requests in x86_64 programs for certain hostnames

[jvb127 at gmail dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=14307

Jeroen van Bemmel <jvb127 at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |kjp at ci dot uchicago.edu

--- Comment #6 from Jeroen van Bemmel <jvb127 at gmail dot com> 2012-06-30 15:08:51 UTC ---
*** Bug 13904 has been marked as a duplicate of this bug. ***

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[Bug network/14307] getaddrinfo() sends superfluous DNS requests in x86_64 programs for certain hostnames

[siddhesh at redhat dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=14307

--- Comment #7 from Siddhesh Poyarekar <siddhesh at redhat dot com> 2012-07-02 02:21:46 UTC ---
Here's a review of the patch. Please post the patch with modifications to the
libc-alpha mailing list for further review and approval and use the following
wiki link as a guideline for the submission:

http://sourceware.org/glibc/wiki/Contribution%20checklist


-          size_t tmpbuflen = 512;
+          // MAX_NR_ALIASES=48 in resolv/nss_dns/dns-host.c 
+          size_t host_data_size = 48 * sizeof(char*) + 16;

Put MAX_NR_ALIASES one of the headers and use it here.

+              size_t tmpbuflen = 512;
           assert (tmpbuf == NULL);
-          tmpbuf = alloca_account (tmpbuflen, alloca_used);
+          tmpbuf = alloca_account (tmpbuflen+host_data_size, alloca_used);

Adjust the tmpbuflen size instead of adding it separately here.

           int rc;
           struct hostent th;
           struct hostent *h;
@@ -579,19 +581,19 @@ gaih_inet (const char *name, const struct gaih_service
*service,
           while (1)
         {
           rc = __gethostbyname2_r (name, AF_INET, &th, tmpbuf,
-                       tmpbuflen, &h, &herrno);
+                       tmpbuflen+host_data_size, &h, &herrno);

Likewise.

           if (rc != ERANGE || herrno != NETDB_INTERNAL)
             break;

           if (!malloc_tmpbuf
-              && __libc_use_alloca (alloca_used + 2 * tmpbuflen))
+              && __libc_use_alloca (alloca_used + 2 * tmpbuflen +
host_data_size))

Here too.

             tmpbuf = extend_alloca_account (tmpbuf, tmpbuflen,
-                            2 * tmpbuflen,
+                            2 * tmpbuflen+host_data_size,

And here.

                             alloca_used);
           else
             {
               char *newp = realloc (malloc_tmpbuf ? tmpbuf : NULL,
-                        2 * tmpbuflen);
+                        2 * tmpbuflen+host_data_size);

Here as well.

               if (newp == NULL)
             {
               result = -EAI_MEMORY;


> Also, I noticed that there is no limit imposed on the size of the temp buffer.
> Not sure what the effect would be of receiving a UDP jumbogram over IPv6 in
> response to a DNS query?

The current implementation is broken for it, since it will keep blowing the
buffer up exponentially till it can fit the entire jumbogram into the buffer
and hope that the underlying memory supports it. This needs to be fixed in
future.

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[Bug network/14307] getaddrinfo() sends superfluous DNS requests in x86_64 programs for certain hostnames

[siddhesh at redhat dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=14307

Siddhesh Poyarekar <siddhesh at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED

--- Comment #8 from Siddhesh Poyarekar <siddhesh at redhat dot com> 2012-07-12 06:05:34 UTC ---
Fixed in master:

http://sourceware.org/git/?p=glibc.git;a=commitdiff;h=7b6e99be77c24a79cb07416d81796b45176923c6

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[Bug network/14307] getaddrinfo() sends superfluous DNS requests in x86_64 programs for certain hostnames

[jackie.rosen at hushmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=14307

Jackie Rosen <jackie.rosen at hushmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jackie.rosen at hushmail dot com

--- Comment #9 from Jackie Rosen <jackie.rosen at hushmail dot com> ---
*** Bug 260998 has been marked as a duplicate of this bug. ***
Seen from the domain http://volichat.com
Page where seen: http://volichat.com/adult-chat-rooms
Marked for reference. Resolved as fixed @bugzilla.

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[Bug network/14307] getaddrinfo() sends superfluous DNS requests in x86_64 programs for certain hostnames

[schwab at sourceware dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=14307

Andreas Schwab <schwab at sourceware dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|jackie.rosen at hushmail dot com   |

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[Bug network/14307] getaddrinfo() sends superfluous DNS requests in x86_64 programs for certain hostnames

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=14307

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
              Flags|                            |security-

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