CentOS - Sep 2011 - Slow DNS lookups on CentOS6

Most of my desktops are still running CentOS5, but I have installed CentOS6 on a
few of them.  The users on those desktops are reporting that DNS lookups are
slow, and from my brief tests, that does appear to be the case.  After some
googling, I found a suggestion to disable IPv6, but that didn't help.  So I
tried to figure out where the delay occurs using strace and ltrace, but that
didn't help much.  When running ltrace, the process I'm trying to trace
(host, nslookup) just dies.  And the output of strace does show a 3+ second
delay, but I can't figure out what it's telling me:

10:46:25.317517 futex(0xd1ba94, FUTEX_WAKE_PRIVATE, 2147483647) = 0
10:46:25.317586 open("/usr/lib/locale/locale-archive",
O_RDONLY|O_LARGEFILE) = 6
10:46:25.317635 fstat64(6, {st_mode=S_IFREG|0644, st_size=99158720, ...}) = 0
10:46:25.317702 mmap2(NULL, 2097152, PROT_READ, MAP_PRIVATE, 6, 0) = 0xb569d000
10:46:25.317754 mmap2(NULL, 1171456, PROT_READ, MAP_PRIVATE, 6, 0x19c) =
0xb557f000
10:46:25.317792 mmap2(NULL, 4096, PROT_READ, MAP_PRIVATE, 6, 0x1084) =
0xb557e000
10:46:25.317827 close(6)                = 0
10:46:25.317904 futex(0xb76a5050, FUTEX_WAKE_OP_PRIVATE, 1, 1, 0xb76a504c,
{FUTEX_OP_SET, 0, FUTEX_OP_CMP_GT,
10:46:25.317990 futex(0xb76a5018, FUTEX_WAKE_PRIVATE, 1) = 1
10:46:25.318037 rt_sigaction(SIGHUP, {0x141390, ~[RTMIN RT_1], 0}, NULL, 8) = 0
10:46:25.318145 rt_sigsuspend([])       = ? ERESTARTNOHAND (To be restarted)
10:46:28.510794 --- SIGTERM (Terminated) @ 0 (0) ---
10:46:28.510842 sigreturn()             = ? (mask now [HUP INT TERM])
10:46:28.510926 futex(0xb76a5050, FUTEX_WAKE_OP_PRIVATE, 1, 1, 0xb76a504c,
{FUTEX_OP_SET, 0, FUTEX_OP_CMP_GT,
10:46:28.511015 futex(0xb76a5018, FUTEX_WAKE_PRIVATE, 1) = 1
10:46:28.511070 futex(0xb76a5050, FUTEX_CMP_REQUEUE_PRIVATE, 1, 2147483647,
0xb76a5018, 18) = 1
10:46:28.511144 futex(0xb76a5018, FUTEX_WAKE_PRIVATE, 1) = 1
10:46:28.511256 futex(0xb769fbd8, FUTEX_WAIT, 2371, NULL) = -1 EAGAIN (Resource
temporarily unavailable)
10:46:28.511316 brk(0x920a000)          = 0x920a000

The CentOS5 systems on the same network with the same /etc/resolv.conf file do
not have these delays.  Has anyone else seen this or have some suggestions as to
how to debug this?

Alfred

On Tue, 27 Sep 2011, Alfred von Campe wrote:
> Most of my desktops are still running CentOS5, but I have installed CentOS6
> on a few of them.  The users on those desktops are reporting that DNS
> lookups are slow, and from my brief tests, that does appear to be the case.
> After some googling, I found a suggestion to disable IPv6, but that
didn't
> help.  So I tried to figure out where the delay occurs using strace and
> ltrace, but that didn't help much.  When running ltrace, the process
I'm
> trying to trace (host, nslookup) just dies.  And the output of strace does
> show a 3+ second delay, but I can't figure out what it's telling
me:
>
> The CentOS5 systems on the same network with the same /etc/resolv.conf file
> do not have these delays.  Has anyone else seen this or have some
> suggestions as to how to debug this?
You probably want to do an strace -f host blah rather than a basic strace, or
I think you'll lose what's going on.

Are you running either sssd or nscd on your box?

I've done nothing special, and DNS seems fast enough.

jh

On Tue, 27 Sep 2011 11:04:42 -0400
Alfred von Campe wrote:
> Most of my desktops are still running CentOS5, but I have installed CentOS6
> on a few of them.  The users on those desktops are reporting that DNS
lookups
> are slow, and from my brief tests, that does appear to be the case. 
Have you considered installing dnsmasq on those machines?

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