Help with reading strace

[Nigel Wade]

Dan Track wrote:
> Hi
> 
> I have a problem with one of my server, where basically I get a
> network hang for a few seconds every minute when communicating with
> another server on the same subnet.
> 
> I caught the hang when running tnsping, and here's the strace. The
> space between the strace is where I found the problem to be.
> 
> My question is can someone please help wme figure out what is going
> on, and guide me to where I can read up on these system calls, like
> POLLIN etc. I'd particularly would like to know what is happening
> before and after the space in the strace.
> 

[snip]

> socket(PF_INET, SOCK_DGRAM, IPPROTO_IP) = 4
> connect(4, {sa_family=AF_INET, sin_port=htons(53),

This is the important bit. You've opened a UDP socket on port 53, i.e DNS and 
the file descriptor used is 4.

> sin_addr=inet_addr("10.11.10.1")}, 28) = 0
> fcntl64(4, F_GETFL)                     = 0x2 (flags O_RDWR)
> fcntl64(4, F_SETFL, O_RDWR|O_NONBLOCK)  = 0
> gettimeofday({1143022459, 869414}, NULL) = 0
> poll([{fd=4, events=POLLOUT, revents=POLLOUT}], 1, 0) = 1
> send(4, "\367\201\1\0\0\1\0\0\0\0\0\0\7neptune\2l3\next"..., 43, 0) = 43

here, it looks like it's sent a request to the DNS server...

 > poll(
 >

and now it's polling for the response.

My guess would be it's a delay in resolving ip/hostnames. Check you DNS settings.

-- 
Nigel Wade, System Administrator, Space Plasma Physics Group,
             University of Leicester, Leicester, LE1 7RH, UK
E-mail :    nmw at ion.le.ac.uk
Phone :     +44 (0)116 2523548, Fax : +44 (0)116 2523555