Best strategy for overcoming excessive gethostbyname timeout.

r0g aioe.org at technicalbloke.com
Sat Nov 28 04:20:57 CET 2009


John Bokma wrote:
> r0g <aioe.org at technicalbloke.com> wrote:
> 
>> It seems gethostbyname asks the OS to resolve the address and the OS
>> uses it's own timeout value ( 25 seconds ) rather than the one provided
>> in setdefaulttimeout. 25 seconds of blocking is way too long for me, I
>> want the response within 5 seconds or not at all but I can see no
>> reasonable way to do this without messing with the OS which naturally I
>> am loathe to do!
> 
> use signal.alarm(time) to send SIGALRM to your process:
> http://docs.python.org/library/signal.html#signal.alarm
> See example at bottom.
> 
> John


Ahh so close. I set the alarm for 3 seconds and it raises the exception,
but only after spending 25 seconds seemingly blocked in gethostbyname.

Here's a snippet, just in case I'm doing it wrong!...

def dns_timeout(a,b):
    raise Exception("DNS timeout")

def send_one_ping(my_socket, dest_addr, ID):
    signal.signal(signal.SIGALRM, dns_timeout)
    signal.alarm(3)
    try:
        dest_addr  =  socket.gethostbyname(dest_addr)
    except Exception, exc:
        print "Exception caught:", exc
    signal.alarm(0)


Oh well, even if it doesn't work in this case it's really useful to know
about the signal module, I'd never stumbled across it til now, thanks!

Roger.



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