socket.gethostbyaddr() question

[Harlin Seritt]

I have a list of IP addresses I am testing with socket.gethostb yaddr().
For the most part, I am able to get a hostname returned to me when I
run gethostbyaddr() . I am also hoping this will allow me to tell if a
computer is up or down. However, in my environment, I am finding that I
am able to get a hostname even though I am unable to actually ping that
server (when I ping a server i like this, I get 'request timed out'
messages telling me that the server is not up).

Other times, I am able to ping a server with success but the
gethostbyaddr() request will not be able to return a hostname for me
making me think that machine is truly down when I know it's not. Can
anyone give me any pointers as to why this happens? Is there anyway I
can do a reliable ping to another server with Python? I am not
interested in doing system calls with a system Ping client (I have to
ping way too many machines and this either takes too long or causes
severe memory leakage due to MS's horrible ping client). Also, I am
unable to use Jeremy Hylton's Python ping client because it does little
more than call gethostbyaddr() . Thanks for taking a look at this.

Harlin Seritt

[Roy Smith]

Re: socket.gethostb yaddr() question

"Harlin Seritt" <harlinseritt@y ahoo.com> wrote:[color=blue]
> I have a list of IP addresses I am testing with socket.gethostb yaddr().
> For the most part, I am able to get a hostname returned to me when I
> run gethostbyaddr() . I am also hoping this will allow me to tell if a
> computer is up or down.[/color]

Gethostbyaddr() simply does a lookup in some sort of name resolution
database (typically DNS, but could be YP/NIS, hostfile, etc). The ability
to look a name up in the database has absolutely no correlation with
whether that node is up, down, or unreachable.

To tell if a machine is up or down, you need to probe the machine directly.
Typically, this is done by sending it an ICMP Echo Request (commonly known
as ping), but that's not foolproof. A machine could be up, but unreachable
because of network or routing problems. A machine could be intentionally
not answering pings. A firewall in the way could be filtering out ping
packets. Or, the Echo Request or Echo Reply could have simply gotten lost
somewhere on the network.

What, exactly, are you trying to do?

[Steve Holden]

Re: socket.gethostb yaddr() question

Harlin Seritt wrote:[color=blue]
> I have a list of IP addresses I am testing with socket.gethostb yaddr().
> For the most part, I am able to get a hostname returned to me when I
> run gethostbyaddr() . I am also hoping this will allow me to tell if a
> computer is up or down. However, in my environment, I am finding that I
> am able to get a hostname even though I am unable to actually ping that
> server (when I ping a server i like this, I get 'request timed out'
> messages telling me that the server is not up).
>
> Other times, I am able to ping a server with success but the
> gethostbyaddr() request will not be able to return a hostname for me
> making me think that machine is truly down when I know it's not. Can
> anyone give me any pointers as to why this happens? Is there anyway I
> can do a reliable ping to another server with Python? I am not
> interested in doing system calls with a system Ping client (I have to
> ping way too many machines and this either takes too long or causes
> severe memory leakage due to MS's horrible ping client). Also, I am
> unable to use Jeremy Hylton's Python ping client because it does little
> more than call gethostbyaddr() . Thanks for taking a look at this.
>[/color]
If you have the ability to run code on the machines you are interested
in you might want to consider Nicola Larosa's heartbeat recipe:

http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/52302

You may also find something useful in

http://pynms.sourceforge.net/ping.html

regards
Steve
--
Steve Holden +44 150 684 7255 +1 800 494 3119
Holden Web LLC www.holdenweb.com
PyCon TX 2006 www.python.org/pycon/