Re: discovering a service behind a nated network

["P. Deelman" <p.deelman () hccnet nl>]

linux user wrote:

> Hiya All,
>
> I would like to discover if a service that is behind a NATed network
> is still working, for
> example if a web server is in a private network, Nated behind a
> gateway, how could i from an external network check if the server is 
> down/ or there are network problems between the server and the
> gateway? is there a way to use a tool such as traceroute for
> NATed/Firewalled network from an external link?
>
>  
If a webserver is behind a gateway, then the only way to check if it's 
down is to telnet to the specific port and see it the webserver give you 
any output. Traceroutes and pings are ICMP based and handeled by the 
gateway. These also could be denied or forwarded. This is only solvable 
if you could log onto the gateway and chekc from there.

> The reason i am asking this is because i have been asked that
> question on a job interview, and i did  not know what the correct
> answer was, it was related to a web cluster farm then.
>
>  
If it's a cluster then the gateway would probably do some kind of 
loadbalancing and without any extra tools at your disposal on the 
gateway you probably wouldn't even know something went wrong. A good 
cluster is redundant all the way. 2 switches and 2 nics in every box. 
The gateway would notice that a webserver in the farm is down (due to 
the heartbeat software on the gateway which regularly checks if a 
machines service if running, or the box at all) and will remove it from 
the forwarding table.

> another reason is howto troubleshoot a service that has been port forwarded from
> the gateway, the port forwarding works for other services, but this
> specific service is not reachable, and you can not tell whether the NATed box
> was down, or the route was down, or what, you could debate that you
> can use ssh to
> the gateway server, but then that is run by a different dept. and you
> have no access to that.
>
>  
A gateway that is run for a webcluster and do some kind of balancing. 
Then it would be run by another dept? this is not good. The only way to 
see then if the gateway is up or not is to ping to it and maybe check 
other forwarded services that are routed to other boxes. That way you 
could see if the gateway is down of just the webserver(s).

> sorry if my English langauge is a bit rusty
>
> TIA
>
> Anst
>  
From a technical point of view. To run all services from 1 IP that is a 
webcluster and propably a mailcluster too is not good. A decent ISP has 
spreaded it's services across serveral IP's and a gateway that does 
loadbalancing for a webcluster should be reachable for the sysops. Or at 
least some tools on the gateway to check what is down and also the 
ability to ssh to specific boxes to check what's wrong with an 
individual machine. Offcourse configuration should be centrally managed 
by some box which holds all config files with CVS capabilities.

If all of the above is not possible, then the only way then i could 
think of is to go and visit the box wasting precious time driving to the 
colo provider, checking into security, log into the box, maybe reset it 
(thank god for remote powerswitches) and drive back to office wasting AT 
LEAST an hour. Also keep in mind that working at you colo is more 
unpleasant then from behind your desk at office.

There is also another way and that is to set up an external box and let 
all (web)servers connect to it using an reverse ssh tunnel, but then the 
gateway and it's firewall is rendered useless if that external box is 
compromised.

Conclusion: the question asked to you raises a lot of questions of good 
system management. But non of the less, good questions to test knowledge.

Patrick


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