Force host assume up when services are up

[Jakob Curdes]

Am 22.10.2012 15:02, schrieb Leonardo Bacha Abrantes:
> Hello,
>
> I know that is possible to use another plugin to check if host is up, 
> however, I never did it on old version of nagios.
> now I'm using nmap, and it's working.
>
> http://systhread.net/texts/2010q3nmap_ping.php
>
It depends on the firewall if this solution works. NMAP tries ICMP and 
if that does not work it sends a "probe" TCP packet to an arbitrary port 
of the host, if it gets back anything (e.g. the host responds with RST- 
no service on this port), nmap sees the host as "up".  If you have a 
firewall that is acutally dropping incoming packets (and not sending 
back a RST/SYN/ACK), then also this technique will fail in a similar 
manner as the ICMP ping. A Firewall might also ALWAYS send an RST packet 
for a certain port, so nmap would assume that the host is up while it 
was the firewall that sent the answer.

So, in short: it is very difficult to determine the actual status of a 
host behind an arbitrary firewall/using packets sent from the monitoring 
server/; in this case it is much safer to use passive checks (i.e. 
sending results from the host side).

jc

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