<html>
<head>
<meta content="text/html; charset=ISO-8859-1"
http-equiv="Content-Type">
</head>
<body bgcolor="#FFFFFF" text="#000000">
<div class="moz-cite-prefix">Am 22.10.2012 15:02, schrieb Leonardo
Bacha Abrantes:<br>
</div>
<blockquote
cite="mid:CAG+8EEYKtbFaBwRen+18G_eY-Y0g_8tEWaQ-j-U3cL+25oe7RA@mail.gmail.com"
type="cite">Hello,<br>
<br>
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.<br>
now I'm using nmap, and it's working.<br>
<br>
<a moz-do-not-send="true"
href="http://systhread.net/texts/2010q3nmap_ping.php">http://systhread.net/texts/2010q3nmap_ping.php</a><br>
<br>
</blockquote>
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. <br>
<br>
So, in short: it is very difficult to determine the actual status of
a host behind an arbitrary firewall<i> using packets sent from the
monitoring server</i>; in this case it is much safer to use
passive checks (i.e. sending results from the host side). <br>
<br>
jc <br>
<br>
</body>
</html>