[clug] Re: Determining availability of a remote machine

Alex Satrapa grail at goldweb.com.au
Sat Apr 5 04:05:32 EST 2003

On Friday, April 4, 2003, at 11:13 , Kim Holburn wrote:

> I assumed that when Michael said "can't use ICMP" he meant he couldn't 
> use echo request.  ICMP is part of all IP traffic.  Things won't work 
> properly without it.  Most modern routers might stop ping (ICMP echo 
> request) but they won't stop ICMP messages that are part of tcp unless 
> they are seriously broken.

Pardon me if I'm repeating myself - but I'm sure you'll find that there 
are plenty of broken networks that do foolish things like block "Must 
Fragment" packets (or simply not send them) - for policy or technical 
reasons, which is why Roaring Penguin includes MSS clamping as a 
standard feature, and why you must use it to clamp MSS to something like 
1472 bytes (IP + PPP headers in there somewhere).

Some network administrators block all ICMP simply because it's easier to 
be broken than to be correct (eg: blocking ALL ICMP takes one rule - 
blocking PING takes two or more). Others (such as - ahem - yours truly) 
block the wrong ICMP messages because they're too STUP... sorry, they're 
not fully conversant in ICMP.

Iin answer to the original question though - I'd either consider using 
UDP based ping (which would require the appropriate listener to be 
running on the remote machine), or pestering the network admin of the 
network that's preventing you from using ICMP ping.

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