Determining availability of a remote machine

Alex Satrapa grail at
Thu Apr 3 04:35:03 EST 2003

On Wednesday, April 2, 2003, at 08:46 , Martin Pool wrote:

> ICMP is a non-optional part of TCP/IP.  Blocking it altogether (if
> that is in fact what is being done) is not a very good idea.

There are many ISPs who insist on doing foolish things like blocking 
ICMP at their gateways, which leads to problems such as "Must Fragment" 
packets not reaching their intended destination.

It is foolishness such as this which causes the need for MSS clamping in 
Roaring Penguin PPPoE - or in fact in the Linux IPTABLES these days.

Admittedly, sometimes the ISPs have no say in the matter, because 
they're using broken equipment (such as some ADSL equipment used by 
Telstra - is it the DSLAM?) which they cannot work around (except by 
putting compliant equipment on either side and having the end-point MTU 
less than the broken equipment's MTU).

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