>> Could ICMP packets not getting through be to blame for my proxy server >> problem? My laptop can't seem to ping anyone (blocked at the firewall >> in this hotel I suppose) and certainly the proxy server can't ping my >> laptop. > > Not all ICMP packets are relevant to detecting the MTU of a node. A correctly > implemented node will return an ICMP Fragmentation Needed (Type 3, Code 4) > packet, with its MTU value. This kind of ICMP packets should not be blocked > at firewalls. Use ping with the do not fragment option to see if packets > above a certain size time out, i.e. they are dropped by some offending node on > the way. > > ping -c 6 -n -M do -s 1472 <server_address>
I get "Frag needed and DF set (mtu = 1492)" when pinging google.com. I get normal replies with -s 1464. ifconfig shows my WAN interface at MTU 1500 so PMTUD must change the MTU for communication with google.com if I understand correctly. > Of course, if the hotel's firewall is blocking all outgoing/incoming pings > this sort of diagnostic test will not be useful. I actually only lose pings to my own remote system so I've started a new thread about that. I tried down to -s 1 but still 100% packet loss there. - Grant
