Strange MTU-ish problem

[Timothy Murphy]

Mikkel L. Ellertson wrote:

>> Under Windows XP Run=>cmd I get
>> 
>> ping www.google.com -f -l 1490
>> ...
>> Packet needs to be fragmented but DF set
>> <repeated several times>
...
>> Is there a similar option for ping under Fedora?
>> 
> You may want to take a look at the ping man page:
> 
> The -f and I believe the -s options.

I did "man ping".

On my system (standard Fedora-10) "ping -s <n> www.google.com"
gives rather strange results (as repeated below).
If n >= 1470 then ping does not return.
If n = 1464 it returns as usual.
If n = 1466 or 1468 then I am told the packets are fragmented,
[I did not try these numbers before.]

Adding the -f flag (as root) does not seem to give any more information.

--------------------------------------------------
[tim at mary tmp]$ ping -s 1464 www.google.com
PING www.l.google.com (74.125.39.147) 1464(1492) bytes of data.
64 bytes from fx-in-f147.google.com (74.125.39.147): icmp_seq=1 ttl=242 (truncated)
64 bytes from fx-in-f147.google.com (74.125.39.147): icmp_seq=2 ttl=242 (truncated)
^C
...
[tim at mary tmp]$ ping -s 1466 www.google.com
PING www.l.google.com (74.125.39.99) 1466(1494) bytes of data.
>From homegate.homenet.telecomitalia.it (192.168.1.1) icmp_seq=1 Frag needed and DF set (mtu = 1492)
^C
...
[tim at mary tmp]$ ping -s 1468 www.google.com
PING www.l.google.com (74.125.39.147) 1468(1496) bytes of data.
>From homegate.homenet.telecomitalia.it (192.168.1.1) icmp_seq=1 Frag needed and DF set (mtu = 1492)
^C
...
[tim at mary tmp]$ ping -s 1470 www.google.com
PING www.l.google.com (74.125.39.99) 1470(1498) bytes of data.
^C
--------------------------------------------------

It seems from my experiment that one can get the information with ping,
but only in a rather bizarre way.

> You can add "MTU=1492" to /etc/sysconfig/network-scripts/ifcfg-eth1.

I did try this on one (very old) laptop
but it did not seem to improve matters.
I'll try it again now, and re-boot
(not sure if I did that before).