MTU not regrowing?

[John Hay]

On Tue, Jun 24, 2014 at 08:43:28PM +0200, Andrea Venturoli wrote:
> Hello.
> 
> Today I experienced something weird (at least for me) on a 8.4 system:
> 
> _ the system had vlan3 interface, with default MTU (1500 bytes);
> _ "ping -D -s 1400 somehost" would work, but "ping -D -s 1500 somehost" 
> would yield "frag needed and DF set" (forgive me if the message is not 
> exact, I don't have it anymore);
> 
> _ to make some tests I reduced MTU size with "ifconfig vlan3 mtu 500";
> _ now, of course, "ping -D -s 400 somehost" would work, but "ping -D -s 
> 500 somehost" would yield "frag needed and DF set";
> 
> _ then I raised MTU again with "ifconfig vlan3 mtu 1500" (notice 
> ifconfig would actually report this as "mtu 1500" was shown);
> _ however the results were as before, i.e. "ping -D -s 400 somehost" 
> would work, but "ping -D -s 500 somehost" would yield "frag needed and 
> DF set";
> 
> _ no way I could ping with a packet bigger than 500 bytes until I rebooted.
> 
> Is this expected behaviour? Any way to get around this?

Do a "route get somehost" and see what mtu is returned. You might be
able to delete or tweak that route.

John
-- 
John Hay -- jhay at meraka.csir.co.za / jhay at meraka.org.za