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From: Mick <michaelkintzios@gmail.com>
To: gentoo-user@lists.gentoo.org
Subject: Re: [gentoo-user] PMTUD
Date: Sun, 1 Sep 2013 16:43:50 +0100
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References: <CAN0CFw1NOD-cfwwOZuU8geHbLP7kzgc9FRGa+3nRFp9sbBGowA@mail.gmail.com> <201309011357.12792.michaelkintzios@gmail.com> <CAN0CFw1vuFeOBrLYWbrVG_gUaDwypMKa8rmiY0pUqoTDYR+3UA@mail.gmail.com>
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On Sunday 01 Sep 2013 14:59:19 Grant wrote:
> >> 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.
> >=20
> > 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.
> >=20
> > ping -c 6 -n -M do -s 1472 <server_address>
>=20
> I get "Frag needed and DF set (mtu =3D 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.
The hotel's router/modem may be using PPPoE to authenticate with their ISP,=
=20
which has a larger header size and requires an MTU of 1492 (1464+28=3D1492)
So, although your NIC is configured to the full ethernet MTU size, the rout=
er=20
drops the size down to 1492 to be able to squeeze it out through the ISP's=
=20
network. That's all good and proper and will not cause the timeout problem=
=20
you have been experiencing.
> > Of course, if the hotel's firewall is blocking all outgoing/incoming
> > pings this sort of diagnostic test will not be useful.
>=20
> 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.
Have you checked that the firewall at your server is not set to drop all IC=
MP=20
packets and that you don't have something like this set up on it:
net.ipv4.icmp_echo_ignore_all =3D 0
(use sysctl to check)
=2D-=20
Regards,
Mick
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