Re: [Openvpn-users] Tun interface and mtu issues

[Timothy B. <T.E...@me...>]

In message <20100828222407.388b1242@jimbo>, Nikita Koshikov 
<kos...@gm...> wrote:

> Setup:
> 
> openvpn-client	-->	openvpn-server	-->	local-net
> tun0(mtu 1500)		tun0(mtu 1300)		eth0(mtu1500)
> eth0(mtu 1500)
> 
> I set mtu manually on openvpn server by 'ip link set tun0 mtu 1300'.

MTU = Maximum Transmission Unit

> Then I'm trying to ping machines from openvpn-client to local-net with
> command:

> ping -M do -s 1400 192.168.1.1
> ^^
> DF bit is on!
> And I still got fragmented replies. However, I must got icmp(Frag needed
> and DF set).

It's working as it should, your ICMP echo request packets are not being 
affected by the MTU setting on tun0 on openvpn-server, as they are not being 
transmitted on that interface, they are received on it.



> 22:07:40.973890 IP (tos 0x0, ttl  64, id 8411, offset 0, flags [none],
> proto: ICMP (1), length: 1428) 192.168.1.1 > 172.16.0.127: ICMP echo
> reply, id 15958, seq 1, length 1408
> 
> Can someone explain me, why tun interface not honor DF flag? Is it
> expected or it's a bug?

It's not set on the ICMP echo replies.