netdev - Re: [PATCH ipsec/vti 0/2] Fragmentation of IPv4 in VTI

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Message-ID: <20190321151630.GA25855@gauss3.secunet.de>
Date:   Thu, 21 Mar 2019 16:16:30 +0100
From:   Steffen Klassert <steffen.klassert@...unet.com>
To:     Bram Yvahk <bram-yvahk@...l.wizbit.be>
CC:     <herbert@...dor.apana.org.au>, <davem@...emloft.net>,
        <netdev@...r.kernel.org>
Subject: Re: [PATCH ipsec/vti 0/2] Fragmentation of IPv4 in VTI

On Sun, Mar 17, 2019 at 11:37:55PM +0000, Bram Yvahk wrote:
> We've experienced an issue with VTI when the path-mtu is smaller than the size
> of the "client" packet.
> 
> What happens: IPv4 packet from the client (i.e. another system in the LAN)
> attempts to transmit some data; IPv4 header shows that 'DF' bit is not set but
> still the client receives ICMPv4 "need-to-frag" message [which the client does
> not expect and ignores].
> 
> Example: $ ping -s 1300 -M dont -c5 192.168.235.2
>     PING 192.168.235.3 (192.168.235.3) 1300(1328) bytes of data.
>     From 192.168.236.254 icmp_seq=1 Frag needed and DF set (mtu = 1214)
>     From 192.168.236.254 icmp_seq=2 Frag needed and DF set (mtu = 1214)
>     From 192.168.236.254 icmp_seq=3 Frag needed and DF set (mtu = 1214)
>     From 192.168.236.254 icmp_seq=4 Frag needed and DF set (mtu = 1214)
>     From 192.168.236.254 icmp_seq=5 Frag needed and DF set (mtu = 1214)
> 
>     --- 192.168.235.3 ping statistics ---
>     5 packets transmitted, 0 received, +5 errors, 100% packet loss, time 3999ms

Hm, this works here. Can you show how you setup the vti device?
Some tunnel configuration options (set ttl etc.) force to have
the DF bit set.