IPSec transport mode, mtu, fragmentation...

Andrey V. Elsukov bu7cher at yandex.ru
Mon Dec 23 09:46:01 UTC 2019

On 23.12.2019 12:39, Andrey V. Elsukov wrote:
> On 20.12.2019 19:22, Victor Sudakov wrote:
>>> What's the root of the problem? ESP packets cannot get fragmented or
>>> what? 
>> Wireshark has shown that the "Don't Fragment" flag is set on all ESP
>> (protocol 50) packets. Who does this, why, and how can I switch it off
>> globally?
> Hi,
> I think this DF flag is originally from TCP packet.
> ESP xform for transport mode just replaces protocol in IP header and
> adds some info to the end of a packet.

This is controlled by net.inet.tcp.path_mtu_discovery variable.
TCP won't set IP_DF flag if you disable this feature.

WBR, Andrey V. Elsukov

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