[strongSwan] XFRM fragmentation before encapsulation

Noel Kuntze noel.kuntze+strongswan-users-ml at thermi.consulting
Mon Oct 21 20:41:16 CEST 2019

Hello André,

Please double check if you have before and after right in that email.
I understand it as such that the behaviour I desire is what the kernel already does?

Kind regards


Am 21.10.19 um 11:34 schrieb André Valentin:
> Hi Noel,
> I did some tests with copy_df set. In all cases the fragmentation was done before encryption.
> Even with namespaces and net.ipv4.ip_no_pmtu_disc=0 it was not possible to get fragmentation after encryption (like cisco is able to).
> In my tests, I always used xfrm interfaces.
> But if you find other possibilities, please let me know.
> Kind regards,
> André
> Am 19.10.19 um 23:42 schrieb Noel Kuntze:
>> Hello list,
>> Does the kernel support IP fragmentation before encapsulation in any way? Even with XFRM interfaces or VTIs?
>> I looked at the XFRM code but did not find any code that deals with fragmenting any packets. If the packet is too large,
>> it is just discarded with an error. If the MTU of the network path is large enough and the packet is pre fragmented by
>> having an XFRM interface with a sufficiently low MTU, then do fragments get encapsulated?
>> Any enlightement would be very appreciated!
>> Kind regards
>> Noel

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