Re: if_enc(4) and net.inet.ipcomp.ipcomp_enable

From: Matteo Riondato <>
Date: Tue, 08 Mar 2022 01:02:13 UTC
> On Mar 1, 2022, at 5:52 AM, Andrey V. Elsukov <> wrote:
> 28.02.2022 02:54, Matteo Riondato пишет:
>> Hello net@,
>> I am trying to use pf to filter packets in ipsec tunnels by filtering
>> on enc0 from if_enc(4).
>> I have the following values for the net.enc sysctl subtree: net.enc.out.ipsec_bpf_mask: 1 net.enc.out.ipsec_filter_mask: 1 2 2
>> and I have
>> net.inet.ipsec.filtertunnel: 1
>> Everything works well when the tunnel does not use ipcomp, but when
>> it does, the incoming packets seem to ignore the value of the sysctl, thus they show up in pf “twice”:
>> once with both external and internall headers, and once only with
>> internal (the value of 2 for this sysctl should make these packets
>> show up only with internal headers). The same can be observed with
>> tcpdump on enc0. This behavior makes it hard to do filtering.
>> Is this behavior expected?
> Hi,
> are you sure that it is not just on ingress and egress? You can use -Q flag for tcpdump to make sure.
> The first time when you see IPcomp packet in PF, it is when it arrives into IP stack on a physical interface (em, igb, ix, etc.). The second time is after decompression on if_enc interface, it is called from IPsec stack.

Hi Andrey,

Sorry for the late reply, somehow your message went into my spam folder. :/
It'll take me a few days to check, but I'll try your suggestion of using -Q to better understand what I’m seeing.