IPSec transport mode, mtu, fragmentation...

[Eugene Grosbein]

16.01.2020 23:07, Victor Sudakov wrote:

>>> What beats me is that I cannot reproduce this problem in bhyve. In this
>>> packet dump: http://admin.sibptus.ru/~vas/ipsec1.pcap.gz I'm scp-ing a
>>> 50M file from 192.168.246.10 (bhyve guest) to 192.168.246.1 (bhyve
>>> host), and I see no fragments, and the largets packet is 1466 bytes, and
>>> the scp never stalls nor fails.
>>>
>>> Why is it NOT broken this time?
>>>
>>> Both hosts are 12.1-RELEASE-p1.
>>
>> I could not reproduce the problem with unpatched recent stable/11, either :-)
> 
> Is there a way to view the MSS in the TCP segments before encryption or
> after decryption? I want to compare them in situations with IPSec
> enabled and disabled.
> 
> I've never been able to see anything in "tcpdump -i enc0", probably it
> cannot do transport mode IPSec because the man page talks about "outer
> and inner header."

enc0 does what you need but before you use it, remember:

1) before starting, you better change sysctls to:

net.enc.in.ipsec_filter_mask=0
net.enc.out.ipsec_filter_mask=0

so using enc0 does not pass packets over netpfilter rules extra time;

2) don't forget: ifconfig enc0 up

3) tcpdump has no means to filter by inner attributes in case of tunnel mode;
it still shows decoded IPSec transport mode packets correctly.