NAT before IPSEC - reply packets stuck at enc0

[Muenz, Michael]

Am 25.07.2017 um 15:04 schrieb Andrey V. Elsukov:
> On 25.07.2017 15:17, Muenz, Michael wrote:
>>>> * 10.26.2.N sends ICMP request to 10.24.66.25
>>>>
>>>> * 10.26.1.1 handles it by tunnel mode IPsec security policy,
>>>> something like:
>>>>      spdadd -4 10.26.2.0/24 10.24.66.0/24 any -P out ipsec \
>>>>          esp/tunnel/213.244.192.191-81.24.74.3/require;
>>>> * IPsec code does lookup for IPsec SA and uses something like:
>>>>      add 213.244.192.191 81.24.74.3 esp 0x2478d746 -m tunnel -E ...;
>>> Thanks for the detailed explaination! I only know the insights with
>>> Linux, but what I try to achieve is, not to build a SA fpr 10.26.2.0
>>> to 10.24.66.0.
>>> So IMHO the address rewriting from 10.26.2 to 10.26.1 should be done
>>> before getting to the IPSEC process.
>>> In Linux a packet not matching a SA would simply be dropped by kernel
>>> or throw a "NO PROPOSAL CHOSEN" since there's no known SA for
>>> 10.26.2.0 to 10.24.66.0.
> As I said already, the NAT thinks that both packets are inbound and does
> translation for source address each time. You need to do translation for
> both directions on enc0 interface like I described, or you need to
> somehow hack/modify ipfw_nat.
>
> You do not need to create SA for 10.26.2.0->10.24.66.0, you only need
> create security policy, that will "route" such packets into the IPsec
> tunnel. The translation will be done inside IPsec before IP
> encapsulation and encryption. Since you are using tunnel mode IPsec,
> replies will be returned to your external IP address, and this SA is
> exists already. After decryption and IP decapsulation the destination
> address of packet will be translated back to 10.26.2.N on if_enc(4).

Can I use this spdadd command also when using strongswan? (Please excuse 
stupid questions)

>
>> 14:02:53.960436 (authentic,confidential): SPI 0xdeda7104: IP (tos 0x0,
>> ttl 63, id 6287, offset 0, flags [none], proto ICMP (1), length 28, bad
>> cksum b07 (->c07)!)
>>      10.26.1.1 > 10.24.66.25: ICMP echo request, id 38600, seq 0, length 8
>              ^^^ - this address must be 10.26.2.N, and it will be
> translated on "out xmit enc0".
>
>> 14:02:53.960460 (authentic,confidential): SPI 0xdeda7104: IP (tos 0x0,
>> ttl 64, id 32607, offset 0, flags [none], proto IPIP (4), length 48, bad
>> cksum 0 (->c99b)!)
>>      213.244.192.191 > 81.24.74.3: IP (tos 0x0, ttl 63, id 6287, offset
>> 0, flags [none], proto ICMP (1), length 28)
>>      10.26.1.1 > 10.24.66.25: ICMP echo request, id 38600, seq 0, length 8
>              ^^^ - and here it will become 10.26.1.1 after translation.
>
>> 14:02:53.968634 (authentic,confidential): SPI 0xcdea472d: IP (tos 0x0,
>> ttl 58, id 18352, offset 0, flags [none], proto IPIP (4), length 48)
>>      81.24.74.3 > 213.244.192.191: IP (tos 0x0, ttl 63, id 38328, offset
>> 0, flags [none], proto ICMP (1), length 28)
>>      10.24.66.25 > 10.26.1.1: ICMP echo reply, id 38600, seq 0, length 8
>                           ^^^ - here your gateway receives the reply and
> will do IP decapsulaton.
>
>> 14:02:53.968653 (authentic,confidential): SPI 0xcdea472d: IP (tos 0x0,
>> ttl 63, id 38328, offset 0, flags [none], proto ICMP (1), length 28)
>>      10.26.1.1 > 10.26.1.1: ICMP echo reply, id 44919, seq 0, length 8
>        ^^^ - here packet will be translated back on "in recv enc0" and
> will have the following addresses: 10.24.66.25 > 10.26.2.N
>
>> So the most specific nat rule in order to get the packet into enc0 is:
>>
>> ipfw nat 1 config ip 10.26.1.1 log reverse
>> ipfw add 179 nat 1 log all from 10.26.2.0/24 to 10.24.66.0/24 in recv
>> vtnet1
>> ipfw add 179 nat 1 log all from 10.24.66.0/24 to 10.26.1.1 in recv enc0
> ipfw nat 1 config ip 10.26.1.1 log
> ipfw add 179 nat 1 all from 10.26.2.0/24 to 10.24.66.0/24 out xmit enc0
> ipfw add 179 nat 1 all from 10.24.66.0/24 to 10.26.1.1 in recv enc0
>

Ok so your 3 nat commands will only match when there's a new spd like 
above right?
Since there's nothing on enc0 without it.

Thanks
Michael