Firewall is not working for IPSec NAT-T

[kouhj]

Tested two OpenWRT 24.10 routers to setup a site to site IPSec connection with Strongswan when the WAN interfaces are over NATed network. After the firewall config were modified to accept the incoming ESP protocol and UDP 500, 4500 ports on the WAN interface (The default config of OpenWRT, which accepts redirecting ESP and UDP 500 to LAN, does not seem to work even to establish the connection), the two routers were able to establish the IPSec connection, and the following commands all reported normal results:
swanctl --list-sas
ip xfrm state
ip xfrm policy
ip route show table 220

BUT the normal site to site traffic just can get through when tested with ping.
If the firewall is stopped on the router that initiates the ping, it worked just fine to reach the other router. And unreachable again if the firewall is re-enabled.

Followed these articles,
https://docs.strongswan.org/docs/latest/howtos/forwarding.html
https://docs.strongswan.org/docs/latest/howtos/trafficDumps.html
and found that the LAN traffic can pass the postrouting chain of the mangle table with this command iptables -t mangle -I POSTROUTING -m policy --pol ipsec --dir out -j NFLOG --nflog-group 5. The key seems how to add a nftables rule in POSTROUTING chain of the nat table like the command iptables -t nat -I POSTROUTING -m policy --pol ipsec --dir out -j ACCEPT pointed out in the Strongswan doc mentioned above.

Tried the following and no luck:
nft insert rule inet fw4 srcnat oif WAN ipsec out reqid 1 counter accept
nft insert rule inet fw4 mangle_postrouting oif WAN ipsec out reqid 1 counter accept

The reqid 1 is confirmed with swanctl --list-sas

Any ideas how to make IPSec NAT-T work with fw4? Thanks!

[brada4]

Does iptables-translate (or iptables-nft) do the conversion for you?
https://git.netfilter.org/iptables/tree/extensions/libxt_policy.txlate

[kouhj]

No. this tools does not work:
iptables-translate -t mangle -I POSTROUTING -m policy --pol ipsec --dir out -j ACCEPT
nft # -t mangle -I POSTROUTING -m policy --pol ipsec --dir out -j ACCEPT

I did the conversion by myself as in the previous post by adding a rule to mangle_postrouting chain or srcnat before the masquerading. The rule can be added but it did not ACCEPT the packet before it SNAT the sender IP, so the original unencapsulated packet was sent to the WAN device. So the IPSec communication failed.

[brada4]

Maybe something like meta ipsec exists meta ipsec missing ?

[brada4]

Also meta secpol 0|1 might be related, check if combination works.
Proper chain-prepend is to make a file like this, repeating hook spec of particular chain. The dynamic zone chains are not guaranteed to be there.

chain input {
 policy idem hook idem priority idem;
 iif "lo" meta idunno exists accept/return
}

And save it to /etc/nftables.d/something.nft , kept over sysupgrade unlike other options.
Probably best practice dummy is some general use linux latest version, OpenWrt has every rarely used ipt or nft keyword as a separate kmos/iptables-mod/nft-mod, so first get syntax right then add modules to match.

[kouhj]

Following this article: https://thermalcircle.de/doku.php?id=blog:linux:nftables_demystifying_ipsec_expressions, I stopped firewall4 and import the following nft rules and it worked just fine.

table ip nat {
        chain postrouting {
                type nat hook postrouting priority srcnat; policy accept;
                oif  $WANDEV ipsec out reqid 1 accept
                oif $WANDEV masquerade
        }
}
table ip filter {
        chain forward {
                type filter hook forward priority filter; policy drop;
                iif br-lan oif $WANDEV accept
                iif $WANDEV oif br-lan ct state established,related accept
                iif $WANDEV oif br-lan ipsec in reqid 1 ct state new accept
        }
}

So next step is how to integrate with firewall...

[kouhj]

Finally managed to make it work by adding a custom nft rule:

cat  > /etc/nftables.d/20-ipsec-natt.nft <<EOF
chain srcnat_wan {
        ipsec out reqid 1 counter accept
}

chain forward_wan {
        ipsec in reqid 1 ct state new accept
}
EOF

Good for a simple setup, but still something to do for multiple connections.
Thanks a lot!

[brada4]

Probably worth researching how to integrate rules into firewalld and amending strongswan docs with Your translation.

[brada4]

multiple interfaces - prepend rulw iif {$wan_devices, $wwan_devices } filter your way. There is no "multiple source zones" option in fw4