Firewall: Use `meta secpath exists` to match xfrm associations.

Marking incoming ESP packets and matching decapsulated packets doesn't
work with NAT traverslate (UDP encapsulation aka MOBIKE).

1 files changed, 12 insertions, 9 deletions

diff --git a/roles/common/templates/etc/nftables.conf.j2 b/roles/common/templates/etc/nftables.conf.j2
index 3104a4a..2261994 100755
--- a/roles/common/templates/etc/nftables.conf.j2
+++ b/roles/common/templates/etc/nftables.conf.j2
@@ -56,13 +56,15 @@ table inet filter {
 
         iif lo accept
 
-        # XXX Bullseye: this is a hack for the lack of reqid matches in
-        # nftables: we mark the esp packet and accept after decapsulation
+        # XXX Bullseye: this is a rather crude match as nftables 0.9.0 lacks support for ipsec expressions
+        # to match match inbound resp. outbound policies and source resp. destination tunnel addresses.
         # https://serverfault.com/questions/971735/how-to-match-reqid-in-nftables
         # https://blog.fraggod.net/2016/09/25/nftables-re-injected-ipsec-matching-without-xt_policy.html
-        define IPsec.mark = 0x220
-        meta l4proto esp mark set mark | $IPsec.mark accept
-        ip saddr 172.16.0.0/24 ip daddr {{ ipsec[inventory_hostname_short] }} mark & $IPsec.mark == $IPsec.mark accept
+        # (We can't use marks to match post-ESP decapsulation here because that doesn't work well with UDP
+        # encapsulation.)
+        meta l4proto esp accept
+        # ipsec in ip saddr {{ ipsec_subnet }} ip daddr {{ ipsec[inventory_hostname_short] }} accept
+        ip saddr {{ ipsec_subnet }} ip daddr {{ ipsec[inventory_hostname_short] }} meta secpath exists accept
 
         # rate-limiting is done directly by the kernel (net.ipv4.icmp_{ratelimit,ratemask} runtime options)
         icmp   type { echo-reply, echo-request, destination-unreachable } counter accept
@@ -104,11 +106,12 @@ table inet filter {
 
         oif lo accept
 
-        # XXX Bullseye: unlike for input we can't use marks here,
-        # because by the time we see a packet to 172.16.0.0/24 we don't
-        # know if it'll be encapsulated
+        # XXX Bullseye: unlike for input we can't use marks or test for
+        # secpath existence here, because by the time we see a packet to
+        # 172.16.0.0/24 we don't know if it'll be encapsulated
         meta l4proto esp accept
-        ip saddr {{ ipsec[inventory_hostname_short] }} ip daddr 172.16.0.0/24 accept
+        # ipsec out ip saddr {{ ipsec[inventory_hostname_short] }} ip daddr {{ ipsec_subnet }} accept
+        ip saddr {{ ipsec[inventory_hostname_short] }} ip daddr {{ ipsec_subnet }} accept
 
         meta l4proto { icmp, icmpv6 } accept