wg-quick: linux: raise priority for mangle nft chain

Setting mark must be done as early as possible in case there are ipv6 rpfilter rules in the mangle table (a nft filter could be done later but with ip6tables this is the latest it can be checked). Mark must be set before the return path check for it to work correctly.

priority -160 gets rendered as "mangle - 10" in nft list table, and will correctly set the mark before other mangle prerouting rules if there are any and same as before if there aren't.
---

I've been discussing with firewalld folks about the ipv6_rpfilter rules they add which block wireguard setup with wg-quick on ipv6 endpoints by default ; see https://github.com/firewalld/firewalld/issues/603 for recap issue.
Long story short: they'll move the rpfilter rule to take effect later (after mark is set) and add mark matching so that it works always with nft backend, and sometimes with iptables backend.

Sometimes, because the ip6tables rpfilter module can only be used up to mangle's prerouting table, and that one's priority with nft hook prerouting priority mangle is "random" (depends on module load order?)

Changing the priority to load a bit earlier is harmless in the default case and should just work for most people once they get a fixed firewalld; I believe that is easier to change than try to document the issue (e.g. telling people to use nft backend or disable ipv6_rpfilter if they have issues as that is hard to debug)

Thanks!

 src/wg-quick/linux.bash | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 7c2c002..9001c6a 100755
@@ -222,7 +222,7 @@ add_default() {
 	local marker="-m comment --comment \"wg-quick(8) rule for $INTERFACE\"" restore=$'*raw\n' nftable="wg-quick-$INTERFACE" nftcmd 
 	printf -v nftcmd '%sadd table %s %s\n' "$nftcmd" "$pf" "$nftable"
 	printf -v nftcmd '%sadd chain %s %s preraw { type filter hook prerouting priority -300; }\n' "$nftcmd" "$pf" "$nftable"
-	printf -v nftcmd '%sadd chain %s %s premangle { type filter hook prerouting priority -150; }\n' "$nftcmd" "$pf" "$nftable"
+	printf -v nftcmd '%sadd chain %s %s premangle { type filter hook prerouting priority -160; }\n' "$nftcmd" "$pf" "$nftable"
 	printf -v nftcmd '%sadd chain %s %s postmangle { type filter hook postrouting priority -150; }\n' "$nftcmd" "$pf" "$nftable"
 	while read -r line; do
 		[[ $line =~ .*inet6?\ ([0-9a-f:.]+)/[0-9]+.* ]] || continue

2.26.2

This patch is missing a Signed-off-by line.

Jason A. Donenfeld wrote on Mon, Apr 27, 2020:

Sorry, will add and resend without RFC after some feedback.

Good question, the only two chains I'm aware of conflicting are wg-quick's premangle and ip6tables's mangle/PREROUTING rpfilter (e.g. a rule like:
ip6tables -t mangle -A PREROUTING -m rpfilter --validmark --invert -j DROP )

As I understand rpfilter only makes sense on prerouting or broadly speaking input (firewalld's nft backend will move the rpfilter rule all the way down to filter input table, but that's not possible with ip6tables) - it checks the incoming packet came through the same interface we would send back to.
For postrouting the kernel already picked an interface for us so there would be little point in checking the kernel would pick the same interface again? So no rpfilter and marks aren't used in these rules.

If someone ever comes in with an ip6tables rule that relies on mark checking in mangle POSTROUTING then it would help to move postmangle to -160 as well, I'm just not aware of any.
Letting you decide on this one, I'd tend not to bother until a usecase shows up, but I guess there's no harm in moving it anyway....

Dominique | Asmadeus

Setting mark must be done as early as possible in case there are ipv6 rpfilter rules in the mangle table (a nft filter could be done later but with ip6tables this is the latest it can be checked). Mark must be set before the return path check for it to work correctly.

priority -160 gets rendered as "mangle - 10" in nft list table, and will correctly set the mark before other mangle prerouting rules if there are any and same as before if there aren't.

---
Well no more answer to RFC thread so resending as proper patch with sob.

Regarding the postrouting hook I'll stick to the same -150 as described in my answer in other thread: I do not see any use case for it so safer to leave it untouched for me.

Thanks,

 src/wg-quick/linux.bash | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 7c2c002..9001c6a 100755
@@ -222,7 +222,7 @@ add_default() {
 	local marker="-m comment --comment \"wg-quick(8) rule for $INTERFACE\"" restore=$'*raw\n' nftable="wg-quick-$INTERFACE" nftcmd 
 	printf -v nftcmd '%sadd table %s %s\n' "$nftcmd" "$pf" "$nftable"
 	printf -v nftcmd '%sadd chain %s %s preraw { type filter hook prerouting priority -300; }\n' "$nftcmd" "$pf" "$nftable"
-	printf -v nftcmd '%sadd chain %s %s premangle { type filter hook prerouting priority -150; }\n' "$nftcmd" "$pf" "$nftable"
+	printf -v nftcmd '%sadd chain %s %s premangle { type filter hook prerouting priority -160; }\n' "$nftcmd" "$pf" "$nftable"
 	printf -v nftcmd '%sadd chain %s %s postmangle { type filter hook postrouting priority -150; }\n' "$nftcmd" "$pf" "$nftable"
 	while read -r line; do
 		[[ $line =~ .*inet6?\ ([0-9a-f:.]+)/[0-9]+.* ]] || continue

2.26.2

Setting mark must be done as early as possible in case there are ipv6 rpfilter rules in the mangle table (a nft filter could be done later but with ip6tables this is the latest it can be checked). Mark must be set before the return path check for it to work correctly.

priority -160 gets rendered as "mangle - 10" in nft list table, and will correctly set the mark before other mangle prerouting rules if there are any and same as before if there aren't.

---
bump ?
I don't mind being plain refused, but no answer is annoying :)

Cheers,

 src/wg-quick/linux.bash | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 7c2c002..9001c6a 100755
@@ -222,7 +222,7 @@ add_default() {
 	local marker="-m comment --comment \"wg-quick(8) rule for $INTERFACE\"" restore=$'*raw\n' nftable="wg-quick-$INTERFACE" nftcmd 
 	printf -v nftcmd '%sadd table %s %s\n' "$nftcmd" "$pf" "$nftable"
 	printf -v nftcmd '%sadd chain %s %s preraw { type filter hook prerouting priority -300; }\n' "$nftcmd" "$pf" "$nftable"
-	printf -v nftcmd '%sadd chain %s %s premangle { type filter hook prerouting priority -150; }\n' "$nftcmd" "$pf" "$nftable"
+	printf -v nftcmd '%sadd chain %s %s premangle { type filter hook prerouting priority -160; }\n' "$nftcmd" "$pf" "$nftable"
 	printf -v nftcmd '%sadd chain %s %s postmangle { type filter hook postrouting priority -150; }\n' "$nftcmd" "$pf" "$nftable"
 	while read -r line; do
 		[[ $line =~ .*inet6?\ ([0-9a-f:.]+)/[0-9]+.* ]] || continue

2.26.2