doc: tproxy is non-terminal in nftables

iptables TPROXY issues NF_ACCEPT while nftables tproxy allows for
post-processing. Update examples. For more info, see:

https://lore.kernel.org/netfilter-devel/ZuSh_Io3Yt8LkyUh@orbyte.nwl.cc/T/

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 38 insertions, 7 deletions

diff --git a/doc/statements.txt b/doc/statements.txt
index 5becf0cb..74af1d1a 100644
--- a/doc/statements.txt
+++ b/doc/statements.txt
@@ -583,27 +583,58 @@ this case the rule will match for both families.
 table ip x {
     chain y {
         type filter hook prerouting priority mangle; policy accept;
-        tcp dport ntp tproxy to 1.1.1.1
-        udp dport ssh tproxy to :2222
+        tcp dport ntp tproxy to 1.1.1.1 accept
+        udp dport ssh tproxy to :2222 accept
     }
 }
 table ip6 x {
     chain y {
        type filter hook prerouting priority mangle; policy accept;
-       tcp dport ntp tproxy to [dead::beef]
-       udp dport ssh tproxy to :2222
+       tcp dport ntp tproxy to [dead::beef] accept
+       udp dport ssh tproxy to :2222 accept
     }
 }
 table inet x {
     chain y {
         type filter hook prerouting priority mangle; policy accept;
-        tcp dport 321 tproxy to :ssh
-        tcp dport 99 tproxy ip to 1.1.1.1:999
-        udp dport 155 tproxy ip6 to [dead::beef]:smux
+        tcp dport 321 tproxy to :22 accept
+        tcp dport 99 tproxy ip to 1.1.1.1:999 accept
+        udp dport 155 tproxy ip6 to [dead::beef]:smux accept
     }
 }
 -------------------------------------
 
+Note that the tproxy statement is non-terminal to allow post-processing of
+packets. This allows packets to be logged for debugging as well as updating the
+mark to ensure that packets are delivered locally through policy routing rules.
+
+.Example ruleset for tproxy statement with logging and meta mark
+-------------------------------------
+table inet x {
+    chain y {
+        type filter hook prerouting priority mangle; policy accept;
+        udp dport 9999 goto {
+            tproxy to :1234 log prefix "packet tproxied: " meta mark set 1 accept
+            log prefix "no socket on port 1234 or not transparent?: " drop
+        }
+    }
+}
+-------------------------------------
+
+As packet headers are unchanged, packets might be forwarded instead of delivered
+locally. As mentioned above, this can be avoided by adding policy routing rules
+and the packet mark.
+
+.Example policy routing rules for local redirection
+----------------------------------------------------
+ip rule add fwmark 1 lookup 100
+ip route add local 0.0.0.0/0 dev lo table 100
+----------------------------------------------------
+
+This is a change in behavior compared to the legacy iptables TPROXY target
+which is terminal. To terminate the packet processing after the tproxy
+statement, remember to issue a verdict as in the example above.
+
 SYNPROXY STATEMENT
 ~~~~~~~~~~~~~~~~~~
 This statement will process TCP three-way-handshake parallel in netfilter