NF_ACCEPT is a terminal verdict in nftables — when a packet returns
from NFQUEUE with NF_ACCEPT, it exits the current chain immediately
and continues to the next hook priority. Rules placed after the queue
statement in the same chain are never evaluated.
This meant that the `ct mark set meta mark` rule (which saves the
bypass decision to conntrack for subsequent packets) was dead code.
The first SYN packet received the correct mark from NFQUEUE, but
conntrack never stored it, so all subsequent packets of the same
connection were redirected to sing-box userspace.
Fix: use NF_REPEAT instead of NF_ACCEPT for bypass and reset verdicts.
NF_REPEAT re-enters the chain from the beginning with the mark already
set on skb->mark. Reorder the prematch chain rules so mark-checking
rules (ct mark set, reject) come before the queue statement:
1. meta mark == outputMark → ct mark set meta mark, return
2. meta mark == resetMark → reject with tcp reset
3. ct mark == outputMark → return
4. TCP SYN → queue to NFQUEUE
This is the standard pattern used by Suricata and other NFQUEUE-based
systems (NF_REPEAT + mark-based skip).
Tested on Orange Pi Zero 3 (arm64, kernel 6.12.58) with sing-box 1.13.3.
Bypass correctly saves ct mark, subsequent packets skip NFQUEUE entirely.
REDIRECT in the OUTPUT chain rewrites the destination to 127.0.0.1,
then ip_route_me_harder() reroutes with the socket's bound interface
constraint (flowi4_oif). Since 127.0.0.1 is only reachable via lo,
the routing lookup fails and the packet is silently dropped.
Add a fallback routing table with `local 127.0.0.1` entries for each
non-loopback interface. When the local table lookup fails due to OIF
mismatch, the fallback table provides a matching RTN_LOCAL route.
The kernel then overrides dev_out to loopback (route.c:2857), so the
packet is delivered locally to the redirect server as intended.
This fixes NetworkManager connectivity checks and other tools that
use SO_BINDTODEVICE (e.g. curl --interface).
