fix: use NF_REPEAT for NFQUEUE bypass/reset verdicts

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.

nfqueue_linux.go

@@ -214,11 +214,15 @@ func (h *nfqueueHandler) handlePacket(attr nfqueue.Attribute) int {
 
 	_, pErr := h.handler.PrepareConnection(N.NetworkTCP, srcAddr, dstAddr, nil, 0)
 
+	// Use NfRepeat for bypass/reset so the packet re-enters the chain
+	// from the beginning, allowing mark-checking rules to save the mark
+	// to conntrack. NfAccept is a terminal verdict in nftables — it exits
+	// the chain immediately, skipping any rules after the queue statement.
 	switch {
 	case errors.Is(pErr, ErrBypass):
-		h.setVerdict(packetID, nfqueue.NfAccept, h.outputMark)
+		h.setVerdict(packetID, nfqueue.NfRepeat, h.outputMark)
 	case errors.Is(pErr, ErrReset):
-		h.setVerdict(packetID, nfqueue.NfAccept, h.resetMark)
+		h.setVerdict(packetID, nfqueue.NfRepeat, h.resetMark)
 	case errors.Is(pErr, ErrDrop):
 		h.setVerdict(packetID, nfqueue.NfDrop, 0)
 	default:

redirect_nftables.go

@@ -423,16 +423,39 @@ func (r *autoRedirect) nftablesAddPreMatchRules(nft *nftables.Conn, table *nftab
 		},
 	})
 
+	// Bypass mark: save to conntrack and return.
+	// When the NFQUEUE handler returns NF_REPEAT with the output mark,
+	// the packet re-enters this chain from the beginning. This rule
+	// catches it, saves the mark to conntrack (so subsequent packets
+	// of the same connection are bypassed via ct mark check below),
+	// and returns.
 	nft.AddRule(&nftables.Rule{
 		Table: table,
 		Chain: chain,
 		Exprs: []expr.Any{
 			&expr.Meta{Key: expr.MetaKeyMARK, Register: 1},
 			&expr.Cmp{Op: expr.CmpOpEq, Register: 1, Data: binaryutil.NativeEndian.PutUint32(r.effectiveOutputMark())},
+			&expr.Ct{Key: expr.CtKeyMARK, Register: 1, SourceRegister: true},
+			&expr.Counter{},
 			&expr.Verdict{Kind: expr.VerdictReturn},
 		},
 	})
 
+	// Reset mark: reject with TCP RST.
+	// When the NFQUEUE handler returns NF_REPEAT with the reset mark,
+	// the packet re-enters this chain and is rejected here.
+	nft.AddRule(&nftables.Rule{
+		Table: table,
+		Chain: chain,
+		Exprs: []expr.Any{
+			&expr.Meta{Key: expr.MetaKeyMARK, Register: 1},
+			&expr.Cmp{Op: expr.CmpOpEq, Register: 1, Data: binaryutil.NativeEndian.PutUint32(r.effectiveResetMark())},
+			&expr.Counter{},
+			&expr.Reject{Type: unix.NFT_REJECT_TCP_RST},
+		},
+	})
+
+	// Already-tracked bypass connections: return immediately.
 	nft.AddRule(&nftables.Rule{
 		Table: table,
 		Chain: chain,
@@ -443,6 +466,7 @@ func (r *autoRedirect) nftablesAddPreMatchRules(nft *nftables.Conn, table *nftab
 		},
 	})
 
+	// TCP SYN: send to NFQUEUE for pre-match evaluation.
 	nft.AddRule(&nftables.Rule{
 		Table: table,
 		Chain: chain,
@@ -469,26 +493,4 @@ func (r *autoRedirect) nftablesAddPreMatchRules(nft *nftables.Conn, table *nftab
 			},
 		},
 	})
-
-	nft.AddRule(&nftables.Rule{
-		Table: table,
-		Chain: chain,
-		Exprs: []expr.Any{
-			&expr.Meta{Key: expr.MetaKeyMARK, Register: 1},
-			&expr.Cmp{Op: expr.CmpOpEq, Register: 1, Data: binaryutil.NativeEndian.PutUint32(r.effectiveResetMark())},
-			&expr.Counter{},
-			&expr.Reject{Type: unix.NFT_REJECT_TCP_RST},
-		},
-	})
-
-	nft.AddRule(&nftables.Rule{
-		Table: table,
-		Chain: chain,
-		Exprs: []expr.Any{
-			&expr.Meta{Key: expr.MetaKeyMARK, Register: 1},
-			&expr.Cmp{Op: expr.CmpOpEq, Register: 1, Data: binaryutil.NativeEndian.PutUint32(r.effectiveOutputMark())},
-			&expr.Ct{Key: expr.CtKeyMARK, Register: 1, SourceRegister: true},
-			&expr.Counter{},
-		},
-	})
 }