util/linuxfw,wgengine/router: add connmark rules for rp_filter workaround (#18860)

When a Linux system acts as an exit node or subnet router with strict reverse path filtering (rp_filter=1), reply packets may be dropped because they fail the RPF check. Reply packets arrive on the WAN interface but the routing table indicates they should have arrived on the Tailscale interface, causing the kernel to drop them. This adds firewall rules in the mangle table to save outbound packet marks to conntrack and restore them on reply packets before the routing decision. When reply packets have their marks restored, the kernel uses the correct routing table (based on the mark) and the packets pass the rp_filter check. Implementation adds two rules per address family (IPv4/IPv6): - mangle/OUTPUT: Save packet marks to conntrack for NEW connections with non-zero marks in the Tailscale fwmark range (0xff0000) - mangle/PREROUTING: Restore marks from conntrack to packets for ESTABLISHED,RELATED connections before routing decision and rp_filter check The workaround is automatically enabled when UseConnmarkForRPFilter is set in the router configuration, which happens when subnet routes are advertised on Linux systems. Both iptables and nftables implementations are provided, with automatic backend detection. Fixes #3310 Fixes #14409 Fixes #12022 Fixes #15815 Fixes #9612 Signed-off-by: Mike O'Driscoll <mikeo@tailscale.com>
2026-03-04 14:09:11 -05:00
parent dab8922fcf
commit 26ef46bf81
6 changed files with 814 additions and 12 deletions
@@ -527,6 +527,104 @@ func (i *iptablesRunner) DelStatefulRule(tunname string) error {
 	return nil
 }

+// AddConnmarkSaveRule adds conntrack marking rules to save and restore marks.
+// These rules run in mangle/PREROUTING (to restore marks from conntrack) and
+// mangle/OUTPUT (to save marks to conntrack) before rp_filter checks, enabling
+// proper routing table lookups for exit nodes and subnet routers.
+func (i *iptablesRunner) AddConnmarkSaveRule() error {
+	// Check if rules already exist (idempotency)
+	for _, ipt := range []iptablesInterface{i.ipt4, i.ipt6} {
+		rules, err := ipt.List("mangle", "PREROUTING")
+		if err != nil {
+			continue
+		}
+		// Look for existing connmark restore rule
+		for _, rule := range rules {
+			if strings.Contains(rule, "CONNMARK") &&
+				strings.Contains(rule, "restore-mark") &&
+				strings.Contains(rule, "ctmask 0xff0000") {
+				// Rules already exist, skip adding
+				return nil
+			}
+		}
+	}
+
+	// mangle/PREROUTING: Restore mark from conntrack for ESTABLISHED/RELATED connections
+	// This runs BEFORE routing decision and rp_filter check
+	for _, ipt := range []iptablesInterface{i.ipt4, i.ipt6} {
+		args := []string{
+			"-m", "conntrack",
+			"--ctstate", "ESTABLISHED,RELATED",
+			"-j", "CONNMARK",
+			"--restore-mark",
+			"--nfmask", fwmarkMask,
+			"--ctmask", fwmarkMask,
+		}
+		if err := ipt.Insert("mangle", "PREROUTING", 1, args...); err != nil {
+			return fmt.Errorf("adding %v in mangle/PREROUTING: %w", args, err)
+		}
+	}
+
+	// mangle/OUTPUT: Save mark to conntrack for NEW connections with non-zero marks
+	for _, ipt := range []iptablesInterface{i.ipt4, i.ipt6} {
+		args := []string{
+			"-m", "conntrack",
+			"--ctstate", "NEW",
+			"-m", "mark",
+			"!", "--mark", "0x0/" + fwmarkMask,
+			"-j", "CONNMARK",
+			"--save-mark",
+			"--nfmask", fwmarkMask,
+			"--ctmask", fwmarkMask,
+		}
+		if err := ipt.Insert("mangle", "OUTPUT", 1, args...); err != nil {
+			return fmt.Errorf("adding %v in mangle/OUTPUT: %w", args, err)
+		}
+	}
+
+	return nil
+}
+
+// DelConnmarkSaveRule removes conntrack marking rules added by AddConnmarkSaveRule.
+func (i *iptablesRunner) DelConnmarkSaveRule() error {
+	for _, ipt := range []iptablesInterface{i.ipt4, i.ipt6} {
+		// Delete PREROUTING rule
+		args := []string{
+			"-m", "conntrack",
+			"--ctstate", "ESTABLISHED,RELATED",
+			"-j", "CONNMARK",
+			"--restore-mark",
+			"--nfmask", fwmarkMask,
+			"--ctmask", fwmarkMask,
+		}
+		if err := ipt.Delete("mangle", "PREROUTING", args...); err != nil {
+			if !isNotExistError(err) {
+				return fmt.Errorf("deleting connmark rule in mangle/PREROUTING: %w", err)
+			}
+			// Rule doesn't exist - this is fine for idempotency
+		}
+
+		// Delete OUTPUT rule
+		args = []string{
+			"-m", "conntrack",
+			"--ctstate", "NEW",
+			"-m", "mark",
+			"!", "--mark", "0x0/" + fwmarkMask,
+			"-j", "CONNMARK",
+			"--save-mark",
+			"--nfmask", fwmarkMask,
+			"--ctmask", fwmarkMask,
+		}
+		if err := ipt.Delete("mangle", "OUTPUT", args...); err != nil {
+			if !isNotExistError(err) {
+				return fmt.Errorf("deleting connmark rule in mangle/OUTPUT: %w", err)
+			}
+			// Rule doesn't exist - this is fine for idempotency
+		}
+	}
+	return nil
+}
+
 // buildMagicsockPortRule generates the string slice containing the arguments
 // to describe a rule accepting traffic on a particular port to iptables. It is
 // separated out here to avoid repetition in AddMagicsockPortRule and