util/linuxfw,wgengine/router: enable IPv6 configuration when netfilter is disabled

Updates #11434

Signed-off-by: James Tucker <james@tailscale.com>

util/linuxfw/iptables_runner.go

@@ -59,7 +59,7 @@ func newIPTablesRunner(logf logger.Logf) (*iptablesRunner, error) {
 	}
 
 	supportsV6, supportsV6NAT := false, false
-	v6err := checkIPv6(logf)
+	v6err := CheckIPv6(logf)
 	ip6terr := checkIP6TablesExists()
 	var ipt6 *iptables.IPTables
 	switch {

util/linuxfw/linuxfw.go

@@ -130,7 +130,7 @@ func errCode(err error) int {
 // missing.  It does not check that IPv6 is currently functional or
 // that there's a global address, just that the system would support
 // IPv6 if it were on an IPv6 network.
-func checkIPv6(logf logger.Logf) error {
+func CheckIPv6(logf logger.Logf) error {
 	_, err := os.Stat("/proc/sys/net/ipv6")
 	if os.IsNotExist(err) {
 		return err

util/linuxfw/nftables_runner.go

@@ -546,7 +546,7 @@ func newNfTablesRunner(logf logger.Logf) (*nftablesRunner, error) {
 	}
 	nft4 := &nftable{Proto: nftables.TableFamilyIPv4}
 
-	v6err := checkIPv6(logf)
+	v6err := CheckIPv6(logf)
 	if v6err != nil {
 		logf("disabling tunneled IPv6 due to system IPv6 config: %v", v6err)
 	}

wgengine/router/router_linux.go

@@ -56,6 +56,7 @@ type linuxRouter struct {
 
 	// Various feature checks for the network stack.
 	ipRuleAvailable bool // whether kernel was built with IP_MULTIPLE_TABLES
+	v6Available     bool // whether the kernel supports IPv6
 	fwmaskWorks     bool // whether we can use 'ip rule...fwmark <mark>/<mask>'
 
 	// ipPolicyPrefBase is the base priority at which ip rules are installed.
@@ -142,6 +143,8 @@ func newUserspaceRouterAdvanced(logf logger.Logf, tunname string, netMon *netmon
 		r.logf("mwan3 on openWRT detected, switching policy base priority to 1300")
 	}
 
+	r.v6Available = linuxfw.CheckIPv6(r.logf) == nil
+
 	r.fixupWSLMTU()
 
 	return r, nil
@@ -416,7 +419,7 @@ func (r *linuxRouter) UpdateMagicsockPort(port uint16, network string) error {
 	case "udp4":
 		magicsockPort = &r.magicsockPortV4
 	case "udp6":
-		if !r.nfr.HasIPV6() {
+		if !r.getV6Available() {
 			return nil
 		}
 		magicsockPort = &r.magicsockPortV6
@@ -523,7 +526,7 @@ func (r *linuxRouter) setNetfilterMode(mode preftype.NetfilterMode) error {
 					return fmt.Errorf("could not add magicsock port rule v4: %w", err)
 				}
 			}
-			if r.magicsockPortV6 != 0 && r.nfr.HasIPV6() {
+			if r.magicsockPortV6 != 0 && r.getV6Available() {
 				if err := r.nfr.AddMagicsockPortRule(r.magicsockPortV6, "udp6"); err != nil {
 					return fmt.Errorf("could not add magicsock port rule v6: %w", err)
 				}
@@ -563,7 +566,7 @@ func (r *linuxRouter) setNetfilterMode(mode preftype.NetfilterMode) error {
 					return fmt.Errorf("could not add magicsock port rule v4: %w", err)
 				}
 			}
-			if r.magicsockPortV6 != 0 && r.nfr.HasIPV6() {
+			if r.magicsockPortV6 != 0 && r.getV6Available() {
 				if err := r.nfr.AddMagicsockPortRule(r.magicsockPortV6, "udp6"); err != nil {
 					return fmt.Errorf("could not add magicsock port rule v6: %w", err)
 				}
@@ -602,6 +605,9 @@ func (r *linuxRouter) setNetfilterMode(mode preftype.NetfilterMode) error {
 }
 
 func (r *linuxRouter) getV6Available() bool {
+	if r.netfilterMode == netfilterOff {
+		return r.v6Available
+	}
 	return r.nfr.HasIPV6()
 }