util/linuxfw: add improved health message for bad kernels

Based on recent bug activity it appears a good number of major distros
have just shipped Linux 6.8.0, a no longer maintained branch, with
backports from other branches but not their corresponding fixes.

Add an explicit check for this output, including a pointer to the
relevant kernel commit, and the suggested workaround.

Updates #13863

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

util/linuxfw/iptables_runner.go

@@ -455,7 +455,14 @@ func (i *iptablesRunner) addBase6(tunname string) error {
 
 	args = []string{"-i", tunname, "-j", "MARK", "--set-mark", TailscaleSubnetRouteMark + "/" + TailscaleFwmarkMask}
 	if err := i.ipt6.Append("filter", "ts-forward", args...); err != nil {
-		return fmt.Errorf("adding %v in v6/filter/ts-forward: %w", args, err)
+		if strings.Contains("MARK: bad value for option \"--set-mark\", or out of range (0-4294967295)", err.Error()) {
+			err = fmt.Errorf(
+				"The Linux kernel release you are running contains a bug that prevents setting packet marks via iptables in IPv6\n" +
+					"Please upgrade your kernel / seek a fix from your kernel vendor.\n" +
+					"See https://github.com/torvalds/linux/commit/306ed1728e8438caed30332e1ab46b28c25fe3d8\n" +
+					"To work around this issue, you can set TS_DEBUG_FIREWALL_MODE=nftables in /etc/default/tailscale")
+		}
+		return err
 	}
 	args = []string{"-m", "mark", "--mark", TailscaleSubnetRouteMark + "/" + TailscaleFwmarkMask, "-j", "ACCEPT"}
 	if err := i.ipt6.Append("filter", "ts-forward", args...); err != nil {