From 91d28e71555bfffc90a21074493b79fcf2477346 Mon Sep 17 00:00:00 2001
From: Andrew Dunham <andrew@du.nham.ca>
Date: Mon, 19 Sep 2022 10:57:36 -0400
Subject: [PATCH] wgengine/router: handle DisableIPv4 in Linux router

---
 wgengine/router/router_linux.go | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/wgengine/router/router_linux.go b/wgengine/router/router_linux.go
index b67d4ddc1..4a81d6461 100644
--- a/wgengine/router/router_linux.go
+++ b/wgengine/router/router_linux.go
@@ -112,6 +112,7 @@ type linuxRouter struct {
 	v6Available     bool
 	v6NATAvailable  bool
 	fwmaskWorks     bool // whether we can use 'ip rule...fwmark <mark>/<mask>'
+	hasV4Prefix     bool // false when 'DisableIPv4' is set on the tailnet
 
 	// ipPolicyPrefBase is the base priority at which ip rules are installed.
 	ipPolicyPrefBase int
@@ -418,6 +419,26 @@ func (r *linuxRouter) Set(cfg *Config) error {
 		cfg = &shutdownConfig
 	}
 
+	// Because the tailnet may have IPv4 disabled, check if we have any v4
+	// prefixes from addresses, routes, or local routes.
+	r.hasV4Prefix = false
+	findV4 := func(arr []netip.Prefix) {
+		// Skip useless loop if we've already found a v4 prefix
+		if r.hasV4Prefix {
+			return
+		}
+		for _, pref := range arr {
+			if pref.Addr().Is4() {
+				r.hasV4Prefix = true
+				return
+			}
+		}
+	}
+	findV4(cfg.LocalAddrs)
+	findV4(cfg.Routes)
+	findV4(cfg.LocalRoutes)
+	findV4(cfg.SubnetRoutes)
+
 	if err := r.setNetfilterMode(cfg.NetfilterMode); err != nil {
 		errs = append(errs, err)
 	}