bolt more stuff onto tun to help auto-assign snat addresses

overlay/tun.go

@@ -1,6 +1,7 @@
 package overlay
 
 import (
+	"crypto/rand"
 	"fmt"
 	"net"
 	"net/netip"
@@ -129,3 +130,53 @@ func selectGateway(dest netip.Prefix, gateways []netip.Prefix) (netip.Prefix, er
 
 	return netip.Prefix{}, fmt.Errorf("no gateway found for %v in the list of vpn networks", dest)
 }
+
+func prepareSnatAddr(d Device, l *logrus.Logger, c *config.C, routes []Route) netip.Prefix {
+	if !d.Networks()[0].Addr().Is6() {
+		return netip.Prefix{} //if we have an IPv4 assignment within the overlay, we don't need a snat address
+	}
+
+	addSnatAddr := false
+	for _, un := range d.UnsafeNetworks() { //if we are an unsafe router for an IPv4 range
+		if un.Addr().Is4() {
+			addSnatAddr = true
+			break
+		}
+	}
+	for _, route := range routes { //or if we have a route defined into an IPv4 range
+		if route.Cidr.Addr().Is4() {
+			addSnatAddr = true //todo should this only apply to unsafe routes?
+			break
+		}
+	}
+	if !addSnatAddr {
+		return netip.Prefix{}
+	}
+
+	var err error
+	out := netip.Addr{}
+	if a := c.GetString("tun.snat_address_for_4over6", ""); a != "" {
+		out, err = netip.ParseAddr(a)
+		if err != nil {
+			l.WithField("value", a).WithError(err).Warn("failed to parse tun.snat_address_for_4over6, will use a random value")
+		} else if !out.Is4() || !out.IsLinkLocalUnicast() {
+			l.WithField("value", out).Warn("tun.snat_address_for_4over6 must be an IPv4 address")
+		}
+	}
+	if !out.IsValid() {
+		octets := []byte{169, 254, 0, 0}
+		_, _ = rand.Read(octets[2:4])
+		if octets[3] == 0 {
+			octets[3] = 1 //please no .0 addresses
+		} else if octets[2] == 255 && octets[3] == 255 {
+			octets[3] = 254 //please no broadcast addresses
+		}
+		ok := false
+		out, ok = netip.AddrFromSlice(octets)
+		if !ok {
+			l.Error("failed to produce a valid IPv4 address for tun.snat_address_for_4over6")
+			return netip.Prefix{}
+		}
+	}
+	return netip.PrefixFrom(out, 32)
+}