Probe a tunnel's other remotes when its current one is unreachable

udp/unreachable_windows.go

@@ -0,0 +1,18 @@
+package udp
+
+import (
+	"errors"
+
+	"golang.org/x/sys/windows"
+)
+
+// IsUnreachable reports whether a send failed because this host has no way to reach the destination from where it
+// is now, rather than for something transient. Moving from a dual stack network to one without IPv6 turns every send
+// to an IPv6 remote into one of these.
+func IsUnreachable(err error) bool {
+	return errors.Is(err, windows.WSAENETUNREACH) ||
+		errors.Is(err, windows.WSAEHOSTUNREACH) ||
+		errors.Is(err, windows.WSAEADDRNOTAVAIL) ||
+		errors.Is(err, windows.WSAEAFNOSUPPORT) ||
+		errors.Is(err, ErrInvalidIPv6RemoteForSocket)
+}