net/tsaddr: add MapViaAddr function

Change-Id: I45122fd65c7133fb3d2cbc7eec6f1de89d768ef4
Signed-off-by: Andrew Dunham <andrew@tailscale.com>

net/tsaddr/tsaddr.go

@@ -309,3 +309,17 @@ func MapVia(siteID uint32, v4 netip.Prefix) (via netip.Prefix, err error) {
 	copy(a[12:], ip4a[:])
 	return netip.PrefixFrom(netip.AddrFrom16(a), v4.Bits()+64+32), nil
 }
+
+// MapViaAddr returns an IPv6 "via" address for an IPv4 address in a given siteID.
+func MapViaAddr(siteID uint32, v4 netip.Addr) (netip.Addr, error) {
+	if !v4.Is4() {
+		return netip.Addr{}, errors.New("want IPv4 address with a site ID")
+	}
+	viaRange16 := TailscaleViaRange().Addr().As16()
+	var a [16]byte
+	copy(a[:], viaRange16[:8])
+	binary.BigEndian.PutUint32(a[8:], siteID)
+	ip4a := v4.As4()
+	copy(a[12:], ip4a[:])
+	return netip.AddrFrom16(a), nil
+}

net/tsaddr/tsaddr_test.go

@@ -105,3 +105,36 @@ func TestUnmapVia(t *testing.T) {
 		}
 	}
 }
+
+func TestMapViaAddr(t *testing.T) {
+	tests := []struct {
+		ip   string
+		site uint32
+		want string
+	}{
+		{"1.2.3.4", 0, "fd7a:115c:a1e0:b1a::102:304"},
+		{"1.2.3.4", 7, "fd7a:115c:a1e0:b1a:0:7:102:304"},
+	}
+	for _, tt := range tests {
+		got, err := MapViaAddr(tt.site, netip.MustParseAddr(tt.ip))
+		if err != nil {
+			t.Errorf("for %q @ %d: error: %v", tt.ip, tt.site, err)
+			continue
+		}
+
+		if got.String() != tt.want {
+			t.Errorf("for %q @ %d: got %q, want %q", tt.ip, tt.site, got.String(), tt.want)
+		}
+	}
+
+	t.Run("Error", func(t *testing.T) {
+		addr, err := MapViaAddr(9, netip.MustParseAddr("::1"))
+		want := "want IPv4 address with a site ID"
+		if err == nil || err.Error() != want {
+			t.Errorf("got err=%v; want %q", err, want)
+		}
+		if addr.IsValid() {
+			t.Errorf("expected invalid Addr")
+		}
+	})
+}