cmd/tailscale: add "debug via" subcommand to do CIDR math for via ranges

$ tailscale debug via 0xb 10.2.0.0/16
fd7a:115c:a1e0:b1a:0🅱️a02:0/112
$ tailscale debug via fd7a:115c:a1e0:b1a:0🅱️a02:0/112
site 11 (0xb), 10.2.0.0/16

Previously: 3ae701f0ebe053a1f7b6a3fa345a56b3132c818f

This adds a little debug tool to do CIDR math to make converting between
those ranges easier for now.

Updates #3616

Change-Id: I98302e95d17765bfaced3ecbb71cbd43e84bff46
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/debug.go

@@ -8,6 +8,7 @@ import (
 	"bufio"
 	"bytes"
 	"context"
+	"encoding/binary"
 	"encoding/json"
 	"errors"
 	"flag"
@@ -21,9 +22,11 @@ import (
 	"time"
 
 	"github.com/peterbourgon/ff/v3/ffcli"
+	"inet.af/netaddr"
 	"tailscale.com/client/tailscale"
 	"tailscale.com/hostinfo"
 	"tailscale.com/ipn"
+	"tailscale.com/net/tsaddr"
 	"tailscale.com/paths"
 	"tailscale.com/safesocket"
 )
@@ -106,6 +109,11 @@ var debugCmd = &ffcli.Command{
 				return fs
 			})(),
 		},
+		{
+			Name:      "via",
+			Exec:      runVia,
+			ShortHelp: "convert between site-specific IPv4 CIDRs and IPv6 'via' routes",
+		},
 	},
 }
 
@@ -348,3 +356,46 @@ func runDaemonMetrics(ctx context.Context, args []string) error {
 		time.Sleep(time.Second)
 	}
 }
+
+func runVia(ctx context.Context, args []string) error {
+	switch len(args) {
+	default:
+		return errors.New("expect either <site-id> <v4-cidr> or <v6-route>")
+	case 1:
+		ipp, err := netaddr.ParseIPPrefix(args[0])
+		if err != nil {
+			return err
+		}
+		if !ipp.IP().Is6() {
+			return errors.New("with one argument, expect an IPv6 CIDR")
+		}
+		if !tsaddr.TailscaleViaRange().Contains(ipp.IP()) {
+			return errors.New("not a via route")
+		}
+		if ipp.Bits() < 96 {
+			return errors.New("short length, want /96 or more")
+		}
+		v4 := tsaddr.UnmapVia(ipp.IP())
+		a := ipp.IP().As16()
+		siteID := binary.BigEndian.Uint32(a[8:12])
+		fmt.Printf("site %v (0x%x), %v\n", siteID, siteID, netaddr.IPPrefixFrom(v4, ipp.Bits()-96))
+	case 2:
+		siteID, err := strconv.ParseUint(args[0], 0, 32)
+		if err != nil {
+			return fmt.Errorf("invalid site-id %q; must be decimal or hex with 0x prefix", args[0])
+		}
+		if siteID > 0xff {
+			return fmt.Errorf("site-id values over 255 are currently reserved")
+		}
+		ipp, err := netaddr.ParseIPPrefix(args[1])
+		if err != nil {
+			return err
+		}
+		via, err := tsaddr.MapVia(uint32(siteID), ipp)
+		if err != nil {
+			return err
+		}
+		fmt.Println(via)
+	}
+	return nil
+}

net/tsaddr/tsaddr.go

@@ -6,6 +6,8 @@
 package tsaddr
 
 import (
+	"encoding/binary"
+	"errors"
 	"sync"
 
 	"inet.af/netaddr"
@@ -280,3 +282,17 @@ func UnmapVia(ip netaddr.IP) netaddr.IP {
 	}
 	return ip
 }
+
+// MapVia returns an IPv6 "via" route for an IPv4 CIDR in a given siteID.
+func MapVia(siteID uint32, v4 netaddr.IPPrefix) (via netaddr.IPPrefix, err error) {
+	if !v4.IP().Is4() {
+		return via, errors.New("want IPv4 CIDR with a site ID")
+	}
+	viaRange16 := TailscaleViaRange().IP().As16()
+	var a [16]byte
+	copy(a[:], viaRange16[:8])
+	binary.BigEndian.PutUint32(a[8:], siteID)
+	ip4a := v4.IP().As4()
+	copy(a[12:], ip4a[:])
+	return netaddr.IPPrefixFrom(netaddr.IPFrom16(a), v4.Bits()+64+32), nil
+}