cmd/natc: add test and fix for ip exhaustion

This is a very dumb fix as it has an unbounded worst case runtime. IP
allocation needs to be done in a more sane way in a follow-up.

Updates #15367

Signed-off-by: James Tucker <james@tailscale.com>

cmd/natc/natc.go

@@ -94,18 +94,24 @@ func main() {
 		}
 		ignoreDstTable.Insert(pfx, true)
 	}
-	var v4Prefixes []netip.Prefix
+	var (
+		v4Prefixes    []netip.Prefix
+		numV4DNSAddrs int
+	)
 	for _, s := range strings.Split(*v4PfxStr, ",") {
 		p := netip.MustParsePrefix(strings.TrimSpace(s))
 		if p.Masked() != p {
 			log.Fatalf("v4 prefix %v is not a masked prefix", p)
 		}
 		v4Prefixes = append(v4Prefixes, p)
+		numIPs := 1 << (32 - p.Bits())
+		numV4DNSAddrs += numIPs
 	}
 	if len(v4Prefixes) == 0 {
 		log.Fatalf("no v4 prefixes specified")
 	}
 	dnsAddr := v4Prefixes[0].Addr()
+	numV4DNSAddrs -= 1 // Subtract the dnsAddr allocated above.
 	ts := &tsnet.Server{
 		Hostname: *hostname,
 	}
@@ -153,12 +159,13 @@ func main() {
 	}
 
 	c := &connector{
-		ts:         ts,
-		lc:         lc,
-		dnsAddr:    dnsAddr,
-		v4Ranges:   v4Prefixes,
-		v6ULA:      ula(uint16(*siteID)),
-		ignoreDsts: ignoreDstTable,
+		ts:            ts,
+		lc:            lc,
+		dnsAddr:       dnsAddr,
+		v4Ranges:      v4Prefixes,
+		numV4DNSAddrs: numV4DNSAddrs,
+		v6ULA:         ula(uint16(*siteID)),
+		ignoreDsts:    ignoreDstTable,
 	}
 	c.run(ctx)
 }
@@ -177,6 +184,11 @@ type connector struct {
 	// v4Ranges is the list of IPv4 ranges to advertise and assign addresses from.
 	// These are masked prefixes.
 	v4Ranges []netip.Prefix
+
+	// numV4DNSAddrs is the total size of the IPv4 ranges in addresses, minus the
+	// dnsAddr allocation.
+	numV4DNSAddrs int
+
 	// v6ULA is the ULA prefix used by the app connector to assign IPv6 addresses.
 	v6ULA netip.Prefix
 
@@ -502,6 +514,7 @@ type perPeerState struct {
 	mu           sync.Mutex
 	domainToAddr map[string][]netip.Addr
 	addrToDomain *bart.Table[string]
+	numV4Allocs  int
 }
 
 // domainForIP returns the domain name assigned to the given IP address and
@@ -547,17 +560,25 @@ func (ps *perPeerState) isIPUsedLocked(ip netip.Addr) bool {
 
 // unusedIPv4Locked returns an unused IPv4 address from the available ranges.
 func (ps *perPeerState) unusedIPv4Locked() netip.Addr {
+	// All addresses have been allocated.
+	if ps.numV4Allocs >= ps.c.numV4DNSAddrs {
+		return netip.Addr{}
+	}
+
 	// TODO: skip ranges that have been exhausted
-	for _, r := range ps.c.v4Ranges {
-		ip := randV4(r)
-		for r.Contains(ip) {
+	// TODO: implement a much more efficient algorithm for finding unused IPs,
+	// this is fairly crazy.
+	for {
+		for _, r := range ps.c.v4Ranges {
+			ip := randV4(r)
+			if !r.Contains(ip) {
+				panic("error: randV4 returned invalid address")
+			}
 			if !ps.isIPUsedLocked(ip) && ip != ps.c.dnsAddr {
 				return ip
 			}
-			ip = ip.Next()
 		}
 	}
-	return netip.Addr{}
 }
 
 // randV4 returns a random IPv4 address within the given prefix.
@@ -583,6 +604,7 @@ func (ps *perPeerState) assignAddrsLocked(domain string) []netip.Addr {
 	if !v4.IsValid() {
 		return nil
 	}
+	ps.numV4Allocs++
 	as16 := ps.c.v6ULA.Addr().As16()
 	as4 := v4.As4()
 	copy(as16[12:], as4[:])