cmd/{containerboot,k8s-operator}: add 4via6 support in singleton egress (#19983)

Add support for configuring egress to destinations reachable via 4via6
subnet routes, using either the synthesized 4via6 address or the MagicDNS
name (in the form <IPv4-with-hyphens>-via-<siteID>[.*]).

Also update the Connector to validate and advertise 4via6 subnet routes.

Export net/netutil.ValidateViaPrefix so it can be reused by the Connector
validation logic.

This change only affects standalone egress proxies — ProxyGroup egress
requires IPv6 support before it can use 4via6.

Updates #19334

Change-Id: I6faecd6eb61ab55fc0cd97fe417af6b6a12fe7fc

Signed-off-by: Becky Pauley <becky@tailscale.com>

cmd/containerboot/main.go

@@ -1114,7 +1114,8 @@ func runHTTPServer(mux *http.ServeMux, addr string) (close func() error) {
 }
 
 // resolveTailnetFQDN resolves a tailnet FQDN to a list of IP prefixes, which
-// can be either a peer device or a Tailscale Service.
+// can be either a peer device, a Tailscale Service, or a 4via6 synthesized
+// DNS name (e.g. "10-1-0-5-via-7.tailnet.ts.net").
 func resolveTailnetFQDN(nm netmapState, fqdn string) ([]netip.Prefix, error) {
 	dnsFQDN, err := dnsname.ToFQDN(fqdn)
 	if err != nil {
@@ -1137,8 +1138,20 @@ func resolveTailnetFQDN(nm netmapState, fqdn string) ([]netip.Prefix, error) {
 	if svcIPs := serviceIPsFromNetMap(nm, dnsFQDN); len(svcIPs) != 0 {
 		return svcIPs, nil
 	}
+	// If not found yet, check for a matching 4via6 DNS name.
+	if addr, ok := kubeutils.ResolveViaDomain(dnsFQDN.WithTrailingDot()); ok {
+		prefix := netip.PrefixFrom(addr, addr.BitLen())
+		for nn := range nm.peers() {
+			for _, allowedIP := range nn.AllowedIPs().All() {
+				if allowedIP.Contains(addr) {
+					return []netip.Prefix{prefix}, nil
+				}
+			}
+		}
+		return nil, fmt.Errorf("resolved 4via6 address %v for %q but no peer advertises a route containing it", addr, fqdn)
+	}
 
-	return nil, fmt.Errorf("could not find Tailscale node or service %q; it either does not exist, or not reachable because of ACLs", fqdn)
+	return nil, fmt.Errorf("could not find Tailscale node, service or 4via6 address %q; it either does not exist, or not reachable because of ACLs", fqdn)
 }
 
 // serviceIPsFromNetMap returns all IPs of a Tailscale Service if its FQDN is