tailcfg,ipn,appc: add c2n endpoint for appc domain routes

This change introduces a c2n endpoint that returns a map of domains to a
slice of resolved IP addresses for the domain.

Fixes tailscale/corp#15657

Signed-off-by: Charlotte Brandhorst-Satzkorn <charlotte@tailscale.com>

appc/appconnector.go

@@ -81,6 +81,20 @@ func (e *AppConnector) Domains() views.Slice[string] {
 	return views.SliceOf(xmaps.Keys(e.domains))
 }
 
+// DomainRoutes returns a map of domains to resolved IP
+// addresses.
+func (e *AppConnector) DomainRoutes() map[string][]netip.Addr {
+	e.mu.Lock()
+	defer e.mu.Unlock()
+
+	drCopy := make(map[string][]netip.Addr)
+	for k, v := range e.domains {
+		copy(drCopy[k], v)
+	}
+
+	return drCopy
+}
+
 // ObserveDNSResponse is a callback invoked by the DNS resolver when a DNS
 // response is being returned over the PeerAPI. The response is parsed and
 // matched against the configured domains, if matched the routeAdvertiser is

ipn/ipnlocal/c2n.go

@@ -121,6 +121,15 @@ func (b *LocalBackend) handleC2N(w http.ResponseWriter, r *http.Request) {
 			return
 		}
 		writeJSON(w, res)
+	case "/appconnector/routes":
+		switch r.Method {
+		case httpm.GET:
+			b.handleC2NAppConnectorDomainRoutesGet(w, r)
+			return
+		default:
+			http.Error(w, "bad method", http.StatusMethodNotAllowed)
+			return
+		}
 	case "/sockstats":
 		if r.Method != "POST" {
 			http.Error(w, "bad method", http.StatusMethodNotAllowed)
@@ -139,6 +148,26 @@ func (b *LocalBackend) handleC2N(w http.ResponseWriter, r *http.Request) {
 	}
 }
 
+// handleC2NAppConnectorDomainRoutesGet handles returning the domains
+// that the app connector is responsible for, as well as the resolved
+// IP addresses for each domain. If the node is not configured as
+// an app connector, an empty map is returned.
+func (b *LocalBackend) handleC2NAppConnectorDomainRoutesGet(w http.ResponseWriter, r *http.Request) {
+	b.logf("c2n: GET /appconnector/routes received")
+
+	var res tailcfg.C2NAppConnectorDomainRoutesResponse
+	if b.appConnector == nil {
+		w.Header().Set("Content-Type", "application/json")
+		json.NewEncoder(w).Encode(res)
+		return
+	}
+
+	res.Domains = b.appConnector.DomainRoutes()
+
+	w.Header().Set("Content-Type", "application/json")
+	json.NewEncoder(w).Encode(res)
+}
+
 func (b *LocalBackend) handleC2NUpdateGet(w http.ResponseWriter, r *http.Request) {
 	b.logf("c2n: GET /update received")
 

tailcfg/c2ntypes.go

@@ -5,6 +5,8 @@
 
 package tailcfg
 
+import "net/netip"
+
 // C2NSSHUsernamesRequest is the request for the /ssh/usernames.
 // A GET request without a request body is equivalent to the zero value of this type.
 // Otherwise, a POST request with a JSON-encoded request body is expected.
@@ -64,3 +66,12 @@ type C2NPostureIdentityResponse struct {
 	// device posture collection.
 	PostureDisabled bool `json:",omitempty"`
 }
+
+// C2NAppConnectorDomainRoutesResponse contains a map of domains to
+// slice of addresses, indicating what IP addresses have been resolved
+// for each domain.
+type C2NAppConnectorDomainRoutesResponse struct {
+	// Domains is a map of lower case domain names with no trailing dot,
+	// to a list of resolved IP addresses.
+	Domains map[string][]netip.Addr
+}