chore: add Coder service prefix to tailnet (#14943)

re: #14715

This PR introduces the Coder service prefix: `fd60:627a:a42b::/48` and refactors our existing code as calling the Tailscale service prefix explicitly (rather than implicitly).

Removes the unused `Addresses` agent option. All clients today assume they can compute the Agent's IP address based on its UUID, so an agent started with a custom address would break things.

coderd/coderd_test.go

@@ -83,7 +83,7 @@ func TestDERP(t *testing.T) {
 			},
 		},
 	}
-	w1IP := tailnet.IP()
+	w1IP := tailnet.TailscaleServicePrefix.RandomAddr()
 	w1, err := tailnet.NewConn(&tailnet.Options{
 		Addresses: []netip.Prefix{netip.PrefixFrom(w1IP, 128)},
 		Logger:    logger.Named("w1"),
@@ -92,7 +92,7 @@ func TestDERP(t *testing.T) {
 	require.NoError(t, err)
 
 	w2, err := tailnet.NewConn(&tailnet.Options{
-		Addresses: []netip.Prefix{netip.PrefixFrom(tailnet.IP(), 128)},
+		Addresses: []netip.Prefix{tailnet.TailscaleServicePrefix.RandomPrefix()},
 		Logger:    logger.Named("w2"),
 		DERPMap:   derpMap,
 	})

coderd/tailnet.go

@@ -61,7 +61,7 @@ func NewServerTailnet(
 ) (*ServerTailnet, error) {
 	logger = logger.Named("servertailnet")
 	conn, err := tailnet.NewConn(&tailnet.Options{
-		Addresses:           []netip.Prefix{netip.PrefixFrom(tailnet.IP(), 128)},
+		Addresses:           []netip.Prefix{tailnet.TailscaleServicePrefix.RandomPrefix()},
 		DERPForceWebSockets: derpForceWebSockets,
 		Logger:              logger,
 		BlockEndpoints:      blockEndpoints,
@@ -352,7 +352,7 @@ func (s *ServerTailnet) ReverseProxy(targetURL, dashboardURL *url.URL, agentID u
 	// "localhost:port", causing connections to be shared across agents.
 	tgt := *targetURL
 	_, port, _ := net.SplitHostPort(tgt.Host)
-	tgt.Host = net.JoinHostPort(tailnet.IPFromUUID(agentID).String(), port)
+	tgt.Host = net.JoinHostPort(tailnet.TailscaleServicePrefix.AddrFromUUID(agentID).String(), port)
 
 	proxy := httputil.NewSingleHostReverseProxy(&tgt)
 	proxy.ErrorHandler = func(w http.ResponseWriter, r *http.Request, theErr error) {

coderd/tailnet_test.go

@@ -186,7 +186,9 @@ func TestServerTailnet_ReverseProxy(t *testing.T) {
 		// Ensure the reverse proxy director rewrites the url host to the agent's IP.
 		rp.Director(req)
 		assert.Equal(t,
-			fmt.Sprintf("[%s]:%d", tailnet.IPFromUUID(a.id).String(), workspacesdk.AgentHTTPAPIServerPort),
+			fmt.Sprintf("[%s]:%d",
+				tailnet.TailscaleServicePrefix.AddrFromUUID(a.id).String(),
+				workspacesdk.AgentHTTPAPIServerPort),
 			req.URL.Host,
 		)
 	})