cmd/serve: don't convert localhost to 127.0.0.1

This is not valid in many situations, specifically when running a local astro site that listens on localhost, but ignores 127.0.0.1

Fixes: https://github.com/tailscale/tailscale/issues/12201

Signed-off-by: Josh McKinney <joshka@users.noreply.github.com>

ipn/serve.go

@@ -538,7 +538,7 @@ func ExpandProxyTargetValue(target string, supportedSchemes []string, defaultSch
 		return "", fmt.Errorf("invalid port %q", u.Port())
 	}
 
-	u.Host = fmt.Sprintf("%s:%d", host, port)
+	u.Host = fmt.Sprintf("%s:%d", u.Hostname(), port)
 
 	return u.String(), nil
 }

ipn/serve_test.go

@@ -137,14 +137,13 @@ func TestExpandProxyTargetDev(t *testing.T) {
 		wantErr          bool
 	}{
 		{name: "port-only", input: "8080", expected: "http://127.0.0.1:8080"},
-		{name: "hostname+port", input: "localhost:8080", expected: "http://127.0.0.1:8080"},
-		{name: "convert-localhost", input: "http://localhost:8080", expected: "http://127.0.0.1:8080"},
+		{name: "hostname+port", input: "localhost:8080", expected: "http://localhost:8080"},
 		{name: "no-change", input: "http://127.0.0.1:8080", expected: "http://127.0.0.1:8080"},
 		{name: "include-path", input: "http://127.0.0.1:8080/foo", expected: "http://127.0.0.1:8080/foo"},
-		{name: "https-scheme", input: "https://localhost:8080", expected: "https://127.0.0.1:8080"},
-		{name: "https+insecure-scheme", input: "https+insecure://localhost:8080", expected: "https+insecure://127.0.0.1:8080"},
-		{name: "change-default-scheme", input: "localhost:8080", defaultScheme: "https", expected: "https://127.0.0.1:8080"},
-		{name: "change-supported-schemes", input: "localhost:8080", defaultScheme: "tcp", supportedSchemes: []string{"tcp"}, expected: "tcp://127.0.0.1:8080"},
+		{name: "https-scheme", input: "https://localhost:8080", expected: "https://localhost:8080"},
+		{name: "https+insecure-scheme", input: "https+insecure://localhost:8080", expected: "https+insecure://localhost:8080"},
+		{name: "change-default-scheme", input: "localhost:8080", defaultScheme: "https", expected: "https://localhost:8080"},
+		{name: "change-supported-schemes", input: "localhost:8080", defaultScheme: "tcp", supportedSchemes: []string{"tcp"}, expected: "tcp://localhost:8080"},
 
 		// errors
 		{name: "invalid-port", input: "localhost:9999999", wantErr: true},