listeners: Add support for named socket activation (#7243)

listeners.go

@@ -38,6 +38,10 @@ import (
 	"github.com/caddyserver/caddy/v2/internal"
 )
 
+// listenFdsStart is the first file descriptor number for systemd socket activation.
+// File descriptors 0, 1, 2 are reserved for stdin, stdout, stderr.
+const listenFdsStart = 3
+
 // NetworkAddress represents one or more network addresses.
 // It contains the individual components for a parsed network
 // address of the form accepted by ParseNetworkAddress().
@@ -305,6 +309,64 @@ func IsFdNetwork(netw string) bool {
 	return strings.HasPrefix(netw, "fd")
 }
 
+// getFdByName returns the file descriptor number for the given
+// socket name from systemd's LISTEN_FDNAMES environment variable.
+// Socket names are provided by systemd via socket activation.
+//
+// The name can optionally include an index to handle multiple sockets
+// with the same name: "web:0" for first, "web:1" for second, etc.
+// If no index is specified, defaults to index 0 (first occurrence).
+func getFdByName(nameWithIndex string) (int, error) {
+	if nameWithIndex == "" {
+		return 0, fmt.Errorf("socket name cannot be empty")
+	}
+
+	fdNamesStr := os.Getenv("LISTEN_FDNAMES")
+	if fdNamesStr == "" {
+		return 0, fmt.Errorf("LISTEN_FDNAMES environment variable not set")
+	}
+
+	// Parse name and optional index
+	parts := strings.Split(nameWithIndex, ":")
+	if len(parts) > 2 {
+		return 0, fmt.Errorf("invalid socket name format '%s': too many colons", nameWithIndex)
+	}
+
+	name := parts[0]
+	targetIndex := 0
+
+	if len(parts) > 1 {
+		var err error
+		targetIndex, err = strconv.Atoi(parts[1])
+		if err != nil {
+			return 0, fmt.Errorf("invalid socket index '%s': %v", parts[1], err)
+		}
+		if targetIndex < 0 {
+			return 0, fmt.Errorf("socket index cannot be negative: %d", targetIndex)
+		}
+	}
+
+	// Parse the socket names
+	names := strings.Split(fdNamesStr, ":")
+
+	// Find the Nth occurrence of the requested name
+	matchCount := 0
+	for i, fdName := range names {
+		if fdName == name {
+			if matchCount == targetIndex {
+				return listenFdsStart + i, nil
+			}
+			matchCount++
+		}
+	}
+
+	if matchCount == 0 {
+		return 0, fmt.Errorf("socket name '%s' not found in LISTEN_FDNAMES", name)
+	}
+
+	return 0, fmt.Errorf("socket name '%s' found %d times, but index %d requested", name, matchCount, targetIndex)
+}
+
 // ParseNetworkAddress parses addr into its individual
 // components. The input string is expected to be of
 // the form "network/host:port-range" where any part is
@@ -336,9 +398,27 @@ func ParseNetworkAddressWithDefaults(addr, defaultNetwork string, defaultPort ui
 		}, err
 	}
 	if IsFdNetwork(network) {
+		fdAddr := host
+
+		// Handle named socket activation (fdname/name, fdgramname/name)
+		if strings.HasPrefix(network, "fdname") || strings.HasPrefix(network, "fdgramname") {
+			fdNum, err := getFdByName(host)
+			if err != nil {
+				return NetworkAddress{}, fmt.Errorf("named socket activation: %v", err)
+			}
+			fdAddr = strconv.Itoa(fdNum)
+
+			// Normalize network to standard fd/fdgram
+			if strings.HasPrefix(network, "fdname") {
+				network = "fd"
+			} else {
+				network = "fdgram"
+			}
+		}
+
 		return NetworkAddress{
 			Network: network,
-			Host:    host,
+			Host:    fdAddr,
 		}, nil
 	}
 	var start, end uint64