Allow passing in an existing net.Listener and a public ip resolver.

server/client_handler.go

@@ -86,6 +86,11 @@ func (c *clientHandler) RemoteAddr() net.Addr {
 	return c.conn.RemoteAddr()
 }
 
+// LocalAddr returns the local network address.
+func (c *clientHandler) LocalAddr() net.Addr {
+	return c.conn.LocalAddr()
+}
+
 func (c *clientHandler) end() {
 	c.daddy.driver.UserLeft(c)
 	c.daddy.clientDeparture(c)

server/driver.go

@@ -74,6 +74,9 @@ type ClientContext interface {
 
 	// Client's address
 	RemoteAddr() net.Addr
+
+	// Servers's address
+	LocalAddr() net.Addr
 }
 
 // FileStream is a read or write closeable stream
@@ -90,11 +93,17 @@ type PortRange struct {
 	End   int // Range end
 }
 
+// PublicIPResolver takes a ClientContext for a connection and returns the public IP
+// to use in the response to the PASV command, or an error if a public IP cannot be determined.
+type PublicIPResolver func(ClientContext) (string, error)
+
 // Settings defines all the server settings
 type Settings struct {
-	ListenAddr                string     // Listening address
-	PublicHost                string     // Public IP to expose (only an IP address is accepted at this stage)
-	DataPortRange             *PortRange // Port Range for data connections. Random one will be used if not specified
-	DisableMLSD               bool       // Disable MLSD support
-	NonStandardActiveDataPort bool       // Allow to use a non-standard active data port
+	Listener                  net.Listener     // Allow providing an already initialized listener. Mutually exclusive with ListenAddr
+	ListenAddr                string           // Listening address
+	PublicHost                string           // Public IP to expose (only an IP address is accepted at this stage)
+	PublicIPResolver          PublicIPResolver // Optional function that can perform a public ip lookup for the given CientContext.
+	DataPortRange             *PortRange       // Port Range for data connections. Random one will be used if not specified
+	DisableMLSD               bool             // Disable MLSD support
+	NonStandardActiveDataPort bool             // Allow to use a non-standard active data port
 }

server/server.go

@@ -94,7 +94,7 @@ func (server *FtpServer) loadSettings() error {
 		return err
 	}
 
-	if s.ListenAddr == "" {
+	if s.Listener == nil && s.ListenAddr == "" {
 		s.ListenAddr = "0.0.0.0:2121"
 	}
 
@@ -112,14 +112,15 @@ func (server *FtpServer) Listen() error {
 		return fmt.Errorf("could not load settings: %v", err)
 	}
 
-	server.listener, err = net.Listen(
-		"tcp",
-		server.settings.ListenAddr,
-	)
+	if server.settings.Listener != nil {
+		server.listener = server.settings.Listener
+	} else {
+		server.listener, err = net.Listen("tcp", server.settings.ListenAddr)
 
-	if err != nil {
-		level.Error(server.Logger).Log(logKeyMsg, "Cannot listen", "err", err)
-		return err
+		if err != nil {
+			level.Error(server.Logger).Log(logKeyMsg, "Cannot listen", "err", err)
+			return err
+		}
 	}
 
 	level.Info(server.Logger).Log(logKeyMsg, "Listening...", logKeyAction, "ftp.listening", "address", server.listener.Addr())

server/transfer_pasv.go

@@ -86,7 +86,19 @@ func (c *clientHandler) handlePASV() {
 
 		// If we don't have an IP address, we can take the one that was used for the current connection
 		if ip == "" {
-			ip = strings.Split(c.conn.LocalAddr().String(), ":")[0]
+			// Defer to the user provided resolver.
+			if c.daddy.settings.PublicIPResolver != nil {
+				var err error
+				ip, err = c.daddy.settings.PublicIPResolver(c)
+				if err != nil {
+					// Not sure if there is better desired behavior than this.
+					// If we can't resolve the public ip to return to the client, is there any actual
+					// fallback that is better than erroring.
+					panic(err)
+				}
+			} else {
+				ip = strings.Split(c.conn.LocalAddr().String(), ":")[0]
+			}
 		}
 
 		quads := strings.Split(ip, ".")