feat: allow dynamic pasv_url depending on remote address (#269)

Co-authored-by: Matt Forster <hey@mattforster.ca>

README.md

@@ -73,7 +73,31 @@ _Note:_ The hostname must be the external IP address to accept external connecti
 __Default:__ `"ftp://127.0.0.1:21"`
 
 #### `pasv_url`
-This **must** be the external WAN **IP address** that the FTP server is bound to. `FTP-srv` provides this IP address to the client when a `PASV` command is received in the handshake for a passive connection. Reference [PASV verb](https://cr.yp.to/ftp/retr.html#pasv).
+`FTP-srv` provides an IP address to the client when a `PASV` command is received in the handshake for a passive connection. Reference [PASV verb](https://cr.yp.to/ftp/retr.html#pasv). This can be one of two options:
+- A function which takes one parameter containing the remote IP address of the FTP client. This can be useful when the user wants to return a different IP address depending if the user is connecting from Internet or from an LAN address.
+ ```js
+const {Netmask} = require('netmask');
+const networks = {
+  '$GATEWAY_IP/32': `${public_ip}`, 
+  '10.0.0.0/8'    : `${lan_ip}`
+} 
+
+const resolverFunction = (address) => {
+    for (const network in networks) {
+        try {
+            const mask = new Netmask(network);
+            if (mask.contains(address)) return networks[network];
+        }
+        catch (err) {
+            logger.error('Error checking source network', err);
+        }
+    }
+    return '127.0.0.1';
+}
+
+new FtpSrv({pasv_url: resolverFunction});
+- A static IP address (ie. an external WAN **IP address** that the FTP server is bound to). In this case, only connections from localhost are handled differently returning `127.0.0.1` to the client. 
+
 If not provided, clients can only connect using an `Active` connection.
 
 #### `pasv_min`

src/commands/registration/pasv.js

@@ -1,3 +1,4 @@
+const Promise = require('bluebird');
 const PassiveConnector = require('../../connector/passive');
 const {isLocalIP} = require('../../helpers/is-local');
 
@@ -11,12 +12,17 @@ module.exports = {
     this.connector = new PassiveConnector(this);
     return this.connector.setupServer()
     .then((server) => {
-      let address = this.server.options.pasv_url;
-      // Allow connecting from local
-      if (isLocalIP(this.ip)) {
-        address = this.ip;
-      }
       const {port} = server.address();
+      let pasvAddress = this.server.options.pasv_url;
+      if (typeof pasvAddress === "function") {
+        return Promise.try(() => pasvAddress(this.ip))
+          .then((address) => ({address, port}));
+      }
+      // Allow connecting from local
+      if (isLocalIP(this.ip)) pasvAddress = this.ip;
+      return {address: pasvAddress, port};
+    })
+    .then(({address, port}) => {
       const host = address.replace(/\./g, ',');
       const portByte1 = port / 256 | 0;
       const portByte2 = port % 256;