"local port ... cannot be bound" when forwarding port using JSch in Java

Question

I need to connect to MongoDB via SSH tunnel and the JSch port forwarding is throwing an error:

local port 127.0.0.1:27017 cannot be bound

I've to connect to MongoDB via SSH tunnel. I found the code here on stackoverflow and it solved my problem. But after few weeks, the same code stopped working. I'm unable to identify the reason for it. Also, using the same credentials that I'm using in Java code, I also tried to connect to the same via CMD using SSH -L command and it worked properly. I'm also able to connect to MongoDB to using same credentials from NoSQLBrowser. I have tried and searched for everything on google and the problem still remains unsolved. Below is the code for it -

java.util.Properties config = new java.util.Properties();
config.put("StrictHostKeyChecking", "no");
JSch jsch = new JSch();
SSH_SESSION = null;
SSH_SESSION = jsch.getSession(SSH_USER, SSH_HOST, SSH_PORT);
SSH_SESSION.setPassword(SSH_PASSWORD);
SSH_SESSION.setConfig(config);
SSH_SESSION.connect();
SSH_SESSION.setPortForwardingL(27017, "localhost", 22);

MongoClient mongoClient = new MongoClient(LOCAL_HOST, LOCAL_PORT);
mongoClient.setReadPreference(ReadPreference.nearest());
MongoCursor<String> dbNames = mongoClient.listDatabaseNames().iterator();
while (dbNames.hasNext()) {
System.out.println(dbNames.next());```

I'm getting the following error on port forwarding line:-

com.jcraft.jsch.JSchException: PortForwardingL: local port 127.0.0.1:27017 cannot be bound.
    at com.jcraft.jsch.PortWatcher.<init>(PortWatcher.java:158)
    at com.jcraft.jsch.PortWatcher.addPort(PortWatcher.java:110)
    at com.jcraft.jsch.Session.setPortForwardingL(Session.java:1847)
    at com.jcraft.jsch.Session.setPortForwardingL(Session.java:1828)
    at com.jcraft.jsch.Session.setPortForwardingL(Session.java:1809)
    at com.jcraft.jsch.Session.setPortForwardingL(Session.java:1792)
    at com.schenker.boot.bootdemo.controller.TestMongoAgain.main(TestMongoAgain.java:35)
Caused by: java.net.BindException: Address already in use: JVM_Bind
    at java.net.DualStackPlainSocketImpl.bind0(Native Method)
    at java.net.DualStackPlainSocketImpl.socketBind(Unknown Source)
    at java.net.AbstractPlainSocketImpl.bind(Unknown Source)
    at java.net.PlainSocketImpl.bind(Unknown Source)
    at java.net.ServerSocket.bind(Unknown Source)
    at java.net.ServerSocket.<init>(Unknown Source)
    at com.jcraft.jsch.PortWatcher.<init>(PortWatcher.java:150)
    ... 6 more
com.jcraft.jsch.JSchException: PortForwardingL: local port 127.0.0.1:27017 is not registered.
    at com.jcraft.jsch.PortWatcher.delPort(PortWatcher.java:118)
    at com.jcraft.jsch.Session.delPortForwardingL(Session.java:1876)
    at com.jcraft.jsch.Session.delPortForwardingL(Session.java:1865)

Also, a very strange thing that if instead of PortForwardingL if I use PortForwardingR, it doesn't throw any error but in the next line it gets connected to my local MongoDB. Can anyone please help me with this?

Martin Prikryl · Accepted Answer · 2019-06-11T09:21:31.227

The port 27017 is probably used by some application/service running on your local machine – Probably your local Mongo DB.

Anyway, just pick any other port number. It can be anything.

Though if you use the port for internal use without your application only, you should not rely on a fixed port number. You never know when some other application blocks that port. Have JSch auto-pick a free port:

int forwardedPort = SSH_SESSION.setPortForwardingL(0, "localhost", 22);
MongoClient mongoClient = new MongoClient(LOCAL_HOST, forwardedPort);

Though in general, your code seems wrong. You forward the port to 22, while you probably should forward it to a remote Mongo DB port.

See a full working example (while it's for MySQL, for MongoDB it is the same, just with a different port):
Connect to remote MySQL database through SSH using Java

Obligatory warning: Do not use StrictHostKeyChecking=no to blindly accept all host keys. That is a security flaw. You lose a protection against MITM attacks.

For a correct (and secure) approach, see:
How to resolve Java UnknownHostKey, while using JSch SFTP library?

Hey Martin, Thank you so much. It worked. And I checked your answer on the link you shared, I understood your point. Thank you. I guess now I better understand how this SSH thing works. — thelearner, Jun 11 '19 at 09:17

"local port ... cannot be bound" when forwarding port using JSch in Java

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