Jenkins comes with an embedded HTTP server, which you can reverse proxy with nginx - but what do you need to lock down and where do you need to do it?

## The Problem

Here's a demonstration of the ports that Jenkins listens on, out of the box:

$ java -jar jenkins.war &
[...time passes...]
$ netstat -npl | grep java | sort | tr -s ' '
tcp6 0 0 :::37510 :::* LISTEN 22736/java
tcp6 0 0 :::49635 :::* LISTEN 22736/java
tcp6 0 0 :::8009  :::* LISTEN 22736/java
tcp6 0 0 :::8080  :::* LISTEN 22736/java
udp6 0 0 :::33848 :::* 22736/java
udp6 0 0 :::5353  :::* 22736/java

And if we kill java and immediately start it up again (bear with me), here's the result of those same checks:

$ java -jar jenkins.war &
[...time passes...]
$ netstat -npl | grep java | sort | tr -s ' '
tcp6 0 0 :::21319 :::* LISTEN 22797/java
tcp6 0 0 :::59271 :::* LISTEN 22797/java
tcp6 0 0 :::8009  :::* LISTEN 22797/java
tcp6 0 0 :::8080  :::* LISTEN 22797/java
udp6 0 0 :::33848 :::* 22797/java
udp6 0 0 :::5353  :::* 22797/java

So we have 2 ports that seem to change (tcp/37510/21319 & tcp/49635/59271), along with the following:


The Solution

I've chosen to try and disable as many of these listeners as possible, and to leave HTTP traffic as the sole route into the server. This will probably cause some problems when bringing slave instances online, later, but it'll do as a single-node setup until then.

Predicable Ports


This is the AJP port, used by an alternative protocol to HTTP on Java servers. Disable it with --ajp13Port=-1.


This is the default port that Jenkins (or, rather the embedded Winstone HTTP server) uses for HTTP traffic. There's nothing wrong with that, per se, but given that we're going to hide Jenkins behind the more robust nginx, we probably want to tell Jenkins only to listen on localhost, not

We do this with --httpListenAddress=

Additionally, if you don't have available for Jenkins to use, the port can be changed with (for example) --httpPort=9005.


This appears to be some sort of well-known slave-related port which, after receiving traffic, tell anyone connected the value of one of the randomly selected ports we saw, above. To disable this, we're going to have to be a bit sneaky, because there isn't a way to disable it at the CLI (or inside Jenkins config) as far as I could tell.

We make the essential assumption here that you're not running Jenkins/Java as root. If you are doing this, please take a refesher class at Sysadmin School.

In order to disable this listener, we merely have to tell Jenkins it should use a privileged port below 1024, which will then fail to bind as we're not running as root. I like to use 1023, since it's so close to 1024 that it can't be a coincidence, which tells the next Sysadmin looking at this something. Hopefully.

Do this with -Dhudson.udp=1023, but make sure it comes before the -jar jenkins.war CLI parameter or it won't work.


If your first thought on seeing those "53"s was "DNS?", then give yourself a cookie. It can be disabled with -Dhudson.DNSMultiCast.disabled=true, but only when placed before the -jar parameter.


Whilst it doesn't appear Jenkins tries to set up an HTTPS listener out of the box, that behaviour might change, and might be affected by other configuration settings in your environment. To definitively disable HTTPS, use --httpsPort=-1.

Non-deterministic ports

That's the end of the predictable ports. Now for the 2 random ports Jenkins opens.

Unfortunately, they don't appear to be controlled at the command line, but from config files and settings. That means you need to run Jenkins once, poke it slightly to cause it to create its config files, and then shut it down and edit them.

Using the command line we've built up thus far, run Jenkins and wait for it to say "INFO: Jenkins is fully up and running":

$ java -Dhudson.DNSMultiCast.disabled=true -Dhudson.udp=1023 -jar \
jenkins.war --httpListenAddress= --httpPort=9005 \
--ajp13Port=-1 --httpsPort=-1

Now hit the Jenkins web UI on whatever address you've told it to listen on. Then:

  • Go to "Manage Jenkins"
  • Go to "Configure"
  • Hit the "Save" button at the bottom of the page

Wait a couple of seconds to make sure the config gets written to disk.

Back at the server's CLI, hit Ctrl-C and check Jenkins shuts down.

In your $HOME/.jenkins/ directory, edit the file config.xml and change the setting for <slaveAgentPort> from 0 to -1.

Finally, put the following contents into $HOME/.jenkins/org.jenkinsci.main.modules.sshd.SSHD.xml:

<?xml version='1.0' encoding='UTF-8'?>

The Result

Let's start up Jenkins with these changes, and see what's now listening for network traffic:

$ java -Dhudson.DNSMultiCast.disabled=true -Dhudson.udp=1023 -jar \
jenkins.war --httpListenAddress= --httpPort=9005 \
--ajp13Port=-1 --httpsPort=-1 &
[...time passes...]
$ sudo netstat -npl | grep java | tr -s ' ' 
tcp6 0 0 :::* LISTEN 23662/java

Only the single HTTP port, on localhost, that we need for reverse proxying. Success!

Though you should probably think about putting some authorisation on that at the nginx layer now …