How to access jails via hostname using mDNS

[Jack828]

EDIT: This is now also available on my website https://jackburgess.dev/blog/truenas-jails-access-via-mdns

Hello folks!

I've done quite a bit of googling duckduckgo-ing on this topic and couldn't find a solution that didn't require adding entries to my hosts file or one that was as plug'n'play as I wanted.

So, I figured out my own way!

This uses zeroconf mDNS to advertise the jail on your local internal network - e.g. you can access your netdata jail at http://netdata.local

I've written a kickstart script that has a handy one-liner in it to get you up and running faster, but I'll explain what the script does in detail here.

Quite a major caveat (for me and my network at least) is that this may not allow resolution of .local domains on Android phones, for a couple of reasons that don't belong here.
This guide also assumes you've got a jail up and running, and a root shell in there already - and that you've set the hostname to something you want.

We'll be using a popular mDNS daemon service Avahi. It is supported on most distributions of pretty much anything.
We'll also require socat, a multipurpose relay tool. This will allow us to access our services on port 80 instead of whatever port they use.

Code:

root@jail:/ # pkg install -y avahi-app socat

Once those are done, go ahead and enable the services for them all. dbus is required by avahi.

Code:

root@jail:/ # sysrc dbus_enable="YES"
root@jail:/ # sysrc avahi_daemon_enable="YES"
root@jail:/ # sysrc socat_enable="YES"

Now, avahi comes with two services enabled by default - which I never use. So to keep the network nice and tidy we'll go ahead and remove them.

Code:

root@jail:/ # rm /usr/local/etc/avahi/services/*.service

This directory is where avahi will look for service definitions - if you do want to add other ones, this is where you put it.
We'll add the definition for our service in here too.

Code:

root@jail:/ # nano /usr/local/etc/avahi/services/http.service

And put in

Code:

<?xml version="1.0" standalone='no'?><!--*-nxml-*-->
<!DOCTYPE service-group SYSTEM "avahi-service.dtd">
<service-group>
  <name replace-wildcards="yes">%h</name>
  <service>
    <type>_http._tcp</type>
    <port>80</port>
  </service>
</service-group>

Now avahi knows to respond to mDNS queries with our jail's hostname + what type of service it is running.

To redirect traffic to the correct port, lets setup socat. Like avahi, it already has some instances defined, so open its config file:

Code:

root@jail:/ # nano /usr/local/etc/socat-instances.conf

And add to the bottom, replacing the word PORT with the numerical value of your port, e.g. 3000

Code:

[jailredirect]
daemonuser=root
flags="tcp-listen:80,reuseaddr,fork tcp:localhost:PORT"

Finally, we can enable all the services.

Code:

root@jail:/ # service dbus start
root@jail:/ # service avahi-daemon start
root@jail:/ # service socat start jailredirect

And give it a test by connecting to http://hostname.local.

Further reading / tools:

GitHub - alteholz/mdns-scan: repository for mdns-scan

repository for mdns-scan. Contribute to alteholz/mdns-scan development by creating an account on GitHub.

github.com

See what's broadcasting in your network by running avahi-browse --resolve _http._tcp on another device
Or you can use dig dig @224.0.0.251 -p 5353 -t ptr +short _http._tcp.local
Dig also supports single hostname lookup dig @224.0.0.251 -p 5353 +short hostname.local