Enable MDNS on a FreeNAS/BSD jail. That will allow a Avahi name resolution of the jail in the local network.

mdns.sh

#!/bin/sh
# This script is to be executed on the FreeNAS/BSD host itself (and not inside the jail)
# usage : ./mnds.sh <name_of_the_jail>

jexec -l ${1} pkg install --yes openmdns
jexec -l ${1} sysrc mdnsd_enable=yes
IFACE=$(jexec -l  ${1} ifconfig | grep epair | head -n 1 | cut -f1 -d:)
jexec -l ${1} sysrc mdnsd_flags=${IFACE}
jexec -l ${1} service mdnsd start

MDNS_jail_FreeNAS

freenas# jls
   JID  IP Address      Hostname                      Path
 ...
     5  -               etherpad                      /mnt/basin/jails/etherpad
...

freenas# ./mdns.sh 5
Updating FreeBSD repository catalogue...
[etherpad] Fetching meta.txz: 100%    944 B   0.9kB/s    00:01    
[etherpad] Fetching packagesite.txz: 100%    6 MiB   5.9MB/s    00:01    
Processing entries: 100%
FreeBSD repository update completed. 25811 packages processed.
New version of pkg detected; it needs to be installed first.
The following 1 package(s) will be affected (of 0 checked):

Installed packages to be UPGRADED:
	pkg: 1.9.4 -> 1.9.4_1

Number of packages to be upgraded: 1

2 MiB to be downloaded.
[etherpad] Fetching pkg-1.9.4_1.txz: 100%    2 MiB   2.6MB/s    00:01    
Checking integrity... done (0 conflicting)
[etherpad] [1/1] Upgrading pkg from 1.9.4 to 1.9.4_1...
[etherpad] [1/1] Extracting pkg-1.9.4_1: 100%
Updating FreeBSD repository catalogue...
FreeBSD repository is up-to-date.
All repositories are up-to-date.
The following 2 package(s) will be affected (of 0 checked):

New packages to be INSTALLED:
	openmdns: 0.6.15
	libopenbsd: r298107

Number of packages to be installed: 2

The process will require 4 MiB more space.
2 MiB to be downloaded.
[etherpad] Fetching openmdns-0.6.15.txz: 100%    2 MiB   2.6MB/s    00:01    
[etherpad] Fetching libopenbsd-r298107.txz: 100%   21 KiB  21.2kB/s    00:01    
Checking integrity... done (0 conflicting)
[etherpad] [1/2] Installing libopenbsd-r298107...
[etherpad] [1/2] Extracting libopenbsd-r298107: 100%
[etherpad] [2/2] Installing openmdns-0.6.15...
===> Creating groups.
Creating group '_mdnsd' with gid '177'.
===> Creating users
Creating user '_mdnsd' with uid '177'.
[etherpad] [2/2] Extracting openmdns-0.6.15: 100%
mdnsd_enable:  -> yes
mdnsd_flags:  -> epair4b
Starting mdnsd.

ubuntu:~$ avahi-browse -a | grep etherpad
+ wlp7s0 IPv4 etherpad [epair4b:76:50:04:0a:98:fb]          Workstation          local