Table of Contents
mDNS and DNS-SD for my lab environment
What is mDNS and DNS-SD ? I’ve pasted the following paragraph below from wikipedia which has a nice description.
DNS-SD (DNS Service Discovery[16]) allows clients to discover a named list of service instances and to resolve those services to hostnames using standard DNS queries. The specification is compatible with existing unicast DNS server and client software, but works equally well with mDNS in a zero-configuration environment. Each service instance is described using a DNS SRV[17] and DNS TXT[18] record. A client discovers the list of available instances for a given service type by querying the DNS PTR[18] record of that service type’s name; the server returns zero or more names of the form
. , each corresponding to a SRV/TXT record pair. The SRV record resolves to the domain name providing the instance, while the TXT can contain service-specific configuration parameters. A client can then resolve the A/AAAA record for the domain name and connect to the service.
I wanted to learn about this as I wanted a quicker way to access hosts in my LAN without modifying each machines /etc/hosts file. and mDNS coupled with DNS-SD will allow me to do this with the small limited hosts in my network and give me the oppertunity to play around with some network stuff in between studying for my RHCSA.
Intro
In my lab , here are the steps i’ve taken to get this up and running across all my RHEl and Debian hosts. For the first part of this setup , I decided that I wanted to also install everything on all hosts using ansible. After modifying the ansible config and enabling token-based auth on all hosts. I ran into an issue with the machines I use for my RHCSA lab work.
In this lab I have 3 machines , one is a dedicated repository server. After searching I found that one of the two packages I need to install is nss-mdns hosted in the epel repo. Therefore, my first task is to host this repo on my repo server and install the package on all machines.
Adding EPEL repo to repo-server
Get files for EPEL repo
- Installed epel-release on the repo-server
sudo dnf install -y \ https://dl.fedoraproject.org/pub/epel/epel-release-latest-9.noarch.rpm - Used reposync to get repo files
sudo reposync --repoid=epel --download-path=/var/www/html/repo/epel --download-metadata
Host repo
To accomplish this , I created the EPEL repo file and used ansible with a script to add the repo file to every host
First I created the Client repo file and copied it to a newly made directory /var/www/html/repofiles
[epel]
name=EPEL RHEL 9 self hosted repo
baseurl=http://192.168.3.180/repo/epel
gpgcheck=0
Copy repo file to all hosts
Created this small script download-repo.sh which requests the file from the web server and pipes the output into the /etc/yum.repos.d/ directory under the name “epel.repo”
#!/bin/bash
# download-repo.sh
curl http://192.168.3.180/repofiles/epel.repo > /etc/yum.repos.d/epel.repo
On the ansible master host, I used this ansible command to execute it across all hosts in my lab
ansible rhel-hosts -m script -a /root/download-repo.sh -b
Then executed another ansible command to download the ‘nss-mdns’ package!
ansible rhel-hosts -m shell -a 'sudo dnf install -y nss-mdns' -b
Checked all hosts now have the installed package ( with ansible of course )
root@ansible-master:~# ansible rhel-hosts -m shell -a 'sudo dnf list | grep nss-mdns' -b
[WARNING]: Invalid characters were found in group names but not replaced, use -vvvv to see details
192.168.3.111 | CHANGED | rc=0 >>
nss-mdns.x86_64 0.15.1-3.1.el9 @epel
192.168.3.112 | CHANGED | rc=0 >>
nss-mdns.x86_64 0.15.1-3.1.el9 @epel
192.168.3.180 | CHANGED | rc=0 >>
nss-mdns.x86_64 0.15.1-3.1.el9 @epel
Also checked to make sure it was using the epel repo file copied ( the repo-server already had and epel repo from cisco , the main goal was to ensure only server1 and server2 are using the epel file )
root@ansible-master:~# ansible rhel-hosts -m shell -a 'dnf repolist'
[WARNING]: Invalid characters were found in group names but not replaced, use -vvvv to see details
192.168.3.111 | CHANGED | rc=0 >>
repo id repo name
AppStream Red hat AppStream repo
BaseOS Red hat BaseOS repo
192.168.3.112 | CHANGED | rc=0 >>
repo id repo name
AppStream Red hat AppStream repo
BaseOS Red hat BaseOS repo
192.168.3.180 | CHANGED | rc=0 >>
repo id repo name
AppStream Red Hat Enterprise Linux 9.0 AppStream RPMs (DVD)
BaseOS Red Hat Enterprise Linux 9.0 BaseOS RPMs (DVD)
epel EPEL RHEL 9 self hosted repo
epel-cisco-openh264 Extra Packages for Enterprise Linux 9 openh264 (From Cisco) - x86_64
Setting up Avahi & mDNS on all hosts
Installation
ansible rhel-hosts -m shell -a 'sudo dnf install avahi avahi-tools -y' -b
Setting up avahi and mDNS
Enabled the service
ansible rhel-hosts -m shell -a 'systemctl enable --now avahi-daemon'
Added service into firewall
ansible rhel-hosts -m shell -a 'firewall-cmd --add-service=mdns --permanent && firewall-cmd --reload' -b
Make sure the /etc/nsswitch.conf file contains the following line
hosts: files mdns4_minimal [NOTFOUND=return] dns
ansible rhel-hosts -m shell -a 'cat /etc/nsswitch.conf | grep mdns4'
Test to make sure it works
root@ansible-master:~# ansible rhel-hosts -m shell -a 'ping -c 4 server2.local'
[WARNING]: Invalid characters were found in group names but not replaced, use -vvvv to see details
192.168.3.180 | CHANGED | rc=0 >>
PING server2.local (192.168.3.112) 56(84) bytes of data.
64 bytes from 192.168.3.112 (192.168.3.112): icmp_seq=1 ttl=64 time=0.312 ms
64 bytes from 192.168.3.112 (192.168.3.112): icmp_seq=2 ttl=64 time=0.331 ms
64 bytes from 192.168.3.112 (192.168.3.112): icmp_seq=3 ttl=64 time=0.464 ms
64 bytes from 192.168.3.112 (192.168.3.112): icmp_seq=4 ttl=64 time=0.375 ms
--- server2.local ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 3004ms
rtt min/avg/max/mdev = 0.312/0.370/0.464/0.058 ms
192.168.3.112 | CHANGED | rc=0 >>
PING server2.local (192.168.3.112) 56(84) bytes of data.
64 bytes from server2 (192.168.3.112): icmp_seq=1 ttl=64 time=0.038 ms
64 bytes from server2 (192.168.3.112): icmp_seq=2 ttl=64 time=0.016 ms
64 bytes from server2 (192.168.3.112): icmp_seq=3 ttl=64 time=0.022 ms
64 bytes from server2 (192.168.3.112): icmp_seq=4 ttl=64 time=0.038 ms
--- server2.local ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 3057ms
rtt min/avg/max/mdev = 0.016/0.028/0.038/0.009 ms
192.168.3.111 | CHANGED | rc=0 >>
PING server2.local (192.168.3.112) 56(84) bytes of data.
64 bytes from server2 (192.168.3.112): icmp_seq=1 ttl=64 time=0.378 ms
64 bytes from server2 (192.168.3.112): icmp_seq=2 ttl=64 time=0.296 ms
64 bytes from server2 (192.168.3.112): icmp_seq=3 ttl=64 time=0.293 ms
64 bytes from server2 (192.168.3.112): icmp_seq=4 ttl=64 time=0.244 ms
--- server2.local ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 3098ms
rtt min/avg/max/mdev = 0.244/0.302/0.378/0.048 ms
root@ansible-master:~#
And voila! We have successfully enabled mDNS and DNS-SD on my network for all rhel hosts in my lab , that way if I add more hosts , I won’t need to add their hostnames manually in the /etc/hosts file.
Installation Script
This is a small script that will install the necessary tools and configuration for nss-mdns & avahi for both Debian , RHEL based systems.
#!/bin/bash
# Simple script that setups tools and configuration for setting up nss-mdns & avahi
PLATFORM=$(awk -F= '/^ID=/{gsub(/"/,"",$2); print $2}' /etc/os-release)
error_exit() {
echo "Error: $1";
exit 1;
}
warning(){
echo "WARNING: $1"
}
firewall_config() { # Detects firewall being used Adds mdns service/ports to firewall
if systemctl is-active --quiet firewalld 2>/dev/null; then
sudo firewall-cmd --add-service=mdns --permanent && firewall-cmd --reload;
elif systemctl is-active --quiet ufw 2>/dev/null; then
echo "ufw"
for x in 5353 51570 40200; do sudo ufw allow $x/udp && sudo ufw reload; done
else
echo "No active firewall detected"
fi
}
enable_avahi() {
systemctl enable --now avahi-daemon && systemctl start avahi-daemon;
}
install() {
case "$1" in # Install nss-mdns & avahi, method based on platform
rhel|centos|fedora|rocky|almalinux)
(sudo dnf install avahi-tools avahi nss-mdns -y || dnf install -y epel-release && dnf install -y avahi-tools avahi nss-mdns) && sudo sed -i '/^hosts:/ s/files dns/files mdns4_minimal [NOTFOUND=return] dns/' /etc/nsswitch.conf
;;
debian|ubuntu|linuxmint)
sudo apt install avahi-daemon avahi-utils libnss-mdns -y && \
sudo sed -i '/^hosts:/ s/files dns/files mdns4_minimal [NOTFOUND=return] dns/' /etc/nsswitch.conf
;;
arch|manjaro)
sudo pacman -Syu avahi nss-mdns && \
sudo sed -i '/^hosts:/ s/files dns/files mdns4_minimal [NOTFOUND=return] dns/' /etc/nsswitch.conf
;;
*)
echo 'ERROR: Distro not detected';
exit 1;
;;
esac
}
install $PLATFORM || error_exit "Unable to install required tools"
firewall_config $PLATFORM || warning "Unable to configure firewall" # Setup firewall config
enable_avahi || error_exit "Unable to start avahi-daemon"; # Start and enable avahi