Quick Tip: A script to resolve local network hostnames without extra services (Linux)

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I was in desperate need to find a way to not always have to scan the network or go over to another device to know its IP so I could ssh into it or use a service it was serving. This is what I found.

The internet is just a lot of devices, which can be addressed with an IP address. Those are unique in every network. To make the network more usable we use domain names with logical, easy to remember names like www.google.com or www.facebook.com.

This works by contactin a server with a fixed IP which provides DNS that resolves your domain name in an IP that your browser can go to. DNS-servers for the global internet are managed by governments and maintained by big companies like Google. This is why, If you want a domain name, it comes with a fee…

On local networks, its overkill (and quite some setup) to have a DNS-server to resolve the IP of devices but other programs like zeroconf¬†and mdns¬†tried to have a more low-maintenance take on this problem. Short story: Still not ideal. You still have to have services running on all devices to ensure that your device gets registered…

The solution

The ideal solution consists of:
– No changes needed to the network (so it works everywhere)
– Reliable
– Very little setup on the device itself and no services need to be running
– Cross-platform

The idea is to maintain a list of MAC-addresses of the devices I use (and sync those with Dropbox or Git), scan the network for these MAC-addresses and if found, add them to the /etc/hosts file. This is reliable because MAC-addresses are hard-coded in your hardware. You only need to know the MAC of your other devices and you can identify it. Every device has to have one. Adding to the /etc/hosts file is something that is supported by the core of linux so it is quite cross-platform on any device you can get linux working on.

So whenever my network-setup changes, or it has been a while and adresses may have switched, I run the following command;

bash ./HOSTS.sh ./HOSTS_MACS

HOSTS.sh script:

#DEPENDENCIES: arp-scan
#take $1 (first argument of the script) -> file with mac adresses + hostname

tmpfile=$(mktemp /tmp/DYNAMIC-RESOLVE.XXXXXX) #tmp file for temporary results

#clear lines form the previous run in /etc/hosts file
sed -i.bak '/DYNAMIC_RESOLVE/d' /etc/hosts

#use the tool arp-scan to find all the devices on the network
if ! [ -z $2 ]; then
 arp-scan -l --localnet --interface=$2 >> $tmpfile
else
 for line in $(ip link | cut -d " " -f 2); do
 interface=${line::-1}
 arp-scan -l --localnet --interface=$interface 2>/dev/null >> $tmpfile
 done
 #TODO interfaceoption -> default wlan
fi
echo Arpscan Finished... Filtering results...

#reading the results one by one
while read -r line
do
 MAC=$(echo $line | cut -d " " -f 1)
 NAME=$(echo $line | cut -d " " -f 2)
 #grep MAC from ARP cache
 IP=$(cat $tmpfile | grep $MAC | cut -d$'\t' -f 1)

if ! [ -z "${IP}" ]; then
 echo Found $NAME at $IP! Adding to /etc/hosts...
 echo "$IP $NAME $NAME #DYNAMIC_RESOLVE">>/etc/hosts #If the MAC is found, add to the /etc/hosts file
 fi 
done < "$1"
echo Done with discovering hosts

HOSTS_MACS file:

00:90:f5:d6:5b:05 c1
c0:ee:fb:59:fc:23 s1
10:02:b5:d6:08:8a o1
b8:27:eb:4e:0c:42 kh1

The second file is just a list of MAC-addresses with a name you chose behind it. You can find your MAC-address in various ways, but the easiest is just running the ifconfig command on linux. (look next to “ether”)

You will need the tool arp-scan to run this script, you can find it in nearly all repositories. I run this script on an android phone, a desktop, chromebook and Raspberry Pi. So no worries.

If you check the /etc/hosts file you can see the following has changed:

#
# /etc/hosts: static lookup table for host names
#

#<ip-address> <hostname.domain.org> <hostname>
127.0.0.1 localhost.localdomain localhost
::1 localhost.localdomain localhost

# End of file
192.168.1.3 s1 s1 #DYNAMIC_RESOLVE
192.168.1.2 kh1 kh1 #DYNAMIC_RESOLVE

You can now use the hostnames in most commands and have them automatically resolved;

ssh s1 #resolves to the IP adress of my phone
ssh root@kh1 -p 22 #same but with more arguments

You can alias the HOSTS.sh script command in you bashrc to make it easier to use. That’s it! have fun!

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