How to write a Device Discovery Tool using Bash

I have an embedded PC running Linux server and a simple web server which is used in agriculture industry, my customers buy these PCs and use them in their fields. ( they install it on a pole 6 or 7 meters above ground level and it is very hard to access) sometimes they change the range of these PCs IP address from original one to their own desired IP Range. for example I ship them with 192.168.10.10/24 but they might change it to something like 172.16.12.19/29 or 10.10.200.10/8 or ....

the problem is that after a while they forget the PC IP Address and we want to give them a tool to detect and show them the current IP Address.

I thought maybe using Broadcast IP address and sending a message over local network might help to solve this problem. but after some search I realized I have to know the range at least for example (172.16.12.X or 10.10.200.X) which is not possible for me because the farmers forgot the network range too. Is there any tool like "NC" in Linux which can help me to write a bash script to find their IP ? I have network access to the PC but I cannot connect a Monitor to the PC so the only way is to find the IP using the local network.

I want something like this :

echo "What is your IP Address ? " | nc -w1 -u 4545

and I put something on the PC which answers to this message whenever they got the exact phrase What is your IP Address ? "

nc -l -u -p 4545

but for the nc I have to specify a network range which is not possible for me.

Edit:

if you have worked with Device Discovery tool (DDT) for IP Cameras you know that they will find the camera IP independent of the PC which has the DDT on it. I want something exactly like those program but with bash commands.

I guess the farmers are reconfiguring it because their local network address changes. In which case, why not allow the server to get it's address over DHCP, you could make a note of the Mac address (which won't change) and then search the DHCP servers tables or even try a lookup on the DNS server to find the pc. Unpluging the cable from the end at the switch and plugging it back in should initiate a DHCP configuration session. Most DHCP servers allow you to assign an address based on a Mac address so you have some control over it at the dhcp server end.

As posted, the problem looks too wide. As I understand, you have two different problems:

1. Generate a method to avoid the problem in future cases.
2. Access to the servers that currently have the problem, to implement solution 1.

For problem #1, instead of getting a way to know the new IP, you could have the servers recover the old IP to a fixed value. You could set a crontab task that checks the current IP, and whenever it deviates from a set value, it changes the IP back to the target. You will of course have time windows of "network-blindness", depending on the frequency set in crontab.

Alternatively, a crontab task could broadcast/send email/etc. the new IP. The right mechanisms depend very much on what are you willing to operate, how are you accessing the servers.

All this seems a workaround. If correctly configured, they should not "forget the PC IP address". This is the point to fix, I guess. If you could give more information on the current configuration, that will narrow the problem.

For problem #2, if that is your target please mention it. I have a "similar" problem. A server that I am accessing remotely changes IP once in a while, since it is assigned by DHCP. Whenever I have problems sshing to the server, I execute a small script that, by a combination of ping and ssh (within a loop of IPs), and checking on the response, can get the current IP. Depending on many aspects of your network configuration this may work for you or not.

With netcat you could do something like a cronjob on your "pole-mounted servers" that does something like this every X minutes:

#!/bin/bash

# replace eth0 with the name of your interface or get the IP via other means

ip="$(ifconfig | grep -A 1 'eth0' | tail -1 | cut -d ' ' -f 10)"
datum=$(date)
echo "This is my IP: "$ip | netcat -q 2 -b -u 255.255.255.255 12001

The User/Farmer then only has to listen like this:

echo | nc -ulp 12001

And after X minutes they should get something like this:

This is my IP: 192.168.1.121

This only works if the "Farmers PC" and your "pole-mounted servers" are in the same subnet, though.

Explanation:

The script will send a UDP broadcast on Port 12001 and exit nc, after the payload (our echo command) + 2 seconds. The IP doesn't have to be got via ifconfig obviously, but its quite easy to parse..

Sending it to 255.255.255.255 means broadcast to the whole subnet, regardless of size.

So we don't have to know which network we are on.

On the client side, we listen with nc until we get a payload. I found nc terminates cleanly in combination with a line-break (via the echo).