Embedded device configured with bad IP address, can I still connect?
My usual solution for problems like that is to:
- Get a device onto the same subnet as the device
- Configure the device with compatible network settings
- Connect to the embedded device
- Change the bad network settings on the embedded device
Step 1 is usually the hard part. It can be a laptop that you just drag to the right location, or a device with two connected network ports; one for you to connect to, one for the bad network data.
If the device has an unknown networking config, it gets hairier and results aren't as good. However, I've done it. You need physical access to the device and its networking connections. What I've done:
- Connect your laptop and the device to a switch connected to nothing else.
- Configure your laptop with no IP address, just leave it with the raw Ethernet addresses.
- Start a promiscuous mode network trace from that laptop.
- Power-cycle the device.
- Watch packets.
If you're lucky, the device will squawk some packets. Probably ARPing for the gateway device's MAC addresses. This should reveal the IP it considers itself to have. If it is squawking for a gateway, that should provide some clues as to what it considers its netmask to be though not always. Once you have the IP, configure your laptop to be one-adjacent to it IP-wise and attempt to ping it. Keep the network trace running.
If the network trace shows that when your machine attempts to ARP the target's IP address and the device does not respond, you may need to set a static ARP-table entry for that IP. I have run into devices where this step was needed to kick the device into working. If pinging does not work even after setting the arp-entry, chances are very high that it's fundamentally unreachable from the network.
If you configure a computer with the IP: 172.16.1.235/8 you'll have the best chance of making it work immediately. If both devices think they're on the same subnet they'll just communicate.
Does it support IPv6? Use IPv6 to connect to the device. You can probably find the MAC and work out the link-local address.
If neither works, likely the interface is down. sysadmin1138's advice is excellent, do exactly that.