This post only references Dell PowerEdge servers, so if this isn't in your sandbox, please ignore.
Dell servers are equipped with a Remote Access Client called iDRAC. One can access it through a web browser and perform administrative tasks remotely. In Dell's infinite wisdom, iDRAC is configured to listen on an IP address different from one set through the OS. This one is set in BIOS.
I have two Dell servers, one in my home office and one in my work office. The one at work is mounted on a rack and has no monitor or keyboard attached. I use XRDP for my work. Except now I've seen a message on the LCD front panel and wish to investigate. The LCD panel displays log messages from iDRAC.
Here's the issue: I don't know what the iDRAC IP address is. The LCD panel doesn't seem to respond, I do not have a license for Dell's RACADM package, and without a monitor attached, I can't drop into BIOS on boot-up so read the address. So, what to do?
I think I've found an answer, but I'm hoping someone can confirm this. First, what DOESN'T work:
ping. Ping is really trying to establish an ICMP connection to a server, and iDRAC doesn't conform to this.
nc/netcat. I thought I was getting close, but the only time I get nc to work on my local MATE (20.04.3) system is by pointing it directly at the address, which doesn't help if I don't know it. I've tried a script to loop through all local IPs, but nc stalls right out of the gate.
nmap. Now we're getting somewhere. But there's a wrinkle: nmap will for some reason not detect the IP address on the same hardware. I can successfully use nmap from a PC (nmap -Pn -p80 --open 192.168.1.0/24) and get a response from every machine on the LAN with an open port 80 -- which is all I want.
So, is the answer that nmap only works from a different physical machine? Both my server address and iDRAC address are on the same subnet, but somehow Dell makes it "invisible" to the host machine.