Josh Barratt's Blog

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Instantly finding a headless Raspberry Pi April 18, 2013

My brother got me a Raspberry Pi for my birthday (thanks!) – a device I’d wanted to get for some time, but hadn’t quite yet been able to justify. I think those sorts of things make the best gifts.

You need just a few things to get a Pi up and running, but 2 things on the list (an HDMI-speaking display and an Ethernet port) don’t appear in the same room in my house. Thankfully, the latest releases of Raspbian (the rPi Debian Distribution) come with DHCP and ssh pre-configured, so if you plug the pi into a network, you’ll be able to connect to it.

At first, I was doing an nmap scan for -p 22 --open, but that’s not actually that quick. I was able to speed it up by tweaking some options:

$ nmap -T5 -n -p 22 --open --min-parallelism 200
  • -T5: ‘Insane’ timing profile, very agressive scan rate and low delays.
  • -n: Turn off reverse DNS lookup
  • -p22 --open: Only look at port 22, and find open ones
  • --min-paralellism 200: Scan in large (almost subnet-sized) chunks

And it works; you get:

Nmap done: 256 IP addresses (3 hosts up) scanned in 1.32 seconds

But there’s an even quicker way, that’s also more precise. After running that 1.32 second scan, I still have to decide which of the port-22-open devices is the pi. It turns out the Raspberry Pi Foundation actually has a range of MAC addresses all to themselves! And arp -a runs almost instantly, and dumps the device’s local arp table:

$ arp -a
? ( at b8:27:eb:5:63:2c on en0 ifscope [ethernet]

This means we can do:

$ arp -a | grep b8:27:eb | grep -Eo '[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}'

which can also be made into a nice .bashrc/.zshrc alias:

alias rpi_ip = "arp -a | grep b8:27:eb | grep -Eo '[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}'"

To make it even easier, why not skip one more step:

alias key_rpi="ssh-copy-id pi@`rpi_ip`"

So now when I put a new SD card into my pi, boot it up, all I have to do is:

$ key_rpi
pi@'s password:

and it works. I’m keyed, and can ssh in now as easily as ssh pi@`rpi_ip. (Until I use Ansible to set up mDNS` seconds later, which is probably good to leave for another post.)


This will need modifications if you’re going to have more than 1 pi on the same network. rpi_ip will still run, but it’ll output multiple lines.