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This is raspbian stretch.
I have net-tools and iproute2 packages installed. That is so far all ive found that looks like might contain the tool from apt-get.
I just want to use the raspi as an always-on linux machine on my network that can send WoL-packets to other machines on my network. I am looking to use arp to discover IP's. I have all the MAC addresses of the machines on my network that I might want to wake up. Also, nmap may be useful, and is also conspicuously absent.
Good news in case you are looking for the "arp scanning and fingerprinting tool": it's in the Raspbian repository and sudo apt-get update; sudo apt-get install arp-scan should help you.
If you don't know the correct or complete name of a package you can search for it:
To query the APT cache for "arp" you can use apt-cache search arp | more. This works even offline. I recommend sudo apt-get update prior to avoid outdated metadata.
Another option is to use the bash autocomplete feature: in a terminal window just type (without pressing return) sudo apt-get install arp and hit tab twice. Autocompletion will show (not install) all package names starting with "arp".
Sometimes it helps to query the Debian repository with an ordinary browser.
Like @goldilocks commented you can use the APT package searching utility apt-file (it's not installed by default) to search targeted for apt-file search -x 'bin/arp$'. This will result in net-tools: /usr/sbin/arp which implies the package net-tools is required.
To locate a binary you can use find /usr/sbin -name arp (like @Seamus commented) or whereis -b arp.
apt install apt-file you can apt-file search -x 'bin/arp$'. Which implies the required package here is net-tools, not arp-scan (although one may require the other or vice versa). The former includes /usr/sbin/arp, the latter /usr/bin/arp-scan.
Assuming you already have the MAC addresses of the host(s) you want to wake, you don't need nmap at all. There are two packages available on RPi that will send the required WoL "magic packet" to the host(s) you choose: etherwake is one, and the other one has the unlikely name of wakeonlan :)
Which one is best? My preference is wakeonlan because it doesn't require root privileges to run (no sudo required). You may install them both, and determine your own preferences:
$ apt-cache search etherwake
etherwake - tool to send magic Wake-on-LAN packets
wakeonlan - Sends 'magic packets' to wake-on-LAN enabled ethernet adapters
$ sudo apt-get update
$ sudo apt-get install etherwake wakeonlan
And now you're ready! man etherwake and man wakeonlan will fill you in on the details for using either of these packages.
As you already know, the host(s) to be awakened will need to have the "Wake-on-LAN" feature enabled. If they happen to be Raspberry Pi devices (or "adult-sized" Debian hosts), there's a good tutorial/how-to that covers that in the Debian wiki.
apt-get install nmap, facepalm. question remains for arp:Unable to locate package arparpwas included with raspbian; i.e. it's not necessary to install it. But clearly, you've tried to run it... would you mind running this & reporting the result:find /usr/sbin -name arp/usr/sbinis not in my path, and i do have the program.