This is a tiny collection of tools for leveraging Multicast DNS (aka Zeroconf or Bonjour) on a mixed network of Apple Macs and other Unix machines. The focus is on making it easier to get around (find and login remotely to other machines) starting from a Mac workstation, since Apple's mDNS is less command-line friendly than the Linux version (avahi).
My setup tends to be that I work from a Mac and login to other macs and non-mac unix machines using Remote Desktop, file sharing and ssh a lot. I like the fact that Apple's Finder automatically finds (using mDNS) all local hosts that advertise their presence. Launching a remote desktop or file sharing session to another Mac directly from Finder is very convenient. This is so convenient that I even get my non-Mac Unix hosts to advertise themselves so that they show up in Finder too (Linux has a fine zeroconf implementation too). This is particularly handy for headless hosts that come and go on DHCP-assigned IP addresses, for example my growing collection of Raspberry Pis.
However, I was finding it annoying if I want to ssh to local hosts I have to type their exact hostname, which is often long on a Mac. I wanted some kind of substring matching, so I only need to provide something unambiguous and let the machine work it out.
Further, using raw mDNS to ssh to Macs, the .local hostname is not even the same string as the displayed name, thus "Christopher's MacBook Pro" is converted to "Christophers-MacBook-Pro" in mDNS (the apostrophe is removed and spaces are converted to hyphens). You can see this in action on the System Preference Sharing menu where "Computer Name" and the hostname in .local have to both be shown.
This all makes ssh a bit of a poor relation (IMO) compared to the GUI tools.
I started to look into scripting easier use of mDNS from the command-line to allow me to get into my machines more easily, but another irritation arose. Apple's documentation for mDNS service discovery (see man dns-sd) tries to scare you away from calling it from scripts. Firstly the output format is not stable. Second, the commands are meant to be consumed as a stream. That's ok for a GUI application such as Finder which runs for a relatively long time. Not ok for a quick and dirty comand-line lookup tool.
Apple instead recommends using language-specific bindings for the language of your choice to call the underlying DNS-SD API. All very well for some serious application, but overkill for a small utility. So I built Z Utils instead.
Z Utils for now consists of a lookup tool which looks up those hosts which advertise ssh, a clone of the Unix "host" command which leverages the lookup for simple queries and also an SSH wrapper which looks up the desired host and then invokes ssh to the resulting hostname or IP address to easily connect you to the target machine.
- zeroconf-ssh-host
This utility finds a host or hosts that advertise ssh locally and returns their details from mDNS as a series of lines. Bash sub-processes and timeouts are used to hide the fact that dns-sd does not terminate, and the output of Apple's tool is parsed with simple easy to hack unix editing pipelines. This is the meat of the Z utils project.
- zhost
This utility approximates the behavior of host, but with the substring matching described previously.
- zssh
This utility provides a way to connect to any ssh-advertising host by providing any substring of its name provided that that host can be resolved in mDNS. The provided name is looked up using zeroconf-ssh-host and the first host that matches will be the one connected to.
In the following examples there are two hosts, my MacBook Pro on wifi, and a raspberry pi, hostname raspberrypi1 which is connected by both ethernet and wifi. The rpi is advertising ssh via mDNS, see below for how to configure that.
- Lookup all ssh-advertising hosts
Christophers-MacBook-Pro:zu cmorgan$ zeroconf-ssh-host
15:48:45.731 Add 2 4 raspberrypi1.local. 10.0.1.38 120
15:48:45.731 Add 3 4 raspberrypi1.local. 10.0.1.7 120
15:48:44.721 Add 2 4 Christophers-MacBook-Pro.local. 10.0.1.13 120
Christophers-MacBook-Pro:zu cmorgan$
- Perform a lookup on a single host, with verbose tracing
chris-mba:zutils.git cmorgan$ zeroconf-ssh-host -v pi3
Looking up "pi3"
Finding ssh advertisers...done
Compare to "Chriss-MacBook-Air" ... no match
Compare to "lemaker" ... no match
Compare to "raspberrypi2" ... no match
Compare to "pi3" ... found!
ssh is exported by pi3
Looking up the IPV4 address of "pi3" in ZeroConf... done
13:35:48.549 Add 2 4 pi3.local. 10.0.1.9 120
- Run a single command on the raspberry pi
Christophers-MacBook-Pro:zu cmorgan$ zssh pi hostname
raspberrypi1
Christophers-MacBook-Pro:zu cmorgan$
- Log into the raspberry pi interactively
Christophers-MacBook-Pro:zu cmorgan$ zssh pi
Linux raspberrypi1 3.18.11+ #781 PREEMPT Tue Apr 21 18:02:18 BST 2015 armv6l
The programs included with the Debian GNU/Linux system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.
Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
permitted by applicable law.
No mail.
Last login: Sat Dec 26 20:47:27 2015 from christophers-macbook-pro.local
cmorgan@raspberrypi1 ~ $
- Log in using a different username
Christophers-MacBook-Pro:zu cmorgan$ zssh pi@pi3
[email protected]'s password:
The programs included with the Debian GNU/Linux system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.
Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
permitted by applicable law.
No mail.
Last login: Sun Jan 17 19:10:42 2016 from 10.0.1.32
The full syntax for invoking zssh is
zssh <flags> <hostname> <ssh params>
The flags are as follows :
-v Verbose
-h Login via hostname
So to verbosely ssh to the rasperry pi using its hostname in .local. (instead of merely connecting to its IP address) you would do the following:
Christophers-MacBook-Pro:zu cmorgan$ ./zssh -v -h pi /sbin/iwconfig
Found raspberrypi1.local. in local domain
Output of "ssh raspberrypi1.local. /sbin/iwconfig" :
lo no wireless extensions.x
eth0 no wireless extensions.
wlan0 IEEE 802.11bgn ESSID:"Chris Morgan's Upstate Network"
Mode:Managed Frequency:2.437 GHz Access Point: 28:CF:DA:B4:03:6D
Bit Rate=65 Mb/s Tx-Power=20 dBm
Retry short limit:7 RTS thr:off Fragment thr:off
Power Management:off
Link Quality=70/70 Signal level=-19 dBm
Rx invalid nwid:0 Rx invalid crypt:0 Rx invalid frag:0
Tx excessive retries:2 Invalid misc:102 Missed beacon:0
Connecting via hostname instead of IP address will be more stable than using the IP address for machines which get their addresses dynamically. I may change this to be the default.
Assuming you copy the provided ssh.service file onto a Linux machine, and assuming it's a system with apt-get (Ubuntu, or raspbian as I use on my Raspberry Pi machines) the following commands would configure that machine to advertise ssh via mDNS :
sudo apt-get install libnss-mdns avahi-daemon
sudo cp ssh.service /etc/avahi/services
sudo service avahi-daemon restart
on systemd based distributions, restarting avahi looks like:
sudo systemctl restart avahi-daemon.service
Explanation:
- libnss-mdns adds mDNS as a name resolution backend to GNU libC
- avahi-daemon is the mDNS (zeroconf) implementation for Linux
- ssh.service is a service fragment that tells avahi to advertise ssh on port 22
- restarting the daemon forces it to notice the new service it should advertise
N.B. Also install avahi-utils if you want to be able to browse the mDNS fields from the linux host, e.g. avahi-browse --all
This project started off as utilities for connecting from Apple Macs to assorted Linux machines. On Linux the situation is much better (it is easier to probe and parse mDNS info), so a zssh would be easier to implement, but I haven't made this work at all yet. Right now it just complains that this is not supported and gives up.