mDNS relay discovery

[johnsonjh]

@schollz

Hi there! First, thanks for croc, I use it all the time.

A while back, I had added support for discovering a relay via mDNS.

I was thinking, I could clean it up and send you a PR, but I wanted to make sure that this would be something that you'd be interested in first.

The way it works is that you'd announce a croc relay as available with your favorite mDNS server (something like this for Avahi):

<?xml version="1.0" standalone='no'?>
<!DOCTYPE service-group SYSTEM "avahi-service.dtd">
<service-group>
  <name>croc relay</name>
  <service>
    <type>_croc._tcp</type>
    <port>9009</port>
    <txt-record>description=croc relay service</txt-record>
    <txt-record>version=10.2.2-g12345</txt-record>
    <txt-record>ip=1.2.3.4:8888</txt-record>
  </service>
</service-group>

Let's say that this is announcing from my router, at 192.168.1.1:9009. I'd do a croc send file, and croc would automatically use the discovered relay (192.168.1.1:9009, but tell the user to use the IP from the txt-record, like:

Sending 'file' (666 MB)
Code is: 3518-enrico-couple-brandy
On the other computer run

croc --relay 1.2.3.4:8888 3518-enrico-couple-brandy

This solves the problem I had of having different aliases for croc (or not using my aliases so it ended up just using your relay). It should also make it easier for others run and use their own relays. If this is something desirable, I'd have to re-do the support and also clean things up.

I'll leave it up to you to decide if you want this kind of thing, and if so:

• should croc use a discovered relay by default (it adds a short bit of time to starting up) or should the user need to specify an option (or possibly an environment variable)?
• how do you want to handle the case of multiple relays being broadcast?
  • should the user be able to select them, and if so, whats the UI for that? (Currently, my implementation was naive and picked the first one it found.)
• OPTIONAL, but nice to have: croc relay probably should have builtin support for advertising via mdns, so users don't have to mess with Avahai.
  • For that to work right, croc relay would need support discovery of its own external IP somehow. So for that you'd probably need to query an external service like https://api.ipify.org/ or https://icanhazip.com/ or something. How do you handle finding multiple IPs, etc? Like I said, this can be optional.

Anyway, let me know what you think!

FYI my implementation used https://github.com/grandcat/zeroconf which seems dead. I used this only because go-chromecast uses it. I think a rewrite would be using https://github.com/hashicorp/mdns, but I'm open to any suggestions.

[schollz]

Hi @johnsonjh I sincerely welcome the addition! If it works for you, it works for me. I'd just want to make sure there are no changes to the default behavior/flags, but feel free to add flags or additional behavior.

[johnsonjh]

@schollz Thanks, I haven't forgotten about this and plan to get to it soon.

[rowend36]

Auto-discovery of relays seems really nice. Disclaimer: relative noob in systems programming.

I am looking at it more from a local/internal IP perspective - using relays on my local computer without specifying them.

There are already ways to enumerate IP addresses using ipconfig or ifconfig. In that regard, I think the idea of discovering relays can be done using a flag --local which tells croc to ping a specified port e.g. 2762 on all interfaces for a specific protocol. FTP, SSH etc. all do this and there are a lot of IANA unassigned ports to choose from.

I could wrap croc with a bash script to do this for me - but if you also think it is nice and you can kindly point me in the right direction - I'm willing to give it a stab at a pull request also.

[johnsonjh]

I haven't forgotten - just been busy. I hope to look at this now in the next few days. I added mDNS to another Go program I was working on in the meantime, so I jaber an idea how to make it happen.