Tailscale: cmd/tailscale: add "check-path" subcommand

[mr-karan]

Is your feature request related to a problem? Please describe.

I am sorry if this is a noob question, but how do I know my packets are flowing through the DERP relay or not? Is there any way to get that information? I tried sudo tailscale netcheck and it just shows the nearest DERPs.

❯ sudo tailscale netcheck        
[sudo] password for karan: 

Report:
        * UDP: true
        * IPv4: yes, <PublicIP>:51163
        * IPv6: no
        * MappingVariesByDestIP: false
        * HairPinning: false
        * PortMapping: 
        * Nearest DERP: 3 (sin)
        * DERP latency:
                - 1, nyc = 299.6ms
                - 2, sfo = 262.9ms
                - 3, sin = 76.5ms
                - 4, fra = 163.5ms
                - 5, syd = 437.7ms

Describe the solution you'd like

Probably a DERP: yes/no indication in netcheck ?

Describe alternatives you've considered

Tried to ping the server with/without Tailscale to notice the latencies. If the result is similar then I assume that the traffic is not going via DERP. Assumption could be wrong in case DERP relay is pretty close by which might not add significant latency, so it's not a reliable way.

Additional context

I have seen UDP: true on the above command, so does that indicate it is not going via DERP?

Thanks!

[bradfitz]

Whether DERP is used is not a function of just your computer's network properties. It's based on the combined properties of the two machines trying to talk to each other.

(But MappingVariesByDestIP: false and UDP: true are the easy case... if both sides have that, it'll almost certainly work.)

The way you check today is ping a peer (or do something to make it active) and then run:

$ tailscale status --active

And look to see where the *asterisks* are around which route. If you see it around an IP:port, then it's using direct UDP.

If you see it around an airport code, then it's using DERP.

We could probably combine that all into one command, like:

$ tailscale check-path <hostname-or-IP>

And tell you which path it took.

[bradfitz]

@mr-karan, done. See https://github.com/tailscale/tailscale/pull/663 (currently in review)

[mr-karan]

@bradfitz That was fast :open_mouth: Thanks a lot!