I'm pretty sure my router (Apple Airport Extreme) supports hairpinning, and netcheck used to agree with me, but no longer does. Checking the telemetry sent by user devices, only 3 devices currently report hairpinning=true, and all of them are running 0.95-405-gbbe69fa4.

git bisect says my netcheck hairpin result changed from true to false (both results are consistent across several runs) in this commit:

commit ff6b3c2
Author: Brad Fitzpatrick [email protected]
Date: Wed Mar 4 13:40:29 2020 -0800

netcheck: include two more DERP nodes, show more in CLI report

Although offhand it's not obvious to me why that would have harmed the results.

Log output from failing netcheck:

2020/03/17 19:20:48 derp1.tailscale.com:3478 says we are 66.130.147.6:37015 (in 28.422465ms)
2020/03/17 19:20:48 derp2.tailscale.com:3478 says we are 66.130.147.6:37015 (in 90.422885ms)
2020/03/17 19:20:48 stunner: slow STUN response from derp2.tailscale.com:3478: 1 retries
2020/03/17 19:20:48 stunner: slow STUN response from derp2.tailscale.com:3478: 1 retries
2020/03/17 19:20:48 derp4.tailscale.com:3478 says we are 66.130.147.6:37015 (in 108.991938ms)
2020/03/17 19:20:48 stunner: slow STUN response from derp4.tailscale.com:3478: 1 retries
2020/03/17 19:20:48 derp2.tailscale.com:3478 says we are 66.130.147.6:37015 (in 99.433961ms)
2020/03/17 19:20:48 derp4.tailscale.com:3478 says we are 66.130.147.6:37015 (in 113.194194ms)
2020/03/17 19:20:48 derp3.tailscale.com:3478 says we are 66.130.147.6:37015 (in 291.324466ms)
2020/03/17 19:20:48 stunner: slow STUN response from derp3.tailscale.com:3478: 3 retries
2020/03/17 19:20:48 derp3.tailscale.com:3478 says we are 66.130.147.6:37015 (in 287.378637ms)
2020/03/17 19:20:48 derp3.tailscale.com:3478 says we are 66.130.147.6:37015 (in 292.846779ms)
2020/03/17 19:20:48 derp3.tailscale.com:3478 says we are 66.130.147.6:37015 (in 287.821991ms)
2020/03/17 19:20:53 stunner: slow STUN response from derp4.tailscale.com:3478: 8 retries
2020/03/17 19:20:53 stunner: slow STUN response from derp3.tailscale.com:3478: 8 retries

Report:
* UDP: true
* IPv6: false
* MappingVariesByDestIP: false
* HairPinning: false
* Nearest DERP: 1 (New York)
* DERP latency:
- derp1.tailscale.com:3478 = 28.422465ms
- derp2.tailscale.com:3478 = 99.433961ms
- derp3.tailscale.com:3478 = 287.821991ms
- derp4.tailscale.com:3478 = 113.194194ms

Output from netcheck in the preceding commit (correct result):

2020/03/17 19:22:15 derp1.tailscale.com:3478 says we are 66.130.147.6:44121 (in 26.738468ms)
2020/03/17 19:22:15 derp2.tailscale.com:3478 says we are 66.130.147.6:44121 (in 91.501479ms)

Report:
* UDP: true
* IPv6: false
* MappingVariesByDestIP: false
* HairPinning: true
* DERP latency:
- derp1.tailscale.com:3478 = 26.738468ms
- derp2.tailscale.com:3478 = 91.501479ms

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