[Field Notes] Solved: docker pull from mcr.microsoft.com failing

Background

Recently had an issue where some of our developers couldn’t pull Docker images from mcr.microsoft.com. This happened over time, and sometimes when they changed locations.

Troubleshooting

Finally narrowed it down to those using Verizon, using Verizon DNS, on IPv6.

ISP	IPv6 Enabled	Machine DNS	Docker DNS	Result
Comcast	Any	Any	Any	✅
Verizon	False	Verizon Default	Default	✅
Verizon	True	1.1.1.1	Default	✅
Verizon	True	Verizon Default	1.1.1.1	✅
Verizon	True	Verizon Default	Default	❌

How to Fix

As far as I can tell, there are a few ways to resolve this issue.

• You can move off of Verizon 😉
• You can disable IPv6 on your machine, allowing only IPv4. However, this seems a little drastic
• You can set your machine’s DNS server away from Verizon’s DNS to something like Cloudflare’s 1.1.1.1 or similar.
• You can update the Docker Engine settings in Docker for Windows to add "dns": ["1.1.1.1"] to the top-level JSON. This will change the DNS settings only within docker, so pulling images should work once more without any changes affecting your wider machine.

Final Thoughts

Still not sure why this is happening, or why it seems to be happening only in this specific scenario. DNS isn’t my strong suit, but I hope someone smarter than me comes along and adds more information here.

For now, just happy to be able to unblock my team.

Happy coding!