Create Blog “2025-09-30-field-notes-solved-docker-pull-from-mcr-microsoft-com-failing”

_posts/2025-09-30-field-notes-solved-docker-pull-from-mcr-microsoft-com-failing.md

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+---
+title: "[Field Notes] Solved: docker pull from mcr.microsoft.com failing"
+comments: true
+tags:
+  - field-notes
+  - dns
+  - docker
+  - verizon
+date: 2025-09-30 15:51 -0400
+---
+## 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!