diff --git a/_posts/2025-09-30-field-notes-solved-docker-pull-from-mcr-microsoft-com-failing.md b/_posts/2025-09-30-field-notes-solved-docker-pull-from-mcr-microsoft-com-failing.md new file mode 100644 index 00000000..877d231f --- /dev/null +++ b/_posts/2025-09-30-field-notes-solved-docker-pull-from-mcr-microsoft-com-failing.md @@ -0,0 +1,42 @@ +--- +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!