TCP DNS is not hard. It's part of the spec. Normally, that argument doesn't mean much to me --- lots of things are parts of specs that I think are silly and not worth doing --- but TCP DNS seems like a basic necessity for DNS to work at all.
What's holding this up? TCP DNS is just UDP DNS, but over a TCP connection, with the packet length sent before the packet itself. It's the simplest thing you could possibly come up with to make TCP DNS work. It's been there since the 1980s. They should add it.
https://datatracker.ietf.org/doc/rfc9210/
This document updates RFCs 1123 and 1536. This document requires the
operational practice of permitting DNS messages to be carried over
TCP on the Internet as a Best Current Practice.This is something networks and DNS client libraries should fix, but we live in a world where PathMTU only mostly works, so realism gets your service working.
By not supporting TCP DNS you are the odd one out.
Part of the request query is included in the response, IPv6 AAAA records are considerably larger than IPv4 A records, DNSSEC adds size, RFC 1035 message compression comes into play, EDNS0 comes into play, etc.
Which is all to say that you're right that for maximum compatibility you should endeavor to fit all responses into a UDP packet, but most places lack the sophistication to ensure that is always the case.
Supporting TCP DNS on the client side should not be considered optional if you want to successfully resolve records that you don't control.
A while ago I evaluated Alpine Linux. I wanted to like it, I really did, it ticked so many boxes.
But time and time again, I kept on running into issues with their adoption of musl libc.
The last straw for me was when I discovered packages in their package repo (some of which were well-known names) that were compiled against musl when the upstream developers quite clearly wrote in their docs that "if you compile X against anything other than glibc, you're on your own". For me, the fact that Alpine ignored this and compiled against musl anyway, was a big red flag. (And yes I raised some of these as bug reports, but the cases got closed and nothing done about it).
(I assume you meant "glibc", not "libc".)
That's how every software works. The software developer cares about A, B, C distros at most, so other distros are on their own. The maintainer of distro D takes responsibility themselves to make the package work on D. The maintainer needs to understand the software well enough to be able to assert that it will work on their distro, patch it as necessary to make that happen, and maintain those patches in the light of bug reports from the distro users.
>(And yes I raised some of these as bug reports, but the cases got closed and nothing done about it).
Well, yeah. Unless you find something that is irrecoverably broken against musl such that it can only be fixed by compiling against glibc, your bug report is pointless.
Yes, corrected.
> The maintainer of distro D takes responsibility themselves to make the package work on D.
Yeah, but Alpine are not doing that, or at least not at the time I evaluated. I got tired of finding distro supplied packages that just didn't work as expected because things broke unexpectedly because of musl.
Still, Alpine is on top of everyone else when it comes to Docker Images sizes. Thats why it will stick.
Blindly switching to alpine to save space is opening yourself up to exactly these sorts of musl-libc quirks and issues--you better have solid test coverage and a plan to make sure there aren't unexpected edge cases.
Indeed. And that's fine. As long as you're willing to support that difference.
But the "Alpine compiling XYZ against musl" thing is/was just being done blindly by Alpine (i.e. load X into auto-build and let it rip). Sure it compiled without errors. But it never ran properly.
There was discussion yesterday [0] that pointed out that there are 29MB Debian [1] and Ubuntu [2] images.
[0] https://news.ycombinator.com/item?id=30633670
[1] https://hub.docker.com/layers/debian/library/debian/stable-s...
> - Increasing the size of the UPD packet above 512 bytes via the Extension Mechanism for DNS (EDNS)
> - Switching the protocol from UDP to TCP
> Alpine Linux, or rather musl libc, doesn’t support either of those options.
It still seems weird to me that such details are decided by libc. My reflex idea when designing a system would be to put DNS functionality in a system service, while libraries would only query the service, without troubling themselves with system caches, TCP vs UDP etc. Then possibly the service could be even swapped for another with a compatible interface, but making different decisions, without perturbing the applications. It sounds like systemd-resolved is a move in that direction, but I still don't understand why putting all that in libc, essentially making all applications perform their own independent DNS work, was the original choice.
First time it took a lot of effort to pinpoint the problem.
Second time too, since it appeared because of a non-relevant code change (which lead to slighty more DNS requests).
In both cases, a simple switch to Debian slim saved the day.
Alpine is since banned from any env I'm working in :-)
dnsConfig: options: - name: ndots value: '1'
cc: https://support.cloudbees.com/hc/en-us/articles/360040999471...
There are also plenty of dormant issue, enough so that I won't be using Alpine ever again imo :'(
1. WFH from VPN, firstly I had to lower mtu from 1500 to 1392 (My VPN specific issue) https://github.com/microsoft/WSL/issues/4698
2. Next, I had to run some powershell script that updates /etc/resolv.conf to use my VPN DNS (WSL specific stuff) https://github.com/microsoft/WSL/issues/1350
3. And I still don't know if apk works properly. Kind of works in Docker build, but I have a feeling something not quite right.
See this example. Why does it "hang"? Docker command not exiting
docker run -it alpine:3.15 apk update
fetch https://dl-cdn.alpinelinux.org/alpine/v3.15/main/x86_64/APKINDEX.tar.gz
docker run -it alpine:3.15 sh
/ # apk update
fetch https://dl-cdn.alpinelinux.org/alpine/v3.15/main/x86_64/APKINDEX.tar.gz
fetch https://dl-
cdn.alpinelinux.org/alpine/v3.15/community/x86_64/APKINDEX.tar.gz
v3.15.0-342-g4fee739486 [https://dl-cdn.alpinelinux.org/alpine/v3.15/main]
v3.15.0-340-g4ed6115e99 [https://dl-cdn.alpinelinux.org/alpine/v3.15/community]
OK: 15859 distinct packages available
/ # exit
The solution to an intentionally broken resolver is to use a third party library.
• musl should support EDNS and DNS over TCP/IP without issues
• People should be smart enough to use DNS services that don't have stupid edge cases
For the latter, if you use Google for resolving DNS, you get what you deserve. Run your own resolver if DNS resolution matters.