Consider backporting glibc bug 12154 to ubuntu 20.04 - DNS resolution fails if a CNAME target begins with a hyphen

[Manuel Bucher]

See:
* https://bugzilla.mozilla.org/show_bug.cgi?id=1870497
* https://sourceware.org/bugzilla/show_bug.cgi?id=12154

[Simon Chopin (schopin)]

Hi, thanks for taking the time to report this.

According to the upstream bug, the fix is present from 2.37 onward, and has been backported to the release branches down to 2.34, while Focal has 2.31. Given the sheer size of the diff, I'm wary of backporting it that far. Jammy should be possible, though.

It would make things much easier if you could provide us with a test case for this :)

[Manuel Bucher (manuel-moz)]

Hi, I've set up this now:

* `ping ok-bug1870497.neon.rocks`

This is a cname to `-bug1870497.neon.rocks` and resolves to one of my servers. Running the commands currently shows this log (on 22.04):

```
$ ping ok-bug1870497.neon.rocks
PING bug1870497.neon.rocks [...]
```

* `ping error-bug1870497.neon.rocks`: is a cname to `-bug1870497.neon.rocks`

This is a cname to `bug1870497.neon.rocks` and then resolves to one of my servers. Running the commands currently shows this log (on 22.04):

```
$ ping error-bug1870497.neon.rocks
ping: error-bug1870497.neon.rocks: System error
```

Edit: just seeing that the reporter on bugzilla.mozilla.org also has setup some publicly accessible test sites: https://gist.github.com/nomis/e4d32435f199aeffbf64197dc55b5ba9

* good: `ping cname1.test.uuid.uk`
* bad: `ping cname5.test.uuid.uk`