macOS & DNS integration

[hynek]

So, it looks like my beloved https://orbstack.dev just made a change that forces me to use their local where a container called foo has the locally-resolvable host name foo.orb.local with constantly changing IPs.

Unfortunately, python-build-standalone Pythons can't resolve these. socket.gethostbyname() fails with

gaierror: [Errno 8] nodename nor servname provided, or not known

While it works on Homebrew & the official builds. Is there any chance for this ever working?

[jjhelmus]

I'll try to replicate and debug this issue this week. A minimal reproducer of the issue would be helpful.

At least for me gethostbyname does work for localhost:

import socket
socket.gethostbyname('localhost')

python-build-standalone builds _socket as a static library that is linked into python where as Homebrew + Python.org use a dynamic library. Additionally PBS is not a Framework build. I'm not sure if either of these make a difference these are worth investigating

[hynek]

I wish I could! To my great shame, these parts of macOS are still terra incognita to me. Currently the only way for me to recreate it is installing a test version of Orb. :(

Maybe @kdrag0n would be willing to tell us how to create a simple test case?

P.S. The change was an accident, so my urgent need has gone away, but I would still love to be able to resolve these hostnames. 🤓

[kdrag0n]

No OrbStack needed to repro. *.orb.local is just mDNS, so any .local name reproduces it. On macOS mDNSResponder handles mDNS automatically as long as you're using libc getaddrinfo. Easiest test case:

import socket
socket.gethostbyname("<your computer hostname>.local")

Investigated by Claude:

Root cause is the build config, not the runtime. The symptom (plain DNS works, .local/mDNS fails, EAI_NONAME) is the signature of CPython falling back to its bundled Modules/getaddrinfo.c, which only does /etc/hosts + plain DNS and never talks to mDNSResponder. That fallback is gated on ac_cv_buggy_getaddrinfo: yes → bundled resolver (no mDNS), no → system getaddrinfo (mDNS works).

In this repo ac_cv_buggy_getaddrinfo=no is only forced inside the cross-compiling branch in cpython-unix/build-cpython.sh. But the native arm64-on-arm64 macOS build isn't cross (BUILD_TRIPLE == TARGET_TRIPLE, so CROSS_COMPILING is unset), so the override never applies and configure runs its own runtime probe instead, under the CI sandbox with PATH nerfed to /usr/bin:/bin. In that restricted environment the probe can come back yes, silently selecting the no-mDNS resolver. That's why it's .local-specific and environment-dependent.

Fix is to force ac_cv_buggy_getaddrinfo=no on macOS unconditionally, not just when cross-compiling. macOS's getaddrinfo isn't buggy and is the one you want for mDNS.

[hynek]

Easiest test case:

import socket
socket.gethostbyname("<your computer hostname>.local")

Can confirm that this succeeds on Homebrew and fails on standalone for me.

[jjhelmus]

Is this occuring on an Intel Mac or Apple Silicon?

[hynek]

I’m on Apple Silicon

[jjhelmus]

I tried a variety of ways to reproduce this locally on my macBook but was unable. I am not able to install Orbstack due to restrictions but a colleague did and was able to resolve the local hostnames with python-build-standalone and Homebrew without issue.

I was also able to resolve a local hostname set via dns-sd locally with Homebrew and uv as follows:

Terminal 1

dns-sd -P "test" _http._tcp local. 9 pbs-test.local. 127.0.0.44

Terminal 2

uv run --isolated --no-project --python 3.14 --managed-python python -c "import socket; print(socket.gethostbyname('pbs-test.local'))"

Returns 127.0.0.44

Is there additional information about the setup where this is occurring that might explain this? What Python version is being used? What uv version? Does updating uv and/or pbs change anything (running uv self upgrade; uv python upgrade --reinstall should upgrade everything in question).

[hynek]

hey thanks for digging deeper (and finding that useful dns-sd snippet)!

Together with your previous question I've just noticed what actually happened here: Rosetta

$ uv run --isolated --no-project --python 3.14-arm64 --managed-python python -c "import socket; print(socket.gethostbyname('pbs-test.local'))"
127.0.0.44

$ uv run --isolated --no-project --python 3.14-x86_64 --managed-python python -c "import socket; print(socket.gethostbyname('pbs-test.local'))"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
    import socket; print(socket.gethostbyname('pbs-test.local'))
                         ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
socket.gaierror: [Errno 8] nodename nor servname provided, or not known

I'm terribly sorry for not mentioning it earlier; I just did the research within one of my Intel-based projects because I needed the resolution there and didn't think of it.

That said, this (python.org) works:

$ /Library/Frameworks/Python.framework/Versions/3.14/bin/python3.14-intel64 -c "import socket; print(socket.gethostbyname('pbs-test.local'))"
127.0.0.44

And so does this (Intel homebrew):

$ /usr/local/Cellar/python@3.14/3.14.6/bin/python3 -c "import socket; print(socket.gethostbyname('pbs-test.local'))"
127.0.0.44

[jjhelmus]

Even with Rosetta and an x86_64 interpreter I'm not able to replicate the issue, both arm64 and x86-64 work on my machine:

❯ uv run --isolated --no-project --python 3.14-x86_64 --managed-python python -c "import socket; print(socket.gethostbyname('pbs-test.local'))"
127.0.0.44

❯ uv run --isolated --no-project --python 3.14-x86_64 --managed-python python -c "import socket; print(socket.gethostbyname('pbs-test.local'))"
127.0.0.44

[hynek]

in that case I'm out of ideas :(

I even tried disconnecting from my work VPN and go back all the way to 3.10.

[jjhelmus-oai]

One possible idea is that the OS is prompting for Network permission but the one-off process terminates before the dialog can be acted on.

Does starting a interactive x86-64 Python with:

 uv run --isolated --no-project --python 3.14-x86_64 --managed-python python

And interactively running import socket; socket.gethostbyname('pbs-test.local') behave differently.

macOS starts applications with undetermined local network access that must be selected by the user and is applied to subsequent operations which may be coming into play.

[hynek]

It does not :(

❯ uv run --isolated --no-project --python 3.14-x86_64 --managed-python python
Python 3.14.5 (main, May 10 2026, 19:20:54) [Clang 22.1.3 ] on darwin
Type "help", "copyright", "credits" or "license" for more information.
🦕🦖
>>> import socket; socket.gethostbyname('pbs-test.local')
Traceback (most recent call last):
  File "<python-input-0>", line 1, in <module>
    import socket; socket.gethostbyname('pbs-test.local')
                   ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
socket.gaierror: [Errno 8] nodename nor servname provided, or not known
>>>
⏎
~ took 14s
❯ uv run --isolated --no-project --python 3.14 --managed-python python
Python 3.14.5 (main, May 10 2026, 19:20:57) [Clang 22.1.3 ] on darwin
Type "help", "copyright", "credits" or "license" for more information.
🏎️💨
>>> import socket; socket.gethostbyname('pbs-test.local')
'127.0.0.44'