nixos: provide /etc/hostname #21566

bjornfor · 2017-01-01T16:12:23Z

Motivation for this change

/etc/hostname is the file used by hostnamectl(1) and the
org.freedesktop.hostname1 dbus service (both provided by systemd) to get
the "static hostname". Better provide it so that users of those
tools/services get a proper hostname.

An example of an issue created by the lack of /etc/hostname is that the
bluetooth stack on NixOS identifies itself to peers as "BlueZ $VERSION"
instead of the hostname.

References:
https://www.freedesktop.org/software/systemd/man/hostname.html

Things done

Tested using sandboxing
(nix.useSandbox on NixOS,
or option build-use-sandbox in nix.conf
on non-NixOS)
Built on platform(s)
- NixOS
- macOS
- Linux
Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
Tested execution of all binary files (usually in ./result/bin/)
Fits CONTRIBUTING.md.

Create the file using attrset instead of list, to make it easier to later provide other files in the same module.

mention-bot · 2017-01-01T16:12:25Z

@bjornfor, thanks for your PR! By analyzing the history of the files in this pull request, we identified @wizeman, @wkennington and @ericsagnes to be potential reviewers.

bjornfor · 2017-01-01T19:01:05Z

One side effect I just noticed. Without this PR, hostnamectl prints

Static hostname: n/a
Transient hostname: the_hostname
...

and if I run sudo hostname foo, that "Transient hostname" line gets updated. As expected.

With this PR, only the "Static hostname" line is shown. (I guess that's fine, now that it includes the proper hostname.) BUT, there is no "Transient hostname" line, even after sudo hostname foo. That surprised me.

cc @edolstra

grahamc · 2017-01-01T19:54:27Z

I added "policy discussion" since adding new files to /etc by default seems to require some discussion. Feel free to remove if you disagree :)

Mic92 · 2017-01-01T20:32:50Z

Strange, why does Bluez not just call hostname(2) like any other tool?

bjornfor · 2017-01-01T22:02:49Z

@Mic92: I think it's because they are fetching more than just the "transient" hostname that you get from the kernel. Basically, they get "PrettyHostname", "StaticHostname" and "Chassis", all provided by org.freedesktop.hostname1[1].

See bluez code here: http://git.kernel.org/cgit/bluetooth/bluez.git/tree/plugins/hostname.c#n133

[1] https://www.freedesktop.org/wiki/Software/systemd/hostnamed/

I thought about patching bluez at first. But when I saw that hostnamectl prints "Static hostname: n/a" without an /etc/hostname file, I dropped that thought. (How much energy should we spend to work around/against upstream?)

Another option could be patching systemd to treat the hostname from the kernel as "static hostname". But I kind of like the idea of a static vs transient hostname. Snippet from hostnamed documentation:

The transient (dynamic) host name is the one configured via the kernel's sethostbyname(). It can be different from the static hostname in case DHCP or mDNS have been configured to change the name based on network information.

jokogr · 2017-01-01T23:03:04Z

I use the /etc/hostname file to identify my machine and load a specific configuration file from my centralized configuration repository, e.g. have a look at https://git.joko.gr/joko/etc-nixos/src/master/configuration.nix#L4 (idea taken from @nathan7's configuration files).

Would this change cause any issues on my setups?

bjornfor · 2017-01-02T08:42:17Z

@jokogr: AFAICT, just change networking.hostName = builtins.readFile /etc/hostname; to networking.hostName = "thehostname";.

bjornfor · 2017-01-02T08:49:15Z

BUT, there is no "Transient hostname" line, even after sudo hostname foo. That surprised me.

It turns out that hostnamectl caches the "Transient hostname" for a few minutes (5?). Eventually it picks up the value from "sudo hostname foo".

edolstra · 2017-01-02T10:50:41Z

This looks fine to me.

LnL7 · 2017-01-02T11:55:45Z

I thought we wanted as little mutable stuff in /etc as possible, unless it makes sense. (eg. a service that can reconfigure itself without restarting)
In this case the contents of /etc/hostname might not even match what is returned by hostname(2).

bjornfor · 2017-01-02T13:18:34Z

I thought we wanted as little mutable stuff in /etc as possible, unless it makes sense. (eg. a service that can reconfigure itself without restarting)

This PR creates an immutable /etc/hostname file, like most other files in /etc created by NixOS. (It'll be a symlink to /etc/static/hostname which in turn is a symlink to /nix/store/xybf7pjnnr1nq2j7apbx1110zqhx7qa1-etc-hostname.)

In this case the contents of /etc/hostname might not even match what is returned by hostname(2).

Correct. That's why systemd has the concept of static vs transient hostname. The static name is stored in /etc/hostname and the transient (dynamic) hostname is the one stored in the kernel. See https://www.freedesktop.org/wiki/Software/systemd/hostnamed/.

LnL7 · 2017-01-02T16:28:48Z

With mutable I was referring to the /etc/static symlink that changes with a nixos-rebuild. Anyway it looks fine to me, I just expected there would be more discussion around this.

Mic92 · 2017-01-02T17:59:08Z

nixos/modules/tasks/network-interfaces.nix

+    # org.freedesktop.hostname1 dbus service (both provided by systemd)
+    environment.etc."hostname" = mkIf (cfg.hostName != "")
+      {
+        text = cfg.hostName;


It would be nice to have a trailing newline in this file.

$ cat /etc/hostname turingmachine$

text = "${cfg.hostname}\n";

This would be consistent with hostnamectl's behavior.

I agree. Fixed.

/etc/hostname is the file used by hostnamectl(1) and the org.freedesktop.hostname1 dbus service (both provided by systemd) to get the "static hostname". Better provide it so that users of those tools/services get a proper hostname. An example of an issue created by the lack of /etc/hostname is that the bluetooth stack on NixOS identifies itself to peers as "BlueZ $VERSION" instead of the hostname. References: https://www.freedesktop.org/software/systemd/man/hostname.html Changes v1 -> v2: * ensure /etc/hostname ends with a newline

nixos: cosmetic refactor of environment.etc."hostid"

49d4444

Create the file using attrset instead of list, to make it easier to later provide other files in the same module.

grahamc added the 6.topic: policy discussion label Jan 1, 2017

Mic92 reviewed Jan 2, 2017

View reviewed changes

Mic92 removed the 6.topic: policy discussion label Jan 2, 2017

bjornfor force-pushed the hostname branch from 30890e0 to cb9195b Compare January 2, 2017 18:18

Mic92 merged commit 1cc8b83 into NixOS:master Jan 2, 2017

bjornfor deleted the hostname branch January 2, 2017 20:20

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nixos: provide /etc/hostname #21566

nixos: provide /etc/hostname #21566

bjornfor commented Jan 1, 2017

mention-bot commented Jan 1, 2017

bjornfor commented Jan 1, 2017

grahamc commented Jan 1, 2017

Mic92 commented Jan 1, 2017

bjornfor commented Jan 1, 2017

jokogr commented Jan 1, 2017

bjornfor commented Jan 2, 2017

bjornfor commented Jan 2, 2017

edolstra commented Jan 2, 2017

LnL7 commented Jan 2, 2017

bjornfor commented Jan 2, 2017

LnL7 commented Jan 2, 2017

Mic92 Jan 2, 2017 •

edited

bjornfor Jan 2, 2017

nixos: provide /etc/hostname #21566

nixos: provide /etc/hostname #21566

Conversation

bjornfor commented Jan 1, 2017

Motivation for this change

Things done

mention-bot commented Jan 1, 2017

bjornfor commented Jan 1, 2017

grahamc commented Jan 1, 2017

Mic92 commented Jan 1, 2017

bjornfor commented Jan 1, 2017

jokogr commented Jan 1, 2017

bjornfor commented Jan 2, 2017

bjornfor commented Jan 2, 2017

edolstra commented Jan 2, 2017

LnL7 commented Jan 2, 2017

bjornfor commented Jan 2, 2017

LnL7 commented Jan 2, 2017

Mic92 Jan 2, 2017 • edited

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edited