Nixpkgs: Requiring `hostname` to be a single domain label is fairly heavy handed for some networks

[grahamc]

PR #76542 changed the hostname to be a short domain in https://github.com/NixOS/nixpkgs/commit/993baa587c4b82e791686f6ce711bcd4ee8ef933 and enforce this by validation. This has broken large corporate users whose networks by convention use the FQDN as their hostname, and who also have decades of history and infrastructure built around this.

I think this is a case where "one size fits all" doesn't work so well, and I'm not sure this particular point is something we want to risk breaking / losing users over.

The PR references man 5 hostname, which says:

The hostname may be a free-form string up to 64 characters in length; however, it is recommended that it consists only of 7-bit ASCII lower-case characters and no spaces or dots, and limits itself to the format allowed for DNS domain name labels, even though this is not a strict requirement.

This point about not being a strict requirement, I think, should not be made in to a strict requirement at our level.

[grahamc]

cc @primeos, @flokli, @zimbatm, @vcunat

[andir]

Just allow users to set just the hostname as FQDN.

I think hostname vs fqdn is a relic that should not be used most of the time. Especially with the combination of search names (which usually provide for nice privacy leaks…). I for my part just pretend there is only the hostname and that is always a FQDN. There is no domain name a server belongs to. It might be the wrong thing to do on paper but in reality I do not care what a server thinks it's name is (except for things like SMTP handshakes where another party wants an in-band confirmation).

[grahamc]

I've reverted the relevant commit in https://github.com/NixOS/nixpkgs/pull/94022 -- take a look?

[primeos]

This has broken large corporate users whose networks by convention use the FQDN as their hostname, and who also have decades of history and infrastructure built around this.

This is of course a problem that we'd like to avoid (as with any breaking changes) but tbh I don't really understand that argument. Couldn't they just easily revert the relevant commit in their fork?

This point about not being a strict requirement, I think, should not be made in to a strict requirement at our level.

Anyway, that's certainly a valid argument. And in #76542 it was only ever made into a strict requirement since NixOS also provides networking.domain which makes our case a bit different. But since only networking.hostName affects the kernel's node name I'm ok if we wouldn't want to enforce it for that reason (and to allow additional characters).

But we also need to consider that allowing dots in networking.hostName makes some NixOS implementations and checks more difficult and can lead to non-obvious configurations issues that can be hard to find (I think there was a comment about Postfix in the PR but I couldn't find it anymore).

[primeos]

This has broken large corporate users whose networks by convention use the FQDN as their hostname, and who also have decades of history and infrastructure built around this.

@grahamc just to better understand this (if you have time): What's the main problem here? Is this about the Linux kernel hostname or networking.hostName (and in that case why is reverting the commit in a fork or updating the code not an option).

[primeos]

@grahamc @arianvp and anyone else who wants relaxed hostname checks: I get that you are busy (as we all are) but we really need quicker and more active responses if we want to resolve this discussion before the 20.09 release.

IIRC we still don't know any technical problems apart from the comments that this might be inconvenient for existing users.
Before the final release we should also look at https://github.com/NixOS/nixpkgs/pull/94022#issuecomment-674385613 (NixOps), check/finalize the release-notes, and determine if we want a read-only fqdn option.

[grahamc]

Sorry, I had a baby and fell off the internet a while. I can’t get back to this soon.

On Sat, Sep 5, 2020, at 8:19 AM, Michael Weiss wrote:

@grahamc https://github.com/grahamc @arianvp https://github.com/arianvp and anyone else who wants relaxed hostname checks: I get that you are busy (as we all are) but we really need quicker and more active responses if we want to resolve this discussion before the 20.09 release.

IIRC we still don't know any technical problems apart from the comments that this might be inconvenient for existing users.
Before the final release we should also look at #94022 (comment) https://github.com/NixOS/nixpkgs/pull/94022#issuecomment-674385613 (NixOps), check/finalize the release-notes, and determine if we want a read-only fqdn option.

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[primeos]

@grahamc congratulations then ;) I'm happy for you :)

Regarding this issue: From the comments here and especially in #94022 it seems to me like the current "trend" is to keep the strict checks and don't change anything (IIRC), though we didn't really reach any consensus yet. So my idea would be to simply leave this issue open for further comments and see if we get any feedback/complaints regarding this during the beta release cycle.

[martinetd]

Just to add a data point as a nobody user, I'm also one of these weird users who use the fqdn as their hostnames, and got "surprised" when installing a new system as 20.09pre to test -- upgrades will also all require adjustments.
I don't have a lot that depends on that but it's more than just adjusting hostnames, and I don't have much, so I can relate to whatever org stumbled into this with whatever history they have.
For example, I have attrsets with hostnames and bag of datas for wireguard autosetup and things like that which will need amending. It could quickly be messy at larger scales.

OTOH, I understand "full hostnames" can cause problems, and the error is clear enough, but if I want to shoot myself in the foot I don't see what's wrong with that? :)

Well, either way 20.09 is out soon -- I'll wait this long to decide if I want to update my scripts or not :D
Keep up the good work everyone and congratulations @grahamc!

EDIT: after reading the comments in #94022 I can understand it's difficult -- places with explicit checks in nixpkgs are annoying for everyone. Well. Happens what will happen, but a step through with a warning as suggested there would probably be appreciable for a few people.

[jonringer]

Just as a reminder, the 20.09 release is scheduled to happen this monday, the 28th.

If this is still relevant to blocking the release, then there should be some forward movement.

A blocker meeting has still yet to be scheduled. But, if you consider this item to still warrant blocking the entirety of the nixos-20.09 release, then please post on the Feature freeze discussion issue. A template for proposing an item can be found https://github.com/NixOS/nixpkgs/issues/95475#issuecomment-699218336

[0x4A6F]

man 7 hostname states:

Each element of the hostname must be from 1 to 63 characters long and the entire hostname, including the dots, can be at most 253 characters long.  Valid  characters  for
hostnames are ASCII(7) letters from a to z, the digits from 0 to 9, and the hyphen (-).  A hostname may not start with a hyphen.

And references some rather old RFCs:

• IETF RFC 1123 1990-08
• IETF RFC 1178 1989-10

RFC1123:

   2.1  Host Names and Numbers

      The syntax of a legal Internet host name was specified in RFC-952
      [DNS:4].  One aspect of host name syntax is hereby changed: the
      restriction on the first character is relaxed to allow either a
      letter or a digit.  Host software MUST support this more liberal
      syntax.

      Host software MUST handle host names of up to 63 characters and
      SHOULD handle host names of up to 255 characters.

The current implementation violates this:

"^$|^[[:alpha:]]([[:alnum:]_-]{0,61}[[:alnum:]])?$";

Are there reasons for this implementation?

[vcunat]

I'd rather restrict this particular thread just to the question whether it should/can contain dots. What exact characters to allow... doesn't seem to be a real problem right now.

[primeos]

The current implementation violates this:

Yes, this is known and the main reason why this issue exists.
Though man 5 hostname (form systemd) is a better reference as Linux only supports up to 64 characters for the entire hostname (including the terminating newline).

Are there reasons for this implementation?

The main discussion was in #76542 (but also #94022 and this issue).

I'd rather restrict this particular thread just to the question whether it should/can contain dots.

Yeah, I completely agree. The Linux kernel network node hostname can contain dots and this issue is about whether we want to allow this using networking.hostName or not. The reason why it currently isn't allowed is because we have networking.domain for this (and because it isn't recommended to use a FQDN, etc.).

Personally I feel like a grace period with a warning might've been a safer choice but this also comes it's own downsides.

Anyway, basically this issue lacks feedback (e.g. from beta testes) for why this is a real problem (i.e. not I used a FQDN for networking.hostName and now this doesn't work anymore / sucks; instead we're interested why the combination of networking.hostName and networking.domain doesn't work as a replacement [e.g. sysctl kernel.hostname should still be overridable via kernel.sysctl."kernel.hostname"]).

[0x4A6F]

Sorry, the length of hostname is limited to 64, but that is not my point.
This implementation introduces too strict type requirements, if dots are disallowed.

Specifically the limitation to alphabetical characters at the start, which must be relaxed as stated in RFC 1123.
RFC 1123 updates RFC 952 and was published as Internet Standard exactly 31 years ago.
Limiting the start of hostname to alphabetic character is stated in man 5 hosts, but it is utterly outdated and not a reference on this topic (no meaningful changes as far as 2004-11-03, only referencing RFC 952).

[grahamc]

My inability to provide details is mostly due to time (baby) and client confidentiality. There are Perl libraries with bug reports a decade old having to do with not handling the correct approach properly.

On Thu, Oct 1, 2020, at 5:23 PM, 0x4A6F wrote:

Sorry, the length of hostname is limited to 64, but that is not my point.
This implementation introduces too strict type requirements, if dots are disallowed.

Specifically the limitation to alphabetical characters at the start, which must be relaxed as stated in RFC 1123.
RFC 1123 updates RFC 952 and was published as Internet Standard exactly 31 https://datatracker.ietf.org/doc/rfc1123/history/ years ago.
Limiting the start of hostname to alphabetic character is stated in man 5 hosts, but it is utterly outdated and not a reference on this topic (no meaningful changes as far as 2004-11-03, only referencing RFC 952).

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[primeos]

This implementation introduces too strict type requirements, if dots are disallowed.

Again, this is known and not ideal but it was accepted as a compromise for its advantages. AFAIK it would be way more relevant to know why this is a problem and which effect of networking.hostName (which is only an abstraction) causes this problem (and if this breaks anything that it shouldn't).

But I also want to point out that I'm only trying to moderate this issue (though I'll try to reduce my participation here as we don't seem to make much progress / reach any consensus). Also I'm basically fine with any outcome (but a bit biased as #94022 was IIRC mostly rejected). Would it maybe help to do another vote here (e.g. keep the strict requirement, only make it a warning, or relax the requirement)?

My inability to provide details is mostly due to time (baby) and client confidentiality.

Yeah, that's unfortunate (but obviously not your fault).

There are Perl libraries with bug reports a decade old having to do with not handling the correct approach properly.

Not sure what this means. Do they need the FQDN and cannot get it if the Linux kernel hostname doesn't contain the domain (in which case kernel.sysctl."kernel.hostname" might be a good workaround)?

[flokli]

@grahamc do you think setting kernel.sysctl."kernel.hostname", or setting a transient hostname via hostnamectl would be suffient to work this around?

I'd assume this mostly breaks "enterprise tooling" outside the NixOS ecosystem reading the hostname directly, not from the module system.

[grahamc]

This is a great question, let me get that tested.

[grahamc]

Okay, I've confirmed this works and fixes the concerns from the Kerberos / perl side:

  boot.kernel.sysctl."kernel.hostname" = "${config.networking.hostName}.${config.networking.domain}";

I wonder if this snippet should either be in the release notes, or a networking.hostnameIncludesDomain option?

[flokli]

Let's add this to the release notes. Setting the sysctl is already exposed as an option - hostnameIncludesDomain could be misunderstood...

[jonringer]

Seems like there's three action items:

• [x] Add mention about boot.kernel.sysctl."kernel.hostname" being a work-around if a FQDN is needed. https://github.com/NixOS/nixpkgs/pull/100151 https://github.com/NixOS/nixpkgs/pull/100166
• [x] Add mention to release notes https://github.com/NixOS/nixpkgs/pull/100151 https://github.com/NixOS/nixpkgs/pull/100166
• [ ] Add a read only option to get FQDN https://github.com/NixOS/nixpkgs/pull/100155

If this seems acceptable, then I think we can remove this as a blocker

[primeos]

@jonringer I just drafted #100151. Could you take a look?

[primeos]

Maybe #100155 will also be helpful for some (but it's only indirectly related to this PR in that it helps to obtain the FQDN via a read-only NixOS option).

[flokli]

I feel like the initial issue has been addressed sufficiently, there's workarounds that were found, documented and added to the release notes.

There's some ongoing discussion on https://github.com/NixOS/nixpkgs/pull/100155, but that's about adding a new convenience option, which is only loosely related to this issue, and certainly not blocking 20.09.

Let's close this one.

[grahamc]

Another case where this has bitten me is provisioning machines in Packet where we only get "hostname" from the Packet API, and customers can only specify "hostname". However, the API-provided hostname will often include dots without intending to actually specify the domain. This is particularly true in the case of a default name. This means I can't do any "best" thing and have to manipulate the user input and potentially set the hostname to something they did not ask for.

[arianvp]

I already brought up the Packet issue before (I dont know where though; maybe it was during the go no-go meeting). Because packet was behind our release process anyway we decided to not make that a blocker if I recall correctly. (Though of course that's a bit of chicken egg ; given you are the one maintaining those images and I suppose this issue is blocking you from creating newer ones :P )

[grahamc]

Thanks. I sorted it by just replacing .s with -s, but since Packet's validation may be more or less strict than our validation, it is essentially unsafe for me to use the user-provided hostname in the system configuration.