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hostnamed does not like fqdns with trailing dots #6369
systemd version the issue has been seen with
Fedora 26, x86_64
Expected behaviour you didn't see
When using a fully qualified domain name (FQDN), the FQDN is discarded by systemd. A FQDN name ends in a dot (
For a lone hostname,
And the same test for a network name using my internal network of
In case of bug report: Steps to reproduce the problem
See command above for both a hostname and a network name.
Thanks to Rabban for looking into the issue, it appears to be due to the following in
Validating hostnames is a science of its own. Do note that while DNS is frequently assumed to be the one and only defining body of hostnames, things are more complicated, and other naming schemes exist. Moreover the hostname the kernel maintains for the system is used for a verity of things: bluetooth identification for example and tons of other things. hostnamed is relatively conservative in what it accepts, in order to maintain the hostname in some form that is relatively universally useful: it doesn't permit the full DNS syntax (in particular when it comes to non-ASCII), and it does assume the name is in the usual, "normalized" form, that people usually write names as (yes, you may write DNS names with a trailing dot if you follow the RFCs, but it isn't really how normal people write them, for example URLs pretty much never include the trailing dot). Requiring normalization (i.e. no trailing dot) is done so that hostnames remain easily comparable, i.e. to avoid confusion that if some app uses the hostname as is to identify an instance that sometimes it might be written without and sometimes with the dot, but equality is assumed. Moreover an overall size limit of 64 is enforced, which is directly conflicting with DNS (which permits 256), as the kernel enforces such a limit too, and we don't accept data the kernel wouldn't accept. Then, we don't accept empty names (or specifically, empty names are treated magically, and cause a "revert to defaults").
Now, this validation is done by hostnamed, which is supposed to be the gateway to potentially less privileged clients to change the hostname (if they have enough PK privs). Being strict for such clients is really important I believe to make sure things remain well-defined and clean even in the prospect of non-technical users. If you want to use names that do not follow these rules that's entirely OK, but in that case please just use the traditional way to configure the hostname: "vim /etc/hostname" as root, and set it to whatever you need it to be.
I hope that makes some sense?
Before this is closed due to trolls I'd like to add that the trailing dot is actually not that uncommon if you've ever worked with DNS (and not via dumbed down GUIs), AFAIK in the zone file you must use trailing dots: https://en.wikipedia.org/wiki/Zone_file
The real question for me: how can we assure that the hostname is considered an FQDN and not a relative hostname below a domain name without using the trailing dot?
@poettering Yes... I don't know you but to be honest I feel sorry, you may have (had) a tendency for over-engineering but this does not excuse that you have a bunch of people following you for the rest of your life and looking for flaws in all your code or your comments :(
But nonetheless, to go back on topic: If you set up a bigger (internal) network as a sysadmin you may really want hostnames which are FQDNs, to not clutter your (internal) domain with those hostnames.
As @EarthlingKira pointed out, working with FQDNs that are dot-terminated is not uncommon. (For example, my
Please read my explanations above before commenting.
Also, if you really care that much about full DNS semantics for Linux hostnames, please start working with the kernel folks, and ensure the kernel actually accepts all full fqdns, because right now it does not, as the kernel ensures a 64char limit, while DNS requires 256 chars.