openssh unix dev - Feb 2021 - [openssh-commits] [openssh] 02/02: upstream: hostname is not specified by POSIX but uname -n is, so use

On 09.02.21 01:25, Nico Kadel-Garcia wrote:
> On Mon, Feb 8, 2021 at 3:59 PM Bob Proulx <bob at proulx.com> wrote:
>> The hostname -f option is a "new-ish" Linux specific option. 
It's not
>> portable.  And because it works by doing a reverse DNS lookup it
>> depends upon live network connectivity at that moment working for the
>> network lookup and the results are spotty depending upon how DNS is
>> set up and how many IP addresses are configured on the host.
> 
> It looks in /etc/hosts first, which works very well when DNS is
> unavailable and when the host his publishing a dynamic DNS entry.
Suffice to say that I added a check to our monitoring so as to detect
machines entering production where `hostname`, not to even mention
/etc/hosts, still returns "localhost.localdomain" or even just
"localhost" instead of something unique.

Out of interest, what *purpose* is the obtained hostname being used for?
Does OpenSSH actually *need* it to be a) unique, b) reproducible, and/or
c) a proper FQDN, or does it merely enter the comment of autogenerated
host keypairs? All the KnownHosts checking AFAIR happens on the *client*
side and uses whatever name(s) of the server are known *there* ...

Regards,
-- 
Jochen Bern
Systemingenieur

Binect GmbH

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On Tue, 9 Feb 2021 at 19:05, Jochen Bern <Jochen.Bern at binect.de> wrote:
[...]
> Out of interest, what *purpose* is the obtained hostname being used for?
For the case that started this thread, it's used for expanding "%"
tokens in the config file.  From ssh_config(5):

TOKENS
     Arguments to some keywords can make use of tokens, which are expanded at
     runtime:
[...]
           %L    The local hostname.
           %l    The local hostname, including the domain name.

These can be used by various keywords, typically for file paths (for
example, if you want to make a ControlPath unique to a hostname).  I'd
have to check the code history to figure out where each token
originally came from, but I made them all more or less consistent
between the 8.2 and 8.3 releases.
> Does OpenSSH actually *need* it to be a) unique, b) reproducible, and/or
> c) a proper FQDN
Depends on what the user uses them for, but typically as long as
they're reproducible and unique within a set of machines that share a
filesystem it would likely be sufficient.

-- 
Darren Tucker (dtucker at dtucker.net)
GPG key 11EAA6FA / A86E 3E07 5B19 5880 E860  37F4 9357 ECEF 11EA A6FA (new)
    Good judgement comes with experience. Unfortunately, the experience
usually comes from bad judgement.