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Fixes: #4683
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This unifies the suggested nsswitch.conf configuration for our four NSS modules to this:
hosts: files mymachines resolve [!UNAVAIL=return] dns myhostname
Note that this restores "myhostname" to the suggested configuration of
nss-resolve for the time being, undoing 4484e1792b64b01614f04b7bde97bf019f601bf9.
"myhostname" should probably be dropped eventually, but when we do this we
should do it in full, and not only drop it from the suggested nsswitch.conf
for one of the modules, but also drop it in source and stop referring to it
altogether.
Note that nss-resolve doesn't replace nss-myhostname in full: the former only
works if D-Bus/resolved is available for resolving the local hostname, the
latter works in all cases even if D-Bus or resolved are not in operation, hence
there's some value in keeping the line as it is right now. Note that neither
dns nor myhostname are considered at all with the above configuration unless
the resolve module actually returns UNAVAIL. Thus, even though handling of
local hostname resolving is implemented twice this way it is only executed once
for each lookup.
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It needs to be possible to tell apart "the nss-resolve module does not exist"
(which can happen when running foreign-architecture programs) from "the queried
DNS name failed DNSSEC validation" or other errors. So return NOTFOUND for these
cases too, and only keep UNAVAIL for the cases where we cannot handle the given
address family.
This makes it possible to configure a fallback to "dns" without breaking
DNSSEC, with "resolve [!UNAVAIL=return] dns". Add this to the manpage.
This does not change behaviour if resolved is not running, as that already
falls back to the "dns" glibc module.
Fixes #4157
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resolve includes myhostname functionality, so there is no need to add it again.
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With this NSS module all dynamic service users will be resolvable via NSS like
any real user.
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So far we recommended placing "nss-mymachines" after "nss-resolve" in the order
of preference in /etc/nsswitch.conf. This change reverse this order.
Rationale: single-label names are resolved via LLMNR by resolved, which has to
time out if no peer by that name exists. By placing "nss-mymachines" first
(which always responds immediately) we avoid running into this timeout for most
containers. Both modules should return the same data if LLMNR is used by the
container anyway.
While we are at it, improve the man pages of the three NSS modules in other
ways a bit.
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Show the same recommended example file in all three man pages, just
highlight the different, relevant parts.
This should be less confusing for users, and clarify what we actually
recommend how /etc/nsswitch.conf is set up.
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