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nixos/networking: Add the FQDN and hostname to /etc/hosts

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This fixes the output of "hostname --fqdn" (previously the domain name
was not appended). Additionally it's now possible to use the FQDN.

This works by unconditionally adding two entries to /etc/hosts:
127.0.0.1 localhost
::1 localhost

These are the first two entries and therefore gethostbyaddr() will
always resolve "127.0.0.1" and "::1" back to "localhost" [0].
This works because nscd (or rather the nss-files module) returns the
first matching row from /etc/hosts (and ignores the rest).

The FQDN and hostname entries are appended later to /etc/hosts, e.g.:
127.0.0.2 nixos-unstable.test.tld nixos-unstable
::1 nixos-unstable.test.tld nixos-unstable
Note: We use 127.0.0.2 here to follow nss-myhostname (systemd) as close
as possible. This has the advantage that 127.0.0.2 can be resolved back
to the FQDN but also the drawback that applications that only listen to
127.0.0.1 (and not additionally ::1) cannot be reached via the FQDN.
If you would like this to work you can use the following configuration:
```nix
networking.hosts."127.0.0.1" = [
  "${config.networking.hostName}.${config.networking.domain}"
  config.networking.hostName
];
```

Therefore gethostbyname() resolves "nixos-unstable" to the FQDN
(canonical name): "nixos-unstable.test.tld".

Advantages over the previous behaviour:
- The FQDN will now also be resolved correctly (the entry was missing).
- E.g. the command "hostname --fqdn" will now work as expected.
Drawbacks:
- Overrides entries form the DNS (an issue if e.g. $FQDN should resolve
  to the public IP address instead of 127.0.0.1)
  - Note: This was already partly an issue as there's an entry for
    $HOSTNAME (without the domain part) that resolves to
    127.0.1.1 (!= 127.0.0.1).
- Unknown (could potentially cause other unexpected issues, but special
  care was taken).

[0]: Some applications do apparently depend on this behaviour (see
c578924) and this is typically the expected behaviour.

Co-authored-by: Florian Klink <flokli@flokli.de>
This commit is contained in:
Michael Weiss 2019-12-23 15:44:14 +01:00
parent bfdb43f1c3
commit 234d95a6fc
No known key found for this signature in database
GPG key ID: 5BE487C4D4771D83
2 changed files with 34 additions and 15 deletions
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16
nixos/doc/manual/release-notes/rl-2009.xml
View file

@ -415,6 +415,22 @@ systemd.services.nginx.serviceConfig.ReadWritePaths = [ "/var/www" ];
continue to work through Breezy.
</para>
</listitem>
<listitem>
<para>
In addition to the hostname, the fully qualified domain name (FQDN),
which consists of <literal>${cfg.hostName}</literal> and
<literal>${cfg.domain}</literal> is now added to
<literal>/etc/hosts</literal>, to allow local FQDN resolution, as used by the
<literal>hostname --fqdn</literal> command and other applications that
try to determine the FQDN. These new entries take precedence over entries
from the DNS which could cause regressions in some very specific setups.
Additionally the hostname is now resolved to <literal>127.0.0.2</literal>
instead of <literal>127.0.1.1</literal> to be consistent with what
<literal>nss-myhostname</literal> (from systemd) returns.
The old behaviour can e.g. be restored by using
<literal>networking.hosts = lib.mkForce { "127.0.1.1" = [ config.networking.hostName ]; };</literal>.
</para>
</listitem>
</itemizedlist>
</section>

33
nixos/modules/config/networking.nix
View file

@ -8,9 +8,6 @@ let
cfg = config.networking;
localhostMapped4 = cfg.hosts ? "127.0.0.1" && elem "localhost" cfg.hosts."127.0.0.1";
localhostMapped6 = cfg.hosts ? "::1" && elem "localhost" cfg.hosts."::1";
localhostMultiple = any (elem "localhost") (attrValues (removeAttrs cfg.hosts [ "127.0.0.1" "::1" ]));
in
@ -147,12 +144,6 @@ in
config = {
assertions = [{
assertion = localhostMapped4;
message = ''`networking.hosts` doesn't map "127.0.0.1" to "localhost"'';
} {
assertion = !cfg.enableIPv6 || localhostMapped6;
message = ''`networking.hosts` doesn't map "::1" to "localhost"'';
} {
assertion = !localhostMultiple;
message = ''
`networking.hosts` maps "localhost" to something other than "127.0.0.1"
@ -161,22 +152,34 @@ in
'';
}];
networking.hosts = {
"127.0.0.1" = [ "localhost" ];
} // optionalAttrs (cfg.hostName != "") {
"127.0.1.1" = [ cfg.hostName ];
# These entries are required for "hostname -f" and to resolve both the
# hostname and FQDN correctly:
networking.hosts = let
hostnames = # Note: The FQDN (canonical hostname) has to come first:
optional (cfg.hostName != "" && cfg.domain != null) "${cfg.hostName}.${cfg.domain}"
++ optional (cfg.hostName != "") cfg.hostName; # Then the hostname (without the domain)
in {
"127.0.0.2" = hostnames;
} // optionalAttrs cfg.enableIPv6 {
"::1" = [ "localhost" ];
"::1" = hostnames;
};
networking.hostFiles = let
# Note: localhostHosts has to appear first in /etc/hosts so that 127.0.0.1
# resolves back to "localhost" (as some applications assume) instead of
# the FQDN! By default "networking.hosts" also contains entries for the
# FQDN so that e.g. "hostname -f" works correctly.
localhostHosts = pkgs.writeText "localhost-hosts" ''
127.0.0.1 localhost
${optionalString cfg.enableIPv6 "::1 localhost"}
'';
stringHosts =
let
oneToString = set: ip: ip + " " + concatStringsSep " " set.${ip} + "\n";
allToString = set: concatMapStrings (oneToString set) (attrNames set);
in pkgs.writeText "string-hosts" (allToString (filterAttrs (_: v: v != []) cfg.hosts));
extraHosts = pkgs.writeText "extra-hosts" cfg.extraHosts;
in mkBefore [ stringHosts extraHosts ];
in mkBefore [ localhostHosts stringHosts extraHosts ];
environment.etc =
{ # /etc/services: TCP/UDP port assignments.