resolved,nss-myhostname: use _gateway for the gateway

This changes the symbolic name for the default gateway from "gateway" to
"_gateway". A new configuration option -Dcompat-gateway-hostname=true|false
is added. If it is set, the old name is also supported, but the new name
is used as the canonical name in either case. This is intended as a temporary
measure to make the transition easier, and the option should be removed
after a few releases, at which point only the new name will be used.

The old "gateway" name mostly works OK, but hasn't gained widespread acceptance
because of the following (potential) conflicts:
- it is completely legal to have a host called "gateway"
- there is no guarantee that "gateway" will not be registered as a TLD, even
  though this currently seems unlikely. (Even then, there would be no
  conflict except for the case when the top-level domain itself was being resolved.
  The "gateway" or "_gateway" labels have only special meaning when the
  whole name consists of a single label, so resolution of any subdomain
  of the hypothetical gateway. TLD would still work OK. )
Moving to "_gateway" avoids those issues because underscores are not allowed
in host names (RFC 1123, §2.1) and avoids potential conflicts with local or
global names.

v2:
- simplify the logic to hardcode "_gateway" and allow
  -Dcompat-gateway-hostname=true as a temporary measure.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/test/test-nss.c b/src/test/test-nss.c
index 57eeb8e40c..44570caa6c 100644
--- a/src/test/test-nss.c
+++ b/src/test/test-nss.c
@@ -491,7 +491,7 @@ static int parse_argv(int argc, char **argv,
                 if (!hostname)
                         return -ENOMEM;
 
-                names = strv_new("localhost", "gateway", "foo_no_such_host", hostname, NULL);
+                names = strv_new("localhost", "_gateway", "foo_no_such_host", hostname, NULL);
                 if (!names)
                         return -ENOMEM;