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/resolve/resolved-dns-synthesize.c b/src/resolve/resolved-dns-synthesize.c
index e3003411f7..c454f64049 100644
--- a/src/resolve/resolved-dns-synthesize.c
+++ b/src/resolve/resolved-dns-synthesize.c
@@ -334,7 +334,7 @@ static int synthesize_gateway_ptr(Manager *m, int af, const union in_addr_union
         if (n < 0)
                 return n;
 
-        return answer_add_addresses_ptr(answer, "gateway", addresses, n, af, address);
+        return answer_add_addresses_ptr(answer, "_gateway", addresses, n, af, address);
 }
 
 int dns_synthesize_answer(