resolve: reject host names with leading or trailing dashes in /etc/hosts

https://tools.ietf.org/html/rfc1035#section-2.3.1 says (approximately)
that only letters, numbers, and non-leading non-trailing dashes are allowed
(for entries with A/AAAA records). We set no restrictions.

hosts(5) says:
> Host names may contain only alphanumeric characters, minus signs ("-"), and
> periods (".").  They must begin with an alphabetic character and end with an
> alphanumeric character.

nss-files follows those rules, and will ignore names in /etc/hosts that do not
follow this rule.

Let's follow the documented rules for /etc/hosts. In particular, this makes us
consitent with nss-files, reducing surprises for the user.

I'm pretty sure we should apply stricter filtering to names received over DNS
and LLMNR and MDNS, but it's a bigger project, because the rules differ
depepending on which level the label appears (rules for top-level names are
stricter), and this patch takes the minimalistic approach and only changes
behaviour for /etc/hosts.

Escape syntax is also disallowed in /etc/hosts, even if the resulting character
would be allowed. Other tools that parse /etc/hosts do not support this, and
there is no need to use it because no allowed characters benefit from escaping.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/resolve/resolved-dnssd.c b/src/resolve/resolved-dnssd.c
index ea96255dc1..2c28ec227a 100644
--- a/src/resolve/resolved-dnssd.c
+++ b/src/resolve/resolved-dnssd.c
@@ -228,10 +228,10 @@ int dnssd_update_rrs(DnssdService *s) {
         if (r < 0)
                 return r;
 
-        r = dns_name_concat(s->type, "local", &service_name);
+        r = dns_name_concat(s->type, "local", 0, &service_name);
         if (r < 0)
                 return r;
-        r = dns_name_concat(n, service_name, &full_name);
+        r = dns_name_concat(n, service_name, 0, &full_name);
         if (r < 0)
                 return r;