resolved: fix NSEC proofs for missing TLDs

For the wildcard NSEC check we need to generate an "asterisk" domain, by
prepend the common ancestor with "*.". So far we did that with a simple
strappenda() which is fine for most domains, but doesn't work if the
common ancestor is the root domain as we usually write that as "." in
normalized form, and "*." joined with "." is "*.." and not "*." as it
should be.

Hence, use the clean way out, let's just use dns_name_concat() which
only exists precisely for this reason, to properly concatenate labels.

There's a good chance this actually fixes #5029, as this NSEC proof is
triggered by lookups in the TLD "example", which doesn't exist in the
Internet.

1 files changed, 6 insertions, 2 deletions

diff --git a/src/resolve/resolved-dns-dnssec.c b/src/resolve/resolved-dns-dnssec.c
index 51327105d0..eddab58a81 100644
--- a/src/resolve/resolved-dns-dnssec.c
+++ b/src/resolve/resolved-dns-dnssec.c
@@ -1710,7 +1710,8 @@ static int dnssec_nsec_covers(DnsResourceRecord *rr, const char *name) {
 }
 
 static int dnssec_nsec_covers_wildcard(DnsResourceRecord *rr, const char *name) {
-        const char *common_suffix, *wc;
+        _cleanup_free_ char *wc = NULL;
+        const char *common_suffix;
         int r;
 
         assert(rr);
@@ -1734,7 +1735,10 @@ static int dnssec_nsec_covers_wildcard(DnsResourceRecord *rr, const char *name)
         if (r <= 0)
                 return r;
 
-        wc = strjoina("*.", common_suffix);
+        r = dns_name_concat("*", common_suffix, &wc);
+        if (r < 0)
+                return r;
+
         return dns_name_between(dns_resource_key_name(rr->key), wc, rr->nsec.next_domain_name);
 }