resolve: print a noisy warning if we show crypto keys that could not be authenticated

Doing DNS retrieval on non-authenticated crypt keys is useless, hence warn
loudly about it.

src/resolve/dns-type.c

@@ -193,6 +193,23 @@ bool dns_type_is_obsolete(uint16_t type) {
                       DNS_TYPE_NULL);
 }
 
+bool dns_type_needs_authentication(uint16_t type) {
+
+        /* Returns true for all (non-obsolete) RR types where records are not useful if they aren't
+         * authenticated. I.e. everything that contains crypto keys. */
+
+        return IN_SET(type,
+                      DNS_TYPE_CERT,
+                      DNS_TYPE_SSHFP,
+                      DNS_TYPE_IPSECKEY,
+                      DNS_TYPE_DS,
+                      DNS_TYPE_DNSKEY,
+                      DNS_TYPE_TLSA,
+                      DNS_TYPE_CDNSKEY,
+                      DNS_TYPE_OPENPGPKEY,
+                      DNS_TYPE_CAA);
+}
+
 int dns_type_to_af(uint16_t t) {
         switch (t) {
 

src/resolve/dns-type.h

@@ -132,6 +132,7 @@ bool dns_type_is_dnssec(uint16_t type);
 bool dns_type_is_obsolete(uint16_t type);
 bool dns_type_may_wildcard(uint16_t type);
 bool dns_type_apex_only(uint16_t type);
+bool dns_type_needs_authentication(uint16_t type);
 int dns_type_to_af(uint16_t t);
 
 bool dns_class_is_pseudo(uint16_t class);

src/resolve/resolve-tool.c

@@ -339,6 +339,7 @@ static int resolve_record(sd_bus *bus, const char *name, uint16_t class, uint16_
         uint64_t flags;
         int r;
         usec_t ts;
+        bool needs_authentication = false;
 
         assert(name);
 
@@ -421,6 +422,10 @@ static int resolve_record(sd_bus *bus, const char *name, uint16_t class, uint16_
                         log_warning_errno(errno, "Failed to resolve interface name for index %i: %m", ifindex);
 
                 printf("%s%s%s\n", s, isempty(ifname) ? "" : " # interface ", ifname);
+
+                if (dns_type_needs_authentication(t))
+                        needs_authentication = true;
+
                 n++;
         }
         if (r < 0)
@@ -441,6 +446,18 @@ static int resolve_record(sd_bus *bus, const char *name, uint16_t class, uint16_
 
         print_source(flags, ts);
 
+        if ((flags & SD_RESOLVED_AUTHENTICATED) == 0 && needs_authentication) {
+                fflush(stdout);
+
+                fprintf(stderr, "\n%s"
+                       "WARNING: The resources shown contain cryptographic key data which could not be\n"
+                       "         authenticated. It is not suitable to authenticate any communication.\n"
+                       "         This is usually indication that DNSSEC authentication was not enabled\n"
+                       "         or is not available for the selected protocol or DNS servers.%s\n",
+                       ansi_highlight_red(),
+                       ansi_normal());
+        }
+
         return 0;
 }