resolved: rework how we handle truncation in the stub resolver

When we a reply message gets longer than the client supports we need to
truncate the response and set the TC bit, and we already do that.
However, we are not supposed to send incomplete RRs in that case, but
instead truncate right at a record boundary. Do that.

This fixes the "Message parser reports malformed message packet."
warning the venerable "host" tool outputs when a very large response is
requested.

See: #6520

1 files changed, 12 insertions, 2 deletions

diff --git a/src/resolve/resolved-dns-packet.h b/src/resolve/resolved-dns-packet.h
index d4c6b3c9cb..b873c0f745 100644
--- a/src/resolve/resolved-dns-packet.h
+++ b/src/resolve/resolved-dns-packet.h
@@ -73,7 +73,7 @@ struct DnsPacketHeader {
 struct DnsPacket {
         int n_ref;
         DnsProtocol protocol;
-        size_t size, allocated, rindex;
+        size_t size, allocated, rindex, max_size;
         void *_data; /* don't access directly, use DNS_PACKET_DATA()! */
         Hashmap *names; /* For name compression */
         size_t opt_start, opt_size;
@@ -187,7 +187,7 @@ static inline unsigned DNS_PACKET_RRCOUNT(DnsPacket *p) {
                 (unsigned) DNS_PACKET_ARCOUNT(p);
 }
 
-int dns_packet_new(DnsPacket **p, DnsProtocol protocol, size_t min_alloc_dsize);
+int dns_packet_new(DnsPacket **p, DnsProtocol protocol, size_t min_alloc_dsize, size_t max_size);
 int dns_packet_new_query(DnsPacket **p, DnsProtocol protocol, size_t min_alloc_dsize, bool dnssec_checking_disabled);
 
 void dns_packet_set_flags(DnsPacket *p, bool dnssec_checking_disabled, bool truncated);
@@ -303,3 +303,13 @@ static inline uint64_t SD_RESOLVED_FLAGS_MAKE(DnsProtocol protocol, int family,
                 return f;
         }
 }
+
+static inline size_t dns_packet_size_max(DnsPacket *p) {
+        assert(p);
+
+        /* Why not insist on a fully initialized max_size during DnsPacket construction? Well, this way it's easy to
+         * allocate a transient, throw-away DnsPacket on the stack by simple zero initialization, without having to
+         * deal with explicit field initialization. */
+
+        return p->max_size != 0 ? p->max_size : DNS_PACKET_SIZE_MAX;
+}