Re: [PATCH 06/13] resolv: Add DNS packet parsing helpers geared towards wire format

[Florian Weimer <fweimer@redhat.com>]

* Siddhesh Poyarekar:

>> +bool
>> +__ns_rr_cursor_next (struct ns_rr_cursor *c, struct ns_rr_wire *rr)
>> +{
>> +  rr->rdata = NULL;
>> +
>> +  /* Extract the record owner name.  */
>> +  int consumed = __ns_name_unpack (c->begin, c->end, c->current,
>> +                                   rr->rname, sizeof (rr->rname));
>> +  if (consumed < 0)
>> +    {
>> +      memset (rr, 0, sizeof (*rr));
>> +      __set_errno (EMSGSIZE);
>> +      return false;
>> +    }
>> +  c->current += consumed;
>> +
>> +  /* Extract the metadata.  */
>> +  struct
>> +  {
>> +    uint16_t rtype;
>> +    uint16_t rclass;
>> +    uint32_t ttl;
>> +    uint16_t rdlength;
>> +  } __attribute__ ((packed)) metadata;
>> +  _Static_assert (sizeof (metadata) == 10, "sizeof metadata");
>> +  if (c->end - c->current < sizeof (metadata))
>> +    {
>> +      memset (rr, 0, sizeof (*rr));
>> +      __set_errno (EMSGSIZE);
>> +      return false;
>> +    }
>> +  memcpy (&metadata, c->current, sizeof (metadata));
>
> Doesn't this go out of sync with the init above?  The initialization
> appears to put current just after rclass (with current += 4).

Do you mean __ns_rr_cursor_init?  The question section has a different
entry layout than the other sections.

Quoting RFC 1035:

| 4.1.2. Question section format
| 
| The question section is used to carry the "question" in most queries,
| i.e., the parameters that define what is being asked.  The section
| contains QDCOUNT (usually 1) entries, each of the following format:
| 
|                                     1  1  1  1  1  1
|       0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
|     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|     |                                               |
|     /                     QNAME                     /
|     /                                               /
|     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|     |                     QTYPE                     |
|     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|     |                     QCLASS                    |
|     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

Versus:

| 4.1.3. Resource record format
| 
| The answer, authority, and additional sections all share the same
| format: a variable number of resource records, where the number of
| records is specified in the corresponding count field in the header.
| Each resource record has the following format:
|                                     1  1  1  1  1  1
|       0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
|     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|     |                                               |
|     /                                               /
|     /                      NAME                     /
|     |                                               |
|     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|     |                      TYPE                     |
|     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|     |                     CLASS                     |
|     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|     |                      TTL                      |
|     |                                               |
|     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|     |                   RDLENGTH                    |
|     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--|
|     /                     RDATA                     /
|     /                                               /
|     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

At least I hope this is what your question is about. 8-)

Thanks,
Florian