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MINOR: dns: update dns response buffer reading pointer due to SRV record

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DNS SRV records uses "dns name compression" to store the target name.
"dns compression" principle is simple. Let's take the name below:
  3336633266663038.red.default.svc.cluster.local.
It can be stored "as is" in the response or it can be compressed like
this:
  3336633266663038<POINTER>
and <POINTER> would point to the string
'.red.default.svc.cluster.local.' availble in the question section for
example.
This mechanism allows storing much more data in a single DNS response.

This means the flag "record->data_len" which stores the size of the
record (hence the whole string, uncompressed) can't be used to move the
pointer forward when reading responses. We must use the "offset" integer
which means the real number of bytes occupied by the target name.

If we don't do that, we can properly read the first SRV record, then we
loose alignment and we start reading unrelated data (still in the
response) leading to a false negative error treated as an "invalid"
response...
This commit is contained in:
Baptiste Assmann 2017-08-11 10:37:20 +02:00 committed by Willy Tarreau
parent ddc8ce6d29
commit 63a2811077
1 changed files with 4 additions and 2 deletions
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4
src/dns.c
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@ -1302,7 +1302,6 @@ int dns_validate_dns_response(unsigned char *resp, unsigned char *bufend, struct
free_dns_answer_item(dns_answer_record); free_dns_answer_item(dns_answer_record);
return DNS_RESP_INVALID; return DNS_RESP_INVALID;
} }
reader++;
dns_answer_record->data_len = len; dns_answer_record->data_len = len;
memcpy(dns_answer_record->target, tmpname, len); memcpy(dns_answer_record->target, tmpname, len);
dns_answer_record->target[len] = 0; dns_answer_record->target[len] = 0;
@ -1324,6 +1323,9 @@ int dns_validate_dns_response(unsigned char *resp, unsigned char *bufend, struct
nb_saved_records += 1; nb_saved_records += 1;
/* move forward dns_answer_record->data_len for analyzing next record in the response */ /* move forward dns_answer_record->data_len for analyzing next record in the response */
if (dns_answer_record->type == DNS_RTYPE_SRV)
reader += offset;
else
reader += dns_answer_record->data_len; reader += dns_answer_record->data_len;
/* Lookup to see if we already had this entry */ /* Lookup to see if we already had this entry */