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MEDIUM: dns: Don't use the ANY query type

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Basically, it's ill-defined and shouldn't really be used going forward.
We can't guarantee that resolvers will do the 'legwork' for us and
actually resolve CNAMES when we request the ANY query-type. Case in point
(obfuscated, clearly):

  PRODUCTION! ahayworth@secret-hostname.com:~$
  dig @10.11.12.53 ANY api.somestartup.io

  ; <<>> DiG 9.8.4-rpz2+rl005.12-P1 <<>> @10.11.12.53 ANY api.somestartup.io
  ; (1 server found)
  ;; global options: +cmd
  ;; Got answer:
  ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 62454
  ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 4, ADDITIONAL: 0

  ;; QUESTION SECTION:
  ;api.somestartup.io.                        IN      ANY

  ;; ANSWER SECTION:
  api.somestartup.io.         20      IN      CNAME api-somestartup-production.ap-southeast-2.elb.amazonaws.com.

  ;; AUTHORITY SECTION:
  somestartup.io.               166687  IN      NS      ns-1254.awsdns-28.org.
  somestartup.io.               166687  IN      NS      ns-1884.awsdns-43.co.uk.
  somestartup.io.               166687  IN      NS      ns-440.awsdns-55.com.
  somestartup.io.               166687  IN      NS      ns-577.awsdns-08.net.

  ;; Query time: 1 msec
  ;; SERVER: 10.11.12.53#53(10.11.12.53)
  ;; WHEN: Mon Oct 19 22:02:29 2015
  ;; MSG SIZE  rcvd: 242

HAProxy can't handle that response correctly.

Rather than try to build in support for resolving CNAMEs presented
without an A record in an answer section (which may be a valid
improvement further on), this change just skips ANY record types
altogether. A and AAAA are much more well-defined and predictable.

Notably, this commit preserves the implicit "Prefer IPV6 behavior."

Furthermore, ANY query type by default is a bad idea: (from Robin on
HAProxy's ML):
  Using ANY queries for this kind of stuff is considered by most people
  to be a bad practice since besides all the things you named it can
  lead to incomplete responses. Basically a resolver is allowed to just
  return whatever it has in cache when it receives an ANY query instead
  of actually doing an ANY query at the authoritative nameserver. Thus
  if it only received queries for an A record before you do an ANY query
  you will not get an AAAA record even if it is actually available since
  the resolver doesn't have it in its cache. Even worse if before it
  only got MX queries, you won't get either A or AAAA
This commit is contained in:
Andrew Hayworth 2015-10-19 22:29:51 +00:00 committed by Willy Tarreau
parent 2f63ef4d1c
commit e6a4a329b8
4 changed files with 21 additions and 16 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

3
include/types/dns.h
View File

@ -161,7 +161,8 @@ struct dns_resolution {
unsigned int last_status_change; /* time of the latest DNS resolution status change */
int query_id; /* DNS query ID dedicated for this resolution */
struct eb32_node qid; /* ebtree query id */
int query_type; /* query type to send. By default DNS_RTYPE_ANY */
int query_type;
/* query type to send. By default DNS_RTYPE_A or DNS_RTYPE_AAAA depending on resolver_family_priority */
int status; /* status of the resolution being processed RSLV_STATUS_* */
int step; /* */
int try; /* current resolution try */

8
src/checks.c
View File

@ -2214,11 +2214,15 @@ int trigger_resolution(struct server *s)
resolution->query_id = query_id;
resolution->qid.key = query_id;
resolution->step = RSLV_STEP_RUNNING;
resolution->query_type = DNS_RTYPE_ANY;
resolution->resolver_family_priority = s->resolver_family_priority;
if (resolution->resolver_family_priority == AF_INET) {
resolution->query_type = DNS_RTYPE_A;
} else {
resolution->query_type = DNS_RTYPE_AAAA;
}
resolution->try = resolvers->resolve_retries;
resolution->try_cname = 0;
resolution->nb_responses = 0;
resolution->resolver_family_priority = s->resolver_family_priority;
eb32_insert(&resolvers->query_ids, &resolution->qid);
dns_send_query(resolution);

6
src/dns.c
View File

@ -102,7 +102,11 @@ void dns_reset_resolution(struct dns_resolution *resolution)
resolution->qid.key = 0;
/* default values */
resolution->query_type = DNS_RTYPE_ANY;
if (resolution->resolver_family_priority == AF_INET) {
resolution->query_type = DNS_RTYPE_A;
} else {
resolution->query_type = DNS_RTYPE_AAAA;
}
/* the second resolution in the queue becomes the first one */
LIST_DEL(&resolution->list);

20
src/server.c
View File

@ -2668,7 +2668,7 @@ int snr_resolution_error_cb(struct dns_resolution *resolution, int error_code)
{
struct server *s;
struct dns_resolvers *resolvers;
int qtype_any, res_preferred_afinet, res_preferred_afinet6;
int res_preferred_afinet, res_preferred_afinet6;
/* shortcut to the server whose name is being resolved */
s = (struct server *)resolution->requester;
@ -2692,21 +2692,13 @@ int snr_resolution_error_cb(struct dns_resolution *resolution, int error_code)
case DNS_RESP_TRUNCATED:
case DNS_RESP_ERROR:
case DNS_RESP_NO_EXPECTED_RECORD:
qtype_any = resolution->query_type == DNS_RTYPE_ANY;
res_preferred_afinet = resolution->resolver_family_priority == AF_INET && resolution->query_type == DNS_RTYPE_A;
res_preferred_afinet6 = resolution->resolver_family_priority == AF_INET6 && resolution->query_type == DNS_RTYPE_AAAA;
if ((qtype_any || res_preferred_afinet || res_preferred_afinet6)
if ((res_preferred_afinet || res_preferred_afinet6)
|| (resolution->try > 0)) {
/* let's change the query type */
if (qtype_any) {
/* fallback from ANY to resolution preference */
if (resolution->resolver_family_priority == AF_INET6)
resolution->query_type = DNS_RTYPE_AAAA;
else
resolution->query_type = DNS_RTYPE_A;
}
else if (res_preferred_afinet6) {
if (res_preferred_afinet6) {
/* fallback from AAAA to A */
resolution->query_type = DNS_RTYPE_A;
}
@ -2716,7 +2708,11 @@ int snr_resolution_error_cb(struct dns_resolution *resolution, int error_code)
}
else {
resolution->try -= 1;
resolution->query_type = DNS_RTYPE_ANY;
if (resolution->resolver_family_priority == AF_INET) {
resolution->query_type = DNS_RTYPE_A;
} else {
resolution->query_type = DNS_RTYPE_AAAA;
}
}
dns_send_query(resolution);