dns.config.udp_answer_limit not taken into account if edns is used

[kamaradclimber]

When filing a bug, please include the following:

consul version for both Client and Server

Client: 0.8.5
Server: 0.8.4

consul info for both Client and Server

Client:

agent:
	check_monitors = 0
	check_ttls = 0
	checks = 1
	services = 2
build:
	prerelease = 
	revision = 2c77151
	version = 0.8.5
consul:
	known_servers = 3
	server = false
runtime:
	arch = amd64
	cpu_count = 32
	goroutines = 41
	max_procs = 2
	os = linux
	version = go1.8.3
serf_lan:
	coordinate_resets = 0
	encrypted = false
	event_queue = 0
	event_time = 3006
	failed = 10
	health_score = 0
	intent_queue = 0
	left = 0
	member_time = 1073708
	members = 3625
	query_queue = 0
	query_time = 1935

Server:

agent:
	check_monitors = 0
	check_ttls = 0
	checks = 1
	services = 3
build:
	prerelease = 
	revision = f436077
	version = 0.8.4
consul:
	bootstrap = false
	known_datacenters = 8
	leader = false
	leader_addr = 10.71.0.171:8300
	server = true
raft:
	applied_index = 119308751
	commit_index = 119308751
	fsm_pending = 0
	last_contact = 18.798146ms
	last_log_index = 119308751
	last_log_term = 462751
	last_snapshot_index = 119303064
	last_snapshot_term = 462751
	latest_configuration = [{Suffrage:Voter ID:10.71.0.173:8300 Address:10.71.0.173:8300} {Suffrage:Voter ID:10.71.0.171:8300 Address:10.71.0.171:8300} {Suffrage:Voter ID:10.71.0.187:8300 Address:10.71.0.187:8300}]
	latest_configuration_index = 1
	num_peers = 2
	protocol_version = 2
	protocol_version_max = 3
	protocol_version_min = 0
	snapshot_version_max = 1
	snapshot_version_min = 0
	state = Follower
	term = 462751
runtime:
	arch = amd64
	cpu_count = 24
	goroutines = 14691
	max_procs = 23
	os = linux
	version = go1.8.3
serf_lan:
	coordinate_resets = 0
	encrypted = false
	event_queue = 0
	event_time = 3006
	failed = 9
	health_score = 0
	intent_queue = 0
	left = 0
	member_time = 1073708
	members = 3624
	query_queue = 0
	query_time = 1935
serf_wan:
	coordinate_resets = 0
	encrypted = false
	event_queue = 0
	event_time = 1
	failed = 0
	health_score = 0
	intent_queue = 0
	left = 0
	member_time = 66258
	members = 24
	query_queue = 0
	query_time = 1

Operating system and Environment details

centos 7.2 or centos 6.8

Description of the Issue (and unexpected/desired result)

Consul agent is configured with dns.config.udp_answer_limit to 1.
We configure bind to query consul dns api:

zone "consul" IN {
	type forward;
	forwarders {
	127.0.0.1 port 8600;
	};
	forward only;
};

When asking bind about consul zone, we get more than 1 results.

Reproduction steps

dig consul.service.consul @localhost -p 8600 gives 1 result
dig consul.service.consul @localhost gives 3 results.

Early investiguations

After analyzing a tcpdump, it seems that bind is using edns fields to allow udp payload size to be larger than the default 512.
In this case consul seems to ignore udp_answer_limit parameter and return as many results as possible.

This is confirmed by

consul/agent/dns.go

Line 570 in 496b0bc

if maxSize == defaultMaxUDPSize && numAnswers > maxAnswers {

[hsmade]

Can this please be either corrected or documented?