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Forwarding DNS

By default DNS is served from port 53 which requires root privileges. Instead of running Consul as root, it is possible to instead run Bind and forward queries to Consul as appropriate.

In this example, Bind and Consul are running on the same machine for simplicity but this is not required.

Bind Setup

First, you have to disable DNSSEC so that Consul and Bind can communicate. This is an example configuration:

options {
    listen-on port 53 { 127.0.0.1; };
    listen-on-v6 port 53 { ::1; };
    directory       "/var/named";
    dump-file       "/var/named/data/cache_dump.db";
    statistics-file "/var/named/data/named_stats.txt";
    memstatistics-file "/var/named/data/named_mem_stats.txt";
    allow-query     { localhost; };
    recursion yes;

    dnssec-enable no;
    dnssec-validation no;

    /* Path to ISC DLV key */
    bindkeys-file "/etc/named.iscdlv.key";

    managed-keys-directory "/var/named/dynamic";
};

include "/etc/named/consul.conf";

Zone File

Then we set up a zone for our Consul managed records in consul.conf:

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

Here we assume Consul is running with default settings, and is serving DNS on port 8600.

Testing

First, perform a DNS query against Consul directly to be sure that the record exists:

[root@localhost ~]# dig @localhost -p 8600 master.redis.service.dc-1.consul. A

; <<>> DiG 9.8.2rc1-RedHat-9.8.2-0.23.rc1.32.amzn1 <<>> @localhost master.redis.service.dc-1.consul. A
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 11536
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;master.redis.service.dc-1.consul. IN A

;; ANSWER SECTION:
master.redis.service.dc-1.consul. 0 IN A 172.31.3.234

;; Query time: 4 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Wed Apr  9 17:36:12 2014
;; MSG SIZE  rcvd: 76

Then run the same query against your Bind instance and make sure you get a result:

[root@localhost ~]# dig @localhost -p 53 master.redis.service.dc-1.consul. A

; <<>> DiG 9.8.2rc1-RedHat-9.8.2-0.23.rc1.32.amzn1 <<>> @localhost master.redis.service.dc-1.consul. A
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 11536
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;master.redis.service.dc-1.consul. IN A

;; ANSWER SECTION:
master.redis.service.dc-1.consul. 0 IN A 172.31.3.234

;; Query time: 4 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Wed Apr  9 17:36:12 2014
;; MSG SIZE  rcvd: 76

Troubleshooting

If you don't get an answer from Bind but you do get an answer from Consul then your best bet is to turn on the query log to see what's going on:

[root@localhost ~]# rndc querylog
[root@localhost ~]# tail -f /var/log/messages

In there if you see errors like this:

error (no valid RRSIG) resolving
error (no valid DS) resolving

Then DNSSEC is not disabled properly. If you see errors about network connections then verify that there are no firewall or routing problems between the servers running Bind and Consul.

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