Erlang DNS Server

Serve DNS authoritative responses... with Erlang.

Building

To build:

make

To start fresh:

make fresh
make

Zones

Zones are loaded from JSON. Example JSON files are in the priv/ directory.

You can also write new systems to load zones by writing the zones directly to the zone cache using erldns_zone_cache:put_zone/1.

Configuration

An example configuration file can be found in erldns.example.config. Copy it to erldns.config and modify as needed.

Running

Launch directly:

overmind start

To get an interactive Erlang REPL:

./rebar3 shell

Build a distribution with and run the release:

./rebar3 release
./_build/default/rel/erldns/bin/erldns foreground

Querying

Here are some queries to try:

dig -p 8053 @127.0.0.1 example.com a
dig -p 8053 @127.0.0.1 example.com cname
dig -p 8053 @127.0.0.1 example.com ns
dig -p 8053 @127.0.0.1 example.com mx
dig -p 8053 @127.0.0.1 example.com spf
dig -p 8053 @127.0.0.1 example.com txt
dig -p 8053 @127.0.0.1 example.com sshfp
dig -p 8053 @127.0.0.1 example.com soa
dig -p 8053 @127.0.0.1 example.com naptr

dig -p 8053 @127.0.0.1 -x 127.0.0.1 ptr

Performance

In our environment (DNSimple) we are seeing 30 to 65 µs handoff times to retrieve a packet from the UDP port and give it to a worker for processing. Your performance may vary, but given those measurements erl-dns is capable of handling between 15k and 30k questions per second. Please note: You may need to configure the number of workers available to handle traffic at higher volumes.

Design

The erldns_resolver module will attempt to find zone data in the zone cache. If you're embedding erl-dns in your application the easiest thing to do is to load the zone cache once the zone cache gen_server starts push an updated zone into the cache each time data changes.

To insert a zone, use erldns_zone_cache:put_zone({Name, Records}) where Name is a binary term such as <<"example.com">> and Records is a list of dns_rr records (whose definitions can be found in deps/dns/include/dns_records.hrl). The name of each record must be the fully qualified domain name (including the zone part).

Here's an example:

erldns_zone_cache:put_zone({
  <<"example.com">>, [
    #dns_rr{
      name = <<"example.com">>,
      type = ?DNS_TYPE_A,
      ttl = 3600,
      data = #dns_rrdata_a{ip = {1,2,3,4}}
    },
    #dns_rr{
      name = <<"www.example.com">>,
      type = ?DNS_TYPE_CNAME,
      ttl = 3600,
      data = #dns_rrdata_cname{dname = <<"example.com">>}
    }
  ]}).

Metrics

Folsom is used to gather runtime metrics and statistics.

There is an HTTP API for querying metric data available at https://github.com/dnsimple/erldns-metrics

Tracing

This project uses OpenTelemetry (OTEL) Tracing to provide telemetry data on request processing inside of erldns.

To enable opentelmetry tracing, you need to:

1. Add opentelemetry as dependency of your application.

2. Configure the opentelemetry client:

   Add the following configuration to the erldns.config:

     {opentelemetry,[
       {processors,
           [{otel_batch_processor,
               #{exporter => {opentelemetry_exporter, #{protocol => http_protobuf,
                                                       endpoints => [{http, "127.0.0.1", 55681, []}]}}}}]}
     ]}

NOTE: You will need to have a running OpenTelemetry Collector.

Application traces:

Name	Dimensions
erldns_tcp_worker
handle_tcp_dns_query	status, qr, rd, ad, qname, qtype
handle_decoded_tcp_message	status
send_tcp_message
erldns_udp_worker	host, port, erlang_port_count, erlang_proc_count, erlang_run_queue, erlang_proc_message_queue_len, status
handle_udp_dns_query	status, qr, rd, ad, qname, qtype
handle_decoded_udp_message	status
synthesize_answer
encode_message	rcode, aa, ra, answers

Admin

There is an administrative API for querying the current zone cache and for basic control. You can find it in https://github.com/dnsimple/erldns-admin