# Erlang DNS Server
Serve DNS authoritative responses... with Erlang.
[](https://github.com/dnsimple/erldns/actions/workflows/ci.yml)
[](https://hex.pm/packages/erldns)
## Requirements
Erlang/OTP 27+
## Building
To build:
```shell
make
```
To start fresh:
```shell
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
```shell
overmind start
```
### To get an interactive Erlang REPL
```shell
./rebar3 shell
```
### Build a distribution with and run the release
```shell
./rebar3 release
./_build/default/rel/erldns/bin/erldns foreground
```
## Querying
Here are some queries to try:
```bash
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.
If you want to perform some benchmarks, see [`BENCHMARKING.md`](./BENCHMARKING.md).
## 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:
```erlang
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">>}
}
]}).
```
### AXFR Support
AXFR zone transfers are not currently implemented. The current "implementation" is just a stub.
## 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](https://opentelemetry.io/docs/erlang/) (OTEL) Tracing to provide telemetry data on request processing inside of **erldns**.
To enable opentelmetry tracing, you need to:
1. Add [opentelemetry](https://github.com/open-telemetry/opentelemetry-erlang#including-in-release) as dependency of your application.
2. Configure the opentelemetry client:
Add the following configuration to the [erldns.config](erldns.config.example):
```erlang
{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](https://github.com/open-telemetry/opentelemetry-collector-contrib).
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>.
## Tests
To run automated tests:
```shell
make test
```
This runs both [EUnit](https://www.erlang.org/doc/apps/eunit/chapter.html) tests and [dialyzer](https://www.erlang.org/docs/23/man/dialyzer.html).
