MeaMod.DNS

A simple Multicast Domain Name Service based on RFC 6762. Can be used as both a client (sending queries) or a server (responding to queries).

A higher level DNS Service Discovery based on RFC 6763 that automatically responds to any query for the service or service instance.

DNS data model with serializer/deserializer for the wire and "master file" format.

Credits and Disclaimer

This repository is a fork of Richard Schneider's net-mdns and net-dns repository without any additional dependencies.

Features

• Targets Framework 4.6, .NET Standard 2.0 and .NET 5

MeaMod.DNS.Multicast Features

• Supports IPv6 and IPv4 platforms
• Detects new and/or removed network interfaces
• Supports multicasting on multiple network interfaces
• Supports reverse address mapping
• Supports service subtypes (features)
• Handles legacy unicast queries, see #61

MeaMod.DNS Features

• Serialization for the wire and master file formats
• Pretty printing of messages
• Supports compressed domain names
• Supports multiple strings in TXT records
• Supports the extended 12-bit RCODE
• Future proof: handles unknown resource records and EDNS options
• Graceful truncation of messages
• A name server that answeres DNS questions
• Data models for
  • RFC 1035 Domain Names (DNS)
  • RFC 1183 New DNS RR Definitions
  • RFC 1996 Zone Changes (DNS NOTIFY)
  • RFC 2136 Dynamic Updates (DNS UPDATE)
  • RFC 2845 Secret Key Transaction Authentication for DNS (TSIG)
  • RFC 2930 Secret Key Establishment for DNS (TKEY RR)
  • RFC 3225 Indicating Resolver Support of DNSSEC
  • RFC 3599 DNS Extensions to Support IPv6
  • RFC 4034 Resource Records for the DNS Security Extensions (DNSSEC)
  • RFC 5001 DNS Name Server Identifier (NSID) Option
  • RFC 6672 DNAME Redirection in the DNS
  • RFC 6891 Extension Mechanisms for DNS (EDNS(0))
  • RFC 7828 The edns-tcp-keepalive EDNS0 Option
  • RFC 7830 The EDNS(0) Padding Option
  • RFC 8914 Extended DNS Errors

Getting started

Published releases are available on NuGet. To install, run the following command in the Package Manager Console

PM> Install-Package MeaMod.DNS

or using .NET CLI run the following command in the project folder

> dotnet add package MeaMod.DNS

Usage Service Discovery

Advertising

Always broadcast the service ("foo") running on local host with port 1024.

using MeaMod.DNS.Multicast;

var service = new ServiceProfile("x", "_foo._tcp", 1024);
var sd = new ServiceDiscovery();
sd.Advertise(service);

See the example advertiser for a working program.

Discovery

Find all services running on the local link.

using MeaMod.DNS.Multicast;

var sd = new ServiceDiscovery();
sd.ServiceDiscovered += (s, serviceName) => { // Do something };

Find all service instances running on the local link.

using MeaMod.DNS.Multicast;

var sd = new ServiceDiscovery();
sd.ServiceInstanceDiscovered += (s, e) => { // Do something };

See the example browser for a working program.

Usage Multicast

Event Based Queries

Get all the Apple TVs. The query is sent when a network interface is discovered. The AnsweredReceived callback contains any answer that is seen, not just the answer to the specific query.

using MeaMod.DNS.Multicast;

var mdns = new MulticastService();
mdns.NetworkInterfaceDiscovered += (s, e) => mdns.SendQuery("appletv.local");
mdns.AnswerReceived += (s, e) => { // do something with e.Message };
mdns.Start();

Async Queries

Get the first answer to Apple TVs. Wait 2 seconds for an answer.

using MeaMod.DNS;
using MeaMod.DNS.Multicast;

var service = "appletv.local";
var query = new Message();
query.Questions.Add(new Question { Name = service, Type = DnsType.ANY });
var cancellation = new CancellationTokenSource(2000);

using (var mdns = new MulticastService())
{
    mdns.Start();
    var response = await mdns.ResolveAsync(query, cancellation.Token);
    // Do something
}

Broadcasting

Respond to a query for the service. Note that ServiceDiscovery.Advertise is much easier.

using MeaMod.DNS;
using MeaMod.DNS.Multicast;

var service = "...";
var mdns = new MulticastService();
mdns.QueryReceived += (s, e) =>
{
    var msg = e.Message;
    if (msg.Questions.Any(q => q.Name == service))
    {
        var res = msg.CreateResponse();
        var addresses = MulticastService.GetIPAddresses()
            .Where(ip => ip.AddressFamily == AddressFamily.InterNetwork);
        foreach (var address in addresses)
        {
            res.Answers.Add(new ARecord
            {
                Name = service,
                Address = address
            });
        }
        mdns.SendAnswer(res);
    }
};
mdns.Start();

Name Server

Create a name server that can answer questions for a zone.

using MeaMod.DNS;
using MeaMod.DNS.Server;

var catalog = new Catalog();
catalog.IncludeZone(...);
catalog.IncludeRootHints();
var resolver = new NameServer { Catalog = catalog };

Answer a question

var request = new Message();
request.Questions.Add(new Question { Name = "ns.example.com", Type = DnsType.AAAA });
var response = await resolver.ResolveAsync(request);

Data Model

using MeaMod.DNS;

var msg = new Message
{
	AA = true,
	QR = true,
	Id = 1234
};
msg.Questions.Add(new Question 
{ 
	Name = "emanon.org" 
});
msg.Answers.Add(new ARecord 
{ 
	Name = "emanon.org",
	Address = IPAddress.Parse("127.0.0.1") 
});
msg.AuthorityRecords.Add(new SOARecord
{
	Name = "emanon.org",
	PrimaryName = "erehwon",
	Mailbox = "hostmaster.emanon.org"
});
msg.AdditionalRecords.Add(new ARecord 
{ 
	Name = "erehwon", 
	Address = IPAddress.Parse("127.0.0.1") 
});

Related projects

• net-dns - DNS data model and Name Server with serializer for the wire and master file format
• net-udns - client for unicast DNS, DNS over HTTPS (DOH) and DNS over TLS (DOT)
• net-mdns - client and server for multicast DNS

License

Copyright © 2021 James Weston

Copyright © 2018-2019 Richard Schneider (makaretu@gmail.com)

The package is licensed under the MIT license. Refer to the LICENSE file for more information.