Tmds.MDns

This library allows to find services announced via multicast DNS (RFC6762 and RFC6763).

Version 0.7.0+ is compatible with .NET Core and .NET Framework 4.0+. Version 0.6 and below also supported .NET Framework 2.0, 3.5. Support for these versions was dropped due to a dotnet cli msbuild issue: dotnet/msbuild#1333.

Example

This examples shows how to use the ServiceBrowser class to find _workstation._tcp_ types.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Text;
using System.Threading.Tasks;
using Tmds.MDns;

namespace ServiceFinder
{
    class Program
    {
        static void Main(string[] args)
        {
            string serviceType = "_workstation._tcp";
            if (args.Length >= 1)
            {
                serviceType = args[0];
            }

            ServiceBrowser serviceBrowser = new ServiceBrowser();
            serviceBrowser.ServiceAdded += onServiceAdded;
            serviceBrowser.ServiceRemoved += onServiceRemoved;
            serviceBrowser.ServiceChanged += onServiceChanged;

            Console.WriteLine("Browsing for type: {0}", serviceType);
            serviceBrowser.StartBrowse(serviceType);
            Console.ReadLine();
        }

        static void onServiceChanged(object sender, ServiceAnnouncementEventArgs e)
        {
            printService('~', e.Announcement);
        }

        static void onServiceRemoved(object sender, ServiceAnnouncementEventArgs e)
        {
            printService('-', e.Announcement);
        }

        static void onServiceAdded(object sender, ServiceAnnouncementEventArgs e)
        {
            printService('+', e.Announcement);
        }

        static void printService(char startChar, ServiceAnnouncement service)
        {
            Console.WriteLine("{0} '{1}' on {2}", startChar, service.Instance, service.NetworkInterface.Name);
            Console.WriteLine("\tHost: {0} ({1})", service.Hostname, string.Join(", ", service.Addresses));
            Console.WriteLine("\tPort: {0}", service.Port);
            Console.WriteLine("\tTxt : [{0}]", string.Join(", ", service.Txt));
        }
    }
}

Implementation

The library does not aim to be a reference DNS querier and cache as described in RFC6762. These are the key differences:

• The DNS time to lives are not used. The user can set his own query interval to obtain the responsiveness (time to detect, time to live) he desires. The query interval does not increase exponentially as suggested by the RFC, queries are sent at a regular pace.
• To avoid that each service browser instance queries the network for the same service, an instance will not query when it hears another querrier. As a result (in steady state) only one querrier will be present, while the others will be listening.
• The library is self-contained and does not use a system service (e.g. avahi, bonjour) for mDNS. RFC6762 recommends the use of a single service, but it is not possible to combine this requirement with the differences described in this paragraph.
• The library automatically resolves the IP addresses of the service host. No separate resolve operation is required.
• The library launches events in a SynchronizationContext. This context can be set explicitly or it is captured automatically when the browse operation is executed. Using this library in a WinForms/WPF application is easy as the user can update the UI directly in the event handlers.
• When there is no SynchronizationContext, events for different interfaces may be generated simultaneously. When iterating over the Services, lock the property to ensure the iterator is not invalidated.