ARSoft.Tools.Net | project contains a complete managed | DNS library

Reading the code at https://github.com/mccj/ARSoft.Tools.Net.Core/blob/master/ARSoft.Tools.Net.Core/Dns/DnsClientBase.cs you can see there is an internal constructor that takes a port number...

And here https://github.com/mccj/ARSoft.Tools.Net.Core/blob/master/ARSoft.Tools.Net.Core/Dns/DnsClient.cs on line 81 we can see an example of an accessible constructor that accesses the internal one, passing in 53 as a hard coded value (i.e. you cant set the port number in your code, if you're using the provided DnsClient)

There doesn't appear to be a way to change the port number either upon instantiation of a DnsClient or afterwards (eg via a property) and you can't subclass DnsClientBase yourself because the relevant constructor you want is internal to the other assembly and not accessible. If you're desperate to do this, you might have to clone the ARSoft.Tools.Net.Core repo yourself and add a constructor to DnsClient

It's worth noting that the port setting is client-wide; there isn't the notion of having server A on port 53 and server B on port 5353, and both servers known to the same client. The port is fixed at instantiate time and applies to all the servers in the known list for that DnsClientBase

You may also get some mileage out of writing to the package author and asking for more info on why the code is arranged thus, as it seems to make it deliberately hard to use a custom port externally but internally all the necessary guts are there to make it a variable setting