Author Topic: Anyone into mDNS?  (Read 1511 times)


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Anyone into mDNS?
« on: January 09, 2022, 09:09:15 pm »
Hi @all,

last month, I've completed a task to implement mDNS/Zeroconfig search into an app.
My target was to get the IP addresses of the PLCs in the network (I am working as an instrumentation and controlling engineer). The PLCs we're using, are responding to mDNS queries from the engineering software and I wanted to have this same functionality in my app.
The app is not open source, but I would like to share all my experience worth a month of internet search (not much info found in internet anyway) for someone like me, who didn't have any idea about how is mDNS working (and still has not the whole picture, just the part he needs). The app I wrote can also get and set the parameters of the network adapters (IP, Gateway etc.) and I would also like to share this code in another thread.

I have used two components from Indy 10 for mDNS/zeroconf/Avahi/Bonjour communication:
- IdIPMCastServer to send the mDNS query
- IdUDPServer to catch the answers

The basics of the multicast is that the IdIPMCastServer sends a query to the group with specifying the UDP port (Bound port in Indy terminology) where your app is waiting for the answers. After sending the query, our IdIPMCastServer needs to be deactivated.
Waiting for answers belongs to IdUDPServer, which should now be activated. Because of using the same port, you can't have them both active at the same time.
More instructions are in the code comments.

Code: Pascal  [Select][+][-]
  1. procedure TfrmMain.mnuMulticastClick(Sender: TObject);
  2. var
  3.   //basic communication as captured by Wireshark
  4.   {00 00 - Transaction ID
  5.    00 00 - Standard query
  6.    00 01 - Question count
  7.    00 00 - Answer RRs count
  8.    00 00 - Authority RRs count
  9.    00 00 - Additional RRs count
  10.    xx xx - query string/question string
  11.    00 0c - Type PTR
  12.    00 01 - Class IN}
  13.   query: TIdBytes = ($00, $00, $00, $00, $00, $01, $00, $00, $00, $00,
  14.     $00, $00, $09, $5f, $73, $65, $72, $76, $69, $63, $65, $73, $07,
  15.     $5f, $64, $6e, $73, $2d, $73, $64, $04, $5f, $75, $64, $70, $05,
  16.     $6c, $6f, $63, $61, $6c, $00, $00, $0c, $00, $01);
  17. begin
  18.   //set CastServer
  19.   IdIPMCastServer.MulticastGroup := Id_IPMC_mDNS;
  20.   IdIPMCastServer.BoundIP := String_IP_Address; //Here goes the IP address of your network adapter
  21.   IdIPMCastServer.BoundPort := 49152; //my favorite port. Pick one that is not used by other apps in your network. Other devices will use this port to answer your query
  22.   IdIPMCastServer.Port := 5353; //this is standard
  24.   //set UDPServer
  25.   IdUDPServer.Bindings.Add;
  26.   IdUDPServer.Bindings[0].IP := String_IP_Address; //Here goes the IP address of your network adapter
  27.   IdUDPServer.Bindings[0].Port := 49152;
  29.   //send Query
  30.   IdIPMCastServer.Active := True;
  31.   IdIPMCastServer.Send(query);
  33.   //deactivate CastServer, activate UDPServer and deactivation timer for UDPServer
  34.   IdIPMCastServer.Active := False;
  35.   tmUDPdeactivate.Interval := IdIPMCastServer.TimeToLive * 1000;
  36.   tmUDPdeactivate.Enabled := True; //I use a timer to deactivate the IdUDPServer after 5 seconds of waiting for messages. You need this if you want to send another query with IdIPMCastServer because of shared port
  37.   IdUDPServer.Active := True;
  38. end;    

My IdUDPServer.OnUDPRead looks like this:
Code: Pascal  [Select][+][-]
  1. procedure TfrmMain.IdUDPServerUDPRead(AThread: TIdUDPListenerThread;
  2.   const AData: TIdBytes; ABinding: TIdSocketHandle);
  3. begin
  4.   sgMachines.InsertRowWithValues(sgMachines.RowCount, [IPAddrToName(ABinding.PeerIP), ABinding.PeerIP]);
  5.  ...
  6. end;
I am just adding to a StringGrid the IP address of devices that responded to my query.
The responses from the devices are pretty much alike to the query-structure that I've sent to multicast.
Except for the PLCs that I wanted to search for, my code also finds my printers and NAS in my home network. It means - the query string is not specific for the PLCs, but it seems to be pretty much a universal query (anyone here with some more info about the query strings?).
This is the point where I am asking for help from more experienced programmers - let's make together a unit for this mDNS queries!

For the beginning, I have prepared the basic types:
Code: Pascal  [Select][+][-]
  1. type
  3.   TmDNSQuestion = record
  4.     Query: string; //null terminated
  5.     QueryType: word;
  6.     QueryClass: word;
  7.   end;
  9.   TmDNSQuery = record
  10.     TransactionID: word;
  11.     Flags: word;
  12.     Questions: word;
  13.     AnswerRRs: word;
  14.     AuthorityRRs: word;
  15.     AdditionslRRs: word;
  16.     Q: array of TmDNSQuestion
  17.   end;
  19.   TmDNSAnswer = record
  20.     AnswerName: word;
  21.     AnswerType: word;
  22.     AnswerClass: word;
  23.     CacheFlush: word;
  24.     TimeToLive: word; //in seconds
  25.     DataLength: word; //for DomainName
  26.     DomainName: array of char; //Not null-terminated
  27.   end;
  29.   TmDNSResponse = record
  30.     TransactionID: word;
  31.     Flags: word;
  32.     Questions: word;
  33.     AnswerRRs: word;
  34.     AuthorityRRs: word;
  35.     AdditionslRRs: word;
  36.     Q: array of TmDNSQuestion;
  37.     A: array of TmDNSAnswer;
  38.   end;
This is what I've got till now from reverse-engineering the Wireshark captures.
TmDNSAuthority and TmDNSAdditional are of unknown structure for me because I don't have any capture containing these.

Anyone interested?
Or anyone with some more knowledge, who may eventually find flaws or mistakes here?

I have my app done. I would like to gather here some info for the benefit of wider public.
« Last Edit: January 09, 2022, 09:12:51 pm by bobby100 » - my Lazarus components and units - my open source apps - remainder at my previous life as a web security expert


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Re: Anyone into mDNS?
« Reply #1 on: January 09, 2022, 09:49:21 pm »
Thanks for updating us. I don't think it's immediately useful to the sort of thing I do but I think everybody here appreciates the feedback.

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Re: Anyone into mDNS?
« Reply #2 on: January 09, 2022, 11:07:41 pm »
In your records you cannot use dynamic types like String and array of byte. Typically you reserve enough memory for, let's say, your query. Then you use pointers and record pointer types to fill the memory with intended values.

Also, you need to take care of endianness. IIRC, network records are big endian. That's why, for instance, your third field in your query, Question count, is $00 $01, while 1 on Intel CPUs is $01 $00

For details, you can rely on DNS documentation/implementation as mDNS is based on DNS.


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