Anyone into mDNS?

[bobby100]

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
23.  
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;
28.  
29.   //send Query
30.   IdIPMCastServer.Active := True;
31.   IdIPMCastServer.Send(query);
32.  
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
2.  
3.   TmDNSQuestion = record
4.     Query: string; //null terminated
5.     QueryType: word;
6.     QueryClass: word;
7.   end;
8.  
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;
18.  
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;
28.  
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.

[MarkMLl]

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.

MarkMLl

[engkin]

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.

[MaxM74]

Can you post the full demo?
Thanks

[Thaddy]

@enkin
In system we have BtoN, NtoLE and NtoBE to maniplulate byte order/network byte order (BE), of course.

[Remy Lebeau]

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.

Your approach has a race condition, where the response may arrive before the TIdUDPServer is ready to receive it.

You SHOULD use the same socket for both sending the query AND receiving the response, then there is no race.  Although TIdIPMCastServer is intended for sending only, you CAN read incoming data with it too. You just have to read it manually using the Binding property directly, as TIdIPMCastServer has no reading logic of its own. TIdIPMCastServer.Send() simply calls TIdIPMCastServer.Binding.SendTo(), you can call TIdIPMCastServer.Binding.RecvFrom() afterwards.

Alternatively you could just use TIdUDPServer by itself.  TIdIPMCastServer isn't really doing anything that can't also be done with TIdUDPServer.  TIdIPMCastServer just sends packets to the multicast group IP/port. You can do that with TIdUDPServer too, by calling SendTo() on the desired Binding.

Because of using the same port, you can't have them both active at the same time.

Actually, you can.  Simply set TIdIPMCastServer.ReuseSocket and TIdUDPServer.ReuseSocket both to rsTrue, and then activate them both.  TIdIPMCastServer does not read from its socket by default, so the TIdUDPServer should be able to receive the response to the query that TIdIPMCastServer sends.

[MaxM74]

in TmDNSQuestion Query is NOT a null terminated string (the same for answers) is an Array in the form:

Code: Pascal  [Select][+][-]

1. Query:
2. //Start of Array
3.   StringLength: Byte;
4.   StringData: array of char;
5.   ...
6.   StringLength: Byte;
7.   StringData: array of char;  
8. //$00 to terminate the Array.
9.  
10.   QueryType: word;
11.   QueryClass: word;
12.  

The Response is:

Code: Pascal  [Select][+][-]

1.    TransactionID: word;
2.     Flags: word;
3.     Questions: word;
4.     AnswerRRs: word;
5.     AuthorityRRs: word;
6.     AdditionslRRs: word;
7.     Q: array; //Your Query
8.     A: Answer;
9.  

Code: Pascal  [Select][+][-]

1. //Answer
2. AnswerName: word;
3.     AnswerType: word;
4.     AnswerClass: word;
5.     CacheFlush: word;
6.     TimeToLive: word; //in seconds
7.     DataLength: word;
8.     DomainName:
9. //Start of Array
10.   StringLength: Byte;
11.   StringData: array of char;
12.   ...
13.   StringLength: Byte;
14.   StringData: array of char;  
15. //End of Array.
16.   Data: array[DataLength-2] of Byte; //? or always 14 bytes
17.  
18. //Start of Array
19.   StringLength: Byte;
20.   StringData: array of char;
21.   ...
22.   StringLength: Byte;
23.   StringData: array of char;  
24. //End of Array.
25.   Data: array[DataLength-2] of Byte; //? or always 14 bytes
26.  
27.  
28.  

this is a Response from a Kyocera Printer/Scanner (is an Hex File)

[MaxM74]

this test search for scanners

[bobby100]

in TmDNSQuestion Query is NOT a null terminated string (the same for answers) is an Array in the form:

Code: Pascal  [Select][+][-]

1. Query:
2. //Start of Array
3.   StringLength: Byte;
4.   StringData: array of char;
5.   ...
6.   StringLength: Byte;
7.   StringData: array of char;  
8. //$00 to terminate the Array.
9.  
10.   QueryType: word;
11.   QueryClass: word;
12.  

I didn't see any byte that would be the string length. How did you get to this conclusion?

[MaxM74]

Look the Data

Code: Pascal  [Select][+][-]

1.   query: TIdBytes = (
2.     $00, $00,
3.     $00, $00,
4.     $00, $01,
5.     $00, $00,
6.     $00, $00,
7.     $00, $00,
8.       $09, $5F, $73, $65, $72, $76, $69, $63, $65, $73,  //_services
9.       $07, $5f, $64, $6e, $73, $2d, $73, $64,                 //_dns-sd
10.       $04, $5f, $75, $64, $70,                                       //_udp
11.       $05, $6c, $6f, $63, $61, $6c,                                //local
12.       $00,
13.     $00, $0c,
14.     $00, $01);
15.  

anyway i found this on github, when i have time i'll try it

https://github.com/marsupilami79/mdns4Delphi

[MaxM74]

Updated Version to work with new FPC and Delphi

https://github.com/maxm74/mdns4Delphi