Anyone into mDNS?
lazarus / 38-Networking and Web Programming

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bobby100 · 2022-01-09 20:09

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][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TfrmMain.mnuMulticastClick(Sender: TObject);var  //basic communication
as captured by Wireshark  {00 00 - Transaction ID   00 00 - Standard
query   00 01 - Question count   00 00 - Answer RRs count   00 00 -
Authority RRs count   00 00 - Additional RRs count   xx xx - query
string/question string   00 0c - Type PTR   00 01 - Class IN}  query:
TIdBytes = ($00, $00, $00, $00, $00, $01, $00, $00, $00, $00,    $00,
$00, $09, $5f, $73, $65, $72, $76, $69, $63, $65, $73, $07,    $5f,
$64, $6e, $73, $2d, $73, $64, $04, $5f, $75, $64, $70, $05,    $6c,
$6f, $63, $61, $6c, $00, $00, $0c, $00, $01);begin  //set CastServer 
IdIPMCastServer.MulticastGroup := Id_IPMC_mDNS; 
IdIPMCastServer.BoundIP := String_IP_Address; //Here goes the IP
address of your network adapter  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 
IdIPMCastServer.Port := 5353; //this is standard   //set UDPServer 
IdUDPServer.Bindings.Add;  IdUDPServer.Bindings[0].IP :=
String_IP_Address; //Here goes the IP address of your network adapter 
IdUDPServer.Bindings[0].Port := 49152;   //send Query 
IdIPMCastServer.Active := True;  IdIPMCastServer.Send(query);  
//deactivate CastServer, activate UDPServer and deactivation timer for
UDPServer  IdIPMCastServer.Active := False;  tmUDPdeactivate.Interval
:= IdIPMCastServer.TimeToLive * 1000;  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  IdUDPServer.Active :=
True;end;    
My IdUDPServer.OnUDPRead looks like this:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TfrmMain.IdUDPServerUDPRead(AThread: TIdUDPListenerThread;  const
AData: TIdBytes; ABinding: TIdSocketHandle);begin 
sgMachines.InsertRowWithValues(sgMachines.RowCount,
[IPAddrToName(ABinding.PeerIP), ABinding.PeerIP]); ...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][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i type  
TmDNSQuestion = record    Query: string; //null terminated   
QueryType: word;    QueryClass: word;  end;   TmDNSQuery = record   
TransactionID: word;    Flags: word;    Questions: word;    AnswerRRs:
word;    AuthorityRRs: word;    AdditionslRRs: word;    Q: array of
TmDNSQuestion  end;   TmDNSAnswer = record    AnswerName: word;   
AnswerType: word;    AnswerClass: word;    CacheFlush: word;   
TimeToLive: word; //in seconds    DataLength: word; //for DomainName 
 DomainName: array of char; //Not null-terminated  end;  
TmDNSResponse = record    TransactionID: word;    Flags: word;   
Questions: word;    AnswerRRs: word;    AuthorityRRs: word;   
AdditionslRRs: word;    Q: array of TmDNSQuestion;    A: array of
TmDNSAnswer;  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.

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MarkMLl · 2022-01-09 20:49

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

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engkin · 2022-01-09 22:07

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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MaxM74 · 2025-05-09 08:08

Can you post the full demo?
Thanks

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Thaddy · 2025-05-09 08:53

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

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Remy Lebeau · 2025-05-09 17:53

Quote from: bobby100 on January 09, 2022, 09:09:15 pmI 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.

Quote from: bobby100 on January 09, 2022, 09:09:15 pmBecause 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.

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MaxM74 · 2025-05-14 14:02

in TmDNSQuestion Query is NOT a null terminated string (the same for
answers) is an Array in the form:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i Query:
//Start of Array  StringLength: Byte;  StringData: array of char; 
...  StringLength: Byte;  StringData: array of char;  //$00 to
terminate the Array.   QueryType: word;  QueryClass: word; 
The Response is:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i  
 TransactionID: word;    Flags: word;    Questions: word;   
AnswerRRs: word;    AuthorityRRs: word;    AdditionslRRs: word;    Q:
array; //Your Query    A: Answer; 
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i
//AnswerAnswerName: word;    AnswerType: word;    AnswerClass: word; 
 CacheFlush: word;    TimeToLive: word; //in seconds    DataLength:
word;    DomainName://Start of Array  StringLength: Byte;  StringData:
array of char;  ...  StringLength: Byte;  StringData: array of char;
 //End of Array.  Data: array[DataLength-2] of Byte; //? or always 14
bytes //Start of Array  StringLength: Byte;  StringData: array of
char;  ...  StringLength: Byte;  StringData: array of char;  //End of
Array.  Data: array[DataLength-2] of Byte; //? or always 14 bytes   
this is a Response from a Kyocera Printer/Scanner (is an Hex File)

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MaxM74 · 2025-05-14 14:51

this test search for scanners

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bobby100 · 2025-05-14 18:31

Quote from: MaxM74 on May 14, 2025, 04:02:53 pmin TmDNSQuestion Query
is NOT a null terminated string (the same for answers) is an Array in
the form:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i Query:
//Start of Array  StringLength: Byte;  StringData: array of char; 
...  StringLength: Byte;  StringData: array of char;  //$00 to
terminate the Array.   QueryType: word;  QueryClass: word; I didn't
see any byte that would be the string length. How did you get to this
conclusion?

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MaxM74 · 2025-05-15 06:29

Look the Data

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i   query:
TIdBytes = (    $00, $00,    $00, $00,    $00, $01,    $00, $00,   
$00, $00,    $00, $00,      $09, $5F, $73, $65, $72, $76, $69, $63,
$65, $73,  //_services      $07, $5f, $64, $6e, $73, $2d, $73, $64,  
             //_dns-sd      $04, $5f, $75, $64, $70,                 
                    //_udp      $05, $6c, $6f, $63, $61, $6c,        
                      //local      $00,    $00, $0c,    $00, $01); 
anyway i found this on github, when i have time i'll try it

https://github.com/marsupilami79/mdns4Delphi

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MaxM74 · 2025-05-16 06:27

Updated Version to work with new FPC and Delphi

https://github.com/maxm74/mdns4Delphi