Delphi: Get MAC of Router

Question

I am using Delphi and I want to determinate the physical MAC address of a network device in my network, in this case the Router itself.

My code:

var
  idsnmp: tidsnmp;
  val:string;
begin
  idsnmp := tidsnmp.create;
  try
    idsnmp.QuickSend('.1.3.6.1.2.1.4.22.1.2', 'public', '10.0.0.1', val);
    showmessage(val);
  finally
    idsnmp.free;
  end;
end;

where 10.0.0.1 is my router.

Alas, QuickSend does always send "Connection reset by peer #10054". I tried to modify the MIB-OID and I also tried the IP 127.0.0.1 which connection should never fail. I did not find any useable Tutorials about TIdSNMP at Google. :-(

Regards Daniel Marschall

Answer 1

You can use the SendARP function to get the Mac Address.

check this sample

uses
 Windows,
 WinSock,
 SysUtils;


function SendArp(DestIP,SrcIP:ULONG;pMacAddr:pointer;PhyAddrLen:pointer) : DWord; StdCall; external 'iphlpapi.dll' name 'SendARP';


function GetMacAddr(const IPAddress: string; var ErrCode : DWORD): string;
var
MacAddr    : Array[0..5] of Byte;
DestIP     : ULONG;
PhyAddrLen : ULONG;
WSAData    : TWSAData;
begin
  Result    :='';
  WSAStartup($0101, WSAData);
  try
    ZeroMemory(@MacAddr,SizeOf(MacAddr));
    DestIP    :=inet_addr(PAnsiChar(AnsiString(IPAddress)));
    PhyAddrLen:=SizeOf(MacAddr);
    ErrCode   :=SendArp(DestIP,0,@MacAddr,@PhyAddrLen);
    if ErrCode = S_OK then
     Result:=Format('%2.2x-%2.2x-%2.2x-%2.2x-%2.2x-%2.2x',[MacAddr[0], MacAddr[1],MacAddr[2], MacAddr[3], MacAddr[4], MacAddr[5]])
  finally
    WSACleanup;
  end;
end;

Answer 2

Not wishing to steal the thunder of RRUZ, I offer the following variant, taken from my codebase, with some observations. I've done this as an answer rather than a comment in order to include code.

type
  TMacAddress = array [0..5] of Byte;

function inet_addr(const IPAddress: string): ULONG;
begin
  Result := ULONG(WinSock.inet_addr(PAnsiChar(AnsiString(IPAddress))));
end;

function SendARP(DestIP, SrcIP: ULONG; pMacAddr: Pointer; var PhyAddrLen: ULONG): DWORD; stdcall; external 'Iphlpapi.dll';

function GetMacAddress(const IPAddress: string): TMacAddress;
var
  MaxMacAddrLen: ULONG;
begin
  MaxMacAddrLen := SizeOf(Result);
  if SendARP(inet_addr(IPAddress), 0, @Result, MaxMacAddrLen)<>NO_ERROR then begin
    raise EMacAddressError.CreateFmt('Unable to do SendARP on address: ''%s''', [IPAddress]);
  end;
end;

There are a couple of points to make.

There is no need to call WSAStartup/WSACleanup.

EDIT As RRUZ points out in a comment, the winsock documentation does not explictly exempt inet_addr from WSAStartup/WSACleanup so I retract this point. On Vista it is simpler just to call RtlIpv4StringToAddress. Having said all that, inet_addr is so easy to implement it may just be easier to roll your own.

Secondly the declaration of inet_addr in WinSock.pas is incorrect. It declares the return value to be of a type u_long which is defined in WinSock.pas as Longint. This is a signed 4 byte integer but it should be an unsigned 4 byte integer, ULONG. Without the explicit cast you can get range errors.