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Simple sockets example for sending and receiving UDP packets

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prodingus:
Hi!

As the title says, I need a very basic UDP send/receive example:

a client loop that  sends continiously a message (to an ip,port),
and a server loop that listens (receives) for any messages

Ps, I am noob at networking.
Side note: I am using 2 windows 7 vms,  ips are 1.1.1.1 and 1.1.1.2, no firewall.

 

Leledumbo:
Unfortunately, apart from BSD sockets unit, there's no pre-shipped UDP packages. fcl-net only contains TCP. However, lNet is just a few clicks away from the online package manager. After installation, check its examples folder for UDP both CLI and GUI. The CLI one is a single program that can be both client and server, just pass -s to be a server instead of a client.

MarkMLl:
This is the simplest I've got to hand. It's not entirely standalone since it's part of my business software, but it relies on a periodic poll rather than a background thread etc.


--- Code: Pascal  [+][-]window.onload = function(){var x1 = document.getElementById("main_content_section"); if (x1) { var x = document.getElementsByClassName("geshi");for (var i = 0; i < x.length; i++) { x[i].style.maxHeight='none'; x[i].style.height = Math.min(x[i].clientHeight+15,306)+'px'; x[i].style.resize = "vertical";}};} ---(* Return a socket suitable for DNS queries, or an error code as a negative  number. This is unprivileged, but is opened here so that the router (ping)  and DNS client code can be kept as similar as possible.*)function DnsClientSocket(): TSocket; begin  result := dnsClientSocketHandleend { DnsClientSocket } ;  (* Send a query to a randomly-selected DNS server, return the saved sequence  number or 0xffff on error. If not in error, the MSB of the sequence number is  set iff the address is local (RFC1918 etc.).*)function SendDnsQuery(): TDnsSequence; (* See e.g. https://www2.cs.duke.edu/courses/fall16/compsci356/DNS/DNS-primer.pdf *) const  standardQuery= $0000;  inverseQuery=  $0800;  statusQuery=   $1000;   query= #1 + '1' +         #1 + '0' +         #1 + '0' +         #3 + '127' +         #7 + 'in-addr' +         #4 + 'arpa' +         #0 +         #$00 + #$0c +                  (* Query type, network-endian           *)         #$00 + #$01;                   (* Query class, network-endian          *) var  server: string;  a: in4_addr;  sockAddr: TSockAddr;  sz: integer;  msg: TDnsMsg;    function localAddress(a: longword): boolean;   begin    result := false;    if a and $ff000000 = (10 << 24) then      exit(true);    if a and $ffff0000 = (192 << 24) + (168 << 16) then      exit(true);    if a and $ff000000 = (172 << 24) then      if a and $00f00000 <> $00000000 then        exit(true);    if a and $ffff0000 = (169 << 24) + (254 << 16) then      exit(true)  end { localAddress } ;  begin  if DnsClientSocket <= 0 then    exit($ffff);  if not Assigned(nameservers) or (nameservers.Count = 0) then    needsReload := true;  if needsReload then    if not loadResolvConf() then begin{$ifdef DEBUG }      WriteLn('DNS query abort -----');{$endif DEBUG }      exit($ffff)    end else      needsReload := false;  server := nameservers[0];  nameservers.Delete(0); (* If we've just deleted the last server from the nameservers list, reread the  *)(* /etc/resolv.conf file. If this returns more than one item then repeat this   *)(* until the first entry in the nameservers list is not the same as the one     *)(* that we're about to use, so that if it fails we're not trying the same       *)(* server twice in succession. This is a noteworthy but justifiable departure   *)(* from randomness.                                                             *)   if nameservers.Count = 0 then    repeat      if not loadResolvConf() then        break;                          (* List is left empty                   *)      if nameservers.Count = 1 then        break;                          (* No alternative to a repeat address   *)    until nameservers[0] <> server;{$ifdef DEBUG }  WriteLn('DNS query ', server, ' -----');{$endif DEBUG }  a := in4_addr(StrToHostAddr(server));  if a = in4_null then    exit($ffff);  FillChar(sockAddr, SizeOf(sockAddr), 0);  sockAddr.sa_family:= AF_INET;  sockAddr.sin_port:= hToNS(53);  sockAddr.sin_addr.s_addr := htoNL(longword(a));  result := Random($7fff);  if localAddress(longword(a)) then    result += $8000;                    (* Maximum possible is $7ffe + $8000    *)  FillChar(msg, SizeOf(msg), 0);  with msg do begin    hdr.id := htons(result);    hdr.flags := htons(standardQuery);    hdr.cnts[2] := htons(1)  end;  Move(query[1], msg.body[0], Length(query));  sz := SizeOf(TDnsHdr) + Length(query);//  while sz mod 4 <> 0 do//    sz += 1;  if fpSendTo(DnsClientSocket, @msg, sz, 0, @sockAddr, SizeOf(sockAddr)) < 0 then    result := $ffff;{$ifdef DEBUG }  WriteLn('DNS seq: ', result){$endif DEBUG }end { SendDnsQuery } ;  (* Retrieve the result of a query returned by a DNS server. Return false if  there is no result or the sequence number doesn't match.*)function PollDnsResult(const seq: TDnsSequence): TDnsResponse; var  sockAddr: TSockAddr;  saLength, sz: integer;  msg: TDnsMsg; begin  result := DnsNoResponse;  if (DnsClientSocket <= 0) or (seq = $ffff) then    exit;  FillChar(sockAddr, SizeOf(sockAddr), 0);  saLength := SizeOf(sockAddr);  FillChar(msg, SizeOf(msg), $ff);  sz := fpRecvFrom(DnsClientSocket, @msg, SizeOf(msg), MSG_DONTWAIT, @sockAddr, @saLength);  if sz <= SizeOf(TDnsHdr) then    exit;{$ifdef DEBUG }  Write('DNS rsp: ', seq);  WriteLn(' got: ', ntohs(msg.hdr.id));{$endif DEBUG } (* Any coherent response is adequate confirmation that we can get through to    *)(* at least one server.                                                         *)   if msg.hdr.id = htons(seq) then    result := DnsOkResponse  else    result := DnsBadResponse;{$ifdef DEBUG }  if result = DnsOkResponse then    WriteLn('DNS OK')  else    WriteLn('DNS Fail'){$endif DEBUG }end { PollDnsResult } ;  // Elsewhere var  dnsClientSocketHandle: TSocket= -32768;   Write(StdErr, 'Creating new Internet domain connectionless client socket... ');  dnsClientSocketHandle := fpSocket(PF_INET, SOCK_DGRAM, 0);  if dnsClientSocketHandle > 0 then    WriteLn(StdErr, 'OK')  else begin    WriteLn(StdErr, 'failed, might need CAP_DAC_OVERRIDE,CAP_NET_BIND_SERVICE,CAP_NET_RAW=p+e');    dnsClientSocketHandle := -Abs(ErrNo)  end;      dnsQuery := SendDnsQuery();         (* Response should turn it green        *)    dnsQueryTime := Now();          case PollDnsResult(dnsQuery) of          DNSWrongResponse: ;          DnsOkResponse:    begin                              if (dnsQuery and $8000) = $8000 then                                DispatchWatchdog(IniFilesLocation.GetHostName() + '/' + SpecialName + '.DNS:0,13')                              else                                DispatchWatchdog(IniFilesLocation.GetHostName() + '/' + SpecialName + '.DNS:0,22');                              dnsQuery := $ffff                            end        otherwise                       (* Timeout on no or bad response        *)          if Now() - dnsQueryTime > 46 * OneSecond then            dnsQuery := $ffff        end 
Most of the complexity there is obviously from setting up the message, and doing something with the result.

MarkMLl

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