I would like to be able to find out the public IP address of the Pi in a Lazarus program.
How can this be accomplished?
It is either connected to a wired or WiFi network router. in my browser I can go to http://checkip.dyndns.com/ and it will print a message with my IP address.
But how is this done in code?

Avoid relying on this in the general case: you can usually use 0.0.0.0 to represent "this host". The problem is that a machine might have multiple physical Ethernet (or other) adapters, and in addition might have aliases set up, and in the general case it's not easy to look at the list of IP addresses and determine which one you want.

http://forum.lazarus.freepascal.org/index.php?topic=20556.0 looks like a good starting point, although it's oriented towards Windows. If you're really looking for "/the/" address then I think you might end up parsing the routing table (left as an exercise :-) and finding which local IP address talks to the default gateway... which itself gets messy since there might be multiple prioritised gateways. And then there's IP6 to consider.

Word of advise: don't try arithmetic operations on IP addresses, and in particular don't speculate on "next hops" etc. based on what you know about your local network.

forexample

http://wiki.lazarus.freepascal.org/Internet_Tools#Get_external_IP_address

or see this
http://forum.lazarus.freepascal.org/index.php/topic,17506.msg110917.html#msg110917

fecske fecske wrote

or see this http://forum.lazarus.freepascal.org/index.php/topic,17506.msg110917.html#msg110917

If you just want to get your external IP using some service like dyndns, you should use pure FCL:

=== begin code ===

uses fphttpclient, RegExpr;
procedure TForm1.Button1Click(Sender: TObject);
begin
  with TRegExpr.Create('[0-9.]+') do try
    if Exec(TFPHTTPClient.SimpleGet('http://checkip.dyndns.org')) then
      ShowMessage(Match[0])
  finally Free; end;
end;

=== end code ==

However, if you can use other service like echoip.com, just:

=== begin code ===

uses  fphttpclient;
procedure TForm1.Button1Click(Sender: TObject);
begin
  ShowMessage(TFPHTTPClient.SimpleGet('http://echoip.com/'));
end;

=== end code ===

HTH

So much clean answer Slivio.

Regards, Ara