Indy10 problem connecting to TCP Server using a host name pointing to an IPv6 address

Question

I've put the following in the Windows HOSTS file:

fe80::a5c6:d354:8e27:9f79%16 ip6test

I can ping ip6test in Windows, and I can connect to a server via TIdTCPClient with the IPv6 address above in square brackets, but trying to connect using the host name [ip6test] using the TCP Client instantly fails with a socket error 11001.

Any ideas why?

Answer 1

The bracket workaround only works in TIdHTTP. Don't use it with TIdTCPClient.

For TIdTCPClient, you have to set its IPVersion property to Id_IPv6 when connecting to an IPv6 server. It is not yet smart enough to detect which IP version an IP address is using, or which IP version(s) a hostname resolves to, so you have to tell it up front (this will be addressed in a future release).

For example, a simple workaround would look like this:

Client.Host := ...; 
Client.Port := ...;

if (Client.Host is an IPv4 address) then
begin
  Client.IPVersion := Id_IPv4;
  Client.Connect;
end
else if (Client.Host is an IPv6 address) then
begin
  Client.IPVersion := Id_IPv6;
  Client.Connect;
end
else
begin
  try
    // try to connect with IPv6 first...
    Client.IPVersion := Id_IPv6;
    Client.Connect;
  except
    // try again with IPv4...
    Client.IPVersion := Id_IPv4;
    Client.Connect;
  end;

  // alternatively, you can use TIdDNSResolver
  // beforehand to resolve the hostname
  // yourself, and then setup TIdTCPClient
  // accordingly... 
end;