Code:

proc test_host_port {host port callback} {  # first resolve the host to an ip (async)  dnslookup $host [list host_resolved $callback $port]}proc host_resolved {callback port ip host success} {  # see if we have a valid ip  if {![string compare $ip "0.0.0.0"]} {    eval $callback [list $host $port 0]    return 1  }  # now open an async socket  set sock [socket -async $ip $port]  fileevent $sock writable [list sock_result $callback $host $port $sock]}proc sock_result {callback host port sock} {  set err [fconfigure $sock -error]  close $sock  eval $callback [list $host $port $err]}