At DFW, assuming you are flying in from Australia:
You have to enter into US through immigration, then you pick up your check luggage to go through US customs. After clearing custom you drop your bags off at onward connection. Then you enter into DFW terminal going through the TSA security check point. If you are fortunate your departing flight will be in the same terminal.
Best if you can go direct to YVR or via an Asia country which has transit area without all these hoops to jump through.