This thread on the Wireshark mailing list brought up the issue of not being able to use Tcpreplay with the loopback interface on FreeBSD. Using tap0 is an alternative: use tap0 to send packets using TcPreplay or Tcpdump. In a second window, sniff with Tcp dump or whatever program you want: tap0: no IPv4 address assigned, use -v or -vv for full protocol decode. Use tap0 on the link-type EN10MB (Ethernet), capture size 1515 bytes.”]
Source: https://taosecurity.blogspot.com/2006/09/using-tap0-with-tcpreplay.html