Force TCPDump to write in PCAP instead of PCAP-NG format

Question 1

The tcpdump man page on Mavericks says:

   -i     Listen on interface.

          If  the  -D flag is supported, an interface number as printed by
          that flag can be used as the interface argument.

          On Darwin systems version 13 or later,  when  the  interface  is
          unspecified,  tcpdump  will  use  a  pseudo interface to capture
          packets  on  a  set  of  interfaces  determined  by  the  kernel
          (excludes by default loopback and tunnel interfaces).

          Alternatively,  to capture on more than one interface at a time,
          one may use "pktap" as the interface parameter  followed  by  an
          optional  list  of  comma  separated interface names to include.
          For example, to capture on the loopback and en0 interface:

                 tcpdump -i pktap,lo0,en0

          An interface argument of "all" or "pktap,all"  can  be  used  to
          capture packets from all interfaces, including loopback and tun-
          nel interfaces.

          A pktap pseudo interface provides for packet metadata using  the
          default  PKTAP data link type and files are written in the Pcap-
          ng file format.  The RAW data link type must be used to force to
          use  the legacy pcap-savefile(5) file format with a ptkap pseudo
          interface.  Note that captures on a ptkap pseudo interface  will
          not be done in promiscuous mode.

          An  interface argument of "iptap" can be used to capture packets
          from at the IP layer.  This capture packets as they  are  passed
          to  the  input and output routines of the IPv4 and IPv6 protocol
          handlers of the networking stack.  Note that captures  will  not
          be done in promiscuous mode.

so you need to specify an interface on which to capture.

Note that versions of OS X dating back to Lion, newer versions of FreeBSD/NetBSD/DragonFly BSD, and newer versions of many Linux distributions include libpcap 1.1.1 or later, which means that programs using libpcap to read capture files can read many pcap-ng files. Wireshark has also been able to read them for several releases.

Question 2

There is also an app, CocoaPacketAnalyzer, which reads pcap files, but not pcap-NG files. There is a web page, https://pcapng.com, that will convert a small pcap-NG file to the older format. However, if you are capturing password or web server interactions, or generally care about security, beware about posting packet captures on the internet!