pcap-linktype - link-layer header types supported by libpcap
For a live capture or ``savefile'', libpcap supplies, as the return value of the
  pcap_datalink(3) routine, a value that indicates the type of link-layer
  header at the beginning of the packets it provides. This is not necessarily
  the type of link-layer header that the packets being captured have on the
  network from which they're being captured; for example, packets from an IEEE
  802.11 network might be provided by libpcap with Ethernet headers that the
  network adapter or the network adapter driver generates from the 802.11
  headers. The names for those values begin with DLT_, so they are
  sometimes called "DLT_ values".
The values stored in the link-layer header type field in the
    savefile header are, in most but not all cases, the same as the values
    returned by pcap_datalink(). The names for those values begin with
    LINKTYPE_.
The link-layer header types supported by libpcap are described at
    https://www.tcpdump.org/linktypes.html.