You created a pcap file with a new version of tcpdump or Wireshark (which supports exotic DLTs) and are now trying to read it with an older version of libpcap or a legacy tool (e.g., an old tcptrace or a deprecated ngrep ). The old library simply has no entry in its switch-case statement for "276."
: The most direct fix is to update your analysis software. For example, upgrading Wireshark to version 3.6 or later typically resolves the issue. -pcap network type 276 unknown or unsupported-