-pcap Network Type 276 Unknown Or Unsupported-

Run capinfos (from Wireshark) on the offending file:

capinfos suspicious.pcap

Look for the line: Link layer header type: Ethernet (1) ← Not your case. Link layer header type: Unknown (276) ← Your case.

Or use od (octal dump) to check the file’s global header:

od -An -j20 -N4 -I yourfile.pcap

If this prints 276, you’ve confirmed it. -pcap network type 276 unknown or unsupported-

Yes. When capturing, disable mpacket mode on your interface (if your driver allows it):

# For some Mellanox NICs
ethtool -K eth0 tx-mpacket off rx-mpacket off
Run basic system checks:
file suspicious.pcap
capinfos suspicious.pcap

Look for the line: "Link-layer header type: Unknown (276)" Run capinfos (from Wireshark) on the offending file:
If you know the real linktype (e.g., Ethernet = 1), you can patch the file:
printf '\x01\x00' | dd of=capture.pcap bs=1 seek=20 count=2 conv=notrunc

Warning: Only do this if you are certain the packet data matches the new linktype; otherwise, dissection will be invalid.
The error "pcap network type 276 unknown or unsupported" manifests in the following typical environments: Look for the line: Link layer header type:
So, what is number 276? According to the official pcap.h definitions and the dlt.h registry maintained by the tcpdump.org community, DLT value 276 is often mapped to DLT_IEEE802_15_4_TAP or a vendor-specific/protocol-specific link type, depending on the build of your libpcap.
In many recent implementations, DLT 276 corresponds to DLT_IPNET (used for Juniper Networks internal encapsulation) or a proprietary radio header. However, the most common source of this error in the open-source community is captures from Bluetooth, ZigBee (802.15.4) , or User-Defined DLTs created by specialized hardware (like GPS receivers or custom FPGA network cards).
The core issue is not the number itself, but the fact that your current libpcap version does not have a decoder registered for DLT 276.
If your file is truly Nordic BLE, use the nRF Sniffer special version of Wireshark, or export to text:
tshark -r capture.pcap -T fields -e btle.advertising.address -e btle.data

But this requires TShark with DLT 276 support. If not available, use Bleak or PyBluez to re-capture.