.PCAP file signature
A pcap capture file file begins with the bytes D4 C3 B2 A1 at the start of the file. The signature is the same whatever the file is named, which is why a renamed or extensionless PCAP file is still recognisable.
Known signatures
| Offset | Hex | ASCII |
|---|---|---|
| 0 | D4 C3 B2 A1Little-endian, microsecond timestamps. | |
| 0 | A1 B2 C3 D4Big-endian. | |
| 0 | 0A 0D 0D 0Apcapng (next-generation) section header block. |
MIME type: application/vnd.tcpdump.pcap · usual extensions: .pcap, .pcapng
How to check a file's signature
Without installing anything: drop the file on our detector. It reads the bytes in your browser and shows the hex dump alongside the verdict; nothing is uploaded.
macOS and Linux (Terminal):
xxd -l 32 file.pcap
file file.pcapWindows (PowerShell):
Format-Hex -Path file.pcap -Count 32If the bytes do not match
A file named .pcap whose first bytes are something else is not a PCAP file. The common explanations are a wrong or deliberately misleading extension, a download that saved an HTML error page instead of the file, or a container such as ZIP wrapping the real content. The detector reports what the bytes say; the PCAP guide explains what a genuine file should contain.