Traces how the initial TTL, TCP window size, MSS, and the order of TCP options in a single SYN packet identify the sending operating system, and why that identity is set by the kernel rather than the browser.
How p0f reads a single SYN packet to name the operating system behind it, traced from Michal Zalewski's 2000 release through the v3 rewrite, the signature grammar, and why TTL, window size, and option order still leak OS identity in 2026.
Traces how the order of TCP options in a SYN packet, the window-scale shift count, the SACK-permitted flag, NOP padding, and the timestamp clock identify an operating system, and how per-connection randomization changed what the timestamp leaks.
How anti-bot systems catch a proxy by comparing the OS in the HTTP User-Agent against the OS inferred from the raw SYN packet, why the two disagree, and where the check breaks down.
How tunnels and VPNs shift MTU and MSS, why a non-standard MSS in a SYN packet betrays an encapsulated path, and how path-MTU discovery behavior turns a packet-size value into a signal.
How servers catch a proxy by comparing where an IP claims to be against how long packets actually take to arrive. The speed-of-light floor, TCP-handshake timing, the TCP-vs-TLS cross-layer split, and JA4L.
Traces the SYN flood from the 1996 Panix attack and the Phrack code that armed it, through the half-open backlog mechanism it exhausts, to SYN cookies and the modern variants that still rank near the top of Layer 3/4 attack vectors.