ICMP Timestamp Type 13/14: Linux Kernel Internals with ftrace
A depth-8 ftrace analysis of how the Linux kernel processes ICMP Timestamp (Type 13/14) requests — covering the full call chain, payload validation, conntrack behavior, and ts_ori echo mechanics.
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Most security tooling treats ICMP as a monolith: ping works, or it doesn’t. ICMP Timestamp (Type 13/14) sits in that overlooked middle ground — defined in RFC 792, implemented in every Linux kernel, but rarely scrutinized at the source level.
This post documents a depth-8 ftrace investigation of icmp_timestamp() on Linux kernel 6.17.13. Beyond cataloguing individual findings, the goal is to map the architectural reality of how the kernel processes ICMP Timestamp requests — every code path, every decision point, every place where the implementation diverges from what RFC 792 specifies.
What emerged was a set of concrete, reproducible behaviors: a silent payload threshold, a Code field that is never read, ts_ori that is echoed without any range check, and a netfilter pipeline that creates a conntrack entry for every single exchange.