My kernel snitches

Boxd operates persistent, fast-starting Linux virtual machines for remote development and lightweight services, and its infrastructure must balance zero trust toward guest software with useful application-level visibility while avoiding intrusive inspection of users’ files or processes. To automatically route each machine’s public URL to the active web service without requiring configuration or polling, Boxd patches its guest kernel to emit small signals before and after successful bind() and listen() calls, sending an eight-byte message through x86 PIO ports to the virtual machine monitor. The VMM converts these notifications into port events and selects a likely public service through a simple priority list of common web ports, ignoring interface-specific bindings and preventing later, lower-priority listeners from replacing a better choice. Although guests can technically forge these unprivileged signals, the system treats them only as hints: a false claim can at worst misroute that guest’s own URL, not grant access, alter firewall rules, or affect another tenant. Detected ports are stored in replicated cluster state, shared with proxies, and preserved across snapshots and forks, while the same guest-to-host signaling channel has since been extended to communicate readiness, exit codes, and file events.

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