A Hacker News experiment asks Claude Code to act as a user space IP stack by developing a command, ping-respond.md, to read, parse, and reply to ICMP echo requests. The command reads a packet from tun0, parses the IPv4 and ICMP headers, constructs a reply by swapping source/dst IPs, setting TTL to 64, and recalculating checksums, then writes the reply. The experiment ran on Haiku 4.5, with a round-trip time of 45 seconds, showing the slow ping response.

The key step is parsing the original packet hex string to extract header fields—like version, IHL, TTL, and protocol (must be 0x01 for ICMP)—then modifying the reply fields. Checksum calculations are manual, using summing 16-bit words, folding carries, and taking one's complement. The reply packet assembly involves concatenating the modified IP and ICMP headers with correct checksums.

The experiment highlights the technical challenge of emulating network protocols in user space without libraries. While the result—45-second ping—is slow, it demonstrates the potential of LLMs for low-level networking tasks. The process involves precise arithmetic for checksums, manual hex string handling, and adherence to IP/ICMP specifications.

Claude's ability to generate a functional ping-respond.md command with correct protocol handling and checksum logic shows its technical depth. The experiment runs on Haiku 4.5, a fast model, yet the slow response time (45 seconds) underscores the complexity of real-time network emulation in user space.