Write a Minimal Traceroute Using Raw Sockets

TTL and ICMP Time Exceeded

Traceroute maps the path to a destination like 8.8.8.8 by exploiting the IP TTL field. Every router decrements TTL by one. When TTL hits zero, the router sends ICMP Time Exceeded (type 11, code 0) back to you.

Incremental TTL Strategy

Send packets with increasing TTL starting at 1:

• TTL 1: expires at first hop (e.g. 192.168.1.1)
• TTL 2: expires at second hop
• TTL 30: reaches destination 8.8.8.8

When the destination is finally reached:

• UDP traceroute: ICMP Port Unreachable (type 3, code 3)
• ICMP traceroute: Echo Reply (type 0)
• TCP SYN traceroute: SYN-ACK or RST

Setting TTL with Raw Sockets

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Standard sockets may not expose TTL control on all platforms. Raw sockets or IPPROTO_ICMP sockets let you set the value directly.

Why This Exercise

This task asks you to document traceroute logic with hop numbers and expected ICMP responses. Van Jacobson's original 1988 traceroute used exactly this TTL-expiry behavior, and network engineers still rely on it to localize latency or packet loss in a path. The mtr tool on every Linux box still increments TTL and reads ICMP Time Exceeded messages the same way.