traceroute

Traceroute is a network diagnostic utility used to determine the path that a packet takes from a source to a destination and to measure transit delays along the way. By listing the intermediate routers, or hops, traceroute helps diagnose routing problems and assess network performance.

How it works: The tool sends packets with progressively increasing time-to-live (TTL) values. Each router decrements the TTL; when TTL reaches zero, the router discards the packet and sends an ICMP Time Exceeded message back to the origin. By sending multiple probes per TTL, traceroute maps the route and estimates per-hop latency. Implementations vary: Unix-like systems typically use UDP probes (or ICMP Echo requests); Windows uses ICMP Echo requests as tracert.

Output interpretation: The result is a list of hops with the router's IP address (and often its hostname) and one or more round-trip times. Some hops may be missing if routers do not respond or block ICMP; network devices such as firewalls and NAT can also affect visibility.

Limitations and variants: Traceroute reveals the path and latency characteristics but may provide an incomplete view due to load balancing, policy filtering, or ICMP rate limiting. It may be less reliable for IPv6 networks or behind certain middleboxes. Several implementations exist, including standard traceroute, tracepath, and Windows tracert, differing in probe type, default ports, and output format. It is commonly used for troubleshooting connectivity and performance issues.

History: Traceroute originated in the 1980s as part of Unix networking tools. It has since become a standard utility across operating systems, with multiple compatible implementations addressing different network stacks and filtering policies.