# traceroute — Trace the Network Path Hop by Hop > Practical guide to traceroute and tracepath — trace network paths hop by hop, pinpoint latency spikes, and work around firewalls with alternative probes. Source: https://www.jpkc.com/db/en/cheatsheets/networking/traceroute/ traceroute shows you the hop-by-hop path that packets take from your machine to a destination host – and pinpoints where latency or packet loss occurs along the way. It sends probes with incrementally increasing TTL values; each router that discards a packet replies with an ICMP message, revealing its address and round-trip time. This lets you tell at a glance whether a problem lies with your ISP, a transit router, or the destination server itself. ## Basic Usage `traceroute HOST` — Trace the route to a host using UDP probes (default). ```bash traceroute 8.8.8.8 ``` `traceroute HOSTNAME` — Trace the route to a hostname with DNS resolution. ```bash traceroute google.com ``` `traceroute -n HOST` — Numeric output only — skip reverse DNS lookups. Much faster. ```bash traceroute -n 8.8.8.8 ``` `traceroute -q N HOST` — Set the number of probe packets per hop (default: 3). ```bash traceroute -q 1 8.8.8.8 ``` `traceroute -w TIMEOUT HOST` — Set the timeout in seconds to wait for a response (default: 5). ```bash traceroute -w 3 8.8.8.8 ``` `traceroute -m MAX_TTL HOST` — Set the maximum number of hops (default: 30). ```bash traceroute -m 20 8.8.8.8 ``` `traceroute -f FIRST_TTL HOST` — Start with a specific TTL value instead of 1. ```bash traceroute -f 5 8.8.8.8 ``` ## Probe Methods `traceroute -I HOST` — Use ICMP Echo probes instead of UDP. Often gives better results through firewalls. ```bash sudo traceroute -I 8.8.8.8 ``` `traceroute -T HOST` — Use TCP SYN probes. Good for tracing through firewalls that block UDP/ICMP. ```bash sudo traceroute -T 8.8.8.8 ``` `traceroute -T -p PORT HOST` — Use TCP SYN probes on a specific port. ```bash sudo traceroute -T -p 443 8.8.8.8 ``` `traceroute -U HOST` — Use UDP probes (default method). ```bash traceroute -U 8.8.8.8 ``` `traceroute -p PORT HOST` — Set the destination port for UDP probes (default: 33434). ```bash traceroute -p 53 8.8.8.8 ``` ## Advanced Options `traceroute -s SOURCE HOST` — Set the source IP address for outgoing probes. ```bash traceroute -s 10.0.0.5 8.8.8.8 ``` `traceroute -i INTERFACE HOST` — Use a specific network interface. ```bash traceroute -i eth0 8.8.8.8 ``` `traceroute -z PAUSE HOST` — Set the pause between probes in milliseconds. ```bash traceroute -z 50 8.8.8.8 ``` `traceroute -A HOST` — Show AS (Autonomous System) numbers at each hop. ```bash traceroute -A 8.8.8.8 ``` `traceroute -e HOST` — Show ICMP extensions (MPLS labels, etc.). ```bash traceroute -e 8.8.8.8 ``` `traceroute -6 HOST` — Force IPv6 traceroute. ```bash traceroute -6 google.com ``` `traceroute6 HOST` — IPv6 traceroute command (alias on many systems). ```bash traceroute6 google.com ``` ## tracepath (No Root Required) `tracepath HOST` — Trace path to host and discover Path MTU. No root required. ```bash tracepath 8.8.8.8 ``` `tracepath -n HOST` — Numeric output only — skip DNS resolution. ```bash tracepath -n 8.8.8.8 ``` `tracepath -b HOST` — Show both hostname and IP address. ```bash tracepath -b 8.8.8.8 ``` `tracepath -m MAX_HOPS HOST` — Set maximum hop count (default: 30). ```bash tracepath -m 20 8.8.8.8 ``` `tracepath -l PACKET_LEN HOST` — Set the initial packet length for MTU discovery. ```bash tracepath -l 1500 8.8.8.8 ``` `tracepath -p PORT HOST` — Set the destination port. ```bash tracepath -p 443 8.8.8.8 ``` `tracepath -4 HOST` — Force IPv4. ```bash tracepath -4 google.com ``` `tracepath -6 HOST` — Force IPv6. ```bash tracepath -6 google.com ``` ## Reading the Output ` 1 192.168.1.1 1.234 ms 0.987 ms 1.102 ms` — Normal hop: hop number, router IP, and three round-trip times. ` 5 * * *` — No response from this hop. The router may block ICMP/UDP or has no reverse route. ` 3 10.0.0.1 2.5 ms !N * *` — !N = Network unreachable. Other codes: !H (host), !P (protocol), !F (fragmentation needed). `Resume: pmtu 1500` — tracepath output: the discovered Path MTU to the destination. ## Common Patterns `traceroute -n -q 1 -w 2 HOST` — Fast traceroute: no DNS, one probe per hop, 2 second timeout. ```bash traceroute -n -q 1 -w 2 8.8.8.8 ``` `sudo traceroute -I -n HOST` — ICMP traceroute without DNS — most reliable through firewalls. ```bash sudo traceroute -I -n 8.8.8.8 ``` `sudo traceroute -T -p 443 -n HOST` — TCP traceroute on HTTPS port — works through most firewalls. ```bash sudo traceroute -T -p 443 -n 8.8.8.8 ``` `traceroute HOST | awk '{print $1, $2}'` — Extract hop numbers and IPs only. ```bash traceroute -n 8.8.8.8 | awk '{print $1, $2}' ``` ## Conclusion For most connectivity problems, `traceroute -n` is the first tool to reach for – you see instantly where the path ends or where latency spikes. If firewalls block UDP probes, switch to `-I` (ICMP) or `-T -p 443` (TCP). For continuous monitoring, `mtr` supersedes traceroute as a more informative alternative. ## Further Reading - [Traceroute – Wikipedia](https://en.wikipedia.org/wiki/Traceroute) – background and how it works - [traceroute(8) – manual page](https://linux.die.net/man/8/traceroute) – every option at a glance ## Related Commands - [mtr](https://www.jpkc.com/db/en/cheatsheets/networking/mtr/) – combines traceroute and ping for continuous network monitoring - [ping](https://www.jpkc.com/db/en/cheatsheets/networking/ping/) – check host reachability and measure round-trip time - [dig](https://www.jpkc.com/db/en/cheatsheets/networking/dig/) – verify DNS resolution when hostnames don't resolve correctly