Trace route monitors

What it checks

A trace route monitor finds the path from one of Mon's locations to a host, one hop at a time. It sends ICMP echo requests that are allowed one router further each time, and records the address of each router that answers and how long it took. The trace succeeds - and the monitor is up - when the host itself replies.

Every check keeps the route it traced, so when a host becomes slow or unreachable you can see where on the way the delay or the break is - proof to take to your host or ISP.

Trace route monitors need a paid plan. They're checked as often as every 5 minutes.

Adding one

Under Monitors, choose Add Monitor, then Trace Route, and enter:

• Host - the machine at the far end of the path, by name or by IP address. A URL is cut down to its host.

Under Advanced options:

• Max Hops - how far the path is followed before the trace gives up, from 1 to 30. 30 by default.
• IP Protocol - Automatic, IPv4 or IPv6. The host name is looked up and the trace follows the path to an address of that family.

To add several at once, choose Add several at once and paste the hosts into Hosts, one per line or separated by commas. Each becomes its own monitor, named after its host, and repeats are dropped.

The rest of the form is shared with every monitor: Monitoring - interval, timeout, maintenance windows and locations, see How checks work - then Alerts, see Alerts and contacts, and the monitor's Name, suggested from the host.

What it reports

What happened	Monitor
The host replied within the hop limit	Up
The trace reached Max Hops without the host replying	Down
The trace ran past the monitor's timeout	Down
The host name couldn't be looked up	Down

The monitor's page shows the Route of its latest check: each hop's number, the address that answered, how long it took, and Reached at the host. A hop that didn't answer shows * and No reply - many routers don't answer traces, and a silent hop in the middle of a route that still reaches the host is normal. The header shows how many Hops the last trace took. Open any trace route check under Checks to see the route that check took.

A failed trace is something another location might not see, so Mon tries the next of the monitor's locations before calling it down - see How checks work.

Timeouts

Each hop is given up to a second to answer, and the whole trace has to finish within the monitor's Timeout. Hops that don't answer use up that time, so a long route with many silent hops can run out of it - raise the timeout, up to 30 seconds, if traces to a working host time out.

The host has to answer ICMP echo requests for a trace to finish. A host behind a firewall that drops them never replies, and the trace ends down. Allow ICMP from Mon's addresses - listed on Locations and IP addresses - or watch that host with a port monitor instead.

On the Free plan

Trace route monitors aren't included in the Free plan. If you move to it, your trace route monitors are paused, and they resume once you're on a paid plan again.

Trace route and ping

A ping monitor tells you a machine is reachable and how quickly it answers. A trace route monitor shows the path to it, so when a ping monitor goes down or slows, you can see where.