jc-path-mtu

Bash script to find the MTU of a remote host, outputs to JSON, conceptually designed for Zabbix or to run as a script on a monitoring server. It requires ICMP connectivity, jq, and jc. Sends JSON output to stdout, errors to stderr, expects jc to be located in /usr/local/bin/jc or /usr/bin/jc.

Usage

Tips

It is best to call the script with the known MTU, to avoid additional tests. When running this via an automated process, such as a Zabbix external script, you amy need to adjust the timeout for the external script, "Timeout=30" in /etc/zabbix/zabbix_server.conf is the maximum with Zabbix.

Arguments

./jc-path-mtu.sh contoso.com
./jc-path-mtu.sh contoso.com 1492
./jc-path-mtu.sh contoso.com 1500 0.2
./jc-path-mtu.sh contoso.com 1450 .05
./jc-path-mtu.sh 1.1.1.1

Raw output

./jc-path-mtu.sh 1.1.1.1 | jq .mtu

Dependencies

bash, jq, and jc

• https://github.com/stedolan/jq
• https://github.com/kellyjonbrazil/jc

Notes

I had attempted to use nmap, but some providers block ICMP types other than echo requests. An MTU change anywhere in the path can destroy IPSec VPNs over UDP, and it is something that chewed away days worth of my time in one instance.