Monitoring

Check Firewall

• UDP

[ ~]$ nc -v -u -z -w 3 targethost 25826
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connected to 192.168.1.2:25826.
Ncat: UDP packet sent successfully
Ncat: 1 bytes sent, 0 bytes received in 2.03 seconds.
[ ~]$ nc -v -u -z -w 3 targethost 25828
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connected to 192.168.1.2:25828.
Ncat: Connection refused.

WAN monitoring - Ethernet OAM - Y.1731

• https://app.netrounds.com/static/2.26/support/task-types/refl/y1731-eth-slm.html
• https://www.cisco.com/c/en/us/support/docs/asynchronous-transfer-mode-atm/operation-administration-maintenance-oam/117457-technote-cfm-00.html
• https://www.juniper.net/documentation/en_US/junos/topics/topic-map/oam-service-overview.html

Essentially a formalised version of Cisco SAA probes You really want to use the two way delay probes but this requires hardware at both ends with accurate clocks and support for it.