Check iptables

[frater]

If one of your colleagues modifies the firewall on one of the servers, you will get a notification which may help when some strange things occur

echo "UserParameter=net.iptables.cksum,sudo iptables-save | grep -v '^[#:]' | md5sum | tr -cd 0-9 | cut -b1-10" >>/etc/zabbix/zabbix_agentd.conf

echo "zabbix ALL = NOPASSWD: `which iptables-save`" >>/etc/sudoers

/etc/init.d/zabbix-agent restart

You only need to create the item: net.iptables.cksum with a refresh of 600 seconds
and the trigger: net.iptables.cksum.diff(0)}>0

[frater]

It didn't work on all my servers, but luckily I found the culprit.
Your /etc/sudoers may contain the line 'Defaults requiretty'
This means any sudo command will fail....

This was in my /var/log/zabbix/zabbix_agentd.log

sudo: sorry, you must have a tty to run sudo

You can comment it out with visudo or just this command:

sed -i -e 's/^Defaults.*requiretty/# &/' /etc/sudoers