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PING WARNING – DUPLICATES FOUND! – Nagios

Lately i was migrating my Nagios monitoring server to a new hardware and also a new CentOS version (CentOS 5.9 -> CentOS 6.4). It was a completely new server with new Linux OS installed and everything was configured perfectly. When i migrated Nagios data to a newly installed server and switched the IP addresses, i started seeing warnings in my Nagios web frontend – “PING WARNING – DUPLICATES FOUND! Packet loss = 0%, RTA = 0.53 ms”. The warning was popping up on different hosts at different times – completely randomly – really weird, i thought!!

I started checking out the problem and browsing Google but i couldn’t find anything that could be related to my server and Nagios configuration. An hour or two passed when an idea popped into my head! On my CentOS 6.4 i configured ethernet NIC bonding – i actually left the bonding configuration default which means that CentOS bonding is configured in load balancing (round-robin) mode!

I dropped one ethernet NIC and the problem disappeared!

The problem was the “Bonding Mode: load balancing (round-robin)”.

Why this happens?Well, i am not exactly a networking expert here so correct me if i am wrong, but in order for load balancing bonding mode to work properly this must be supported by both endpoints. In my case both Ethernet cables from my server are connected to a Cisco switch. This Cisco switch needs to have a special feature enabled to succesfully communicate to my CentOS server load balancing configured Ethernet cards.

PING WARNING - DUPLICATES FOUND! - Nagios
PING WARNING – DUPLICATES FOUND! – Nagios

So before migrating Nagios server check to see how your NIC bonding is configured!

ROUND-ROBIN

# cat /proc/net/bonding/bond0
Ethernet Channel Bonding Driver: v3.6.0 (September 26, 2009)
Bonding Mode: load balancing (round-robin)
Primary Slave: None
Currently Active Slave: eth0
MII Status: up
MII Polling Interval (ms): 100
Up Delay (ms): 0
Down Delay (ms): 0
Slave Interface: eth0
MII Status: up
Link Failure Count: 0
Permanent HW addr: 3c:4a:92:f9:33:18
Slave queue ID: 0
Slave Interface: eth1
MII Status: up
Link Failure Count: 0
Permanent HW addr: 3c:4a:92:f9:33:19
Slave queue ID: 0

 

If it is configured in load balancing (round-robin) be sure to reconfigure it – the endpoint or the Linux bonding!

ACTIVE-BACKUP

# cat /etc/modprobe.d/bonding.conf
alias bond0 bonding
options bond0 miimon=100 mode=1
# cat /proc/net/bonding/bond0
Ethernet Channel Bonding Driver: v3.6.0 (September 26, 2009)
Bonding Mode: fault-tolerance (active-backup)
Primary Slave: None
Currently Active Slave: eth0
MII Status: up
MII Polling Interval (ms): 100
Up Delay (ms): 0
Down Delay (ms): 0
Slave Interface: eth0
MII Status: up
Link Failure Count: 0
Permanent HW addr: 3c:4a:92:f9:33:18
Slave queue ID: 0
Slave Interface: eth1
MII Status: up
Link Failure Count: 0
Permanent HW addr: 3c:4a:92:f9:33:19
Slave queue ID: 0

 

Hope this saves someone an hour of research! Have fun! 🙂

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    • Gene

      I’m getting the same error running nagios 4.06 on Ubuntu. I’m running 64bit Ubuntu on Virtualbox. As far as I know there is no bonding enabled. I’m not sure how I would check for that in Ubuntu?
      Thanks

      • Mitch

        Hi Gene! Hmm this is weird. What host are you getting “duplicates error” on? is it just one or more hosts? Maybe you have an IP conflict, did you check? If you didn’t configure bonding you probably do not have it – it does not make much sense in virtual environments anyway. Regards, Mitch