ntpq: write to ::1 failed: Operation not permitted

 

The other day I got a bug report about check_ntpmon, which was reporting UNKNOWN status back to Nagios even though everything seemed to be working fine. A bit of debugging revealed that it was receiving the message on standard error:

ntpq: write to ::1 failed: Operation not permitted

This was a bit strange, because various links I found indicated that this message is usually due to firewalls:

But this host was not blocking anything, not to mention that check_ntpmon’s use of ntpq only ever uses the loopback interface, which is rarely ever touched by firewalls. A bit of further digging showed that indeed it was not the firewall, but a full conntrack table, with dmesg showing:

Aug 4 03:04:19 hostname kernel: [5226949.016837] nf_conntrack: table full, dropping packet

Increasing the conntrack limit fixed the problem.

(Just thought I’d document this here for posterity, since none of the links I found suggested this issue.)
Source: libertysys.com.au

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