Monitor OVS system interfaces (ie. br-data), which are down (speed file returns "Invalid argument")

[Alvaro Uria]

# ip l l br-data
59: br-data: <BROADCAST,MULTICAST> mtu 9000 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    link/ether MAC_REDACTED brd ff:ff:ff:ff:ff:ff

# cat /sys/class/net/br-data/operstate
down
# cat /sys/class/net/br-data/mtu
9000
# cat /sys/class/net/br-data/speed
cat: /sys/class/net/br-data/speed: Invalid argument

For a customer, we want to monitor br-data MTU size, so we need to skip "speed" check when "operstate" is "down".

[Alvaro Uria (aluria)]

Before the proposed patch:

# sudo -u nagios /usr/local/lib/nagios/plugins/check_netlinks.py -i br-data
CRITICAL: br-data link state is down
# sudo -u nagios /usr/local/lib/nagios/plugins/check_netlinks.py -i br-data -o down
CRITICAL: br-data/mtu is 9000 (target: 1500)
# sudo -u nagios /usr/local/lib/nagios/plugins/check_netlinks.py -i br-data -o down -m 9000
<function check_iface at 0x7f3e89dc72f0> raised unknown exception '<class 'OSError'>'
============================================================
Traceback (most recent call last):
  File "/usr/local/lib/nagios/plugins/nagios_plugin3.py", line 32, in try_check
    function(*args, **kwargs)
  File "/usr/local/lib/nagios/plugins/check_netlinks.py", line 37, in check_iface
    metric_value = fd.readline().strip()
OSError: [Errno 22] Invalid argument
============================================================

[Xav Paice (xavpaice)]

Released in cs:nrpe-50