Iptables

Overview

The iptables plugin gathers packets and bytes counters for rules within a set of table and chain from the Linux’s iptables firewall.

Rules are identified through associated comment. Rules without comment are ignored. Indeed we need a unique ID for the rule and the rule number is not a constant: it may vary when rules are inserted/deleted at start-up or by automatic tools (interactive firewalls, fail2ban, …). Also when the rule set is becoming big (hundreds of lines) most people are interested in monitoring only a small part of the rule set.

Before using this plugin you must ensure that the rules you want to monitor are named with a unique comment. Comments are added using the -m comment --comment "my comment" iptables options.

The iptables command requires CAP_NET_ADMIN and CAP_NET_RAW capabilities. You have several options to grant agent to run iptables:

  • Run agent as root. This is strongly discouraged.
  • Configure systemd to run agent with CAP_NET_ADMIN and CAP_NET_RAW. This is the simplest and recommended option.
  • Configure sudo to grant agent to run iptables. This is the most restrictive option, but require sudo setup.

Using systemd capabilities

You may run systemctl edit circonus-unified-agent.service and add the following:

[Service]
CapabilityBoundingSet=CAP_NET_RAW CAP_NET_ADMIN
AmbientCapabilities=CAP_NET_RAW CAP_NET_ADMIN

Since the agent will fork a process to run iptables, AmbientCapabilities is required to transmit the capabilities bounding set to the forked process.

Using sudo

You will need the following in your config:

[[inputs.iptables]]
  use_sudo = true

You will also need to update your sudoers file:

$ visudo
# Add the following line:
Cmnd_Alias IPTABLESSHOW = /usr/bin/iptables -nvL *
cua  ALL=(root) NOPASSWD: IPTABLESSHOW
Defaults!IPTABLESSHOW !logfile, !syslog, !pam_session

Using IPtables lock feature

Defining multiple instances of this plugin in circonus-unified-agent.conf can lead to concurrent IPtables access resulting in “ERROR in input [inputs.iptables]: exit status 4” messages in the log and missing metrics. Setting ‘use_lock = true’ in the plugin configuration will run IPtables with the ‘-w’ switch, allowing a lock usage to prevent this error.

Configuration

  # use sudo to run iptables
  use_sudo = false
  # run iptables with the lock option
  use_lock = false
  # Define an alternate executable, such as "ip6tables". Default is "iptables".
  # binary = "ip6tables"
  # defines the table to monitor:
  table = "filter"
  # defines the chains to monitor:
  chains = [ "INPUT" ]