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Internal firewalls

While the external firewall regulates network traffic to the OCF from outside the OCF network, internal firewalls are responsible for regulating network traffic between different machines within the OCF.

OCF machines are broadly classed into internal and DMZ. Internal machines are those which are not running user code and are not staff VMs, specifically those in the IP range 5-90 (excluding the ones in the DMZ listed below). The DMZ consists of all other machines, including:

  • Those running user code (i.e. the public SSH login, apphosting, and web servers, and desktops)
  • Staff VMs
  • External machines plugged into the OCF network (e.g. staffers' laptops)

Each server filters input traffic as follows:

  • All servers allow traffic from internal servers, which are considered trusted. For instance, all servers will accept traffic from supernova.
  • All servers disallow traffic from the DMZ by default, as those hosts are considered untrusted. For instance, hal will not accept traffic from tsunami without a special exception. (DMZ servers can't even talk to other DMZ servers by default.)
  • Some servers allow certain traffic from DMZ servers. The precise rules can be found in the puppet code. Generally:
    • DMZ servers can talk to needed production services on the appropriate ports.
    • Staff VMs allow all incoming traffic.
    • Staff VMs and desktops are allowed to directly SSH into internal servers.
    • There are other miscellaneous rules allowing other traffic.

In addition to input rules, it's necessary to have some output rules as well to protect devices that we don't trust to have reliable firewalls, such as printers and IPMI devices. The rules for those devices work a little differently:

  • A handful of "uber-trusted" servers, such as supernova and hypervisors, are allowed to talk to all of these special devices. This allows us to use these servers for the purposes of configuring or debugging those devices.
  • Some other servers can talk to certain special devices. For example, the print server can talk to printers.
  • Most other hosts cannot talk to these special devices.

Output rules are not a perfect solution, since they operate on a voluntary mechanism and can't prevent non-OCF hosts that may be connected to our network from contacting these special devices anyway. Preventing this is a future project.

It's important to note that internal firewalls are set up to only filter traffic from other OCF machines. Traffic from outside the OCF network is the sole responsibility of the external firewall.

Implementation

Internal firewalls are implemented using iptables rules set by Puppet with the puppetlabs-firewall module. We place all of our input rules in the PUPPET-INPUT chain and all of our output rules in the PUPPET-OUTPUT chain.

Firewall rules are added by using firewall_multi and ocf::firewall::firewall46 declarations:

  • ocf::firewall::firewall46 should generally be used in most cases. It inserts IPv4 and IPv6 iptables rules, but only adds the IPv6 iptables rule if the host has a public IPv6 address. This prevents Puppet errors otherwise occurring due to IPv6 addresses not being resolved.
  • firewall_multi should be used if IP addresses need to be manually specified in the firewall rule.
  • ocf::firewall::firewall46 and firewall_multi both internally use the firewall resource. Direct use of the firewall resource should be avoided since such resources wouldn't be subject to the ordering constraints generally placed on firewall resources.

Debugging

iptables commands

Note that all of the following commands need to be run as root.

The iptables command allows you to inspect and debug IPv4 firewall rules:

  • iptables -L: list firewall rules
    • iptables -L PUPPET-INPUT lists our input firewall rules
    • iptables -L PUPPET-OUTPUT lists our output firewall rules
    • You can add a -v option to list more detailed info (like statistics and input/output interface)
    • You can add a -n option to show IP addresses and port numbers instead of hostnames and port names
  • iptables -S: dump firewall rules in machine-parseable format
    • Shows command line options used to add each rule
    • iptables -S <chain>: dump firewall rules for a given chain
  • iptables -F <chain>: deletes all of the rules in the given chain.
    • It seems that in some circumstances iptables may be buggy and not remove a rule that Puppet tells it to remove. This seems to be more likely to occur when the rule is the only rule in its chain. In this case, running iptables -F on that chain and subsequently rerunning Puppet will clear the issue.
  • iptables -D <chain> <rulenum>: Deletes the rulenum-th rule from the given chain (i.e. PUPPET-INPUT or PUPPET-OUTPUT).
  • iptables -A <chain> <rule-specification>: Adds the specified rule to the given chain. Note that this command is not particularly useful for the PUPPET-INPUT and PUPPET-OUTPUT chains, since any added rules will be purged by Puppet.

For IPv6 firewall rules, you need to use the ip6tables command instead. The invocation is the same as for iptables.

Iptables rules are not automatically persisted across reboots. In order for your changes to iptables to be preserved across reboots, you need to additionally run service netfilter-persistent save. This is done automatically after every Puppet run which results in iptables rules being modified, but if you manually fiddle with iptables you may need to run it yourself.

Disabling firewalls through hiera

In an emergency, it's possible to effectively disable firewalling of input traffic on a server by setting ocf::firewall::reject_unrecognized_input to false in that server's hieradata. Turning on this option causes the rules in the PUPPET-INPUT chain which reject traffic to be deleted, effectively disabling the firewall's function of filtering input packets.

An analogous kill switch does not currently exist for output-traffic firewalling, but can be easily added.