Minimal Configuration

Zorp Kernel Module

KZorp is the kernel module of the Zorp application level firewall. The module makes possible to make kernel space decisions about the traffic according to the configured Zorp policy. It also provides some extensions to IPTables so that you can build your own packet filter ruleset that uses Zorp concepts and policy objects.

Rule evaluation

Zorp communicates the policy to KZorp when starting up, so inital policy decisions can be applied to certain traffic in kernel space. As the result of the decision, packets are either dropped or put back to the chain of IPTables where the KZORP target has been called.

IPTables relation

The KZorp kernel and IPTables modules allow using certain Zorp concepts in packet filter rulesets.

It adds support for the following IPTables modules:

  • zone match: you can match on Zorp zones (defined in the Zorp policy) in your IPTables ruleset.
  • service match: matches on either the name or the type of the service that has been selected for the packet based on your Zorp policy.
  • KZORP target: handles DAC checks, transparent proxy redirections and generic processing of packets for PFService services (that is, Zorp services that process packets on the packet filter level, not in a user-space proxy).


The main problem of transparent proxy firewalls is the fact that the traffic does not target the firewall itself, but a host behind the network security device. In a usual case the traffic is forwarded to the originally targeted server, but in case of a firewall the traffic must be delivered locally to the proxy, which will connect to the originally targeted server, or another according to the policy. The divertable packets should be identified somehow in the packet filter rulesets. It can be performed by the means of transparent proxy (TProxy) kernel module of the kernel.

The idea is to identify packets with the destination address matching a local socket on your box, set the packet mark to a certain value, and then match on that value using policy routing to have those packets delivered locally.

—TProxy Kernel Module Documentation

The following sections will describe the IPTables and policy routing rules that are essential to make Zorp operable.


At least the following IPTables ruleset is required for Zorp. Note that this ruleset is fair enough for Zorp, but it is inadequate for even the simplest firewall. The ruleset submits a working example of Zorp, so it must be extended with some other rules that are ordinary in case of a proxy firewall (for example: grant SSH access, handle ICMP messages).


# mark and accept connection already handled by Zorp <1>
-A PREROUTING -m socket --transparent -j MARK --set-mark 0x80000000/0x80000000
-A PREROUTING --match mark --mark 0x80000000/0x80000000 --jump ACCEPT





# chain to collect KZorp related rules <2>
# insert rules here to bypass KZorp <3>
# jump to KZorp and mark the packet <4>
-A DIVERT -j KZORP --tproxy-mark 0x80000000/0x80000000



# accept earlier marked packet <5>
-A INPUT -m mark --mark 0x80000000/0x80000000 -j ACCEPT

# accept connection relates to a packet filter service <6>
-A FORWARD -m conntrack ! --ctstate INVALID -m service --service-type forward -j ACCEPT



[KZorp related IPTables rules]

  1. The socket matcher inspects the traffic by performing a socket lookup on the packet (non-transparent sockets are not counted) and checks if an open socket can be found. It practically means that Zorp (or any other application) has a socket for the traffic, it is already handled by Zorp in the userspace, no kernel-level intervention is required. In this case it is marked with the TProxy mark value (0x80000000), meaning that it should be handled by Zorp.
  2. There are some chains of table mangle where KZorp must be hooked for certain purposes (rule evaluation, NAT handling, ...). In these cases we are jumping to a user-defined chain (DIVERT) where the corresponding rules can placed to pass the traffic to KZorp or even bypass it.
  3. This is the place where we can put rules which match to certain traffic should be hidden from KZorp and accept it.
  4. If no rule has been matched in this chain earlier, this rule jumps to KZorp and also marks the packet. This mark can be used in policy routing rules to divert traffic locally to Zorp instead of forwarding it to its original address. Note that this mark is the same that we use in case of the first rule of the PREROUTING chain.
  5. If the traffic has already been marked in table mangle with the corresponding value (0x80000000), we should accept it. For example the data channel connection of active mode FTP matches the first rule of mangle table PREROUTING chain, so it has been marked, but should be accepted in the INPUT chain of filter table as it is an incoming connection.
  6. The service matcher looks up services specified within KZorp. Services can be identified by name or by type. Type forward means a forwarded session (or PFService). These kind of sessions should be forwarded in the FORWARD chain of the filter table.


The ruleset above contains those and only those rules which are essential to make KZorp and Zorp operable. The ruleset must be extended with other rules that make the firewall operable (for example: accepting incoming SSH connection or particular typed ICMP packets).


The ruleset above is IP version-independent, so it can be used both in case of iptables and ip6tables.

Advanced Routing

Packets have been marked to a certain value in IPTables. Now match on that value using policy routing to have those packets delivered locally to Zorp instead of forwarding it to the original address, and Zorp will connect to a server depending on the policy.

ip -4 rule add fwmark 0x80000000/0x80000000 lookup tproxy #<1>
ip -4 route add local default dev lo table tproxy #<2>

[Zorp related policy routing rules]

  1. Rule instructs the system to lookup route for the traffic from table tproxy if the traffic has been marked with the required value (0x80000000) in the DIVERT chain of the mangle table in IPTables.
  2. Table tproxy has only one route that diverts the traffic locally to Zorp, so it is not forwarded as it would have been done by default.

Table name tproxy can be used only if the following line is added to rt_tables file.

100     tproxy

[Zorp related policy routing table names]


The policy routing rules above must be repeated with options -6 instead of -4 to make IPv6 operable.

comments powered by Disqus