labrea.conf is the configuration file for the labrea(1) program.

Each line consists of a selector field, followed by an action verb.

Whitespace is suppressed. Blank lines are ignored, as are lines beginning with "#".

IPs can be specified as either a single address (e.g. "192.168.0.4") or as a range of addresses (e.g. "192.168.0.1 - 192.168.0.50").

Ports can be specified as either a single port (e.g. 12345) or as a range of ports (e.g. 1-65535).

When labrea sees an ARP request for an unused IP, it does the following:

On an IP by IP basis, store a time and an originating IP address:

1.

For an incoming ARP request, check the current time:

a.

If currently stored time is 0 or the arp comes from a different address than the one stored, then store the current time and the requesting IP and return.

b.

If the stored time is less than "-r" seconds ago, ignore it and return.

c.

If currently stored time is more than a minute ago, store 0, return. (Max timeout)

d.

Otherwise, grab the IP.

2.

See an ARP reply, set stored time to 0.

When an ARP request for a particular IP goes unanswered for longer than its "rate" setting (default: 3 seconds), labrea crafts an ARP reply that routes all traffic destined for the IP to a "bogus" MAC address. labrea listens for TCP/IP traffic routed to that MAC address and then responds to any SYN packet (ie incoming connection) with a SYN/ACK packet.

Excluded IPs: Are those IPs that labrea should never capture. Note that automatic mechanisms are also used to prevent capturing IPs with an active machine on it. See labrea(1) for more details.

Hard captured IPs: The -h --hard-capture option instructs labrea that once it captures an IP address, then it needn't wait for a "-r" timeout the next time around. These IPs are said to be "hard" captured.

Hard excluded IPS: These are IPs that should never be "hard" captured. In other words, each time there is an ARP request for this IP, then labrea will always wait for the timeout -r secs before responding.

Tarpitting: On a captured IP, labrea responds to an incoming SYN connection attempt with a SYN/ACK. This causes the remote machine's stack to initiate the Tcp connection and then waste time fruitlessly trying to continue the conversation.

Persist state capture: labrea can permanently capture connect attempts by closing the TCP window to force the connection into "persist" state. In this state, the connection never times out, and labrea hangs on to the incoming connection until it is closed from the other end.

To accomplish this, short packets are sent every so often to say "keep waiting, my Tcp window is still closed". So a maximum b/w control is implemented to limit the total b/w consumption. (see the -p --max-rate startup option)

Auto hard capturing: This is a startup option that says that unless an IP is excluded or hard-excluded, then mark it as being hard captured. This is normally a risky thing to do and should be used with caution.

Default port behaviour: Incoming connections on any port will be subject to tarpitting / persist capturing.

Since all connections are inbound, there should be no incoming SYN/ACKs. Labrea will respond RST to an incoming SYN/ACK unless the startup option -a --no-resp-synack disables this behaviour.

Excluded ports: Ports that are specifically excluded will not be tarpitted or persist captured.

Incoming connection attempts on an excluded port will receive a RST.

Active ports: When firewalling (i.e. -f --no-resp-excluded-ports) is active, then by default only the most widely used ports are active at startup.

Incoming connections on these active ports will be tarpitted and/or persist captured as usual.

Excluded ports: When firewalling is active, incoming connections on excluded ports will not receive a response. The packets will be dropped.

Among other things, this means that nmap scans take much more time to complete.

Other ports: Ports that are neither active nor excluded are passively monitored for incoming SYN activity. At startup, they behave as an excluded port (i.e. packets are dropped).

However, if there is enough activity on a given port, it will dynamically become active. The threshold is more than 6 SYNs for a given port in an hour. However every 15 minutes, the port's SYN count is reduced by 1 to eliminate noise.

If the SYN count for a port finally reaches 255, then the port is considered permanently active.