Snort 3.0 (SnortSP) Beta 3 -- Inline Bridging Mode in 3 Steps

To complete the updating of my 3-part series on Installing and Configuring Snort 3 Beta 3, I now present to you the final chapter in this trilogy: the updated recipe for Snort 3 Inline Bridging Mode in 3 Steps. Snort 3.0 Beta 3 has changed the syntax for creating a new dsrc, so this has been reflected in the new recipe.

Step 1: Add a new function to your snort.lua file
If you followed the updated Snort 3.0 Beta 3 Installation HOWTO, then the default snort.lua file is in /etc/snortsp/snort.lua. Open the file and add the following function:

function bridge (interface1, interface2)
if interface2 == nil then
error("Two interface strings must be specified ( e.g. bridge(\"eth0\", \"eth1\") )")
end
dsrc1 = {name="src1",
type="afpacket",
intf=interface1..":"..interface2,
flags=10,
snaplen=1514,
display="none",
tcp={maxflows=262144, maxidle=30, flow_memcap=10000000},
other={maxflows=131072, maxidle=30, flow_memcap=1000000},
cksum_mode=0x0,
}
dsrc.new(dsrc1)
eng.new({name="e1"})
eng.link({engine="e1", source="src1"})
eng.start("e1")
end

Step 2: Start SnortSP
Start SnortSP using the following command:

sudo snortsp -L /etc/snortsp/snort.lua

SnortSP will start up and read your new snort.lua file. You will then be greeted by the SnortSP Lua shell.

Step 3: Use the bridge() function
In the Lua shell, type the following (replacing eth0 and eth1 with the correct interfaces on your system):

bridge("eth0", "eth1")

Once the bridge is up and running, you can do the following to display packets traversing the bridge:

eng.set_display({engine="e1", display="none"})
Press the Up arrow and change "none" to "classic".
View the output.
Press the Up arrow twice to retrieve the "none" command and press Enter.

When finished, shut down SnortSP by running the following command:

ssp.shutdown()