Snort Inline on CentOS

Monday, 3. August 2015

I have been wanting to setup Snort on a CentOS based firewall for a while and I finally got around to it.  The good thing is I finally got it working thanks to a blog Dennis Panagiotopoulos here, I have confirmed this works for CentOS 6.6 and 7.1.  The problem is as with getting Snort to run inline.  I was unable to find any thing on getting this to work correctly.  So here is what I had to do in addition to Dennis’s blog post:

First thing is to edit /etc/sysconfig/snort so it has both the internal and external interfaces like the following:


For me this was line 15 in the file


Second thing is to edit /etc/snort/snort.conf uncomment the daq lines and make them like the following:

config daq: afpacket
config daq_dir: /usr/local/lib/daq
config daq_mode: inline
config daq_var: buffer_size_mb=128

For me these lines were 155 though 158 in the file.


The last thing I had to do was to edit the service at /etc/init.d/snort to add -Q to the end of the following lines under start like the following:


For me these lines were 115 and 119 in the file.

Once I made the changes I restarted the Snort service and then tested and confirmed Snort was running inline.


An easy way to test is to edit /etc/snort/rules/icmp.rules and add:

drop icmp any any -> any any (msg:”ICMP Packet”; sid:477; rev:3;)

Once the rule is added restart snort and then ping from a machine that is on the inside out to the outside and it should return “Destination port unreachable”

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