Re: Cannot get Snort listen on a second network interface (creating a gateway)

["Al Lewis (allewi)" <allewi () cisco com>]

Hello Henry,

Use “-i ” to set the monitoring interface. Some other uses with the daq are listed here: 
http://manual.snort.org/node7.html

If you set an IP on that interface snort will only see the packets destined for that host.

You may want to span traffic to the monitoring interface and put it into promiscuous mode so you can see all traffic on 
that subnet.

Also… Hosts on your network wont use your gateway to access each other because based on your subnet mask/network number 
they are all in the same subnet. Once they ARP for the destination they will send the packets directly to the end point 
and not your machine. Hosts will only use a default gateway to get to networks they don’t know about (not within their 
current subnet).


Hope this helps.

Albert Lewis
QA Software Engineer
SOURCEfire, Inc. now part of Cisco
9780 Patuxent Woods Drive
Columbia, MD 21046
Phone: (office) 443.430.7112
Email: allewi () cisco com

From: Henry Collins [mailto:hcol1987 () gmail com]
Sent: Monday, February 23, 2015 5:41 AM
To: snort-users () lists sourceforge net
Subject: [Snort-users] Cannot get Snort listen on a second network interface (creating a gateway)

I have a small subnet that consists of several computers. I want these computers to configure in such a way that they 
would use my gateway to access each other and the external internet.

There is already a gateway working in the subnet, but it doesn't have Snort installed. It's ip is 10.165.17.1

I am working on creating another gateway that would use the upper gateway to serve computers in the subnet, but this 
gateway would receive packets from computers in the subnet. In this way, I want to instruct Snort to listen on for 
example eth1, which would be used for computers in the subnet and eth0 for communication with the subnet's gateway 
(10.165.17.1). How is it done?

Here is a short overview of my network:

Gateways:
10.165.17.1 (has access to external internet and computers in the subnet)
10.165.17.70 (is used by computers in the subnet as gateway and uses 10.165.17.1 as its gateway)

Computers:
10.165.17.60 (gateway: 10.165.17.70)
10.165.17.61 (gateway: 10.165.17.70)
and so on...

This is my /etc/network/interfaces so far. However, eth1 doesn't get an IP. How do I fix this? I want to make Snort 
listen on eth1:

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto eth0
iface eth0 inet static
address 10.165.17.66
gateway 10.165.17.1
dns-nameservers 10.165.0.10 10.165.0.11
netmask 255.255.255.0
broadcast 10.165.17.255

# The secondary network interface
auto eth1
iface eth1 inet static
address 10.165.17.70
gateway 10.165.17.1
dns-nameservers 10.165.0.10 10.165.0.11
netmask 255.255.255.0
broadcast 10.165.17.255

This is output from ifconfig:

eth0      Link encap:Ethernet  HWaddr 00:0c:29:83:e8:ff
          inet addr:10.165.17.66  Bcast:10.165.17.255  Mask:255.255.255.0
          inet6 addr: fe80::20c:29ff:fe83:e8ff/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:137 errors:0 dropped:21 overruns:0 frame:0
          TX packets:12 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:16987 (16.9 KB)  TX bytes:928 (928.0 B)

eth1      Link encap:Ethernet  HWaddr 00:0c:29:83:e8:09
          inet6 addr: fe80::20c:29ff:fe83:e809/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:92 errors:0 dropped:11 overruns:0 frame:0
          TX packets:8 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:9675 (9.6 KB)  TX bytes:648 (648.0 B)

lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

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