[Snort-users] Cannot get Snort listen on a second network interface (creating a gateway)

Al Lewis (allewi) allewi at ...589...
Mon Feb 23 09:20:25 EST 2015


The span should be done on your network switch.  Your switch will have to mirror the traffic to Snort. Snort only processes traffic sent to its interface.

If you have removed the ip from eth1 you wont be able to route traffic through that interface.

I think your best bet is to span traffic to eth1 and set snort to monitor there.

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 at ...589...

From: Henry Collins [mailto:hcol1987 at ...11827...]
Sent: Monday, February 23, 2015 9:10 AM
To: Al Lewis (allewi)
Cc: snort-users at lists.sourceforge.net
Subject: Re: [Snort-users] Cannot get Snort listen on a second network interface (creating a gateway)

I deleted IP address from eth1, but I still cannot see anything on my gateway. I used -i eth1 parameter. How do I make computers that try to use my gateway come through eth1 and reach external internet (outside the subnet) via eth0?

My interfaces file looks like this now:

# 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
dns-nameservers 10.165.0.10 10.165.0.11
netmask 255.255.255.0
broadcast 10.165.17.255

And ifconfig gives this output:

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:695 errors:0 dropped:0 overruns:0 frame:0
          TX packets:178 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:54360 (54.3 KB)  TX bytes:28055 (28.0 KB)

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:232 errors:0 dropped:0 overruns:0 frame:0
          TX packets:8 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:16781 (16.7 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)


> 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.
How do i do that?

I have changed ip addresses in my snort.conf:
ipvar HOME_NET 10.165.17.0/24<http://10.165.17.0/24>
ipvar EXTERNAL_NET !$HOME_NET

Because my subnet is 10.165.17.0 - 10.165.17.255



On Mon, Feb 23, 2015 at 1:05 PM, Al Lewis (allewi) <allewi at ...589...<mailto:allewi at ...589...>> wrote:
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<tel:443.430.7112>
Email: allewi at ...589...<mailto:allewi at ...589...>

From: Henry Collins [mailto:hcol1987 at ...11827...<mailto:hcol1987 at ...11827...>]
Sent: Monday, February 23, 2015 5:41 AM
To: snort-users at lists.sourceforge.net<mailto:snort-users at 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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