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Old 02-16-2006, 08:22 AM   #1
Crito
Senior Member
 
Registered: Nov 2003
Location: Knoxville, TN
Distribution: Kubuntu 9.04
Posts: 1,168

Rep: Reputation: 53
Snort and cable modem setup


OK, I'm trying to setup snort IDS on my network. I installed a second NIC in my PC and a hub between cable modem and router. So first NIC is on internal/private 192.168.x.x and second is on Comcast's external/public network. Second interface I bring up with no IP and then start snort. So far so good, I'm a sniffing fool at this point.

Problem now is I'm apparently sniffing the whole neighborhood and my cable modem uses DHCP. I'd like to filter just packets hitting my house, but the modem's IP might change at any time. How the !@#$ should I set this up? Any advice appreciated.

Code:
cable modem ---- hub ---- router & ---- switch ---- PC2
                  |       firewall         |   ---- PC3
     PC1 ---eth1---                        |   ---- PC4
      |                                    |
      ---------------eth0-------------------
 
Old 02-16-2006, 08:57 AM   #2
TruckStuff
Member
 
Registered: Apr 2002
Posts: 498

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Quote:
Originally Posted by Crito
Problem now is I'm apparently sniffing the whole neighborhood and my cable modem uses DHCP. I'd like to filter just packets hitting my house, but the modem's IP might change at any time. How the !@#$ should I set this up? Any advice appreciated.
Are you just trying to ignore the DHCP messages? If so, you can simply turn off that alert. What's your HOME_NET set to?
 
Old 02-16-2006, 09:44 AM   #3
Crito
Senior Member
 
Registered: Nov 2003
Location: Knoxville, TN
Distribution: Kubuntu 9.04
Posts: 1,168

Original Poster
Rep: Reputation: 53
Right now everything is set to defaults (I'm a snort newbie,) so HOME_NET would be "any" and I'm only sniffing on the one NIC (eth1.) I'm not too concerened about internal attacks, as I'm the only person using my LAN at the moment and I have no public servers/services (and that's why there's no DMZ too.) What I'm concerened about are external attacks against the cable modem, router or firewall devices themselves. My router uses NAT/PAT so from the hub side I can't see the private/internal addresses anyway.

Anyway, I guess my question(s) is/are:

1) Is there a way to set a var like HOME_NET to filter on MAC address of the cable modem instead of IP, which changes because ISP uses DHCP.

2) If #1 above sounds stupid (and being a snort newbie that's entirely possible) what's the right/best way to implement snort with cable modem that uses DHCP?

Perhaps I'm still confused on the proper setup. I did read the snort FAQ, which is where I got the idea for the hub between modem and router (they said could be used instead of a passive ethernet tap.)
 
Old 02-17-2006, 06:37 AM   #4
Crito
Senior Member
 
Registered: Nov 2003
Location: Knoxville, TN
Distribution: Kubuntu 9.04
Posts: 1,168

Original Poster
Rep: Reputation: 53
Geez, don't everyone speak up at once. LOL.. All I was really looking for are some other/alternate diagrams of how peeps have hooked up an NIDS to their cable modem LANs.

Let me elaborate further. The nature of my cable modem service allows me to see broadcast traffic from others on the segment. When I start the sniffer I get bombarded with ARP traffic, burying the packets I want to see. For example, I just captured this:

Code:
02/17-07:21:02.947018 ARP who-has 68.34.194.50 tell 68.34.192.1

02/17-07:21:02.947279 ARP who-has 68.34.194.52 tell 68.34.192.1

02/17-07:21:02.947505 ARP who-has 68.34.194.49 tell 68.34.192.1

02/17-07:21:02.949095 206.37.232.91:30889 -> 68.34.XXX.XXX:1026
UDP TTL:116 TOS:0x0 ID:16606 IpLen:20 DgmLen:908
Len: 880
04 00 28 00 10 00 00 00 00 00 00 00 00 00 00 00  ..(.............
00 00 00 00 00 00 00 00 F8 91 7B 5A 00 FF D0 11  ..........{Z....
A9 B2 00 C0 4F B6 E6 FC F4 24 96 5D 15 AE 33 12  ....O....$.]..3.
5E 2E EC 1C BB 3C F7 BE 00 00 00 00 01 00 00 00  ^....<..........
00 00 00 00 00 00 FF FF FF FF 20 03 00 00 00 00  .......... .....
11 00 00 00 00 00 00 00 11 00 00 00 53 45 43 55  ............SECU
52 49 54 59 20 4D 4F 4E 49 54 4F 52 00 00 00 00  RITY MONITOR....
11 00 00 00 00 00 00 00 11 00 00 00 57 49 4E 44  ............WIND
4F 57 53 20 55 53 45 52 00 00 00 00 00 00 00 00  OWS USER........
D4 02 00 00 00 00 00 00 D4 02 00 00 49 6D 70 6F  ............Impo
72 74 61 6E 74 20 57 69 6E 64 6F 77 73 20 53 65  rtant Windows Se
63 75 72 69 74 79 20 42 75 6C 6C 65 74 69 6E 0D  curity Bulletin.
0A 3D 3D 3D 3D 3D 3D 3D 3D 3D 3D 3D 3D 3D 3D 3D  .===============
3D 3D 3D 3D 3D 3D 3D 0D 0A 42 75 66 66 65 72 20  =======..Buffer
4F 76 65 72 72 75 6E 20 69 6E 20 4D 65 73 73 65  Overrun in Messe
6E 67 65 72 20 53 65 72 76 69 63 65 20 41 6C 6C  nger Service All
6F 77 73 20 52 65 6D 6F 74 65 20 43 6F 64 65 20  ows Remote Code
45 78 65 63 75 74 69 6F 6E 2C 0D 0A 56 69 72 75  Execution,..Viru
73 20 49 6E 66 65 63 74 69 6F 6E 20 61 6E 64 20  s Infection and
55 6E 65 78 70 65 63 74 65 64 20 43 6F 6D 70 75  Unexpected Compu
74 65 72 20 53 68 75 74 64 6F 77 6E 73 0D 0A 0D  ter Shutdowns...
0A 41 66 66 65 63 74 65 64 20 53 6F 66 74 77 61  .Affected Softwa
72 65 3A 20 0D 0A 0D 0A 4D 69 63 72 6F 73 6F 66  re: ....Microsof
74 20 57 69 6E 64 6F 77 73 20 4E 54 20 57 6F 72  t Windows NT Wor
6B 73 74 61 74 69 6F 6E 20 0D 0A 4D 69 63 72 6F  kstation ..Micro
73 6F 66 74 20 57 69 6E 64 6F 77 73 20 4E 54 20  soft Windows NT
53 65 72 76 65 72 20 34 2E 30 20 0D 0A 4D 69 63  Server 4.0 ..Mic
72 6F 73 6F 66 74 20 57 69 6E 64 6F 77 73 20 32  rosoft Windows 2
30 30 30 20 20 20 0D 0A 4D 69 63 72 6F 73 6F 66  000   ..Microsof
74 20 57 69 6E 64 6F 77 73 20 58 50 20 20 0D 0A  t Windows XP  ..
4D 69 63 72 6F 73 6F 66 74 20 57 69 6E 64 6F 77  Microsoft Window
73 20 57 69 6E 39 38 20 20 20 0D 0A 4D 69 63 72  s Win98   ..Micr
6F 73 6F 66 74 20 57 69 6E 64 6F 77 73 20 53 65  osoft Windows Se
72 76 65 72 20 32 30 30 33 0D 0A 0D 0A 4E 6F 6E  rver 2003....Non
20 41 66 66 65 63 74 65 64 20 53 6F 66 74 77 61   Affected Softwa
72 65 3A 20 0D 0A 0D 0A 4D 69 63 72 6F 73 6F 66  re: ....Microsof
74 20 57 69 6E 64 6F 77 73 20 4D 69 6C 6C 65 6E  t Windows Millen
6E 69 75 6D 20 45 64 69 74 69 6F 6E 0D 0A 0D 0A  nium Edition....
59 6F 75 72 20 73 79 73 74 65 6D 20 69 73 20 61  Your system is a
66 66 65 63 74 65 64 2C 20 64 6F 77 6E 6C 6F 61  ffected, downloa
64 20 74 68 65 20 70 61 74 63 68 20 66 72 6F 6D  d the patch from
20 74 68 65 20 61 64 64 72 65 73 73 20 62 65 6C   the address bel
6F 77 20 21 20 0D 0A 46 49 52 53 54 20 54 59 50  ow ! ..FIRST TYP
45 20 54 48 45 20 41 44 44 52 45 53 53 20 42 45  E THE ADDRESS BE
4C 4F 57 20 49 4E 54 4F 20 59 4F 55 52 20 49 4E  LOW INTO YOUR IN
54 45 52 4E 45 54 20 42 52 4F 57 53 45 52 2C 20  TERNET BROWSER,
54 48 45 4E 20 43 4C 49 43 4B 20 27 4F 4B 27 2E  THEN CLICK 'OK'.
0D 0A 54 48 45 20 41 44 44 52 45 53 53 20 57 49  ..THE ADDRESS WI
4C 4C 20 44 49 53 41 50 50 45 41 52 20 4F 4E 43  LL DISAPPEAR ONC
45 20 59 4F 55 20 43 4C 49 43 4B 20 27 4F 4B 27  E YOU CLICK 'OK'
2E 0D 0A 0D 0A 20 20 20 20 20 20 20 20 20 20 20  .....
20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20
20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20
20 20 20 20 20 20 20 20 20 77 77 77 2E 70 61 74           www.pat
63 68 75 70 64 61 74 65 2E 69 6E 66 6F 0D 0A 00  chupdate.info...

=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+

02/17-07:21:02.949199 ARP who-has 68.34.194.44 tell 68.34.192.1

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02/17-07:21:03.063549 ARP who-has 68.34.193.25 tell 68.34.192.1

02/17-07:21:03.063802 ARP who-has 68.34.193.24 tell 68.34.192.1

02/17-07:21:03.064037 ARP who-has 68.34.193.23 tell 68.34.192.1

02/17-07:21:03.064256 ARP who-has 68.34.193.21 tell 68.34.192.1

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02/17-07:21:03.406543 ARP who-has 68.34.195.81 tell 68.34.192.1

02/17-07:21:03.406776 ARP who-has 68.34.195.80 tell 68.34.192.1

02/17-07:21:03.406996 ARP who-has 68.34.195.76 tell 68.34.192.1

02/17-07:21:03.407222 ARP who-has 68.34.195.82 tell 68.34.192.1

02/17-07:21:03.407437 ARP who-has 68.34.195.74 tell 68.34.192.1

02/17-07:21:03.407652 ARP who-has 68.34.195.68 tell 68.34.192.1

02/17-07:21:03.408314 ARP who-has 68.34.195.65 tell 68.34.192.1

02/17-07:21:03.408576 ARP who-has 68.34.195.73 tell 68.34.192.1

02/17-07:21:03.408970 ARP who-has 68.34.195.64 tell 68.34.192.1

02/17-07:21:03.409232 ARP who-has 68.34.195.58 tell 68.34.192.1

02/17-07:21:03.409448 ARP who-has 68.34.195.66 tell 68.34.192.1

02/17-07:21:03.409659 ARP who-has 68.34.195.67 tell 68.34.192.1

02/17-07:21:03.409946 ARP who-has 68.34.195.51 tell 68.34.192.1

02/17-07:21:03.410176 ARP who-has 68.34.195.49 tell 68.34.192.1

02/17-07:21:03.410390 ARP who-has 68.34.195.53 tell 68.34.192.1

02/17-07:21:03.410612 ARP who-has 68.34.195.44 tell 68.34.192.1

02/17-07:21:03.410938 ARP who-has 68.34.195.37 tell 68.34.192.1
The only packet directed at me is the Windows messenger trojan (I blanked out my IP.) The rest won't make it into my LAN anyway, so I really don't want to see them... though I wonder if there's not an ARP DoS attack my Security+ book didn't cover... anyway.
 
Old 02-20-2006, 09:57 PM   #5
Crito
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By the way, if you see ARP traffic like this it's a sure sign of the Nachia/Welchia worm.
 
Old 02-22-2006, 04:17 PM   #6
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Whos your ISP?
 
Old 02-23-2006, 05:59 AM   #7
Crito
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Comcast cable service... anyway, it's apparent I'm going to have to go buy a book just for this one app. There's a whole lot more to chew on here than I initially thought.
 
Old 02-23-2006, 07:00 AM   #8
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Keep a close eye on your ip address, I have Charter and mine has never changed. The only time it ever changes is when there is a different NIC plugged into the modem. So if you have a server plugged into it that is going to be dhcp and whatever else it will never change. I have had a webserver on mine for the last year and its never changed. You can then just set eth1 to your outside ip address and you wont have a problem. Also take a look at www.bleedingsnort.com they have a lot of user submission rule sets that you can download for free and have a forum to ask questions with.
 
Old 02-23-2006, 07:37 AM   #9
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Do you really see packets of others?
From this log, it seems you only see packets from your proxyarp/switch?

I think cable modems provider constantly send this arp requests.

Arp packets won't be seen as an attack under snort, so you won't log them.
And for your sniffer, maybe ignore arp packets or broadcast packets directed to you (FF:FF:FF:FF:FF:FF)

Its quite rare to run a sniffer without a filter. Ethereal is very usefull for this: from a 200Meg file, after clicking on "packet not like this" several times, then clicking on follow tcp stream and select not this stream, you finally end up with a few intersting packets.
 
Old 02-23-2006, 06:44 PM   #10
Crito
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Quote:
Originally Posted by ninjaz
Keep a close eye on your ip address, I have Charter and mine has never changed. The only time it ever changes is when there is a different NIC plugged into the modem.
Hmmm, with Comcast seems to be the cable modem's MAC that's bound to the IP through DHCP. So changing the NIC doesn't do anything (verified with MAC cloning feature in router.) And can't bring up eth1 with same IP either, but just knowing the IP won't change is good enough. Thanks for info!
 
  


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