Snort by gorvam saddar

Introduction to Snort
1
Introduction to Snort
A lightweight Intrusion Detection System by Marty Roesch
Document based extensively on original snort documentation from /usr/local/share/snort

Introduction to Snort
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Using Snort
What is snort?
Snort is a lightweight intrusion detection system that can log packets coming across your network. This

program can be used on smaller networks but on larger ones, with Gigabit Ethernet, snort can become
unreliable. Snort doesn't require that you recompile your kernel or add any software or hardware to your
existing distribution but it does require that you have root privileges.
Why?
Many times it’s far too easy for hackers to scan your network for vulnerable services that could be running or
ports that are available. With this being a fact there isn’t an excuse to ignore security when putting intrusion
detection in is so easy to do. Having snort watch your internal network is important because many of the
security problems actually come from inside your network and in that case you have more of a chance to
correct something before it goes too far. Best of all this tool is free and available on most platforms.
When do I use snort?
Snort can be used anytime you want to have basic security measures in place that allow you to log and
analyze the traffic on your network. Along with a firewall this should be another fundamental part of your
network security.
How do I use snort?
Snort has a few uses: packet sniffer like tcpdump(1), packet logger for network troubleshooting, or an
Intrusion Detection System. Snort can be placed on machines throughout your network and it works in
promiscuous mode to watch all traffic on the wire. Snort can also be used to sift through already-made
tcpdump files.
What platforms does snort run on?
Snort can run on Linux, BSD, MacOS X and Windows. Snort can run on all kinds of variants of these OSs
and you can visit www.snort.org to find more information on it.
x86  Sparc  M68k/PPC Alpha Other
X  X  X  X  X  Linux
X  X  X    OpenBSD
X    X    FreeBSD
X    X    NetBSD
X  X    Solaris
X    SunOS 4.1.X
X  HP-UX
X  AIX
X  IRIX
X    Tru64
X    MacOS X Server
Table from: http://www.snort.org/what_is_snort.htm
Introduction to Snort
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Snort Tutorial
The Output
The simplest way to start snort to see what is does is to use this command:
snort -v -i eth0
This command tells snort to be verbose and display the results to the console using the eth0 interface
(Ethernet).
11/05-18:29:42.554864 192.168.1.2 -> 192.168.1.1
ICMPTTL:64 TOS:0x0 ID:275
ID:63745 Seq:0  ECHO
11/05-18:29:42.554962 192.168.1.1 -> 192.168.1.2
ICMP TTL:255 TOS:0x0 ID:2323
ID:63745 Seq:0  ECHO REPLY
The output contains a lot of information you may find useful. Here we can tell that we have an ICMP packet
coming from 192.168.1.2 going to 192.168.1.1 and that the ICMP packet is an ECHO packet. This is the
result of what you might find from a ping to this address. Then there's the ECHO REPLY from our machine
sent to 192.168.1.2. The packets also contain the date stamp so you can see when they happened. When
we stop snort using CONTROL-C we see this output:
===============================================================================
Snort received 6 packets and dropped 0(0.000%) packets
Breakdown by protocol:
TCP: 0 (0.000%)
UDP: 0 (0.000%)
ICMP: 2 (100.000%)
ARP: 0 (0.000%)
IPX: 0 (0.000%)
OTHER: 0 (0.000%)
===============================================================================
This can be really handy because it tells you your protocol statistics. Of course we have all of ours as ICMP.
Application Layer Data
Our next "trick" will be to display the application layer data as well. The application layer data is what is
actually transmitted across the network. This can be used to sniff for passwords on a network as well
because it displays the text that is passed between machines. You would do this by adding "-d" to the
command:
snort -d -v -i eth0
Ethernet Information
If you want the Ethernet information to be displayed too this can be done using "-e":
snort -d -v -e -i eth0
Introduction to Snort
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Of course you can smash these options together to make it a little easier:
snort -dev -i eth0
ARP
Most of what we initially see are IP packets but we can add “-a” to the command to see ARP packets too:
snort –deva –i eth0
Logging
This is really interesting but not much more useful past real-time monitoring. We want to log these
transactions to a file and that is done through the "-l" option accompanied by the directory you want to log to:
snort -dev -i eth0 -l $HOME/log
Make sure that you have or create the directory you wish to log to otherwise snort will complain! When
packets are logged snort uses the address of the remote computer as it's logging directory.
Identifying Your Network
Sometimes you want to log packets relative to your network. To log packets into directories they are
associated with use the "-h" option with the network address and mask of your home network.
snort -dev -i eth0 -l $HOME/log -h 192.168.1.0/24
Using Rules
Now for one of the most powerful parts of snort. SNORT RULES!. Indeed, snort would be limited if it didn't
have a facility for rules. We'll cover rules syntax in a little bit but for now let's see how see specify what rules
file to use. Snort comes with all sorts of built-in rules files in /usr/local/share/snort/. The rules files are simply
ASCII files with lines of rules snort uses to determine what to log/alert. You'll notice that they can be quite
simple or complicated but also cover some of the more sophisticated attacks and probes and they're pre-made for you. Snort rules can be specified with the simple "-c" option:
snort -dev -i eth0 -l $HOME/log -c rule-file
All together a snort command on the command line might look like this:
snort -dev -i eth0 -l $HOME/log -h 192.168.1.0/24 -c rule-file
This is quite a long command which is why this could be a good candidate for the old /etc/rc.d/ scripts files.
TCPDUMP Files
Another way to use snort is to sift through already created tcpdump files. Using the "-r" option you can start
snort the way you normally would to check a tcpdump file for all the same kinds of things you use snort for
on the network.
snort -d -h 192.168.1.0/24 -l $HOME/log -c rule-file -r tcpdump.file
Introduction to Snort
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Daemon Mode
Finally we want to find out how to put this thing into daemon mode so that it doesn't hog our terminal with the
"-D" option:
snort -d -h 192.168.1.0/24 -l $HOME/log -c rule-file -D
Making Our Lives a Little Easier - Tips
Okay, now that we've covered most of the important snort stuff let's look at better ways to use it. For
example, the "-v" could be left out because if snort is logging then you have that information anyway and the
console would just be cluttered and for speed reasons it's really not required. The Ethernet "-e" data, too, is
probably not really needed for most applications.
Introduction to Snort Rules
Now we'll take a look at some basic rules to see how they work. The basic format of a rule is as follows:
function protocol source_ip/mask source_port -> destination_ip/mask destination_port options
A rules file will contain many lines of rules and it can look a little complicated at first. Here’s an example.
# look for stealth port scans/sweeps
alert tcp any any -> $HOME_NET any (msg:"SYN FIN Scan"; flags: SF;)
alert tcp any any -> $HOME_NET any (msg:"FIN Scan"; flags: F;)
alert tcp any any -> $HOME_NET any (msg:"NULL Scan"; flags: 0;)
alert tcp any any -> $HOME_NET any (msg:"XMAS Scan"; flags: FPU;)
alert tcp any any -> $HOME_NET any (msg:"Full XMAS Scan"; flags: SRAFPU;)
alert tcp any any -> $HOME_NET any (msg:"URG Scan"; flags: U;)
alert tcp any any -> $HOME_NET any (msg:"URG FIN Scan"; flags: FU;)
alert tcp any any -> $HOME_NET any (msg:"PUSH FIN Scan"; flags: FP;)
alert tcp any any -> $HOME_NET any (msg:"URG PUSH Scan"; flags: PU;)
alert tcp any any -> $HOME_NET any (flags: A; ack: 0; msg:"NMAP TCP ping!";)
# IMAP buffer overflow
alert tcp any any -> $HOME_NET 143 (msg:"IMAP Buffer Overflow!"; content:"|E8
C0FF FFFF|"; flags: PA;)
# x86 named buffer overflow
alert tcp any any -> $HOME_NET 53 (msg:"named Buffer Overflow!"; content:"|CD80
E8D7 FFFF FF|"; flags: PA;)
To break it down, let's look at a simple rule:
log tcp any any -> 192.168.1.0/24 23
This rule says to log tcp traffic coming from any IP address and any source port to this network where the
destination port is 23 (telnet).
log tcp any any -> any 22 (msg:”Geez! Someone’s trying to use SSH!”;)
Here we can see how a message can be appended to the packets that we capture. In this case we’re
looking for any incoming traffic going to the standard SSH port.
Rules can also be bi-directional and look for traffic going in either direction:
log tcp any any <> any 23
This rule logs all tcp traffic where the destination port is 23 (telnet).
Introduction to Snort
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Messages can be added to alerts to make the alert logs a bit easier to read:
alert tcp any any -> 192.168.1.0/24 23 (msg:"Hey, someone's telnetting to my
network!";)
Resources
For more information on snort rules go to www.snort.org. They have a really cool rules database that you
can get predefined rules from. There are several sites that support snort rules including whitehats.com.
Enjoy!
http://www.snort.org
http://www.whitehats.com
Output Examples
Ping
-*> Snort! <*-
Version 1.5.1
By Martin Roesch (roesch@clark.net, www.clark.net/~roesch)
Decoding Ethernet on interface ed0
11/12-22:17:48.490312 ARP who-has 192.168.1.1 tell 192.168.1.2
11/12-22:17:48.490409 ARP reply 192.168.1.1 is-at 0:20:18:B8:FE:B
11/12-22:17:48.490746 0:5:2:62:E8:14 -> 0:20:18:B8:FE:B type:0x800 len:0x62
192.168.1.2 -> 192.168.1.1 ICMP TTL:64 TOS:0x0 ID:5087
ID:52519 Seq:0 ECHO
3A 0F 5E 2D 00 09 FA 2F 08 09 0A 0B 0C 0D 0E 0F :.^-.../........
10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F ................
20 21 22 23 24 25 26 27 28 29 2A 2B 2C 2D 2E 2F !"#$%&'()*+,-./
30 31 32 33 34 35 36 37 01234567
11/12-22:17:48.490830 0:20:18:B8:FE:B -> 0:5:2:62:E8:14 type:0x800 len:0x62
192.168.1.1 -> 192.168.1.2 ICMP TTL:255 TOS:0x0 ID:930
ID:52519 Seq:0 ECHO REPLY
3A 0F 5E 2D 00 09 FA 2F 08 09 0A 0B 0C 0D 0E 0F :.^-.../........
10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F ................
20 21 22 23 24 25 26 27 28 29 2A 2B 2C 2D 2E 2F !"#$%&'()*+,-./
30 31 32 33 34 35 36 37 01234567
Traceroute
-*> Snort! <*-
Version 1.5.1
By Martin Roesch (roesch@clark.net, www.clark.net/~roesch)
Decoding Ethernet on interface ed0
11/12-22:18:29.612633 ARP who-has 192.168.1.1 tell 192.168.1.2
11/12-22:18:29.612733 ARP reply 192.168.1.1 is-at 0:20:18:B8:FE:B
11/12-22:18:29.613033 0:5:2:62:E8:14 -> 0:20:18:B8:FE:B type:0x800 len:0x3C
192.168.1.2:42959 -> 192.168.1.1:33435 UDP TTL:1 TOS:0x0 ID:42960
Len: 20
Introduction to Snort
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01 01 00 00 3A 0F 5E 56 00 0B CE 8F 00 00 00 00 ....:.^V........
00 00 ..
11/12-22:18:29.613117 0:20:18:B8:FE:B -> 0:5:2:62:E8:14 type:0x800 len:0x46
192.168.1.1 -> 192.168.1.2 ICMP TTL:255 TOS:0x0 ID:933
DESTINATION UNREACHABLE: PORT UNREACHABLE
00 00 00 00 45 00 00 28 D0 A7 00 00 01 11 8E A1 ....E..(........
C0 A8 01 02 C0 A8 01 01 A7 CF 82 9B 00 14 00 00 ................
11/12-22:18:29.615732 0:5:2:62:E8:14 -> 0:20:18:B8:FE:B type:0x800 len:0x3C
192.168.1.2:42959 -> 192.168.1.1:33436 UDP TTL:1 TOS:0x0 ID:42961
Len: 20
02 01 00 00 3A 0F 5E 56 00 0B DB 35 00 00 00 00 ....:.^V...5....
00 00 ..
11/12-22:18:29.615788 0:20:18:B8:FE:B -> 0:5:2:62:E8:14 type:0x800 len:0x46
192.168.1.1 -> 192.168.1.2 ICMP TTL:255 TOS:0x0 ID:934
DESTINATION UNREACHABLE: PORT UNREACHABLE
00 00 00 00 45 00 00 28 D1 A7 00 00 01 11 8E A0 ....E..(........
C0 A8 01 02 C0 A8 01 01 A7 CF 82 9C 00 14 00 00 ................
11/12-22:18:29.616745 0:5:2:62:E8:14 -> 0:20:18:B8:FE:B type:0x800 len:0x3C
192.168.1.2:42959 -> 192.168.1.1:33437 UDP TTL:1 TOS:0x0 ID:42962
Len: 20
03 01 00 00 3A 0F 5E 56 00 0B DF 3C 00 00 00 00 ....:.^V...<....
00 00 ..
11/12-22:18:29.616798 0:20:18:B8:FE:B -> 0:5:2:62:E8:14 type:0x800 len:0x46
192.168.1.1 -> 192.168.1.2 ICMP TTL:255 TOS:0x0 ID:935
DESTINATION UNREACHABLE: PORT UNREACHABLE
00 00 00 00 45 00 00 28 D2 A7 00 00 01 11 8E 9F ....E..(........
C0 A8 01 02 C0 A8 01 01 A7 CF 82 9D 00 14 00 00 ................
Telnet
-*> Snort! <*-
Version 1.5.1
By Martin Roesch (roesch@clark.net, www.clark.net/~roesch)
Decoding Ethernet on interface ed0
11/12-22:20:09.234983 ARP who-has 192.168.1.1 tell 192.168.1.2
11/12-22:20:09.235079 ARP reply 192.168.1.1 is-at 0:20:18:B8:FE:B
11/12-22:20:09.235384 0:5:2:62:E8:14 -> 0:20:18:B8:FE:B type:0x800 len:0x4A
192.168.1.2:1176 -> 192.168.1.1:23 TCP TTL:64 TOS:0x0 ID:5235 DF
S***** Seq: 0x6FFFFDB7 Ack: 0x0 Win: 0x7D78
TCP Options => MSS: 1460 SackOK TS: 649836 0 NOP WS: 0
11/12-22:20:09.235525 0:20:18:B8:FE:B -> 0:5:2:62:E8:14 type:0x800 len:0x3A
192.168.1.1:23 -> 192.168.1.2:1176 TCP TTL:64 TOS:0x0 ID:1027 DF
S***A* Seq: 0x62E9DFFD Ack: 0x6FFFFDB8 Win: 0x4470
TCP Options => MSS: 1460
…
…
11/12-22:20:09.334644 0:20:18:B8:FE:B -> 0:5:2:62:E8:14 type:0x800 len:0x6A
Introduction to Snort
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192.168.1.1:23 -> 192.168.1.2:1176 TCP TTL:64 TOS:0x10 ID:1036 DF
***PA* Seq: 0x62E9E073 Ack: 0x6FFFFE47 Win: 0x4470
0D 00 0D 0A 46 72 65 65 42 53 44 2F 69 33 38 36 ....FreeBSD/i386
20 28 62 73 64 6D 61 63 68 69 6E 65 29 20 28 74 (bsdmachine) (t
74 79 70 30 29 0D 00 0D 0A 0D 00 0D 0A 6C 6F 67 typ0)........log
69 6E 3A 20 in:
11/12-22:20:09.354517 0:5:2:62:E8:14 -> 0:20:18:B8:FE:B type:0x800 len:0x3C
192.168.1.2:1176 -> 192.168.1.1:23 TCP TTL:64 TOS:0x0 ID:5246 DF
****A* Seq: 0x6FFFFE47 Ack: 0x62E9E0A7 Win: 0x7D78
00 00 00 00 00 00 ......
11/12-22:20:10.388357 0:5:2:62:E8:14 -> 0:20:18:B8:FE:B type:0x800 len:0x3C
192.168.1.2:1176 -> 192.168.1.1:23 TCP TTL:64 TOS:0x0 ID:5248 DF
***PA* Seq: 0x6FFFFE47 Ack: 0x62E9E0A7 Win: 0x7D78
6B 00 00 00 00 00 k.....
11/12-22:20:10.388619 0:20:18:B8:FE:B -> 0:5:2:62:E8:14 type:0x800 len:0x37
192.168.1.1:23 -> 192.168.1.2:1176 TCP TTL:64 TOS:0x10 ID:1037 DF
***PA* Seq: 0x62E9E0A7 Ack: 0x6FFFFE48 Win: 0x4470
6B k
11/12-22:20:10.404587 0:5:2:62:E8:14 -> 0:20:18:B8:FE:B type:0x800 len:0x3C
192.168.1.2:1176 -> 192.168.1.1:23 TCP TTL:64 TOS:0x0 ID:5249 DF
****A* Seq: 0x6FFFFE48 Ack: 0x62E9E0A8 Win: 0x7D78
00 00 00 00 00 00 ......
…
…
11/12-22:20:11.024771 0:20:18:B8:FE:B -> 0:5:2:62:E8:14 type:0x800 len:0x3F
192.168.1.1:23 -> 192.168.1.2:1176 TCP TTL:64 TOS:0x10 ID:1043 DF
***PA* Seq: 0x62E9E0AE Ack: 0x6FFFFE4E Win: 0x4470
50 61 73 73 77 6F 72 64 3A Password:
11/12-22:20:11.044621 0:5:2:62:E8:14 -> 0:20:18:B8:FE:B type:0x800 len:0x3C
192.168.1.2:1176 -> 192.168.1.1:23 TCP TTL:64 TOS:0x0 ID:5260 DF
****A* Seq: 0x6FFFFE4E Ack: 0x62E9E0B7 Win: 0x7D78
00 00 00 00 00 00 ......
11/12-22:20:11.697108 0:5:2:62:E8:14 -> 0:20:18:B8:FE:B type:0x800 len:0x3C
192.168.1.2:1176 -> 192.168.1.1:23 TCP TTL:64 TOS:0x0 ID:5261 DF
***PA* Seq: 0x6FFFFE4E Ack: 0x62E9E0B7 Win: 0x7D78
56 00 00 00 00 00 V.....