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How to use Snort by Martin Roesch


Snort really isn't very hard to use, but there are a lot of command line options to play with, and it's not always obvious which ones go together well. This file aims to make using Snort easier for new users. Before we proceed, there are a few basic concepts you should understand about Snort. There are three main modes in which Snort can be configured: sniffer, packet logger, and network intrusion detection system. Sniffer mode simply reads the packets off of the network and displays them for you in a continuous stream on the console. Packet logger mode logs the packets to the disk. Network intrusion detection mode is the most complex and configurable configuration, allowing Snort to analyze network traffic for matches against a user defined rule set and perform several actions based upon what it sees.


First, let's start with the basics. If you just want to print out the TCP/IP packet headers to the screen (i.e. sniffer mode), try this:

./snort -v

This command will run Snort and just show the IP and TCP/UDP/ICMP headers, nothing else. If you want to see the application data in transit, try the following:

./snort -vd

This instructs Snort to display the packet data as well as the headers. If you want an even more descriptive display, showing the data link layer headers do this :

./snort -vde

As an aside, these switches may be divided up or smashed together in any combination. The last command could also be typed out as:

./snort -d -v -e

and it would do the same thing.


Ok, all of these commands are pretty cool, but if you want to record the packets to the disk, you need to specify a logging directory and Snort will automatically know to go into packet logger mode:

./snort -dev -l ./log

Of course, this assumes you have a directory named "log" in the current directory. If you don't, Snort will exit with an error message. When Snort runs in this mode, it collects every packet it sees and places it in a directory hierarchy based upon the IP address of one of the hosts in the datagram. If you just specify a plain "-l" switch, you may notice that Snort sometimes uses the address of the remote computer as the directory in which it places packets, and sometimes it uses the local host address. In order to log relative to the home network, you need to tell Snort which network is the home network:

./snort -dev -l ./log -h

This rule tells Snort that you want to print out the data link and TCP/IP headers as well as application data into the directory ./log, and you want to log the packets relative to the class C network. All incoming packets will be recorded into subdirectories of the log directory, with the directory names being based on the address of the remote (non-192.168.1) host. Note that if both hosts are on the home network, then they are recorded based upon the higher of the two's port numbers, or in the case of a tie, the source address. If you're on a high speed network or you want to log the packets into a more compact form for later analysis you should consider logging in "binary mode". Binary mode logs the packets in "tcpdump format" to a single binary file in the logging directory:

./snort -l ./log -b

Note the command line changes here. We don't need to specify a home network any longer because binary mode logs everything into a single file, which eliminates the need to tell it how to format the output directory structure. Additionally, you don't need to run in verbose mode or specify the -d or -e switches becuase in binary mode the entire packet is logged, not just sections of it. All that is really required to place Snort into logger mode is the specification of a logging directory at the command line with the -l switch, the -b binary logging switch merely provides a modifier to tell it to log the packets in something other than the default output format of plain ASCII text. Once the packets have been logged to the binary file, you can read the packets back out of the file with any sniffer that supports the tcpdump binary format such as tcpdump or Ethereal. Snort can also read the packets back by using the -r switch, which puts it into playback mode. Packets from any tcpdump formatted file can be processed through Snort in any of its run modes. For example, if you wanted to run a binary log file through Snort in sniffer mode to dump the packets to the screen, you can try something like this:

./snort -dv -r packet.log

You can manipulate the data in the file in a number of ways through Snort's packet logging and intrusion detection modes, as well as with the BPF interface that's available from the command line. For example, if you only wanted to see the ICMP packets from the log file, simply specify a BPF filter at the command line and Snort will only "see" the ICMP packets in the file:

./snort -dvr packet.log icmp

For more info on how to use the BPF interface, read the man page.


To enable network intrusion detection (NIDS) mode (so that you don't record every single packet sent down the wire), try this :

./snort -dev -l ./log -h -c snort.conf

Where snort.conf is the name of your rules file. This will apply the rules set in the snort.conf file to each packet to decide if an action based upon the rule type in the file should be taken. If you don't specify an output directory for the program, it will default to /var/log/snort. One thing to note about the last command line is that if Snort is going to be used in a long term way as an IDS, the "-v" switch should be left off the command line for the sake of speed. The screen is a slow place to write data to, and packets can be dropped while writing to the display. It's also not necessary to record the data link headers for most applications, so the so it's not necessary to specify the -e switch either.

./snort -d -h -l ./log -c snort.conf

This will configure Snort to run in it's most basic NIDS form, logging packets that the rules tell it to in plain ASCII to a hierarchical directory structure (just like packet logger mode).


There are a number of ways to configure the output of Snort in NIDS mode. The default logging and alerting mechanisms are to log in decoded ASCII format and use "full" alerts. The full alert mechanism prints out the alert message in addition to the full packet headers. There are several other alert output modes available at the command line, as well as two logging facilities. Packets can be logged to their default decoded ASCII format or to a binary log file via the -b command line switch. If you wish to disable packet logging all together, use the -N command line switch. Alert modes are somewhat more complex. There are six alert modes available at the command line, full, fast, socket, syslog, smb (winpopup), and none. Four of these modes are accessed with the -A command line switch. The four options are:

    -A fast -   fast alert mode, write the alert in a simple format with a
                timestamp, alert message, source and destination IPs/ports

    -A full -   this is also the default alert mode, so if you specify nothing
                this will automatically be used

    -A unsock - send alerts to a UNIX socket that another program can listen on

    -A none -   turn off alerting

To send alerts to syslog, use the -s switch. The default facilities for the syslog alerting mechanism are LOG_AUTHPRIV and LOG_ALERT. If you want to configure other facilities for syslog output, use the output plugin directives in the rules files (see the snort.conf file for more information). Finally, there is the SMB alerting mechanism. This allows Snort to make calls to the smbclient that comes with Samba and send WinPopup alert messages to Windows machines. To use this alerting mode, you must configure Snort to use it at configure time with the --enable-smbalerts switch. Here are some output configuration examples:

1) Log to default (decoded ASCII) facility and send alerts to syslog

./snort -c snort.conf -l ./log -s -h
2) Log to the default facility in /var/log/snort and send alerts to a fast alert file:
./snort -c snort.conf -s -h
3) Log to a binary file and send alerts to Windows workstation:
./snort -c snort.conf -b -M WORKSTATIONS


If you want Snort to go *fast* (like keep up with a 100 Mbps net fast) use the "-b" and "-A fast" or "-s" (syslog) options. This will log packets in tcpdump format and produce minimal alerts. For example :

./snort -b -A fast -c snort-lib

In this configuration, Snort has been able to log multiple simultaneuos probes and attacks on a 100 Mbps LAN running at a saturation level of approximately 80 Mbps. In this configuration the logs are written in binary format to the snort.log tcpdump-formatted file. To read this file back and break out the data in the familiar Snort format, just rerun Snort on the data file with the "-r" option and the other options you would normally use. For example:

./snort -d -c snort-lib -l ./log -h -r snort.log

Once this is done running, all of the data will be sitting in the log directory in its normal decoded format. Cool, eh?


Some people don't like the default way in which Snort applies it's rules to packets, with the Alert rules applied first, then the Pass rules, and finally the Log rules. This sequence is somewhat counterintuitive, but it's a more foolproof method than allowing the user to write a hundred alert rules and then disable them all with an errant pass rule. For people who know what they're doing, the "-o" switch has been provided to change the default rule applicaition behavior to Pass rules, then Alert, then Log:

./snort -d -h -l ./log -c snort.conf -o


If you are willing to run snort in "daemon" mode, you can add -D switch to any combination above. Please NOTICE that if you want to be able to restart snort by sending SIGHUP signal to the daemon, you will need to use full path to snort binary, when you start it, i.g.:

/usr/local/bin/snort -d -h -l /var/log/snortlogs -c /usr/local/etc/snort-lib -s -D

Relative pathes are not supported due to security concerns.
If you're going to be posting packet logs to public mailing lists you might want to try out the -O switch. This switch "obfuscates" your the IP addresses in the packet printouts. This is handy if you don't want the people on the mailing list to know the IP addresses involved. You can also combine the -O switch with the -h switch to only obfuscate the IP addresses of hosts on the home network. This is useful if you don't care who sees the address of the attacking host. For example :

./snort -d -v -r snort.log -O -h

This will read the packets from a log file and dump the packets to the screen, obfuscating only the addresses from the class C network.

Well, that's about it for now.
If you have any further questions about using Snort, drop me an e-mail at

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