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.
Alert modes are somewhat more complex. There are seven alert modes available at the command line: full, fast, socket, syslog, console, cmg, and none. Six of these modes are accessed with the -A command line switch. These options are:
Fast alert mode. Writes the alert in a simple format with a timestamp, alert message, source and destination IPs/ports.
Full alert mode. This is the default alert mode and will be used automatically if you do not specify a mode.
Sends alerts to a UNIX socket that another program can listen on.
Turns off alerting.
Sends ``fast-style'' alerts to the console (screen).
Generates ``cmg style'' alerts.
Packets can be logged to their default decoded ASCII format or to a binary log file via the -b command line switch. To disable packet logging altogether, use the -N command line switch.
For output modes available through the configuration file, see Section 2.6.
Command line logging options override any output options specified in the configuration file. This allows debugging of configuration issues quickly via the command line.
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 snort.conf. See Section 2.6.1 for more details on configuring syslog output.
For example, use the following command line to log to default (decoded ASCII) facility and send alerts to syslog:
./snort -c snort.conf -l ./log -h 192.168.1.0/24 -s
As another example, use the following command line to log to the default facility in /var/log/snort and send alerts to a fast alert file:
./snort -c snort.conf -A fast -h 192.168.1.0/24