Section: System Administration (8)
Updated: 8 May 2002
scanlogd - detects and logs TCP port scans
detects port scans and writes one line per scan via the
mechanism. If a source address sends multiple
packets to different ports in a short time, the event will be
logged. The format of the messages is:
saddr[:sport] to daddr [and others,] ports port[, port...], ..., flags[, TOS TOS][, TTL TTL] @HH:MM:SS
The fields in square brackets are optional;
sport, TOS, and TTL
will only be displayed if they were constant during the scan.
field represents TCP control bits seen in packets
coming to the system from the address of the scan. It is a
combination of eight characters, with each corresponding to
one of the six defined and two reserved TCP control bits (see
RFC 793). Control bits that were always set are encoded with an
uppercase letter, and a lowercase letter is used if the bit was
always clear. A question mark is used to indicate bits that
changed from packet to packet.
In order to do its job,
needs a way to obtain raw IP packets that either come to the system
is running on, or travel across a network segment that is directly
connected to the system. Current versions of
can be built with support for one of several packet capture interfaces.
As of version 2.0,
is aware of the
interface on Linux,
The use of
alone is discouraged. If you're on a system other than Linux and/or
want to monitor the traffic of an entire network at once, you should
in order to handle fragmented IP packets.
At least 7 different privileged or 21 non-privileged ports, or a
weighted combination of those, have to be accessed with no longer
than 3 seconds between the accesses to be treated as a scan.
If more than 5 scans are detected within 20 seconds, that event
will be logged and logging will be stopped temporarily.
Logging is done with a facility of
and a priority level
should be started as root since it needs access to a packet capture
interface. By default, it switches to running as user
after the packet capture interface is initialized.
If the daemon couldn't start up successfully, it will exit with a
status of 1.
You're expected to create a dummy user for
to run as. Make sure you allocate unique UID and GID to the user.
In most cases,
should be started from a rc.d script on system startup.
In /etc/syslog.conf you may use something like:
As the name indicates,
only logs port scans.
It does not prevent them.
You will only receive summarized information in the system's log.
Obviously, the source address of port scans can be spoofed.
Don't take any action against the source of attacks
unless other evidence is available.
Sometimes IP addresses are shared between many people; this is the
case for ISP shell servers, dynamic dialup pools, and corporate
networks behind NAT (masquerading).
Due to the nature of port scans, both false positives (detecting a
scan when there isn't one) and false negatives (not detecting a scan
when there's one) are possible. In particular, false positives occur
when many small files are transferred rapidly with passive mode FTP.
Solar Designer <solar at openwall.com>
Steffen Dettmer <steffen at dett.de>
wrote the initial version of this manual page.
scanlogd home page: http://www.openwall.com/scanlogd/
Phrack Magazine, issue 53, article 13
- COMPILE-TIME DEFAULTS
- EXIT STATUS
- SECURITY NOTES
- SEE ALSO
This document was created by man2html, using the scanlogd manual page,
and required some editing.