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PF: Logging

Table of Contents

• Introduction
• Reading a Log File
• Filtering Log Output
• Packet Logging Through Syslog

Introduction

Reading a Log File

To view the log file:

# tcpdump -n -e -ttt -r /var/log/pflog

Note that using tcpdump(8) to watch the pflog file does not give a real-time display. A real-time display of logged packets is achieved by using the pflog0 interface:

# tcpdump -n -e -ttt -i pflog0

NOTE: When examining the logs, special care should be taken with tcpdump's verbose protocol decoding (activated via the -v command line option). Tcpdump's protocol decoders do not have a perfect security history. At least in theory, a delayed attack could be possible via the partial packet payloads recorded by the logging device. It is recommended practice to move the log files off of the firewall machine before examining them in this way.

Additional care should also be taken to secure access to the logs. By default, pflogd will record 96 bytes of the packet in the log file. Access to the logs could provide partial access to sensitive packet payloads (like telnet(1) or ftp(1) usernames and passwords).

Filtering Log Output

# tcpdump -n -e -ttt -r /var/log/pflog port 80

This can be further refined by limiting the display of packets to a certain host and port combination:

# tcpdump -n -e -ttt -r /var/log/pflog port 80 and host 192.168.1.3

The same idea can be applied when reading from the pflog0 interface:

# tcpdump -n -e -ttt -i pflog0 host 192.168.4.2

Note that this has no impact on which packets are logged to the pflogd log file; the above commands only display packets as they are being logged.

In addition to using the standard tcpdump(8) filter rules, OpenBSD's tcpdump filter language has been extended for reading pflogd output:

• ip - address family is IPv4.
• ip6 - address family is IPv6.
• on int - packet passed through the interface int.
• ifname int - same as on int.
• rulenum num - the filter rule that the packet matched was rule number num.
• action act - the action taken on the packet. Possible actions are pass and block.
• reason res - the reason that action was taken. Possible reasons are match, bad-offset, fragment, short, normalize, and memory.
• inbound - packet was inbound.
• outbound - packet was outbound.

Example:

# tcpdump -n -e -ttt -i pflog0 inbound and action block and on wi0

This display the log, in real-time, of inbound packets that were blocked on the wi0 interface.

Packet Logging Through Syslog

First we have to create a user, pflogger, with a /sbin/nologin shell. The easiest way to create this user is with adduser(8).

After creating the user pflogger, create the following two scripts:

/etc/pflogrotate

	FILE=/home/pflogger/pflog5min.$(date "+%Y%m%d%H%M")
	kill -ALRM $(cat /var/run/pflogd.pid)
	if [ $(ls -l /var/log/pflog | cut -d " " -f 8) -gt 24 ]; then
	   mv /var/log/pflog $FILE
	   chown pflogger $FILE
	   kill -HUP $(cat /var/run/pflogd.pid)
	fi

/home/pflogger/pfl2sysl

	for logfile in /home/pflogger/pflog5min* ; do
	   tcpdump -n -e -ttt -r $logfile | logger -t pf -p local0.info
	   rm $logfile
	done

Edit root's cron job:

# crontab -u root -e

Add the following two lines:

# rotate pf log file every 5 minutes
0-59/5 * * * * /bin/sh /etc/pflogrotate

Create a cron job for user pflogger:

# crontab -u pflogger -e

Add the following two lines:

# feed rotated pflog file(s) to syslog
0-59/5 * * * * /bin/sh /home/pflogger/pfl2sysl

Add the following line to /etc/syslog.conf:

local0.info     /var/log/pflog.txt

If you also want to log to a remote log server, add the line:

local0.info     @syslogger

Make sure host syslogger has been defined in the hosts(5) file.

Create the file /var/log/pflog.txt to allow syslog to log to that file.

# touch /var/log/pflog.txt

Make syslogd notice the changes by restarting it:

# kill -HUP $(cat /var/run/syslog.pid)

All logged packets are now sent to /var/log/pflog.txt. If the second line is added they are sent to the remote logging host syslogger as well.

The script /etc/pflogrotate now processes and then deletes /var/log/pflog so rotation of pflog by newsyslog(8) is no longer necessary and should be disabled. However, /var/log/pflog.txt replaces /var/log/pflog and rotation of it should be activated. Change /etc/newsyslog.conf as follows:

    #/var/log/pflog       600    3    250    *    ZB /var/run/pflogd.pid
    /var/log/pflog.txt    600    7    *      24

PF will now log in ASCII to /var/log/pflog.txt. If so configured in /etc/syslog.conf, it will also log to a remote server. The logging is not immediate but can take up to about 5-6 minutes (the cron job interval) before the logged packets appear in the file.

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