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PF: Logging

Table of Contents

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

Introduction

Logging Packets

In order to log packets passing through PF, the log keyword must be used. The log keyword causes all packets that match the rule to be logged. In the case where the rule is creating state, only the first packet seen (the one that causes the state to be created) will be logged.

The options that can be given to the log keyword are:

all

Causes all matching packets, not just the initial packet, to be logged. Useful for rules that create state.

to pflogN

Causes all matching packets to be logged to the specified pflog(4) interface. For example, when using spamlogd(8) all SMTP traffic can be logged to a dedicated pflog(4) interface by PF. The spamlogd(8) daemon can then be told to listen on that interface. This keeps the main PF logfile clean of SMTP traffic which otherwise would not need to be logged. Use ifconfig(8) to create pflog(4) interfaces. The default log interface pflog0 is created automatically.

user

Causes the UNIX user-id and group-id that owns the socket that the packet is sourced from/destined to (whichever socket is local) to be logged along with the standard log information.

Options are given in parenthesis after the log keyword; multiple options can be separated by a comma or space.

pass in log (all, to pflog1) on $ext_if inet proto tcp to $ext_if port 22 keep state

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 160 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, the 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.
• ruleset name - the ruleset/anchor that the packet was matched in.
• 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, memory, bad-timestamp, congestion, ip-option, proto-cksum, state-mismatch, state-insert, state-limit, src-limit, and synproxy.
• 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

Create the following script:

/etc/pflogrotate

	#!/bin/sh
	PFLOG=/var/log/pflog
	FILE=/var/log/pflog5min.$(date "+%Y%m%d%H%M")
	pkill -ALRM -u root -U root -t - -x pflogd
	if [ -r $PFLOG ] && [ $(stat -f %z $PFLOG) -gt 24 ]; then
	   mv $PFLOG $FILE
	   pkill -HUP -u root -U root -t - -x pflogd
	   tcpdump -n -e -s 160 -ttt -r $FILE | logger -t pf -p local0.info
	   rm $FILE
	fi

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

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, and give it the same permissions as the pflog file.

# touch /var/log/pflog.txt
# chmod 600 /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 "pkill -HUP -u root -U root -t - -x pflogd"
    /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 it can take up to about 5-6 minutes (the cron job interval) before the logged packets appear in the file.

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