NPFD(8) System Manager's Manual NPFD(8)

npfd — packet filter logging and state synchronization daemon

npfd [-D] [-d delay] [-f filename] [-i interface] [-p pidfile] [-s snaplen] [expression]

npfd is a background daemon which writes to a file in pcap(3) format logged packets read from an npflog interface. The npflog interface is used by npf(7) to log packets as defined in npf.conf(5). The generated pcap(3) files can then be analysed using tools such as tcpdump(8).

npfd closes and then re-opens the log file when it receives SIGHUP, permitting newsyslog(8) to rotate logfiles automatically. SIGALRM causes npfd to flush the current logfile buffers to the disk, thus making the most recent logs available. The buffers are also flushed every delay seconds.

If the log file contains data after a restart or a SIGHUP, new logs are appended to the existing file. If the existing log file was created with a different snaplen, npfd temporarily uses the old snaplen to keep the log file consistent.

npfd tries to preserve the integrity of the log file against I/O errors. Furthermore, integrity of an existing log file is verified before appending. If there is an invalid log file or an I/O error, the log file is moved out of the way and a new one is created. If a new file cannot be created, logging is suspended until a SIGHUP or a SIGALRM is received.

If SIGINFO is received, then npfd logs capture statistics to syslogd(8).

The options are as follows:

Debugging mode. npfd does not disassociate from the controlling terminal.
delay
Time in seconds to delay between automatic flushes of the file. This may be specified with a value between 5 and 3600 seconds. If not specified, the default is 60 seconds.
filename
Log output filename. Default is /var/log/npflog0.pcap.
interface
Specifies the npflog interface to use. By default, npfd will use npflog0.
pidfile
Writes a file containing the process ID of the program. The file name has the form /var/run/npfd.pid. If the option is not given, pidfile defaults to npfd.
snaplen
Analyze at most the first snaplen bytes of data from each packet rather than the default of 116. The default of 116 is adequate for IP, ICMP, TCP, and UDP headers but may truncate protocol information for other protocols. Other file parsers may desire a higher snaplen.
expression
Selects which packets will be dumped, using the regular language of tcpdump(8).

/var/run/npfd.pid
Process ID of the currently running npfd.
/var/log/npflog0.pcap
Default log file.

Log specific tcp packets to a different log file with a large snaplen (useful with a log-all rule to dump complete sessions):

# npfd -s 1600 -f suspicious.log port 80 and host evilhost

Log from another npflog interface, excluding specific packets:

# npfd -i npflog3 -f network3.log "not (tcp and port 23)"

Display binary logs:

# tcpdump -n -e -ttt -r /var/log/npflog0.pcap

Display the logs in real time (this does not interfere with the operation of npfd):

# tcpdump -n -e -ttt -i npflog0

Tcpdump has been extended to be able to filter on the OpenBSD pfloghdr structure defined in sys/net/npf/if_npflog.h. Tcpdump can restrict the output to packets logged on a specified interface, a rule number, a reason, a direction, an IP family or an action.

ip
Address family equals IPv4.
ip6
Address family equals IPv6.
ifname kue0
Interface name equals "kue0".
on kue0
Interface name equals "kue0".
ruleset rules
Ruleset name equals "rules".
rulenum 10
Rule number equals 10.
reason match
Reason equals match.
action pass
Action equals pass. Also accepts "block".
inbound
The direction was inbound.
outbound
The direction was outbound.

Display the logs in real time of inbound packets that were blocked on the wi0 interface:

# tcpdump -n -e -ttt -i npflog0 inbound and action block and on wi0

Each npf(7) rule is marked with an id number, shown using:

# npfctl show
...
        block final all apply "log" # id="45"
...

This id is the rule id shown by tcpdump:

# tcpdump -enr /var/log/npflog0.pcap
...
11:26:02.288199 rule 45.rules.0/0(match): block in on sk0: \
1.2.3.4.46063 > 5.6.7.8.23231: Flags [S], seq 1, win 8192, \
options [mss 1440], length 0
...

pcap(3), npf.conf(5), npf(7), newsyslog(8), npfctl(8), tcpdump(8)

The npfd command appeared in NetBSD 8.0.

This manual page was written by Can Erkin Acar <canacar@openbsd.org>.

Offline analysis of captured data is advised to alleviate issues with malicious data intended to exploit bugs in the packet parsing code of tcpdump(8).

August 7, 2018 NetBSD 11.0