.\"	$OpenBSD: bpflogd.8,v 1.7 2025/05/16 05:47:30 kn Exp $
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.\" Copyright (c) 2001 Can Erkin Acar.  All rights reserved.
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.Dd $Mdocdate: May 16 2025 $
.Dt BPFLOGD 8
.Os
.Sh NAME
.Nm bpflogd
.Nd Berkeley Packet Filter logging daemon
.Sh SYNOPSIS
.Nm bpflogd
.Op Fl dPp
.Op Fl F Ar filterfile
.Op Fl s Ar snaplen
.Op Fl u Ar user
.Op Fl w Ar waitms
.Op Fl y Ar datalinktype
.Fl f Ar filename
.Fl i Ar interface
.Op Ar expression ...
.Sh DESCRIPTION
.Nm
is a daemon which captures packets using
.Xr bpf 4
and writes the packets to a logfile
in
.Xr tcpdump 8
binary format.
These logs can be reviewed later using the
.Fl r
option of
.Xr tcpdump 8 .
.Pp
When starting up,
.Nm
drops privileges before opening its log file.
After receiving a
.Dv SIGHUP
signal it will write any pending packets to the log file, close it,
and then open it again,
permitting
.Xr newsyslog 8
to rotate logfiles automatically.
If the log file contains data after being opened,
the PCAP header is checked before new logs are appended to the existing file.
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl d
Debugging mode.
.Nm
does not daemonise and logs to the terminal.
.It Fl f Ar filename
Log output filename.
The file must already exist, and be readable and writable by the
_pflogd user.
.It Fl F Ar filterfile
Specify a file containing a filter expression as per
.Xr pcap-filter 5 .
.It Fl i Ar interface
Specifies the interface to capture packets on using
.Xr bpf 4 .
This can be specified multiple times to capture packets from multiple
interfaces, but all the interfaces must support the same datalink type.
.It Fl P
Put the interfaces into promiscuous mode.
.It Fl p
Do not put the interfaces into promiscuous mode.
This is the default.
.It Fl s Ar snaplen
Capture at most the first
.Ar snaplen
bytes of data from each packet.
By default
.Nm
captures whole packets.
.It Fl u Ar user
Drop privileges to
.Ar user .
By default
.Nm
drops privileges to the _pflogd user.
.It Fl w Ar waitms
Specify the maximum amount of time in milliseconds between when a
packet is captured and when it will be written to the log file.
The default
.Ar waitms
value is 2000 milliseconds.
.It Fl y Ar datalinktype
Specify the datalink type when capturing packets.
If this is not specified then the default datalink type on the first
interface is used.
.It Ar expression
Specify a filter expression for matching packets as per
.Xr pcap-filter 5 .
.El
.Pp
A filter expression may only be specified by a file with
.Ar -F
or as arguments on the command line; specifying both is unsupported.
If a filter is not provided then all packets are captured.
.Sh SEE ALSO
.Xr pcap_open_live 3 ,
.Xr pcap-filter 5 ,
.Xr newsyslog 8 ,
.Xr tcpdump 8
.Sh HISTORY
The
.Nm
command appeared in
.Ox 7.8 .
.\" .Sh AUTHORS
.\" .Nm
.\" was written by
.\" .An David Gwynne Aq Mt dlg@uq.edu.au .