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The following instructions apply if you have a Linux or FreeBSD platform and
want libpcap to support the DAG range of passive network monitoring cards from
Endace (, see below for further contact details).
1) Install and build the DAG software distribution by following the
instructions supplied with that package. Current Endace customers can download
the DAG software distibution from
2) Configure libcap. To allow the 'configure' script to locate the DAG
software distribution use the '--with-dag' option:
./configure --with-dag=DIR
Where DIR is the root of the DAG software distribution, for example
/var/src/dag. If the DAG software is correctly detected 'configure' will
checking whether we have DAG API... yes
If 'configure' reports that there is no DAG API, the directory may have been
incorrectly specified or the DAG software was not built before configuring
See also the libpcap INSTALL.txt file for further libpcap configuration
Building libpcap at this stage will include support for both the native packet
capture stream (linux or bpf) and for capturing from DAG cards. To build
libpcap with only DAG support specify the capture type as 'dag' when
configuring libpcap:
./configure --with-dag=DIR --with-pcap=dag
Applications built with libpcap configured in this way will only detect DAG
cards and will not capture from the native OS packet stream.
Libpcap when built for DAG cards against dag-2.5.1 or later releases:
Timeouts are supported. pcap_dispatch() will return after to_ms milliseconds
regardless of how many packets are received. If to_ms is zero pcap_dispatch()
will block waiting for data indefinitely.
pcap_dispatch() will block on and process a minimum of 64kB of data (before
filtering) for efficiency. This can introduce high latencies on quiet
interfaces unless a timeout value is set. The timeout expiring will override
the 64kB minimum causing pcap_dispatch() to process any available data and
pcap_setnonblock is supported. When nonblock is set, pcap_dispatch() will
check once for available data, process any data available up to count, then
return immediately.
pcap_findalldevs() is supported, e.g. dag0, dag1...
Some DAG cards can provide more than one 'stream' of received data.
This can be data from different physical ports, or separated by filtering
or load balancing mechanisms. Receive streams have even numbers, e.g.
dag0:0, dag0:2 etc. Specifying transmit streams for capture is not supported.
pcap_setfilter() is supported, BPF programs run in userspace.
pcap_setdirection() is not supported. Only received traffic is captured.
DAG cards normally do not have IP or link layer addresses assigned as
they are used to passively monitor links.
pcap_breakloop() is supported.
pcap_datalink() and pcap_list_datalinks() are supported. The DAG card does
not attempt to set the correct datalink type automatically where more than
one type is possible.
pcap_stats() is supported. ps_drop is the number of packets dropped due to
RX stream buffer overflow, this count is before filters are applied (it will
include packets that would have been dropped by the filter). The RX stream
buffer size is user configurable outside libpcap, typically 16-512MB.
pcap_get_selectable_fd() is not supported, as DAG cards do not support
poll/select methods.
pcap_inject() and pcap_sendpacket() are not supported.
Some DAG cards now support capturing to multiple virtual interfaces, called
streams. Capture streams have even numbers. These are available via libpcap
as separate interfaces, e.g. dag0:0, dag0:2, dag0:4 etc. dag0:0 is the same
as dag0. These are visible via pcap_findalldevs().
libpcap now does NOT set the card's hardware snaplen (slen). This must now be
set using the appropriate DAG coniguration program, e.g. dagthree, dagfour,
dagsix, dagconfig. This is because the snaplen is currently shared between
all of the streams. In future this may change if per-stream slen is
DAG cards by default capture entire packets including the L2
CRC/FCS. If the card is not configured to discard the CRC/FCS, this
can confuse applications that use libpcap if they're not prepared for
packets to have an FCS.
Libpcap now reads the environment variable ERF_FCS_BITS to determine
how many bits of CRC/FCS to strip from the end of the captured
frame. This defaults to 32 for use with Ethernet. If the card is
configured to strip the CRC/FCS, then set ERF_FCS_BITS=0. If used with
a HDLC/PoS/PPP/Frame Relay link with 16 bit CRC/FCS, then set
If you wish to create a pcap file that DOES contain the Ethernet FCS,
specify the environment variable ERF_DONT_STRIP_FCS. This will cause
the existing FCS to be captured into the pcap file. Note some
applications may incorrectly report capture errors or oversize packets
when reading these files.
Please submit bug reports via <>.
Please also visit our Web site at:
For more information about Endace DAG cards contact <>.