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README.dag.md

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Compiling and using libpcap with Endace DAG capture cards

How to build

The following instructions apply if you have a Linux host 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 a recent version of the DAG software distribution by following the instructions supplied with that package. Current Endace customers can download the DAG software distribution from here.

  2. Configure libpcap. The configure script detects a typical DAG software installation automatically. In case you need to use the DAG software distribution from a custom location, 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 report:

    configure: using Endace DAG API headers from /var/src/dag/include
configure: using Endace DAG API libraries from /var/src/dag/lib

    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 libpcap (if the DAG package requires building before use).

  3. Building libpcap at this stage will include support for both the usual Linux libpcap devices (network interfaces, USB, Bluetooth etc.) and for 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 be able to use DAG cards only.

CMake builds support DAG too, see the libpcap installation notes for further libpcap configuration options.

Supported libpcap features

pcap_set_timeout(3PCAP) is supported. pcap_dispatch(3PCAP) 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 64KiB 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 64KiB minimum causing pcap_dispatch() to process any available data and return.

pcap_setnonblock(3PCAP) is supported. When nonblock is set, pcap_dispatch() will check once for available data, process any data available up to cnt packets, then return immediately.

pcap_findalldevs(3PCAP) is supported. At the time of this writing all supported DAG cards implement capturing to multiple logical interfaces, called "streams". This can be data from different physical ports, or separated by filtering or load balancing mechanisms. Receive (capture) streams on a given card have even numbers (0, 2, 4 etc.) and are available via pcap_findalldevs() as separate capture devices (dag0:0, dag0:2, dag0:4 etc.). dag0:0 is the same as dag0. Specifying transmit streams for capture is not supported.

pcap_setfilter(3PCAP) is supported, BPF programs run in userspace.

pcap_setdirection(3PCAP) 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_set_promisc(3PCAP) has no effect because DAG cards always capture packets in promiscuous mode.

pcap_breakloop(3PCAP) is supported.

pcap_datalink(3PCAP) and pcap_list_datalinks(3PCAP) are supported. pcap_activate(3PCAP) attempts to set the correct datalink type automatically when the capture stream supports more than one type.

pcap_stats(3PCAP) 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 16–512MiB.

pcap_get_selectable_fd(3PCAP) is not supported, as DAG cards do not support poll()/select() methods.

pcap_inject(3PCAP) and pcap_sendpacket(3PCAP) are supported under certain conditions. Endace DAG transmit support in libpcap is an experimental feature, it has been tested for Ethernet only and is disabled by default. To enable it, add --enable-dag-tx to ./configure arguments or -DENABLE_DAG_TX=yes to cmake arguments.

The transmit support will work as expected if:

  • the packets are Ethernet frames w/o FCS (the usual in libpcap), and
  • the card's transmitting port is adding 32-bit FCS to outgoing Ethernet frames (usually the default configuration).

The DAG device may be configured to expect FCS in the packets supplied for sending and to strip it before adding a new FCS (usually the default configuration) or to expect the packets to have no FCS. pcap_activate() automatically detects this configuration and subsequently pcap_inject() composes the Ethernet frame to match what the card is expecting. If the card configuration changes after the call to pcap_activate(), things will break.

To transmit packets from an interface (a port on the DAG card) other than the default 0 (port A), set the ERF_TX_INTERFACE environment variable to the number of the required interface before calling pcap_activate(). For example, ERF_TX_INTERFACE=1 uses port B.

This feature has been tested on DAG 7.5G2 and DAG 9.2X2, other models may or may not work. It does not support vDAG devices.

Other considerations

libpcap neither checks nor sets DAG hardware snaplen (slen). It is trivial to tell how many bytes of a captured packet data to return based on the length of the data and the configured snapshot length. But it is not always possible to tell how much packet data to capture such that the amount of input to the packet filter program would be sufficient for correct filtering — how deep the program tries to access into a captured packet data can be a function of both the program and the data. Also on older hardware slen applies to the entire card rather than a particular Rx stream. If necessary, use dagconfig to configure a custom slen value before activating the capture.

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, which usually fits 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 ERF_FCS_BITS=16.

If you wish to create a pcap file that does contain the Ethernet FCS, specify the environment variable ERF_DONT_STRIP_FCS=1. 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.

Endace contact details

Please submit DAG software distribution and hardware bug reports via [email protected].

Please also visit our web site.

For more information about Endace DAG cards contact [email protected].