Erlang packet capture interface using pcap
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c_src epcap: Capture direction support Jan 24, 2019
test epcap: Capture direction support Jan 24, 2019
.gitignore rebar3: update gitignore Sep 1, 2016
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An Erlang port interface to libpcap.

epcap includes a small example program called sniff.


  • 0.05: The packet message format has changed and is now documented.

  • 0.06: epcap doesn't register itself, so multiple instances can be run.

  • 0.7.0: removed epcap_net.erl (moved to


$ rebar3 compile # or: make

To compile the examples:

$ make examples

# Allow your user to epcap with root privs
sudo visudo
youruser ALL = NOPASSWD: /path/to/epcap/priv/epcap
# And if requiretty is enabled, disable it by using one of these
Defaults!/path/to/epcap/priv/epcap !requiretty
Defaults:youruser !requiretty

rebar3 shell

% Start the sniffer process

% Use your interface, or leave it out and trust in pcap
sniff:start([{interface, "eth0"}]).

% To change the filter
sniff:start([{filter, "icmp or (tcp and port 80)"},{interface, "eth0"}]).

% To stop sniffing


epcap:start() -> {ok, pid()}
epcap:start(Args) -> {ok, pid()}
epcap:start_link() -> {ok, pid()}
epcap:start_link(Args) -> {ok, pid()}

    Types   Args = [Options]
            Options = {chroot, string()} | {group, string()} | {interface, string()} | {promiscuous, boolean()} |
                        {user, string()} | {filter, string()} | {progname, string()} | {file, string()} |
                        {monitor, boolean()} | {cpu_affinity, string()} | {cluster_id, non_neg_integer()}} |
                        {inject, boolean()} | {snaplen, non_neg_integer} | {buffer, non_neg_integer()} |
                        {time_unit, microsecond | timestamp} | {direction, in | out | inout}

    Packets are delivered as messages:

        {packet, DataLinkType, Time, Length, Packet}

    The DataLinkType is an integer representing the link layer,
    e.g., ethernet, Linux cooked socket.

    The Time can be either in microseconds or a timestamp in the same
    format as erlang:now/0 depending on the value of the time_unit
    option (default: timestamp):

    {MegaSecs, Secs, MicroSecs}

    The Length corresponds to the actual packet length on the
    wire. The captured packet may have been truncated. To get the
    captured packet length, use byte_size(Packet).

    The Packet is a binary holding the captured data.

    If the version of the pcap library supports it, the pcap buffer
    size can be set to avoid dropped packets by using the 'buffer'
    option. The buffer size must be larger than the snapshot
    length (default: 65535) plus some overhead for the pcap data
    structures. Using some multiple of the snapshot length is

epcap:send(Ref, Packet) -> ok

    Types   Ref = pid()
            Packet = binary()

    Inject a packet on the network interface. To enable sending
    packets, start_link/1 must be called with the {inject, true} option
    (default: {inject, false}). When disabled, any data sent
    to the epcap port is silently discarded.

    Packet injection failures are treated as fatal errors, terminating
    the epcap port. Partial writes are not considered to be errors
    and are ignored (an error message will be printed to stderr if
    the verbose option is used).


    In case you want to compile epcap with PF_RING support,
    just specify the path to the libpfring and modified libpcap libraries
    via shell variable PFRING.

        PFRING=/home/user/pfring make

    To complete the configuration you need to set up the cluster_id option.
    The value of the cluster_id option is integer and should be in range between 0 and 255.

        epcap:start_link([{interface, "lo"}, {cluster_id, 2}]).

    You can also specify the option cpu_affinity to set up CPU affinity for epcap port:

        epcap:start_link([{interface, "lo"}, {cluster_id, 2}, {cpu_affinity, "1,3,5-7"}]).


=INFO REPORT==== 27-Oct-2013::11:47:43 ===
    pcap: [{time,"2013-10-27 11:47:43"},
    ether: [{source_macaddr,"F0:BD:4F:AA:BB:CC"},
    ipv6: [{protocol,tcp},
    tcp: [{source_port,80},
    payload_size: 567
    payload: "HTTP/1.0 301 Moved Permanently..Location: text/html; charset=UTF-8..Date: Sun, 27 Oct 2013 15:47:49 GMT..Expires: Tue, 26 Nov 2013 15:47:49 GMT..Cache-Control: public, max-age=2592000..Server: gws..Content-Length: 218..X-XSS-Protection: 1; mode=block..X-Frame-Options: SAMEORIGIN..Alternate-Protocol: 80:quic....<HTML><HEAD><meta http-equiv=\"content-type\" content=\"text/html;charset=utf-8\">.<TITLE>301 Moved</TITLE></HEAD><BODY>.<H1>301 Moved</H1>.The document has moved.<A HREF=\"\">here</A>...</BODY></HTML>.." 

And a screenshot of the number of packets epcap is processing on a production system:

IPTraf Screenshot


  • return error atoms/tuples instead of using errx

  • add support for retrieving packet statistics using pcap_stats(3PCAP)