epcap_compile

%%% @copyright 2012-2020 Michael Santos <michael.santos@gmail.com>

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-module(epcap_compile).

-export([
    compile/1,
    compile/2
]).

-define(PCAP_NETMASK_UNKNOWN, 16#ffffffff).

-define(DLT_EN10MB, 1).

-on_load(on_load/0).

on_load() ->
    erlang:load_nif(progname(), []).

-spec pcap_compile(
    Filter :: iodata(),
    Optimize :: 0 | 1,
    Netmask :: non_neg_integer() | {byte(), byte(), byte(), byte()},
    Linktype :: integer(),
    Snaplen :: integer()
) -> {ok, [binary()]} | {error, string()}.
pcap_compile(_, _, _, _, _) ->
    erlang:nif_error(not_implemented).

% @doc Compile a PCAP filter to a BPF program
%
% Filter is a string in pcap-filter(7) format.
%
% compile/1 defaults to:
%
% * optimization enabled
%
% * an unspecified netmask (filters specifying the broadcast
%   will return an error)
%
% * datalinktype set to ethernet (DLT_EN10MB)
%
% * a packet length of 65535 bytes
%
% * a limit of 8192 bytes for filters. Filters larger than this limit
%   will return `{error, enomem}'. A limit less than 0 disables the
%   length check.
-spec compile(Filter :: iodata()) -> {ok, [binary()]} | {error, string()}.
compile(Filter) ->
    compile(Filter, []).

-type compile_options() :: [
    {optimize, true | false}
    | {netmask, non_neg_integer()}
    | {dlt, integer()}
    | {snaplen, integer()}
    | {limit, integer()}
].

% @doc Compile a PCAP filter to a BPF program
%
% Filter is a string in pcap-filter(7) format.
%
% See pcap_compile(3PCAP) for documentation about each option.
-spec compile(Filter :: iodata(), compile_options()) -> {ok, [binary()]} | {error, string()}.
compile(Filter, Options) when is_binary(Filter); is_list(Filter) ->
    Optimize = bool(proplists:get_value(optimize, Options, true)),
    Netmask = mask(proplists:get_value(netmask, Options, ?PCAP_NETMASK_UNKNOWN)),
    Linktype = proplists:get_value(dlt, Options, ?DLT_EN10MB),
    Snaplen = proplists:get_value(snaplen, Options, 16#ffff),
    Limit = proplists:get_value(limit, Options, 8192),

    case iolist_size(Filter) < Limit orelse Limit < 0 of
        true ->
            pcap_compile(Filter, Optimize, Netmask, Linktype, Snaplen);
        false ->
            {error, enomem}
    end.

bool(true) -> 1;
bool(false) -> 0.

mask(N) when is_integer(N) -> N;
mask({A, B, C, D}) -> (A bsl 24) bor (B bsl 16) bor (C bsl 8) bor D.

progname() ->
    case code:priv_dir(?MODULE) of
        {error, bad_name} ->
            filename:join([
                filename:dirname(code:which(?MODULE)),
                "..",
                "priv",
                ?MODULE
            ]);
        Dir ->
            filename:join([Dir, ?MODULE])
    end.