From a56fb0bdc6d914fa3d2ffb6cbad301f5215dff7a Mon Sep 17 00:00:00 2001
From: Michael Santos <michael.santos@gmail.com>
Date: Fri, 17 Sep 2010 15:26:12 -0400
Subject: [PATCH] Try to figure out the default interface

Use a dumb algorithm (whether an interface has a self-assigned IP address)
to determine the default network device. Really should check the routing
table instead for the interface associated with the default route.
---
 README       | 23 +++++++++++++++++------
 src/perv.erl | 39 ++++++++++++++++++++++++++++++++-------
 2 files changed, 49 insertions(+), 13 deletions(-)

diff --git a/README b/README
index 2c1893f..65c1f1d 100644
--- a/README
+++ b/README
@@ -25,11 +25,26 @@ HOW TO USE IT
 perv:start() -> ok
 perv:start(Options) -> ok
 
-    Types   Options = list()
+    Types   Options = [EpcapArgs | Keyword]
+            EpcapArgs = list()
+            Keyword = [exclude]
 
     See epcap documentation for options.
 
-    Files are written to priv/files/<mime type>.
+    perv will try to find the default interface to snoop (and will
+    probably get it wrong if there is more than one network interface). To
+    exclude the local host from packet dumps, use:
+
+    > perv:start(exclude).
+
+    To override the defaults, manually specify the device using the
+    epcap options, e.g.:
+
+    > perv:start([
+            {interface, "eth0"},
+            {filter, "tcp and src port 80 and not host 10.11.11.11"}]).
+
+    Content is written to priv/files/<mime type>.
 
     A trace file containing the HTTP response is written to priv/tmp. The
     trace file can be unpacked by using pervon:content/3.
@@ -99,9 +114,5 @@ TODO
 * pervon:content/3 should return a list of tuples containing the name,
   file type and file contents, rather than writing them directly to disk
 
-* since the interface being snooped is known, grab the device's IP
-  address using inet:ifget/2. Provide an option for including/excluding
-  the user's address.
-
 * add option to enable/disable debug messages and tracing
 
diff --git a/src/perv.erl b/src/perv.erl
index 3ae4af5..f668b66 100644
--- a/src/perv.erl
+++ b/src/perv.erl
@@ -37,6 +37,8 @@
 -define(SERVER, ?MODULE).
 
 -export([start/0, start/1, stop/0]).
+-export([dev/0]).
+-export([getip/1]).
 -export([start_link/0, start_link/1]).
 -export([init/1, handle_call/3, handle_cast/2, handle_info/2,
         terminate/2, code_change/3]).
@@ -46,15 +48,14 @@
     }).
 
 -define(EPCAP_DEFAULT_ARG, [{filter, "tcp and src port 80"},
-                {interface, "wlan0"},
                 {promiscuous, true},
                 {chroot, "priv/tmp"}]).
 
 
 start() ->
     start_link().
-start(EpcapArg) ->
-    start_link(EpcapArg).
+start(Options) ->
+    start_link(Options).
 
 stop() ->
     gen_server:call(?SERVER, [stop]).
@@ -62,11 +63,20 @@ stop() ->
 start_link() ->
     start_link([]).
 
-start_link(EpcapArg) ->
-    gen_server:start_link({local, ?SERVER}, ?MODULE, [EpcapArg], []).
+start_link(Options) ->
+    gen_server:start_link({local, ?SERVER}, ?MODULE, [Options], []).
 
-init([EpcapArg]) ->
-    epcap:start(EpcapArg ++ ?EPCAP_DEFAULT_ARG),
+init([Options]) ->
+    {Dev, Address} = dev(),
+
+    EpcapArgs = case Options of
+        exclude ->
+            [{filter, "tcp and src port 80 and not host " ++ inet_parse:ntoa(Address)}];
+        Opt when is_list(Opt) ->
+            Options
+    end,
+
+    epcap:start(EpcapArgs ++ [{interface, Dev}] ++ ?EPCAP_DEFAULT_ARG),
     {ok, #state{c = dict:new()}}.
 
 
@@ -199,3 +209,18 @@ buf(Pid, SeqNo, Payload, PayloadSize) when PayloadSize < byte_size(Payload) ->
     pervon:buf(Pid, SeqNo, <<Payload:PayloadSize/bytes>>).
 
 
+% Try to autostupidly figure out the local host's network device.
+% Guaranteed to annoy.
+dev() ->
+    {ok, Devs} = inet:getiflist(),
+    Ifs = [ {N, getip(N)} || N <- Devs ],
+    hd(lists:filter(
+        fun ({_, {127,_,_,_}}) -> false;
+            ({_, {169,_,_,_}}) -> false;
+            ({_, {_,_,_,_}}) -> true
+        end, Ifs)).
+
+getip(Dev) ->
+    {ok, Addr} = inet:ifget(Dev, [addr]),
+    proplists:get_value(addr, Addr).
+