Fix for "Misultin websocket hixie76 implementation doesn't work behind haproxy" (https://github.com/ostinelli/misultin/issues/98). Misultin is EOL'd, so I thought I'd paste my patch here.

gistfile1.diff

diff --git a/src/misultin_websocket_draft-hixie-76.erl b/src/misultin_websocket_draft-hixie-76.erl
index 6bcd8e1..d691e97 100644
--- a/src/misultin_websocket_draft-hixie-76.erl
+++ b/src/misultin_websocket_draft-hixie-76.erl
@@ -70,9 +70,19 @@ check_websocket(Headers) ->
 % ----------------------------------------------------------------------------------------------------------
 -spec handshake(Req::#req{}, Headers::http_headers(), {Path::string(), Origin::string(), Host::string()}) -> iolist().
 handshake(#req{socket = Sock, socket_mode = SocketMode, ws_force_ssl = WsForceSsl}, Headers, {Path, Origin, Host}) ->
-  % build data
-	Key1 = misultin_utility:header_get_value('Sec-WebSocket-Key1', Headers),
-	Key2 = misultin_utility:header_get_value('Sec-WebSocket-Key2', Headers),
+	% prepare handhsake response
+	WsMode = case SocketMode of
+		ssl -> "wss";
+		http when WsForceSsl =:= true  -> "wss"; % behind stunnel or similar, client is using ssl
+		http when WsForceSsl =:= false -> "ws"
+	end,
+	Handshake = ["HTTP/1.1 101 WebSocket Protocol Handshake\r\n",
+		     "Upgrade: WebSocket\r\n",
+		     "Connection: Upgrade\r\n",
+		     "Sec-WebSocket-Origin: ", Origin, "\r\n",
+		     "Sec-WebSocket-Location: ", WsMode, "://", lists:concat([Host, Path]), "\r\n\r\n"],
+	ok = misultin_socket:send(Sock, Handshake, SocketMode),
+
 	% handshake needs body of the request, still need to read it [TODO: default recv timeout hard set, will be exported when WS protocol is final]
 	misultin_socket:setopts(Sock, [{packet, raw}, {active, false}], SocketMode),
 	Body = case misultin_socket:recv(Sock, 8, 30*1000, SocketMode) of
@@ -84,13 +94,12 @@ handshake(#req{socket = Sock, socket_mode = SocketMode, ws_force_ssl = WsForceSs
 			?LOG_ERROR("tcp error treating data: ~p", [_Other]),
 			<<>>
 	end,
-	?LOG_DEBUG("got content in body of websocket request: ~p", [Body]),	
-	% prepare handhsake response
-	WsMode = case SocketMode of
-		ssl -> "wss";
-		http when WsForceSsl =:= true  -> "wss"; % behind stunnel or similar, client is using ssl
-		http when WsForceSsl =:= false -> "ws"
-	end,
+	?LOG_DEBUG("got content in body of websocket request: ~p", [Body]),
+
+	% build data
+	Key1 = misultin_utility:header_get_value('Sec-WebSocket-Key1', Headers),
+	Key2 = misultin_utility:header_get_value('Sec-WebSocket-Key2', Headers),
+
 	% build challenge
 	Ikey1 = [D || D <- Key1, $0 =< D, D =< $9],
 	Ikey2 = [D || D <- Key2, $0 =< D, D =< $9],
@@ -100,14 +109,9 @@ handshake(#req{socket = Sock, socket_mode = SocketMode, ws_force_ssl = WsForceSs
 	Part2 = erlang:list_to_integer(Ikey2) div Blank2,
 	Ckey = <<Part1:4/big-unsigned-integer-unit:8, Part2:4/big-unsigned-integer-unit:8, Body/binary>>,
 	Challenge = erlang:md5(Ckey),
-	% format
-	["HTTP/1.1 101 WebSocket Protocol Handshake\r\n",
-		"Upgrade: WebSocket\r\n",
-		"Connection: Upgrade\r\n",
-		"Sec-WebSocket-Origin: ", Origin, "\r\n",
-		"Sec-WebSocket-Location: ", WsMode, "://", lists:concat([Host, Path]), "\r\n\r\n",
-		Challenge
-	].
+
+	% return challenge
+	[Challenge].
 
 % ----------------------------------------------------------------------------------------------------------
 % Function: -> websocket_close | {websocket_close, DataToSendBeforeClose::binary() | iolist()} | NewState