shotgun: add minimalistic UI

A slight improvement over the previous UI.

- now it's possible to select protocol and send 100% of traffic over it
  (udp, tcp, dot, doh)
- check dir exists to fail early (instead of after test)
- rename options related to binding to IP, these are likely to change in
  the future
- remove stats_interval, since it's better to always use 1s with updated
  dnsjit

Related #6

shotgun.lua

 #!/usr/bin/env dnsjit
+
+--- Check if a file or directory exists in this path
+local function exists(file)
+   local ok, err, code = os.rename(file, file)
+   if not ok then
+      if code == 13 then
+         -- Permission denied, but it exists
+         return true
+      end
+   end
+   return ok, err
+end
+
+--- Check if a directory exists in this path
+local function isdir(path)
+   -- "/" works on both Unix and Windows
+   return exists(path.."/")
+end
+
 local object = require("dnsjit.core.objects")
 local log = require("dnsjit.core.log")
 local getopt = require("dnsjit.lib.getopt").new({
 	{ "v", "verbose", 2, "Verbosity level (0-5)", "?" },
 	{ "T", "threads", 1, "Number of sender threads", "?" },
 	{ "p", "port", 53, "Target port", "?" },
+	{ "P", "protocol", "udp", "Protocol to use: udp, tcp, dot, doh", "?" },
+	{ "M", "http-method", "GET", "HTTP method for DoH: GET, POST", "?" },
 	{ "s", "server", "::1", "Target IPv6 address", "?" },
 	{ "t", "timeout", 2, "Timeout for requests", "?" },
 	{ "k", "hanshake_timeout", 5, "Timeout for session handshake", "?" },
-	{ "e", "idle_timeout", 10, "Idle timeout for TCP sessions", "?" },
-	{ "P", "priority", "NORMAL", "GnutTLS priority string", "?" },
-	{ "b", "bind", "", "Source IPv6 bind address pattern (example: 'fd00::%x')", "?" },
-	{ "i", "ips", 1, "Number of source IPs per thread (when -b is set)", "?" },
-	{ "d", "drift", 1.0, "Maximum realtime drift (seconds)", "?" },
-	{ "S", "stats_interval", 5, "Log statistics every N seconds", "?" },
-	{ "O", "outdir", ".", "Directory for output files (must exist)", "?" },
+	{ "e", "idle_timeout", 10, "Idle timeout for stateful transports", "?" },
+	{ nil, "tls-priority", "NORMAL", "GnutTLS priority string", "?" },
+	{ nil, "bind-pattern", "", "Source IPv6 bind address pattern (example: 'fd00::%x')", "?" },
+	{ nil, "bind-num", 1, "Number of source IPs per thread (when --bind-pattern is set)", "?" },
+	{ nil, "drift", 1, "Maximum realtime drift (seconds)", "?" },
+	{ "O", "outdir", os.getenv("PWD"), "Directory for output files (must exist)", "?" },
 })
 local pcap = unpack(getopt:parse())
 if getopt:val("help") then
@@ -49,28 +69,81 @@ local TARGET_PORT = getopt:val("p")
 local TIMEOUT = getopt:val("t")
 local HANDSHAKE_TIMEOUT = getopt:val("k")
 local IDLE_TIMEOUT = getopt:val("e")
-local TLS_PRIORITY = getopt:val("P")
-local BIND_IP_PATTERN = getopt:val("b")
-local NUM_BIND_IP = getopt:val("i")
-local REALTIME_DRIFT = getopt:val("d")
-local LOG_INTERVAL = getopt:val("S")
-local OUTDIR = getopt:val("O")
+local BIND_IP_PATTERN = getopt:val("bind-pattern")
+local NUM_BIND_IP = getopt:val("bind-num")
+local REALTIME_DRIFT = getopt:val("drift")
 local MAX_CLIENTS_DNSSIM = 200000
 local CHANNEL_SIZE = 16384  -- empirically tested value suitable for UDP dnssim
 local MAX_BATCH_SIZE = 32  -- libuv default
 
+local PROTOCOLS = {
+	udp = "udp",
+	tcp = "tcp",
+	tls = "tls",
+	dot = "tls",
+	https2 = "https2",
+	doh = "https2",
+}
+local proto = getopt:val("P")
+local PROTOCOL = PROTOCOLS[proto]
+if PROTOCOL == nil then
+	log.fatal("unknown protocol: "..proto)
+end
+local HTTP_METHOD = getopt:val("M")
+if HTTP_METHOD ~= "GET" and HTTP_METHOD ~= "POST" then
+	log.fatal("unsupported HTTP method: "..HTTP_METHOD)
+end
+local TLS_PRIORITY = getopt:val("tls-priority")
+if string.find(TLS_PRIORITY, '"') ~= nil or string.find(TLS_PRIORITY, "'") ~= nil then
+	log.fatal("tls priority string may not contain quotes");
+end
+local OUTDIR = getopt:val("O")
+if not isdir(OUTDIR) then
+	log.fatal("output directory \"" .. OUTDIR .. "\" doesn't exist")
+end
+
 
-local function thread_output(thr)
+local function send_thread_main(thr)
 	local channel = thr:pop()
-	local output = require("dnsjit.output.dnssim").new(thr:pop())
 	local running
 
+	-- output must be global (per thread) to be accesible in loadstring()
+	-- luacheck: globals output, ignore log
+	output = require("dnsjit.output.dnssim").new(thr:pop())
+	local log = require("dnsjit.core.log")
+
 	output:target(thr:pop(), thr:pop())
 	output:timeout(thr:pop())
 	output:handshake_timeout(thr:pop())
 	output:idle_timeout(thr:pop())
-	output:tls(thr:pop())
-	output:stats_collect(thr:pop())
+
+	local protocol = thr:pop()
+	local tls_priority = thr:pop()
+	local http_method = thr:pop()
+	local cmd = "output:" .. protocol
+
+	if protocol == "udp" then
+		if type(output.udp_only) == "function" then
+			-- backward compat with dnsjit 1.0.0
+			-- https://github.com/DNS-OARC/dnsjit/pull/173
+			cmd = "output:udp_only"
+		end
+		cmd = cmd .. "()"
+	elseif protocol == "tcp" then
+		cmd = cmd .. "()"
+	elseif protocol == "tls" then
+		cmd = cmd .. "('" .. tls_priority .. "')"
+	elseif protocol == "https2" then
+		if type(output.https2) ~= "function" then
+			log.fatal("https2 isn't supported with this version of dnsjit")
+		end
+		cmd = cmd .. "({ method = '" .. http_method .. "' }, '" .. tls_priority .. "')"
+	else
+		log.fatal("unknown protocol: " .. protocol)
+	end
+	assert(loadstring(cmd))()
+
+	output:stats_collect(1)
 	output:free_after_use(true)
 
 	local outfile = thr:pop()
@@ -138,7 +211,7 @@ for i = 1, SEND_THREADS do
 	ipsplit:receiver(channels[i])
 
 	threads[i] = thread.new()
-	threads[i]:start(thread_output)
+	threads[i]:start(send_thread_main)
 	threads[i]:push(channels[i])
 	threads[i]:push(MAX_CLIENTS_DNSSIM)
 	threads[i]:push(TARGET_IP)
@@ -146,8 +219,9 @@ for i = 1, SEND_THREADS do
 	threads[i]:push(TIMEOUT)
 	threads[i]:push(HANDSHAKE_TIMEOUT)
 	threads[i]:push(IDLE_TIMEOUT)
+	threads[i]:push(PROTOCOL)
 	threads[i]:push(TLS_PRIORITY)
-	threads[i]:push(LOG_INTERVAL)
+	threads[i]:push(HTTP_METHOD)
 	threads[i]:push(string.format(outname, i))
 	threads[i]:push(MAX_BATCH_SIZE)
 	if BIND_IP_PATTERN ~= "" then