使用Lua脚本为wireshark编写自定义通信协议解析器插件_网络通信/流媒体

在网络通信应用中，我们往往需要自定义应用层通信协议，例如基于UDP的Real-Time Transport Protocol以及基于TCP的RTP over HTTP。鉴于RTP协议的广泛性，wireshark(ethereal)内置了对RTP协议的支持，调试解析非常方便。RTP over HTTP作为一种扩展的RTP协议，尚未得到wireshark的支持。在《RTP Payload Format for Transport of MPEG-4 Elementary Streams over http》中，使用wireshark只能抓获到原始的TCP数据包，需要我们自己解析蕴含其中的RTP报文，剥离RTP over HTTP 16字节的报文头，然后萃取出MP4V_ES码流数据。

怎么样使wireshark支持自定义通信协议的解析呢？基于GPL v2的wireshark强大的插件系统支持用户编写自定义协议解析器插件。wireshark使用C语言编写而成，它支持动态链接库形式的插件扩展。除此之外，wireshark内置了Lua脚本引擎，可以使用Lua脚本语言编写dissector插件。打开C:/Program Files/Wireshark/init.lua，确保disable_lua = false，以开启wireshark的Lua Console。

Lua是一种功能强大的、快速的、轻量的、嵌入式的脚本语言，使用Lua编写wireshark dissector插件是非常方便的。本文例解使用Lua编写ROH（RTP over HTTP）协议解析插件。

以下为GET http://219.117.194.183:60151/rtpOverHttp?Url=nphMpeg4/nil-640×480，获取MP4V_ES的一个片段，在没有ROH解析支持时，Packet 294显示为TCP报文。

使用Lua脚本为wireshark编写自定义通信协议解析器插件

关于RTP over HTTP报文的解析，参考《RTP Payload Format for Transport of MPEG-4 Elementary Streams over http》。关于Lua语法，参考Wireshark User’s Guide的《Lua Support in Wireshark》一节。

以下为自定义RTP over HTTP协议解析器插件代码roh.lua：

–[[

Proto.new(name, desc)

name: displayed in the column of “Protocol” in the packet list

desc: displayed as the dissection tree root in the packet details

–]]

local PROTO_ROH = Proto(“ROH”, “Rtp Over Http”)

–[[

ProtoField:

to be used when adding items to the dissection tree

–]]

–[[

(1)Rtp Over Http Header

–]]

— rtp over http header flag(1 byte)

local f_roh_headerflag = ProtoField.uint8(“ROH.HeaderFlag”, “Header Flag”, base.HEX)

— rtp over http interleaved channel(1 byte)

local f_roh_interleave = ProtoField.uint8(“ROH.InterleavedChannel”, “Interleaved Channel”, base.DEC)

— rtp over http packet length(2 bytes)

local f_roh_packlen = ProtoField.uint16(“ROH.PacketLen”, “Packet Length”, base.DEC)

–[[

(2)RTP Over Http

–]]

— rtp header(1 byte = V:2+P:1+X:1+CC:4)

local f_rtp_header = ProtoField.uint8(“ROH.Header”, “Rtp Header”, base.HEX)

— rtp payloadtype(1 byte = M:1+PT:7)

local f_rtp_payloadtype = ProtoField.uint8(“ROH.PayloadType”, “Rtp Payload Type”, base.HEX)

— rtp sequence number(2 bytes)

local f_rtp_sequence = ProtoField.uint16(“ROH.Sequence”, “Rtp Sequence Number”, base.DEC)

— rtp timestamp(4 bytes)

local f_rtp_timestamp = ProtoField.uint32(“ROH.Timestamp”, “Rtp Timestamp”, base.DEC)

— rtp synchronization source identifier(4 bytes)

local f_rtp_ssrc = ProtoField.uint32(“ROH.SSRC”, “Rtp SSRC”, base.DEC)

— define the fields table of this dissector(as a protoField array)

PROTO_ROH.fields = {f_roh_headerflag, f_roh_interleave, f_roh_packlen, f_rtp_header, f_rtp_payloadtype, f_rtp_sequence, f_rtp_timestamp, f_rtp_ssrc}

–[[

Data Section

–]]

local data_dis = Dissector.get(“data”)

–[[

ROH Dissector Function

–]]

local function roh_dissector(buf, pkt, root)

— check buffer length

local buf_len = buf:len()

if buf_len < 16

then

return false

end

— check header flag

if buf(0,2):uint() ~= 0x2400

then

return false

end

–[[

packet list columns

–]]

pkt.cols.protocol = “ROH”

pkt.cols.info = “Rtp Over Http”

–[[

dissection tree in packet details

–]]

— tree root

local t = root:add(PROTO_ROH, buf(0,16))

— child items

— ROH Header

t:add(f_roh_headerflag, buf(0,1))

t:add(f_roh_interleave, buf(1,1))

t:add(f_roh_packlen, buf(2,2))

— ROH

— (1)header

t:add(f_rtp_header, buf(4,1))

— (2)payloadtype

t:add(f_rtp_payloadtype, buf(5,1))

— (3)sequence number

t:add(f_rtp_sequence, buf(6,2))

— (4)timestamp

t:add(f_rtp_timestamp, buf(8,4))

— (5)ssrc

t:add(f_rtp_ssrc, buf(12,4))

if buf_len > 16

then

local data_len = buf:len()-16;

local d = root:add(buf(16, data_len), “Data”)

d:append_text(“(“..data_len..” bytes)”)

d:add(buf(16, data_len), “Data: “)

d:add(buf(16,0), “[Length: “..data_len..”]”)

local start_code = buf(16,4):uint()

if start_code == 0x000001b0

then

d:add(buf(16,0), “[Stream Info: VOS]”)

elseif start_code == 0x000001b6

then

local frame_flag = buf(20,1):uint()

if frame_flag<2^6

then

d:add(buf(16,0), “[Stream Info: I-Frame]”)

elseif frame_flag<2^7

then

d:add(buf(16,0), “[Stream Info: P-Frame]”)

else

d:add(buf(16,0), “[Stream Info: B-Frame]”)

end

return true

end

–[[

Dissect Process

–]]

function PROTO_ROH.dissector(buf, pkt, root)

if roh_dissector(buf, pkt, root)

then

— valid ROH diagram

else

data_dis:call(buf, pkt, root)

end

–[[

Specify Protocol Port

–]]

local tcp_encap_table = DissectorTable.get(“tcp.port”)

tcp_encap_table:add(60151, PROTO_ROH)

end

通过菜单“ToolsàLuaàEvaluate”打开Lua调试窗口，将以上代码输入Evaluate窗口，然后点击“Evaluate”按钮，若无错误提示，且末尾提示“–[[ Evaluated –]]”，则说明代码无语法错误。这时，通过菜单“HelpàSupported Protocols”可以发现，wireshark已经添加了ROH协议调试支持，在Display Filter中输入“roh”，则可以看到具体解析结果。

将以上代码保存为C:/Program Files/Wireshark/roh.lua。在C:/Program Files/Wireshark/init.lua末尾添加代码行：dofile(“roh.lua”)。这样在以后启动wireshark时，自动加载roh.lua。

增加roh.Lua插件扩展后，Packet 294将解析为RTP over HTTP报文。最后的Data部分为剥离16字节报文头后的码流。

使用Lua脚本为wireshark编写自定义通信协议解析器插件

由于尚未掌握Lua的位域（bitfield）操作，因此上面对于RTP的解析不完整，第一个字节（V:2+P:1+X:1+CC:4）和第二字节（M:1+PT:7）的相关位域没有解析出来。

既然wireshark内置了对RTP协议的支持，我们在解析完RTP over HTTP的头4个字节后，可以将余下的报文交由RTP协议解析器解析。以下为调用内置RTP协议解析器代码roh_rtp.lua：

–[[

Proto.new(name, desc)

name: displayed in the column of “Protocol” in the packet list

desc: displayed as the dissection tree root in the packet details

–]]

local PROTO_ROH = Proto(“ROH”, “Rtp Over Http”)

local PROTO_PANASONIC_PTZ = Proto(“PANASONIC_PTZ”, “Panasonic PTZ Protocol”)

–[[

ProtoField:

to be used when adding items to the dissection tree

–]]

–[[

1.ROH ProtoField

–]]

–rtp over http header flag(1 byte)

local f_roh_headerflag = ProtoField.uint8(“ROH.HeaderFlag”, “Header Flag”, base.HEX)

–rtp over http interleaved channel(1 byte)

local f_roh_interleave = ProtoField.uint8(“ROH.InterleavedChannel”, “Interleaved Channel”, base.DEC)

–rtp over http packet length(2 bytes)

local f_roh_packlen = ProtoField.uint16(“ROH.PacketLen”, “Packet Length”, base.DEC)

–define the fields table of this dissector(as a protoField array)

PROTO_ROH.fields = {f_roh_headerflag, f_roh_interleave, f_roh_packlen}

–[[

2.PANASONIC_PTZ ProtoField

–]]

— panasonic ptz header flag(32 ascii)

local f_panasonic_ptz_flag = ProtoField.bytes(“PANASONIC_PTZ”, “Header Flag”)

— panasonic ptz command(6~17 ascii)

local f_panasonic_ptz_cmd = ProtoField.bytes(“PANASONIC_PTZ”, “Command”)

–define the fields table of this dissector(as a protoField array)

PROTO_PANASONIC_PTZ.fields = {f_panasonic_ptz_flag, f_panasonic_ptz_cmd}

–[[

Data Section

–]]

local data_dis = Dissector.get(“data”)

–[[

ROH Dissector Function

–]]

local function roh_dissector(buf, pkt, root)

— check buffer length

local buf_len = buf:len()

if buf_len < 16

then

return false

end

— check header flag

if buf(0,2):uint() ~= 0x2400

then

return false

end

–[[

packet list columns

–]]

pkt.cols.protocol = “ROH”

pkt.cols.info = “Rtp Over Http”

–[[

dissection tree in packet details

–]]

— tree root

local t = root:add(PROTO_ROH, buf(0,4))

— child items

t:add(f_roh_headerflag, buf(0,1))

t:add(f_roh_interleave, buf(1,1))

t:add(f_roh_packlen, buf(2,2))

return true

end

–[[

PANASONIC_PTZ Dissector Function Helper

–]]

local function get_cmd_len(buf)

local found=0

for i=0,17 do

if buf(i,1):uint() == 0x26

then

found = i

break

end

return found

end

–[[

PANASONIC_PTZ Dissector Function

–]]

local function panasonic_ptz_dissector(buf, pkt, root)

— check buffer length

local buf_len = buf:len()

if buf_len < 32

then

return false

end

— check header flag

if buf(0,32):string() ~= “GET /nphControlCamera?Direction=”

then

return false

end

— check direction

local sub_buf = buf(32, 18):tvb()

local cmd_len = get_cmd_len(sub_buf)

if cmd_len > 0

then

–[[

packet list columns

–]]

pkt.cols.protocol = “PANASONIC_PTZ”

pkt.cols.info = “Panasonic PTZ Protocol”

–[[

dissection tree in packet details

–]]

— tree root

local t = root:add(PROTO_PANASONIC_PTZ, buf(0,buf_len))

— child items

local flag = t:add(f_panasonic_ptz_flag, buf(0,32))

flag:add(buf(0,31), “[“..buf(0,31):string()..”]”)

local cmd = t:add(f_panasonic_ptz_cmd, buf(32,cmd_len))

cmd:add(buf(32,cmd_len), “[“..buf(32,cmd_len):string()..”]”)

else

return false

end

return true

end

–[[

Dissect Process

–]]

function PROTO_ROH.dissector(buf, pkt, root)

if roh_dissector(buf, pkt, root)

then

— skip over rtp over http header

local rtp_buf = buf(4,buf:len()-4):tvb()

— call internal rtp dissector

local rtp_dissector = Dissector.get(“rtp”)

rtp_dissector:call(rtp_buf, pkt, root)

elseif panasonic_ptz_dissector(buf, pkt, root)

then

— valid ptz datagram

else

data_dis:call(buf, pkt, root)

end

–[[

Specify Protocol Port

–]]

local tcp_encap_table = DissectorTable.get(“tcp.port”)

tcp_encap_table:add(60151, PROTO_ROH)

end

此外，wireshark内置了对MP4V_ES码流解析器，选择菜单“EditàPreferencesàProtocolsàMP4V_ES”，在“MP4V_ES dynamic payload type：”中填写“96”。

将以上代码保存为C:/Program Files/Wireshark/roh_rtp.lua。在C:/Program Files/Wireshark/init.lua末尾添加代码行：dofile(“roh_rtp.lua”)，在dofile(“roh.lua”)前添加“—”注释，以免端口冲突（在roh.lua中tcp.port=60151）。保存init.lua后，重启wireshark。

自定义应用层协议基于传输层（UDP/TCP），自定义的协议端口号不能与wireshark内置应用层协议冲突。若将ROH协议指定为TCP 80，则需要选择菜单“AnalyzeàEnabled Protocols”勾掉HTTP，这样TCP 80的报文将按照ROH协议解析。

在Display Filter中输入“roh”可查看RTP+MP4V_ES解析结果，如下图所示：

使用Lua脚本为wireshark编写自定义通信协议解析器插件