Lab Exercise – Protocol Layers
Objective
To learn how protocols and layering are represented in packets. They are key concepts for structuring networks that are covered in the text.
The trace for this lab is here:
https://www.doczj.com/doc/3715077520.html,/~kevin/com320/labs/wireshark/trace-protocol-layers.pcap
(although the main trace you will look at is from a site you pick such as https://www.doczj.com/doc/3715077520.html, in the exam-ples which follow).
Requirements
Wireshark: This lab uses the Wireshark software tool to capture andexamine a packet trace. A packet trace is a record of traffic at a location on the network, as if a snapshot was taken of all the bits that passed across a particular wire. The packet trace records a timestamp for each packet, along with the bits that make up the packet, from the lower-layer headers to the higher-layer contents.Wireshark runs on most operating systems, including Windows, Mac and Linux. It provides a graphical UI that shows the sequence of packets and the meaning of the bits when interpreted as protocol headers and data. It col-or-codes packets by their type, and has various ways to filter and analyze packets to let you investigate the behavior of network protocols. Wireshark is widely used to troubleshoot networks. You can down-load it https://www.doczj.com/doc/3715077520.html, for your personal computer. It is an ideal packet analyzer for our labs –it is stable, has a large user base and well-documented support that includes a user-guide https://www.doczj.com/doc/3715077520.html,/docs/wsug_html_chunked), and a detailed FAQ, rich functionality that in-cludes the capability to analyze hundreds of protocols, and a well-designed user interface. It operates in computers using Ethernet, serial (PPP and SLIP), 802.11 wireless LANs, and many other link-layer tech-nologies (if the OS on which it is running allows Wireshark to do so). It is already installed in the labs.
A quick help guide to Wireshark display filters is here: https://www.doczj.com/doc/3715077520.html,/wireshark_filters.php Wireshark is a core tool for any wireless ‘man in the middle’ or similar snooping attack. It is simply i n-dispensable for those who wish to examine packets being transferred over a network –good or bad…..
Wireshark & Packet Sniffing Background
The basic tool for observing the messages exchanged between executing protocol entities is called a packet sniffer. As the name suggests, a packet sniffer captures (“sniffs”)messages being sent/received from/by your computer; it will also typically store and/ordisplay the contents of the various protocol fields in these captured messages. A packetsniffer itself is passive. It observes messages being sent and received by applications andprotocols running on your computer, but never sends packets itself. Similar-ly, receivedpackets are never explicitly addressed to the packet sniffer. Instead, a packet snifferreceives a copy of packets that are sent/received from/by application and protocolsexecuting on your machine. Figure 1shows the structure of a packet sniffer. At the right of Figure 1are the protocols(in this case, In-ternet protocols) and applications (such as a web browser or ftp client)that normally run on your com-puter. The packet sniffer, shown within the dashedrectangle in Figure 1is an addition to the usual soft-ware in your computer, and consistsof two parts. The packet capture library receives a copy of every link-layer frame thatis sent from or received by your computer. Messages exchanged by higher layer protocols such as HTTP,FTP, TCP, UDP, DNS, or IP all are eventually encapsulated in link-layer frames that aretransmitted over physical media such as an Ethernet cable.
Figure 1: Packet Sniffer Structure
In Figure 1, the assumedphysical media is an Ethernet, and so all upper-layer protocols are eventual-lyencapsulated within an Ethernet frame. Capturing all link-layer frames thus gives you allmessages sent/received from/by all protocols and applications executing in yourcomputer.
The second component of a packet sniffer is the packet analyzer, which displays thecontents of all fields within a protocol message. In order to do so, the packet analyzermust “understand” the struc ture of all messages exchanged by protocols. For example,suppose we are interested in displaying the various fields in messages exchanged by theHTTP protocol in Figure 1. The packet analyzer understands the format of Ethernetframes, and so can identify the IP datagram within an Ethernet frame. It also unders-tandsthe IP datagram format, so that it can extract the TCP segment within the IP datagram.Finally, it understands the TCP segment structure, so it can extract the HTTP messagecontained in the TCP seg-ment. Finally, it understands the HTTP protocol and so, forexample, knows that the first bytes of an HTTP message will contain the string “GET,”“POST,” or “HEAD,”.
Step 1: Capture a Trace
https://www.doczj.com/doc/3715077520.html,unching Wireshark
You can type Wireshark in the run box of main Windows 8 start screen. Press the Windows key on the keyboard and type “wireshark”. If a problem launching it then see here1.
Figure 2: Wireshark in lab
2.Just close the dialog box which prompts you to install a new version. You will then see a startup
screen, as shown next.
Figure 3: Initial Wireshark Screen
3.Take a look at the left hand side of the screen –you’ll see an “Interface list”. This is the list of
network interfaces on your computer. Choose Ethernet.
1It should load but there can be a problem with the new lab configuration for Wireshark and npf driver. Therefore if this is not working…. then please do the next https://www.doczj.com/doc/3715077520.html,unch Wireshark as follows. Click desktop icon on main windows screen & use the file explorer to browse to C:\local Disk (C)\Program Files\Wireshark. Finally, RIGHT CLICK on Wireshark as “Run as administrator”.
4.Next, click Start (as shown below).
5.Wireshark will capture all packets on that interface. Click on the network card on the particular
machine you are working on. In the example above, it is the Ethernet Driver to start packet cap-ture (i.e., for Wireshark to begin capturing all packets being sent to/from that interface), a
screen like the one below will be displayed, showing information about the packets being cap-tured. Once you start packet capture, you can stop it by using the Capture pull down menu and selecting Stop.
We want this trace to look at the protocol structure of packets. A simple Web fetch of a URL from a server of your choice to your computer, which is the client, could also serve as traffic.
6.Open your browser, e.g. Chrome and pick a URL & fetch it e.g.https://www.doczj.com/doc/3715077520.html,”.
7.Close unnecessary browser tabs and windows. By minimizing browser activity you will stop your
computer from fetching unnecessary web content, and avoid incidental traffic in the trace.
8.Now return to Wireshark and you will see a screen similar to below.
That is all for now. Next you will read a little more about Wireshark and tracing packets.
Main Wireshark Interface
The Wireshark interface has five major components:
Figure 4: Wireshark Graphical User Interface, during packet capture and analysis
? The command menus are standard pull down menus located at the top of the window. The File menu allows you to save captured packet data or open a file containing previously captured packet data. The Capture menuallows you to begin packet capture.
? The packet-listing window displays a one-line summary for each packet captured, including the packet number (assigned by Wireshark; this is not a packet number contained in any protocol’s header), the time at which the packet was captured, the packet’s source and destination addresses, the protocol type, and protocol-specific information contained in the packet. The packet listing canbe sorted according to any of these categories by clicking on a column name. The protocol type field lists the highest-level pro-tocol that sent or received this packet, i.e., the protocol that is source or ultimate sink for this packet. ? The packet-header details window provides details about the packet selected (highlighted) in the packet-listing window. (To select a packet in the packet-listing window, place the cursor over the pack-et’s one-line summary in the packet-listing window and click with the left mouse button.). These details include information about the Ethernet frame (assuming the packet was sent/received over an Ethernet interface) and IP datagram that contains this packet. The amount of Ethernet and IP-layer detail dis-played can be expanded or minimized by clicking on the plus minus boxes to the left of the Ethernet frame or IP datagram line in the packet details window.
? The packet-contents window displays the entire contents of the captured frame, in both ASCII and hexadecimal format. Towards the top of the Wireshark graphical user interface, is the packet display fil-ter field, into which a protocol name or other information can be entered in order to filter the informa-tion displayed in the packet-listing window.
Filtering Traffic
Now return to your Wireshark trace captured earlier. We will look to see how we can isolate the Web Page traffic from the other protocol packets.
9.You have two options. You can simply type “tcp.port==80” in the main filter box on the main
screen like below.
10.Instead of the method above, you can also filter by typing “port 80” in the filter box like b e-
low.This filter will record only standard web traffic and not other kinds of packets that your
computer may send. The checking will translate the addresses of the computers sending and re-ceiving packets into names, which should help you to recognize whether the packets are going to or from your computer. Your capture window will be similar to the one pictured in next
screenshot.
Figure 5: Setting up the capture options
11.When the capture is started, visit a site which uses http (not https). This time, the packets will be
recorded by Wireshark as the content is transferred. Try again, https://www.doczj.com/doc/3715077520.html, in your browser.
12.After the fetch is successful, return to Wireshark and use the menus or buttons to stop the
trace.If you have succeeded, the upper Wireshark window will show multiple packets, and most likely it will be full. How many packets are captured will depend on the size of the web page, but there should be at least 8 packets in the trace, and typically 20-100, and many of these packets will be colored green. I recommend you stop recording by pressing the Red Stop button on the interface as this will make it easier for you to scroll back to the start of the request to find the “200 OK” packet. An example is shown below. Congratulations, you have captured a trace!
Figure 6: Packet trace of web browsing traffic
Step 2: Inspect the Trace
Select a packet for which the Protocol column is “HTTP” and the Info column says it is a GET.It is the packet that carries the web (HTTP) request sent from your computer to the server.(You can click the column headings to sort by that value, though it should not be difficult to find an HTTP packet by inspec-tion.) Let’s have a closer look to s ee how the packet structure reflects the protocols that are in use. Since we are fetching a web page, we know that the protocol layers being used are as shown below. That is, HTTP is the application layer web protocol used to fetch URLs. Like many Internet applications, it runs on top of the TCP/IP transport and network layer protocols. The link and physical layer protocols depend on your network, but are typically combined in the form of Ethernet (shown) if your computer is wired, or 802.11 (not shown) if your computer is wireless.
Figure 7: Protocol stack for a web fetch
With the HTTP GET packet selected, look closely to see the similarities and differences between it and our protocol stack as described next.The protocol blocks are listed in the middle panel. You can expand each block (by clicking on the “+” expander or icon) to see its details.
? The first Wireshark block is “Frame”. This is not a protocol, it is a record that describes overall i n-formation about the packet, including when it was captured and how many bits long it is.
? The second block is “Ethernet”. Note that you may have taken a trace on a computer using 802.11 yet still see an Ethernet block instead of an 802.11 block. Why? It happens because we asked Wireshark to capture traffic in Ethernet format on the capture options, so it converted the real 802.11 header into a pseudo-Ethernet header.
? Then come IP, TCP, and HTTP, which are just as we wanted. Note that the order is from the bot-tom of the protocol stack upwards. This is because as packets are passed down the stack, the header information of the lower layer protocol is added to the front of the information from the higher layer protocol, as in Fig. 1-15 of your text. That is, the lower layer protocols come first in the packet “on the wire”.
HTTP
TCP
IP
Ethernet Client
Server HTTP TCP IP Ethernet
packet
Step 3: Inspect the Trace Again
Now find another HTTP packet, the response from the server to your computer, and look at the structure of this packet for the differences compared to the HTTP GET packet.This packet should have “200 OK” in the Info field, denoting a successful fetch. In our trace, there are two extra blocks in the detail panel as seen in the next figure.
?The first extra block says something like “*... reassembled TCP segments …+”. Details in your cap-ture will vary, but this block is describing more than the packet itself. Most likely, the web re-
sponse was sent across the network as a series of packets that were put together after they ar-rived at the computer. The packet labeled HTTP is the last packet in the web response, and the block lists packets that are joined together to obtain the complete web response. Each of these packets is shown as having protocol TCP even though the packets carry part of an HTTP re-
sponse. Only the final packet is shown as having protocol HTTP when the complete HTTP mes-
sage may be understood, and it lists the packets that are joined together to make the HTTP re-
sponse.
?The second extra block says “Line-based text data …”. Details in your capture will vary, but this block is describing the contents of the web page that was fetched. In our case it is of type
text/html, though it could easily have been text/xml, image/jpeg, or many other types. As with the Frame record, this is not a true protocol. Instead, it is a description of packet contents that Wireshark is producing to help us understand the network traffic.
Figure 8: Inspecting a HTTP “200 OK” response
1.Locate the HTTP GET packet which has “200 OK (text/html)” in the Info field, denoting a
successful fetch. See below for instance in the third line in white.
2.Scroll down in the middle pane to [+] Line-based text data: text/html and right
click with your mouse. (See above)
3.From the context menu which now appears, select Copy -→ Bytes → Printable Text Only You
should see a window similar to the following popup window below.
4.This copies all the code for the Ulster home page.
5.Open up a text editor such as notepad++ and paste this HTML code into a new document. Save
as UlsterHomePage.html in a directory such as W: or downloads. Next open up explorer and scroll to where you have saved the file and open it in the default browser by clicking on it.
6.Note how much of the Ulster home page actually gets sent over the wire to your PC.
Step 4: Packet Structure
Examine a HTTP GET packet that shows the position and size in bytes of the TCP, IP and Ethernet proto-col headers. Note the range of the Ethernet header and the Ethernet payload that IP passes to Ethernet to send over the network.Note also the range of the IP header and the IP payload.
To work out sizes, observe that when you click on a protocol block in the middle panel (the block itself, not the “+” expander) then Wireshark will highlight the bytes it corresponds to in the packet in the lower panel and display the length at the bottom of the window. For instance, clicking on the IP version 4 header of a packet in our trace shows us that the length is 20 bytes. (Your trace will be different if it is IPv6, and may be different even with IPv4 depend-ing on various options.) You may also use the overall packet size shown in the Length column or Frame detail block. Answer
Figure 9: Protocol layer structure of the HTTP GET packet
There are several features to note:
? The order of the headers (Ethernet, IP, TCP, HTTP) is the protocol stack from the bottom up-wards because the lower layers are outermost in the packet as it travels through the network. ? Observant students will note some differences between the Ethernet header size in Wireshark and in the text that will be explored in later labs.
? The size of the IP and TCP headers is normally around 20 bytes each, but it may be larger in some cases depending on the OS, e.g., IPv6 instead of IPv4 and optional TCP header fields might double these numbers.
? The size of the HTTP message will vary depending on what tool and URL is used to send the web request. For wget/curl, it is likely to be around 100-300 bytes.
? The Ethernet payload comprises everything beyond the Ethernet header. That is, Ethernet does not understand the IP / TCP / HTTP internal structure; it is up to higher layers to determine their headers and message boundaries.
? Similarly, the IP payload comprises everything beyond the IP header. Note that neither the IP header nor payload covers the Ethernet header. HTTP Ethernet
IP TCP 14 bytes 20 bytes 20 bytes 112 bytes Ethernet header Ethernet payload 166 bytes total
Start of packet
IP header
IP payload
Step 5: Demultiplexing Keys
When an Ethernet frame arrives at a computer, the Ethernet layer must hand the packet that it contains to the next higher layer to be processed. The act of finding the right higher layer to process received packets is called demultiplexing. We know that in our case the higher layer is IP. But how does the Ethernet protocol know this? After all, the higher-layer could have been another protocol entirely (such as ARP).We have the same issue at the IP layer – IP must be able to determine that the contents of IP message is a TCP packet so that it can hand it to the TCP protocol to process. The answer is that proto-cols use information in their header known as a “demultiplexing key” to determine the higher layer. Look at the Ethernet and IP headers of a download packet in detail to answer the following questions:
1.Which Ethernet header field is the demultiplexing key that tells it the next higher layer is IP?
What value is used in this field to indicate “IP”?
Answer. The demultiplexing key for Ethernet is the Type field. It holds 0x800 when the higher
layer is IP.
2.Which IP header field is the demultiplexing key that tells it the next higher layer is TCP? What
value is used in this field to indicate “TCP”?
Answer: The demultiplexing key for IP is the Protocol field. It has value 6 when the higher layer is TCP.
第二章网络通讯选择关于网络协议的正确陈述。（选择三项）正确答案应该是1 3 5 定义特定层PDU 的结构 规定实现协议层功能的方式 概述层与层之间通信所需的功能 限制了硬件兼容性 需要取决于协议层的封装过程 杜绝厂商之间的标准化 2下列哪些陈述正确指出了中间设备在网络中的作用？（选择三项） 确定数据传输路径 发起数据通信 重新定时和重新传输数据信号 发送数据流 管理数据流 数据流最后的终止点 3在封装过程中，数据链路层执行什么操作？ 不添加地址。 添加逻辑地址。 添加物理地址。 添加进程的端口号。 4以下哪种特征正确代表了网络中的终端设备？
管理数据流 发送数据流 重新定时和重新传输数据信号 确定数据传输路径 5什么是PDU？ 传输期间的帧损坏 在目的设备上重组的数据 因通信丢失而重新传输的数据包 特定层的封装 6 请参见图示。所示网络属于哪一类型？ WAN MAN LAN WLAN
7通过检查网络层报头可以确定什么？ 本地介质上的目的设备 用于到达目的主机的路径 将要通过介质传输的比特 创建数据的源应用程序或进程 8哪一层负责将数据段封装成数据包？ 物理层 数据链路层 网络层 传输层 9封装的关键功能是什么？（选择三项） 供传输前修改原始数据 标识数据片段属于同一通信 为通信提供统一的网络路径 确保数据片段可以转发到正确的接收端设备 实现完整消息的重组 跟踪终端设备之间的延迟 10T CP/IP 网络接入层有何作用？ 路径确定和数据包交换 数据表示、编码和控制
可靠性、流量控制和错误检测 详细规定构成物理链路的组件及其接入方法 将数据段划分为数据包 11下列哪三项陈述是对局域网(LAN) 最准确的描述？（选择三项） LAN 通常位于一个地域内。 此类网络由由一个组织管理。 LAN 中的不同网段之间一般通过租用连接的方式连接。 此类网络的安全和访问控制由服务提供商控制。 LAN 为同一个组织内的用户提供网络服务和应用程序访问。 此类网络的每个终端通常都连接到电信服务提供商(TSP)。 12 请参见图示。哪个术语正确判断出了包含于绿色区域中的设备类型？ 源 终端 传输
第一次实验：利用Wireshark软件进行数据包抓取 1.3.2 抓取一次完整的网络通信过程的数据包实验 一，实验目的： 通过本次实验，学生能掌握使用Wireshark抓取ping命令的完整通信过程的数据包的技能，熟悉Wireshark软件的包过滤设置和数据显示功能的使用。 二，实验环境： 操作系统为Windows 7,抓包工具为Wireshark. 三，实验原理： ping是用来测试网络连通性的命令，一旦发出ping命令，主机会发出连续的测试数据包到网络中，在通常的情况下，主机会收到回应数据包，ping采用的是ICMP协议。 四，验步骤： 1.确定目标地址：选择https://www.doczj.com/doc/3715077520.html,作为目标地址。 2.配置过滤器：针对协议进行过滤设置，ping使用的是ICMP协议，抓包前使用捕捉过滤器，过滤设置为icmp,如图 1- 1
图 1-1 3.启动抓包：点击【start】开始抓包，在命令提示符下键入ping https://www.doczj.com/doc/3715077520.html,, 如图 1-2
图 1-2 停止抓包后，截取的数据如图 1-3 图 1-3 4，分析数据包：选取一个数据包进行分析，如图1- 4
图1-4 每一个包都是通过数据链路层DLC协议，IP协议和ICMP协议共三层协议的封装。DLC协议的目的和源地址是MAC地址，IP协议的目的和源地址是IP地址，这层主要负责将上层收到的信息发送出去，而ICMP协议主要是Type和Code来识别，“Type:8,Code：0”表示报文类型为诊断报文的请求测试包，“Type:0,Code:0”表示报文类型为诊断报文类型请正常的包。ICMP提供多种类型的消息为源端节点提供网络额故障信息反馈，报文类型可归纳如下： （1）诊断报文（类型：8，代码0；类型：0代码：0）； (2）目的不可达报文（类型：3，代码0-15）； （3）重定向报文（类型：5，代码：0--4）； （4）超时报文（类型：11，代码：0--1）； （5）信息报文（类型：12--18）。
Lab Exercise – TCP Objective To see the details of TCP (Transmission Control Protocol). TCP is the main transport layer protocol used in the Internet. The trace file is here: https://www.doczj.com/doc/3715077520.html,/~kevin/com320/labs/wireshark/trace-tcp.pcap Requirements Wireshark: This lab uses Wireshark to capture or examine a packet trace. A packet trace is a record of traffic at some location on the network, as if a snapshot was taken of all the bits that passed across a particular wire. The packet trace records a timestamp for each packet, along with the bits that make up the packet, from the low-layer headers to the higher-layer contents. Wireshark runs on most operating systems, including Windows, Mac and Linux. It provides a graphical UI that shows the sequence of pack-ets and the meaning of the bits when interpreted as protocol headers and data. The packets are color-coded to convey their meaning, and Wireshark includes various ways to filter and analyze them to let you investigate different aspects of behavior. It is widely used to troubleshoot networks. You can down-load Wireshark from https://www.doczj.com/doc/3715077520.html, if it is not already installed on your computer. We highly rec-ommend that you watch the short, 5 minute video “Introduction to Wireshark” that is on the site. wget / curl: This lab uses wget (Linux and Windows) and curl (Mac) to fetch web resources. wget and curl are command-line programs that let you fetch a URL. Unlike a web browser, which fetches and executes entire pages, wget and curl give you control over exactly which URLs you fetch and when you fetch them. Under Linux, wget can be installed via your package manager. Under Windows, wget is available as a binary at my site https://www.doczj.com/doc/3715077520.html,/~kevin/com320/labs/wget.exe or look for download information on https://www.doczj.com/doc/3715077520.html,/software/wget/. Both have many options (try “wget --help” or “curl --help” to see) but a URL can be fetched simply with “wget URL” or “curl URL”. Browser: This lab uses a web browser to find or fetch pages as a workload. Any web browser will do.
Wireshark教程带实例 第 1 章介绍 1.1. 什么是Wireshark Wireshark 是网络包分析工具。网络包分析工具的主要作用是尝试捕获网络包，并尝试显示包的尽可能详细的情况。 你可以把网络包分析工具当成是一种用来测量有什么东西从网线上进出的测量工具，就好像使电工用来测量进入电信的电量的电度表一样。（当然比那个更高级） 过去的此类工具要么是过于昂贵，要么是属于某人私有，或者是二者兼顾。 Wireshark出现以后，这种现状得以改变。Wireshark可能算得上是今天能使用的最好的开元网络分析软件。 1.1.1. 主要应用 下面是Wireshark一些应用的举例： ?网络管理员用来解决网络问题 ?网络安全工程师用来检测安全隐患 ?开发人员用来测试协议执行情况 ?用来学习网络协议 除了上面提到的，Wireshark还可以用在其它许多场合。 1.1. 2. 特性 ?支持UNIX和Windows平台 ?在接口实时捕捉包 ?能详细显示包的详细协议信息 ?可以打开/保存捕捉的包 ?可以导入导出其他捕捉程序支持的包数据格式 ?可以通过多种方式过滤包 ?多种方式查找包 ?通过过滤以多种色彩显示包 ?创建多种统计分析 ?…还有许多 不管怎么说，要想真正了解它的强大，您还得使用它才行
图 1.1. Wireshark捕捉包并允许您检视其内 1.1.3. 捕捉多种网络接口 Wireshark 可以捕捉多种网络接口类型的包，哪怕是无线局域网接口。想了解支持的所有网络接口类型，可以在我们的网站上找到https://www.doczj.com/doc/3715077520.html,/CaptureSetup/NetworkMedia. 1.1.4. 支持多种其它程序捕捉的文件 Wireshark可以打开多种网络分析软件捕捉的包，详见??? 1.1.5. 支持多格式输出 Wieshark可以将捕捉文件输出为多种其他捕捉软件支持的格式，详见??? 1.1.6. 对多种协议解码提供支持 可以支持许多协议的解码(在Wireshark中可能被称为解剖)??? 1.1.7. 开源软件 Wireshark是开源软件项目，用GPL协议发行。您可以免费在任意数量的机器上使用它，不用担心授权和付费问题，所有的源代码在GPL框架下都可以免费使用。因为以上原因，人们可以很容易在Wireshark上添加新的协议，或者将其作为插件整合到您的程序里，这种应用十分广泛。 1.1.8. Wireshark不能做的事 Wireshark不能提供如下功能 ?Wireshark不是入侵检测系统。如果他/她在您的网络做了一些他/她们不被允许的奇怪的事情，Wireshark不会警告您。但是如果发生了奇怪的事情，Wireshark可能对察看发生了什么会有所帮助。[3]?Wireshark不会处理网络事务，它仅仅是“测量”(监视)网络。Wireshark不会发送网络包或做其它交互性的事情（名称解析除外，但您也可以禁止解析）。 1.2. 系通需求
一、实验目的 1、熟悉并掌握Wireshark的基本使用； 2、了解网络协议实体间进行交互以及报文交换的情况。 二、实验环境 与因特网连接的计算机，操作系统为Windows，安装有Wireshark、IE等软件。 三、预备知识 要深入理解网络协议，需要观察它们的工作过程并使用它们，即观察两个协议实体之间交换的报文序列，探究协议操作的细节，使协议实体执行某些动作，观察这些动作及其影响。这种观察可以在仿真环境下或在因特网这样的真实网络环境中完成。 观察正在运行的协议实体间交换报文的基本工具被称为分组嗅探器（packet sniffer），又称分组捕获器。顾名思义，分组嗅探器捕获（嗅探）你的计算机发送和接收的报文。 图显示了一个分组嗅探器的结构。 图分组嗅探器的结构 图右边是计算机上正常运行的协议和应用程序（如：Web浏览器和FTP客户端）。分组嗅探器（虚线框中的部分）主要有两部分组成：第一是分组捕获器，其功能是捕获计算机发送和接收的每一个链路层帧的拷贝；第二个组成部分是分组分析器，其作用是分析并显示协议报文所有字段的内容（它能识别目前使用的各种网络协议）。 Wireshark是一种可以运行在Windows, UNIX, Linux等操作系统上的分组
嗅探器，是一个开源免费软件，可以从下载。 运行Wireshark 程序时，其图形用户界面如图所示。最初，各窗口中并无数据显示。Wireshark 的界面主要有五个组成部分： 命令和菜单 协议筛选框 捕获分组 列表 选定分组 首部明细 分组内容 左：十六进制 右：ASCII码 图 Wireshark 主界面 命令菜单（command menus）：命令菜单位于窗口的最顶部，是标准的下拉式菜单。 协议筛选框（display filter specification）：在该处填写某种协议的名称，Wireshark据此对分组列表窗口中的分组进行过滤，只显示你需要的分组。 捕获分组列表（listing of captured packets）：按行显示已被捕获的分组内容，其中包括：分组序号、捕获时间、源地址和目的地址、协议类型、协议信息说明。单击某一列的列名，可以使分组列表按指定列排序。其中，协议类型是发送或接收分组的最高层协议的类型。 分组首部明细（details of selected packet header）：显示捕获分组列表窗口中被选中分组的首部详细信息。包括该分组的各个层次的首部信息，需要查看哪层信息，双击对应层次或单击该层最前面的“＋”即可。 分组内容窗口（packet content）：分别以十六进制（左）和ASCII码（右）两种格式显示被捕获帧的完整内容。
Wireshark抓包分析实验 若惜年 一、实验目的： 1.学习安装使用wireshark软件，能在电脑上抓包。 2.对抓出包进行分析，分析得到的报文，并与学习到的知识相互印证。 二、实验内容： 使用抓包软件抓取HTTP协议通信的网络数据和DNS通信的网络数据，分析对应的HTTP、TCP、IP协议和DNS、UDP、IP协议。 三、实验正文： IP报文分析： 从图中可以看出： IP报文版本号为:IPV4 首部长度为:20 bytes 数据包长度为:40 标识符：0xd74b 标志：0x02 比特偏移：0 寿命：48 上层协议：TCP 首部校验和：0x5c12 源IP地址为：119.75.222.18 目的IP为：192.168.1.108
从图中可以看出： 源端口号：1891 目的端口号：8000 udp报文长度为：28 检验和：0x58d7 数据长度：20 bytes UDP协议是一种无需建立连接的协议，它的报文格式很简单。当主机中的DNS 应用程序想要惊醒一次查询时，它构造一个DNS查询报文段并把它给UDP，不需要UDP之间握手，UDP为报文加上首部字段，将报文段交给网络层。
第一次握手： 从图中看出： 源端口号：56770 目的端口号：80 序列号为:0 首部长为: 32 bytes SYN为1表示建立连接成功当fin为1时表示删除连接。
第二次握手： 从图中看出： 源端口号是：80 目的端口号为：56770 序列号为：0 ack为：1 Acknowledgement为1表示包含确认的报文Syn为1表示建立连接。
第三次握手： 从图中看出： 源端口：56770 目的端口：80 序列号为：1 ACK为：1 首部长为：20bytes Acknowledgement为1表示包含确认的报文 所以，看出来这是TCP连接成功了 Tcp是因特网运输层的面向连接的可靠的运输协议，在一个应用进程可以开始向另一个应用进程发送数据前，这两个进程必须先握手，即它们必须相互发送预备文段，建立确保传输的参数。
Wireshark教程 当前，互联网已经越来越成为人们生活中必不可少的组成部分。面对日益复杂的网络环境，网络管理员必须花费更很多的时间和精力，来了解网络设备的运作情况，以维持系统的正常运行。当网络趋于复杂，就必须借助于专业的工具了。因此，作为一个网络管理人员和网络从业者，熟练掌握和运用一套网络管理软件来对整个网络进行协议分析，是一个必不可少的技能。 当前较为流行的网络协议嗅探和分析软件—Wireshark 。通过使用抓包工具，来准确而快速地判断网络问题的所在，大大缩短寻找问题的时间。 网络管理人员的私人秘书—Wireshark 网络流量分析是指捕捉网络中流动的数据包，并通过查看包内部数据以及进行相关的协议、流量分析、统计等来发现网络运行过程中出现的问题，它是网络和系统管理人员进行网络故障和性能诊断的有效工具。通常，人们把网络分析总结为四种方式：基于流量镜像协议分析，基于 SNMP 的流量监测技术，基于网络探针（ Probe ）技术和基于流（ flow ）的流量分析。而我们下面要向大家介绍的 Wireshark 就是基于流量镜像协议分析。 流量镜像协议分析方式是把网络设备的某个端口（链路）流量镜像给协议分析仪，通过 7 层协议解码对网络流量进行监测。但该方法主要侧重于协议分析，而非用户流量访问统计和趋势分析，仅能在短时间内对流经接口的数据包进行分析，无法满足大流量、长期的抓包和趋势分析的要求。 Wireshark 的前身是著名的 Ethereal 。 Wireshark 是一款免费的网络协议检测程序。它具有设计完美的 GUI 和众多分类信息及过滤选项。下面是 Wireshark 的界面。 用户通过 Wireshark ，同时将网卡插入混合模式，可以用来监测所有在网络上被传送的包，并分析其内容。通过查看每一封包流向及其内容，用来检查网络的工作情况，或是用来发现网络程序的 bugs 。 Wireshark 是一个开放源码的网络分析系统，也是是目前最好的开放源码的网络协议分析器，支持 Unix Linux 和 Windows 平台。由于 Wireshark 是 Open Source ，更新快，支持的协议多，特别是数据包过滤功能灵活强大。 Wireshark 提供了对 TCP 、 UDP 、 SMB 、 telnet 和 ftp 等常用协议的支持。它在很多情况下可以代替价格昂贵的 Sniffer 。 安装好后，双击桌面上的 Wireshark 图标，运行软件。再捕捉数据包之前，首先要对捕获的条件进行设置。点击工具栏里的“Capture à Options”，（图一）或者直接点击快捷按钮（图二），打开选项设置页面（图三）。
Lab Exercise – Protocol Layers Objective To learn how protocols and layering are represented in packets. They are key concepts for structuring networks that are covered in the text. The trace for this lab is here: https://www.doczj.com/doc/3715077520.html,/~kevin/com320/labs/wireshark/trace-protocol-layers.pcap (although the main trace you will look at is from a site you pick such as https://www.doczj.com/doc/3715077520.html, in the exam-ples which follow). Requirements Wireshark: This lab uses the Wireshark software tool to capture andexamine a packet trace. A packet trace is a record of traffic at a location on the network, as if a snapshot was taken of all the bits that passed across a particular wire. The packet trace records a timestamp for each packet, along with the bits that make up the packet, from the lower-layer headers to the higher-layer contents.Wireshark runs on most operating systems, including Windows, Mac and Linux. It provides a graphical UI that shows the sequence of packets and the meaning of the bits when interpreted as protocol headers and data. It col-or-codes packets by their type, and has various ways to filter and analyze packets to let you investigate the behavior of network protocols. Wireshark is widely used to troubleshoot networks. You can down-load it https://www.doczj.com/doc/3715077520.html, for your personal computer. It is an ideal packet analyzer for our labs –it is stable, has a large user base and well-documented support that includes a user-guide https://www.doczj.com/doc/3715077520.html,/docs/wsug_html_chunked), and a detailed FAQ, rich functionality that in-cludes the capability to analyze hundreds of protocols, and a well-designed user interface. It operates in computers using Ethernet, serial (PPP and SLIP), 802.11 wireless LANs, and many other link-layer tech-nologies (if the OS on which it is running allows Wireshark to do so). It is already installed in the labs. A quick help guide to Wireshark display filters is here: https://www.doczj.com/doc/3715077520.html,/wireshark_filters.php Wireshark is a core tool for any wireless ‘man in the middle’ or similar snooping attack. It is simply i n-dispensable for those who wish to examine packets being transferred over a network –good or bad…..
计算机网络实验指导书
目录 实验一Wireshark的安装与使用 (3) 实验二使用Wireshark分析以太网帧与ARP协议 (7) 实验三使用Wireshark分析IP协议 (11) 实验四利用Wireshark分析ICMP (19) 实验五使用Wireshark分析UDP协议 (25) 实验六使用Wireshark分析TCP协议 (29) 实验七利用Wireshark分析协议HTTP (35) 实验八利用Wireshark分析DNS协议 (40) 实验九使用Wireshark分析FTP协议(选作) (44) 实验十使用Wireshark分析SMTP与POP3协议(选作) (48)
实验一Wireshark的安装与使用 一、实验目的 1、熟悉并掌握Wireshark的基本使用; 2、了解网络协议实体间进行交互以及报文交换的情况。 二、实验环境 与因特网连接的计算机,操作系统为Windows,安装有Wireshark、IE等软件。 三、预备知识 要深入理解网络协议,需要观察它们的工作过程并使用它们,即观察两个协议实体之间交换的报文序列,探究协议操作的细节,使协议实体执行某些动作,观察这些动作及其影响。这种观察可以在仿真环境下或在因特网这样的真实网络环境中完成。 观察正在运行的协议实体间交换报文的基本工具被称为分组嗅探器(packet sniffer),又称分组捕获器。顾名思义,分组嗅探器捕获(嗅探)您的计算机发送与接收的报文。 图1显示了一个分组嗅探器的结构。 图1 图1右边就是计算机上正常运行的协议与应用程序(如:Web浏览器与FTP客户端)。分组嗅探器(虚线框中的部分)主要有两部分组成:第一就是分组捕获器,其功能就是捕获计算机发送与接收的每一个链路层帧的拷贝;第二个组成部分就是分组分析器,其作用就是分析并显示协议报文所有字段的内容(它能识别目前使用的各种网络协议)。 Wireshark就是一种可以运行在Windows, UNIX, Linux等操作系统上的分组嗅探器,就是一个开源免费软件,可以从、wireshark、org下载。
Wireshark使用教程 第 1 章介绍 1.1. 什么是Wireshark Wireshark 是网络包分析工具。网络包分析工具的主要作用是尝试捕获网络包，并尝试显示包的尽可能详细的情况。 你可以把网络包分析工具当成是一种用来测量有什么东西从网线上进出的测量工具，就好像使电工用来测量进入电信的电量的电度表一样。（当然比那个更高级） 过去的此类工具要么是过于昂贵，要么是属于某人私有，或者是二者兼顾。 Wireshark出现以后，这种现状得以改变。Wireshark可能算得上是今天能使用的最好的开元网络分析软件。 1.1.1. 主要应用 下面是Wireshark一些应用的举例： ?网络管理员用来解决网络问题 ?网络安全工程师用来检测安全隐患 ?开发人员用来测试协议执行情况 ?用来学习网络协议 除了上面提到的，Wireshark还可以用在其它许多场合。 1.1. 2. 特性 ?支持UNIX和Windows平台 ?在接口实时捕捉包 ?能详细显示包的详细协议信息 ?可以打开/保存捕捉的包 ?可以导入导出其他捕捉程序支持的包数据格式 ?可以通过多种方式过滤包 ?多种方式查找包 ?通过过滤以多种色彩显示包 ?创建多种统计分析 ?…还有许多 不管怎么说，要想真正了解它的强大，您还得使用它才行
图 1.1. Wireshark捕捉包并允许您检视其内 1.1.3. 捕捉多种网络接口 Wireshark 可以捕捉多种网络接口类型的包，哪怕是无线局域网接口。想了解支持的所有网络接口类型，可以在我们的网站上找到https://www.doczj.com/doc/3715077520.html,/CaptureSetup/NetworkMedia. 1.1.4. 支持多种其它程序捕捉的文件 Wireshark可以打开多种网络分析软件捕捉的包，详见??? 1.1.5. 支持多格式输出 Wieshark可以将捕捉文件输出为多种其他捕捉软件支持的格式，详见??? 1.1.6. 对多种协议解码提供支持 可以支持许多协议的解码(在Wireshark中可能被称为解剖)??? 1.1.7. 开源软件 Wireshark是开源软件项目，用GPL协议发行。您可以免费在任意数量的机器上使用它，不用担心授权和付费问题，所有的源代码在GPL框架下都可以免费使用。因为以上原因，人们可以很容易在Wireshark上添加新的协议，或者将其作为插件整合到您的程序里，这种应用十分广泛。 1.1.8. Wireshark不能做的事 Wireshark不能提供如下功能 ?Wireshark不是入侵检测系统。如果他/她在您的网络做了一些他/她们不被允许的奇怪的事情，Wireshark不会警告您。但是如果发生了奇怪的事情，Wireshark可能对察看发生了什么会有所帮助。[3]?Wireshark不会处理网络事务，它仅仅是“测量”(监视)网络。Wireshark不会发送网络包或做其它交互性的事情（名称解析除外，但您也可以禁止解析）。 1.2. 系通需求
思科章节练习第二章到第七章试题及答案 第二章 1.下列哪些陈述正确指出了中间设备在网络中的作用？（选择三项） (135) 确定数据传输路径 发起数据通信 重新定时和重新传输数据信号 发送数据流 管理数据流 数据流最后的终止点 2.请选择关于网络协议的正确陈述。（选择三项。135） 定义特定层 PDU 的结构 规定实现协议层功能的方式 概述层与层之间通信所需的功能 限制了对硬件兼容性的需要 需要取决于协议层的封装过程 杜绝厂商之间的标准化 3.封装的两个功能是什么？（45选择两项） 跟踪终端设备之间的延迟 为通信提供统一的网络路径 在传输前可以修改原始数据 标识属于同一通信的数据片段 确保数据片段可以转发到正确的接收端设备 4.数据链路层封装添加的报尾信息有何主要功能？1 支持差错校验 确保数据按照顺序到达 确保送达正确目的 标识本地网络中的设备 帮助中间设备进行处理和路径选择 5.OSI 模型哪两层的功能与 TCP/IP 模型的网络接入层相同？（选择两项34）网络层 传输层 物理层 数据链路层 会话层 6.什么是 PDU？4 传输期间的帧损坏 在目的设备上重组的数据 因通信丢失而重新传输的数据包 特定层的封装 7.以下哪种特征正确代表了网络中的终端设备？ 2 管理数据流 发送数据流 重新定时和重新传输数据信号
确定数据传输路径 8.IP 地址为 10.0.0.34 的“手机A”已经与 IP 地址为 172.16.1.103 的“IP 电话1”建立了 IP 会话。请根据图示回答，下列哪个设备类型是对无线设备“手机A”功能的最准确描述？ 2 目的设备 终端设备 中间设备 介质设备 9.下列哪三个标签正确标识了图示网段的网络类型？（236选择三项） 网络 A -- WAN 网络 B -- WAN 网络 C -- LAN 网络 B -- MAN 网络 C -- WAN 网络 A -- LAN 10.下列哪三项陈述是对局域网 (LAN) 最准确的描述？（选择三项125） LAN 通常位于一个地域内。 此类网络由由一个组织管理。 LAN 中的不同网段之间一般通过租用连接的方式连接。 此类网络的安全和访问控制由服务提供商控制。 LAN 为同一个组织内的用户提供网络服务和应用程序访问。 此类网络的每个终端通常都连接到电信服务提供商 (TSP)。 11. 哪个网络术语描述了图中所示的数据交替发送过程？4 通道 PDU 流传输 多路复用 封装 12第 4 层端口指定的主要作用是什么？5 标识本地介质中的设备 标识源设备和目的设备之间的跳数 向中间设备标识通过该网络的最佳路径 标识正在通信的源终端设备和目的终端设备 标识终端设备内正在通信的进程或服务 13. 哪种设备被视为中间设备？ 5 文件服务器 IP 电话 笔记本电脑 打印机 交换机 14.哪个术语正确判断出了包含于区域 B 中的设备类型？ 4 源设备 终端设备
Wireshark的捕捉过滤器和显示过滤器 Wireshark两种过滤器使用的语法是完全不同的。我们将在接下来的几页中对它们进行介绍： 1. 捕捉过滤器 捕捉过滤器的语法与其它使用Lipcap（Linux）或者Winpcap（Windows）库开发的软件一样，比如著名的TCPdump。捕捉过滤器必须在开始捕捉前设置完毕，这一点跟显示过滤器是不同的。 设置捕捉过滤器的步骤是： - 选择capture -> options。 - 填写"capture filter"栏或者点击"capture filter"按钮为您的过滤器起一个名字并保存，以便 在今后的捕捉中继续使用这个过滤器。 - 点击开始（Start）进行捕捉。
语法： 例子：tcp dst 10.1.1.1 80 and tcp dst 10.2.2.2 3128 Protocol（协议）: 可能的值: ether, fddi, ip, arp, rarp, decnet, lat, sca, moprc, mopdl, tcp and udp. 如果没有特别指明是什么协议，则默认使用所有支持的协议。 Direction（方向）: 可能的值: src, dst, src and dst, src or dst 如果没有特别指明来源或目的地，则默认使用"src or dst" 作为关键字。 例如，"host 10.2.2.2"与"src or dst host 10.2.2.2"是一样的。Host(s): 可能的值：net, port, host, portrange. 如果没有指定此值，则默认使用"host"关键字。
实验一wireshark抓包工具使用[实验目的] 学习wireshark抓包工具的使用 了解wireshark抓包工具的功能 通过学习，进一步理解协议及网络体系结构思想 [实验原理] Wireshark是网络包分析工具。网络包分析工具的主要作用是尝试捕获网络包，并尝试显示包的尽可能详细的情况。 主要应用： 网络管理员用来解决网络问题 网络安全工程师用来检测安全隐患 开发人员用来测试协议执行情况 用来学习网络协议 [实验内容] 下载WIRESHARK，学习工具的使用和功能。
Wireshark 是网络包分析工具。网络包分析工具的主要作用是尝试捕获网络包，并尝试显示包的尽可能详细的情况。 你可以把网络包分析工具当成是一种用来测量有什么东西从网线上进出的测量工具，就好像使电工用来测量进入电信的电量的电度表一样。（当然比那个更高级） 过去的此类工具要么是过于昂贵，要么是属于某人私有，或者是二者兼顾。 Wireshark出现以后，这种现状得以改变。 Wireshark可能算得上是今天能使用的最好的开元网络分析软件。 工作流程 （1）确定Wireshark的位置。如果没有一个正确的位置，启动Wireshark后会花费很长的时间捕获一些与自己无关的数据。 （2）选择捕获接口。一般都是选择连接到Internet网络的接口，这样才可以捕获到与网络相关的数据。否则，捕获到的其它数据对自己也没有任何帮助。 （3）使用捕获过滤器。通过设置捕获过滤器，可以避免产生过大的捕获文件。这样用户在分析数据时，也不会受其它数据干扰。而且，还可以为用户节约大量的时间。 （4）使用显示过滤器。通常使用捕获过滤器过滤后的数据，往往还是很复杂。为了使过滤的数据包再更细致，此时使用显示过滤器进行过滤。 （5）使用着色规则。通常使用显示过滤器过滤后的数据，都是有用的数据包。如果想更加突出的显示某个会话，可以使用着色规则高亮显示。 （6）构建图表。如果用户想要更明显的看出一个网络中数据的变化情况，使用图表的 形式可以很方便的展现数据分布情况。
第 1 章介绍 1.1. 什么是Wireshark Wireshark 是网络包分析工具。网络包分析工具的主要作用是尝试捕获网络包，并尝试显示包的尽可能详细的情况。 你可以把网络包分析工具当成是一种用来测量有什么东西从网线上进出的测量工具，就好像使电工用来测量进入电信的电量的电度表一样。（当然比那个更高级） 过去的此类工具要么是过于昂贵，要么是属于某人私有，或者是二者兼顾。 Wireshark出现以后，这种现状得以改变。Wireshark可能算得上是今天能使用的最好的开元网络分析软件。 1.1.1. 主要应用 下面是Wireshark一些应用的举例： ?网络管理员用来解决网络问题 ?网络安全工程师用来检测安全隐患 ?开发人员用来测试协议执行情况 ?用来学习网络协议 除了上面提到的，Wireshark还可以用在其它许多场合。 1.1. 2. 特性 ?支持UNIX和Windows平台 ?在接口实时捕捉包 ?能详细显示包的详细协议信息 ?可以打开/保存捕捉的包 ?可以导入导出其他捕捉程序支持的包数据格式 ?可以通过多种方式过滤包 ?多种方式查找包 ?通过过滤以多种色彩显示包 ?创建多种统计分析 ?…还有许多 不管怎么说，要想真正了解它的强大，您还得使用它才行 图 1.1. Wireshark捕捉包并允许您检视其内
1.1.3. 捕捉多种网络接口 Wireshark 可以捕捉多种网络接口类型的包，哪怕是无线局域网接口。想了解支持的所有网络接口类型，可以在我们的网站上找到https://www.doczj.com/doc/3715077520.html,/CaptureSetup/NetworkMedia. 1.1.4. 支持多种其它程序捕捉的文件 Wireshark可以打开多种网络分析软件捕捉的包，详见??? 1.1.5. 支持多格式输出 Wieshark可以将捕捉文件输出为多种其他捕捉软件支持的格式，详见??? 1.1.6. 对多种协议解码提供支持 可以支持许多协议的解码(在Wireshark中可能被称为解剖)??? 1.1.7. 开源软件 Wireshark是开源软件项目，用GPL协议发行。您可以免费在任意数量的机器上使用它，不用担心授权和付费问题，所有的源代码在GPL框架下都可以免费使用。因为以上原因，人们可以很容易在Wireshark上添加新的协议，或者将其作为插件整合到您的程序里，这种应用十分广泛。 1.1.8. Wireshark不能做的事 Wireshark不能提供如下功能 ?Wireshark不是入侵检测系统。如果他/她在您的网络做了一些他/她们不被允许的奇怪的事情，Wireshark不会警告您。但是如果发生了奇怪的事情，Wireshark可能对察看发生了什么会有所帮助。[3]?Wireshark不会处理网络事务，它仅仅是“测量”(监视)网络。Wireshark不会发送网络包或做其它交互性的事情（名称解析除外，但您也可以禁止解析）。 1.2. 系通需求 想要安装运行Wireshark需要具备的软硬件条件... 1.2.1. 一般说明 ?给出的值只是最小需求，在大多数网络中可以正常使用，但不排除某些情况下不能使用。[4] ?在繁忙的网络中捕捉包将很容塞满您的硬盘！举个简单的例子：在100MBIT/s全双工以太网中捕捉数据将会产生750MByties/min的数据！在此类网络中拥有高速的CPU，大量的内存和足够的磁盘空间是十分有必要的。 ?如果Wireshark运行时内存不足将会导致异常终止。可以在 https://www.doczj.com/doc/3715077520.html,/KnownBugs/OutOfMemory察看详细介绍以及解决办法。 ?Wireshark作为对处理器时间敏感任务，在多处理器/多线程系统环境工作不会比单独处理器有更快的速度，例如过滤包就是在一个处理器下线程运行，除了以下情况例外：在捕捉包时“实时更新包列表”，此时捕捉包将会运行在一个处理下，显示包将会运行在另一个处理器下。此时多处理或许会有所帮助。[5] 1.2.2. Microsoft Windows ?Windows 2000,XP Home版,XP Pro版,XP Tablet PC，XP Media Center, Server 2003 or Vista(推荐在XP下使用) ?32-bit奔腾处理器或同等规格的处理器（建议频率：400MHz或更高）,64-bit处理器在WoW64仿真环境下-见一般说明 ?128MB系统内存（建议256Mbytes或更高） ?75MB可用磁盘空间（如果想保存捕捉文件，需要更多空间） 800*600（建议1280*1024或更高）分辨率最少65536(16bit)色，(256色旧设备安装时需要选择”legacy GTK1”) ?网卡需求： o以太网：windows支持的任何以太网卡都可以 o无线局域网卡：见MicroLogix support list, 不捕捉802.11包头和无数据桢。 o其它接口见：https://www.doczj.com/doc/3715077520.html,/CaptureSetup/NetworkMedia
Lab Exercise – DNS Objective DNS (Domain Name System) is the system & protocol that translates domain names to IP addresses . Step 1: Analyse the supplied DNS Trace Here we examine the supplied trace of a browser making DNS requests as follows. The trace is here: https://www.doczj.com/doc/3715077520.html,/~kevin/com320/labs/wireshark/trace-dns.pcap https://www.doczj.com/doc/3715077520.html,unch Wireshark and start a capture with a filter of “udp port 53”.We use this filterbe- cause there is no shorthand for DNS, but DNS is normally carried on UDP port 53. Figure 3: Setting up the capture options
Step 2: Inspect the Trace To explore the details of DNS packets, select a DNS query expand its Domain Name System block (by us-ing the “+” expander or icon). Your display should be similar to the one shown in our figure, with a series of packets with protocol DNS.. We have selected the first DNS message. Figure 3: Trace of DNStraffic showing the details of the DNS header Look for the following details: ?The DNS block follows the IP and UDP blocks. This is because DNS messages are carried in UDP segments within IP packets. You will see that the UDP port used by a nameserver is 53. ?The DNS header starts with a Transaction ID that is used to link a request and the corresponding reply – they both carry the same Transaction ID. ?Next come a set of flags that you can expand. They indicate whether the DNS message is a query or response, amongst other details. ?Then comethe number of query, answer, authority and additional records. These fields conclude the header.