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wireshark练习及答案lab-tcp

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.wendangku.net/doc/3a2799902.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.wendangku.net/doc/3a2799902.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.wendangku.net/doc/3a2799902.html,/~kevin/com320/labs/wget.exe or look for download information on https://www.wendangku.net/doc/3a2799902.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.

Step 1: Capture a Trace

Proceed as follows to capture a trace of a single TCP connection that sends a moderate amount of data; alternatively, you may use a supplied trace. Many applications use TCP as a transport, including web browsers. So we will simply perform a web download to exercise a TCP connection. However, note that TCP is able to transfer data in both directions at the same time, but with a download content is only sent from the remote server the local computer (after the initial request).

1.Find a URL of a single moderately-sized resource, and that you can download using HTTP (rather

than HTTPS). You may use your browser to search, perhaps looking for a picture (.jpg) or PDF

document (.pdf). You want to ensure that it is a single resource and not a web page (e.g., a.html) with many inlined resources.

2.Fetch the URL with wget or curl to check that you are able to retrieve at least 500 KB of con-

tent over at least several of network time seconds. For example, use the command

“wget https://www.wendangku.net/doc/3a2799902.html,/~kevin/com320/papers/macpaper.pdf” or go to

https://www.wendangku.net/doc/3a2799902.html,/sigcomm/2011/conf-program.php in your browser and pick a

PDF to download. Successful examples of fetching are shown in the figure below.

Figure 1: A successful fetch of a web resource with wget (Windows)

https://www.wendangku.net/doc/3a2799902.html,unch Wireshark and start a capture with a filter of “tcp and host xx.xx.xx”, where

xx.xx.xx is the name of the remote server from which you will fetch content, e.g., “confe-

https://www.wendangku.net/doc/3a2799902.html,” in the figure showing our example below. The idea of the filter is to only capture TCP traffic between your computer and the server. Your capture window should be similar to the one pictured below, other than our highlighting. Select the interface from which to capture as the main wired or wireless interface used by your computer to connect to the Inter-

net. If unsure, guess and revisit this step later if you r capture is not successful. Uncheck “capture packets in promiscuous mode”. This mode is useful to overhear packets sent to/from other

computers on broadcast networks. We only want to record packets sent to/from your computer.

Leave other options at their default values. The capture filter, if present, is used to prevent the capture of other traffic your computer may send or receive. On Wireshark 1.8, the capture filter box is present directly on the options screen, but on Wireshark 1.9, you set a capture filter by

double-clicking on the interface.

Figure 2: Setting up the capture options

4.After the capture is started, repeat the wget/curl command above. This time, the packets will

also be recorded by Wireshark.

5.When the command is complete, return to Wireshark and use the menus or buttons to stop the

trace.You should now have a trace similar to that shown in the figure below. We have expanded the detail of the TCP header in our view, since it is our focus for this lab.

Figure 3: Trace of TCP traffic showing the details of the TCP header

Step 2: Inspect the Trace

Select a long packet anywhere in the middle of your trace whose protocol is listed as TCP. Expand the TCP protocol section in the mi ddle panel (by using the “+” expander or icon).All packets except the initial HTTP GET and last packet of the HTTP response should be listed as TCP. Picking a long packet ensures that we are looking at a download packet from the server to your computer. Looking at the protocol layers, you should see an IP block before the TCP block. This is because the TCP segment is carried in an IP. We have shown the TCP block expanded in our figure.

You will see roughly the following fields:

?First comes the source port, then the destination port. This is the addressing that TCP adds beyond the IP address. The source port is likely to be 80 since the packet was sent by a web

server and the standard web server port is 80.

?Then there is the sequence number field. It gives the position in the bytestream of the first payl-oad byte.

?Next is the acknowledgement field. It tells the last received position in the reverse byte stream.

?The header length giving the length of the TCP header.

?The flags field has multiple flag bits to indicate the type of TCP segment. You can expand it and look at the possible flags.

?Next is a checksum, to detect transmission errors.

?There may be an Options field with various options. You can expand this field and explore if you would like, but we will look at the options in more detail later.

?Finally, there may be a TCP payload, carrying the bytes that are being transported.

As well as the above fields, there may be other informational lines that Wireshark provides to help you interpret the packet. We have covered only the fields that are carried across the network.

Step 3: TCP SegmentStructure

To show your understanding of TCP, sketch a figure of the TCP segment youstudied. It should show the position and size in bytes of the TCP header fields you can observe using Wireshark.Do not break down the Flags field, or any Options field, and if you find that some TCP fields share a byte then group them. As usual, your figure can simply show the frame as a long, thin rectangle. Try not to look at the figure of a TCP segment in your text; check it afterwards to note and investigate any differences.

To work out sizes, observe that when you click on a protocol block in the middle panel (the block itself, not the “+” expander) Wireshark will highlight the c orresponding bytes in the packet in the lower panel, and display the length at the bottom of the window. You may also use the overall packet size shown in the Length column or Frame detail block. Note that this method will not tell you sub-byte positions. (Please note answer on next page).

Answers to Step 3: TCP Segment Structure

Figure 4: Structure of a TCP segment

This drawing differs from the text drawing in the book in only minor respects:

?The Header length and Flags fields are combined into a 2 byte quantity. It is not easy to deter-mine their bit lengths with Wireshark.

?The Urgent Pointer field is shown as dotted. This field is typically not used, and so does not show up in Wireshark and we do not expect you to have it in your drawing. You can notice its exis-

tence in Wireshark, however, by observing the zero bytes in the segment that are skipped over as you select the different fields.

?The Options field is shown dotted, as it may or may not be present for the segments in your trace. Most often it will be present, and when it is then its length will be a multiple of four bytes.

?The Payload is optional. It is present for the segment you viewed, but not present on an Ack-only segment, for example.

Step 4: TCP Connection Setup/Teardown

Three-Way Handshake

To see th e “three way handshake” in action, look for a TCP segment with the SYN flag on, most likely at the beginning of your trace, and the packets that follow it. The SYN flag is noted in the Info column. You can also s earch for packets with the SYN flag on using the filter expression “tcp.flags.syn==1”. A “SYN packet” is the start of the three-way handshake. In this case it will be sent from your computer to the remote server. The remote server should reply with a TCP segment with the SYN and ACK flags set, or a “SYN ACK packet”. On receiving this segment, your computer will ACK it, consider the connection set up, and begin sending data, which in this case will be the HTTP request. Your exchange should follow this pattern, though it is possible that it differs slightly if a packet was lost and must be retransmitted.

Draw a time sequence diagram of the three-way handshake in your trace, up to and including the first data packet (the HTTP GET request) sent by your computer when the connection is established Put your computer on the left side and the remote server on the right side.As usual, time runs down the page, and lines across the page indicate segments. The result will be similar to diagrams such as Fig. 6-37.

Include the following features on your diagram:

?The Sequence and ACK number, if present, on each segment. The ACK number is only carried if the segment has the ACK flag set.

?The time in milliseconds,starting at zero, each segment was sent or received at your computer.

?The round-trip time to the server estimated as the difference between the SYN and SYN-ACK segments.

Answers to Step 4: TCP Connection Setup/Teardown

Figure 5: Time sequence diagram for the TCP three-way handshake

There are several features to note:

?The initial SYN has no ACK number, only a sequence number. All subsequent packets have ACK numbers.

?The initial sequence numbers are shown as zero in each direction. This is because our Wireshark is configured to show relative sequence numbers. The actual sequence number is some large 32-bit number, and it is different for each end.

?The ACK number is the corresponding sequence number plus 1.

?Our computer sends the third part of the handshake (the ACK) and then sends data right away in

a different packet. It would be possible to combine these packets, but they are typically sepa-

rate (because one is triggered by the OS and one by the application).

?For the Data segment, the sequence number and ACK stay with the previous values. The se-quence number will advance as the sender sends more data. The ACK number will advance as

the sender receives more data from the remote server.

?The three packets received and sent around 88ms happen very close together compared to the gap between the first and second packet. This is because they are local operations; there is no

network delay involved.

?The RTT is 88ms in our trace. If you use a local web server, the RTT will be very small, likely a few milliseconds. If you use a major web server that may be provided by a content distribution net-work, the RTT will likely be tens of milliseconds. If you use a geographically remote server, the

RTT will likely be hundreds of milliseconds.

Step 5: Connection Options

As well as setting up a connection, the TCP SYN packets negotiate parameters between the two ends us-ing Options. Each end describes its capabilities, if any, to the other end by including the appropriate Op-tions on its SYN. Often both ends must support the behavior for it to be used during data transfer. Answer the following question:

1.WhatTCP Options are carried on the SYN packets for your trace?

Common Options include Maximum Segment Size (MSS) to tell the other side the largest segment that can be received, and Timestamps to include information on segments for estimating the round trip time. There are also Options such as NOP (No-operation) and End of Option list that serve to format the Op-tions but do not advertise capabilities. You do not need to include these formatting options in your an-swer above. Options can also be carried on regular segments after the connection is set up when they play a role in data transfer. This depends on the Option. For example: the MSS option is not carried on each packet because it does not convey new information; timestamps may be included on each packet to keep a fresh estimate of the RTT; and options such as SACK (Selective Acknowledgments) are used on-ly when data is received out of order. For fun, look at the options on data packets in your trace.

Answers to Step 5: Connection Options

1.Our TCP Options are Maximum Segment Size, Window Scale, SACK permitted, and Timestamps.

Each of these Options is used in both directions. There are also the NOP and End of Option List formatting options.

Here is an example of a FIN teardown:

Figure 6: Time sequence diagram for FIN teardown

Points to note:

?The teardown is initiated by the computer; it might also be initiated by the server.

?Like the SYN, the FIN flag occupies one sequence number. Thus when the sequence number of the FIN is 192, the corresponding Ack number is 193.

?Your sequence numbers will vary. Our numbers are relative (as computed by Wireshark) but clearly depend on the resource that is fetched. You can tell that it is around 1 MB long.

?The RTT in the FIN exchange is similar to that in the SYN exchange, as it should be. Your RTT will vary depending on the distance between the computer and server as before.

Step 6: FIN/RST Teardown

Finally, the TCP connection is taken down after the download is complete. This is typically done with FIN (Finalize) segments. Each side sends a FIN to the other and acknowledges the FIN they receive; it is simi-lar to the three-way handshake. Alternatively, the connection may be torn down abruptly when one end sends a RST (Reset). This packet does not need to be acknowledged by the other side.

Draw a picture of the teardown in your trace, starting from when the first FIN or RST is issued until the connection is complete. As before, show the sequence and ACK numbers on each segment. If you have FINs then use the time difference to estimate the round-trip time.

Answers to Step 6: FIN/RST Teardown

Here is an example of a RST teardown:

Figure 7: Time sequence diagram for RST teardown

Points to note:

?The teardown is initiated by the computer; it might also be initiated by the server.

?The teardown is abrupt – a single RST in this case, and then it is closed, which the other end must accommodate.

?The sequence and Ack numbers do not really matter here. They are simply the (relative Wire-shark) values at the end of the connection.

?Since there is no round trip exchange, no RTT can be estimated.

Step 7: TCP Data Transfer

The middle portion of the TCP connection is the data transfer, or download, in our trace. This is the main event. To get an overall sense of it, we will first look at the download rate over time.

U nder the Statistics menu select an “IO Graph”. By default, this graph shows the rate of packets over time. Tweak it to show the download rate with the changes given below. You might be tempted to use the “TCP Stream Graph” tools under the Statistics menu instead. However, these tools are not useful for our case because they assume the trace is taken near the computer sending the data; our trace is taken near the computer receiving the data.

?On the x-axis, adjust the tick interval and pixels per tick. The tick interval should be small enough to see into the behavior over the trace, and not so small that there is no averaging. 0.1 seconds is a good choice for a several second trace. The pixels per tick can be adjusted to make the

graph wider or narrower to fill the window.

?On the y-axis, change the unit to be Bits/Tick. The default is Packet/Tick. By changing it, we can easily work out the bits/secthroughput by taking the y-axis value and scaling as appropriate, e.g., 10X for ticks of 0.1 seconds.

?Add a filter expression to see only the download packets. So far we are looking at all of the packets. Assuming the download is from the usual web server port of 80, you can filter for it

with a filter of “tcp.srcport==80”.Don’t forget to press Enter, and you may need to click

the “Graph” button to cause it to redisplay.

?To see the corresponding graph for the upload traffic, enter a second filter in the next box. Again assuming the usual web server port, the filter is “tcp.dstport==80”. After you press Enter

and click the Graph button, you should have two lines on the graph.

Our graph for this procedure is shown in the figure below. From it we can see thesample download rate quickly increase from zero to a steady rate, with a bit of an exponential curve. This is slow-start. The download rate when the connection is running is approximately 2.5 Mbps. You can check your rate es-timate with the information from wget/curl. The upload rate is a steady, small trickle of ACK traf-fic.Our download also proceeds fairly steadily until it is done. This is the ideal, but many downloads may display more variable behavior if, for example, the available bandwidth varies due to competing down-loads, the download rate is set by the server rather than the network, or enough packets are lost to dis-rupt the transfer. You can click on the graph to be taken to the nearest point in the trace if there is a fea-ture you would like to investigate.

Figure 8: TCP download rate over time via an IO graph

Answer the following questions to show your understanding of the data transfer:

1.What is the rough data rate in the download direction in packets/second and bits/second once

the TCP connection is running well?

2.What percentage of this download rate is content? Show your calculation. To find out, look at a

typical download packet; there should be many similar, large download packets. You can see

how long it is, and how many bytes of TCP payload it contains.

3.What is the rough data rate in the upload direction in packets/second and bits/second due to the

ACK packets?

Inspect the packets in the download in the middle of your trace for these features:

?You should see a pattern of TCP segments received carrying data and ACKs sent back to the server. Typically there will be one ACK every couple of packets. These ACKs are called Delayed

ACKs. By delaying for a short while, the number of ACKs is halved.

?Since this is a download, the sequence number of received segments will increase; the ACK number of subsequently transmitted segments will increase correspondingly.

?Since this is a download, the sequence number of transmitted segments will not increase (after the initial get). Thus the ACK number on received segments will not increase either.

?Each segment carries Window information to tell the other end how much space remains in the buffer. The Window must be greater than zero, or the connection will be stalled by flow control. Answer the following question:

4.If the most recently received TCP segment from the server has a sequence number of X, then

what ACK number does the next transmitted TCP segment carry?

As well as regular TCP segments carrying data, you may see a variety of other situations. You can sort the trace on the Info column and brow se the packets of type “[TCP xxx ...”. Depending on the download, you may see duplicate acks, out of order data, retransmissions, zero windows, window up-dates, and more. These segments are not generally distinguished by flags in the TCP header, like SYN or FIN segments. Instead, they are names for situations that may occur and be handled during transport.

Answers to Step 7: TCP Data Transfer

1.Our rate is 250 packet/sec and

2.5 Mbps. Your rate will differ, but it is likely that the packet and

bit rates will differ by a factor of around 10,000 for packets of size roughly 1000 bytes.

2.Our download packets are1434 bytes long, of which 1368 bytes are the TCP payload carrying

contents. Thus 95% of the download is content. Your number should be similarly high for large packets of around 1 KB, since the TCP header is only 20-40 bytes.

3.Our rate is 120 packets/sec and 60,000 bits/sec. Your rate will vary, but we expect the ACK

packet rate to be around half of the data packet rate for the typical pattern of one delayed ACK per two data packets received. The ACK bit rate will be at least an order of magnitude below

the data bit rate because the packets are much smaller, around 60 bytes.

4.The Ack number tells the next expected sequence number. Thus it will be X plus the number of

TCP payload bytes in the data segment.

按章节练习及答案全部1-11章

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

Wireshark抓包实验报告.

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.wendangku.net/doc/3a2799902.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.wendangku.net/doc/3a2799902.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）。

wireshark练习及答案lab-tcp

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.wendangku.net/doc/3a2799902.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.wendangku.net/doc/3a2799902.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.wendangku.net/doc/3a2799902.html,/~kevin/com320/labs/wget.exe or look for download information on https://www.wendangku.net/doc/3a2799902.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使用教程详解,带实例

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.wendangku.net/doc/3a2799902.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

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）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抓包分析实验报告

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。

实验四wireshark教程

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）。

wireshark练习及答案lab-protocol-layers

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.wendangku.net/doc/3a2799902.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.wendangku.net/doc/3a2799902.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.wendangku.net/doc/3a2799902.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.wendangku.net/doc/3a2799902.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.wendangku.net/doc/3a2799902.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Wireshark

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(精品)

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

思科章节练习第二章到第七章试题及答案

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捕获器使用教程

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。

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.wendangku.net/doc/3a2799902.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.wendangku.net/doc/3a2799902.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.wendangku.net/doc/3a2799902.html,/CaptureSetup/NetworkMedia

wireshark练习及答案lab-dns.doc

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.wendangku.net/doc/3a2799902.html,/~kevin/com320/labs/wireshark/trace-dns.pcap https://www.wendangku.net/doc/3a2799902.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.