Home > Articles

This chapter is from the book

This chapter is from the book

Open Source Network AdministrationOpen Source Network Administration

Learn More Buy

This chapter is from the book

This chapter is from the book

Open Source Network AdministrationOpen Source Network Administration

Learn More Buy

8.5 Examples of Debugging with Tcpdump

The following sections provide specific examples of debugging with tcpdump.

8.5.1 Packet Flooding

Using tcpdump to find the source of a traffic flood is usually straightforward. Start by connecting a machine in a place where it will be able to monitor network traffic. If it is necessary to configure a switch so that packets will be sent to an additional port for monitoring, make sure to do so. Then run tcpdump and look for high talkers. Be sure to disable domain name lookups: 

 Linux# tcpdump -n
 17:36:16.265220 10.255.255.27.1221 > 10.18.0.100.9995: udp 1168 (DF)
 17:36:16.269171 10.255.255.27.1221 > 10.18.0.100.9995: udp 1168 (DF)
 17:36:16.273130 10.255.255.23.1221 > 10.18.0.100.9995: udp 1168 (DF)
 17:36:16.285228 10.255.255.27.1221 > 10.18.0.100.9995: udp 1168 (DF)
 17:36:16.302173 10.255.255.27.1221 > 10.18.0.100.9995: udp 1168 (DF)
 17:36:16.319372 10.255.255.27.1221 > 10.18.0.100.9995: udp 1168 (DF)
 17:36:16.334600 10.7.15.65.7000 > 10.18.1.140.7001: rx ack (66) (DF)
 17:36:16.334975 10.7.15.65.7000 > 10.18.1.140.7001: rx data (36) (DF)
 17:36:16.336606 10.255.255.27.1221 > 10.18.0.100.9995: udp 1168 (DF)
 17:36:16.336623 10.7.1.70.7000 > 10.18.1.140.7001: rx ack (66) (DF)
 17:36:16.336939 10.7.1.70.7000 > 10.18.1.140.7001: rx data (36) (DF)
 17:36:16.352253 10.255.255.27.1221 > 10.18.0.100.9995: udp 1168 (DF)
 17:36:16.356199 10.255.255.27.1221 > 10.18.0.100.9995: udp 1168 (DF)
 17:36:16.396921 10.255.255.27.1221 > 10.18.0.100.9995: udp 1168 (DF)
 17:36:16.398427 10.155.0.153.57195 > 239.255.255.253.427: udp 49
 17:36:16.400831 10.255.255.27.1221 > 10.18.0.100.9995: udp 1168 (DF)
 17:36:16.404805 10.255.255.27.1221 > 10.18.0.100.9995: udp 1168 (DF)
 17:36:16.408749 10.255.255.27.1221 > 10.18.0.100.9995: udp 1168 (DF)
 17:36:16.412705 10.255.255.27.1221 > 10.18.0.100.9995: udp 1168 (DF)
 17:36:16.416750 10.255.255.27.1221 > 10.18.0.100.9995: udp 1168 (DF)

From this short sample of output, we can see that there is a suspiciously large amount of traffic coming from 10.255.255.27 port 1221, directed at 10.18.0.100. Each packet is a UDP datagram with 1168 bytes of UDP payload data. The small difference in time stamps between packets helps convince us of the speed with which they are being sent.

Frequently, there is so much traffic on the network that it will not be so easy to determine who the high talker is. If that is the case, you may wish to rule out certain network traffic or include only certain network traffic in an attempt to focus in on the problem. For example, if you happen to know the flooding is directed at a particular host, use a filter to view traffic destined for that host: 

   Linux# tcpdump -n dst victim.example.com

8.5.2 A More Complicated Example

Imagine several workstations are having trouble accessing your Web server. The Web browser just hangs. Oddly, other machines on the same network have no problem reaching the server in a timely manner, and tests from your own workstation indicate there is no problem in connectivity. So you use a machine running tcpdump to examine the problem. First you may choose to look at the Web traffic originating from a workstation exhibiting the problem. You start tcpdump, instructing it to monitor port 80 (the port used for HTTP transactions) and then try to open the page in a Web browser: 

   Linux# tcpdump host client.example.com and port 80

But you see no traffic. Immediately, you can rule out the Web sever as the problem. If no traffic is sent to the Web server in the first place, the problem is probably not the fault of the server. So you decide to look at more traffic than just Web traffic by removing the port 80 restriction: 

   Linux# tcpdump host client.example.com
   18:06:11.162372 client.example.com.45600 > dns.example.com.doma...

What is of interest is what you did not see. Although client.example.com makes a DNS request to dns.example.com, there is no response. If you look closer: 

   Linux# tcpdump -xls 1500 host client | ./tcpdump-data-filter.pl
   18:14:12.842409 brokenclient.example.com.55313 > dns.example.co...
        4500 0048 058b 4000 ff11 9d80 0a12 0064      E..H..@........d
        0a05 061e d811 0035 0034 8a44 e4ca 0010      .......5.4.D....
        0001 0000 0000 0001 0377 7777 0765 7861      .........www.exa
        6d70 6c65 0363 6f6d 0000 0f00 0100 0029      mple.com.......)
        0800 0000 8000 0000                          ........

In the body of the request, you can see the DNS lookup is for www.example.com. Now the problem is clear: The clients are attempting to look up the IP address of www.example.com before connecting to the Web server, but the DNS server is not responding. This explains why some workstations can connect without difficulty; they already have the IP address of the Web server cached from an earlier transaction.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020