Welcome!

Linux Authors: Sematext Blog, Plutora Blog, Carmen Gonzalez, Pat Romanski, Elizabeth White

Blog Feed Post

The Ins and Outputs of TCPDUMP

The Ins and Outputs of TCPDUMP
By: Nicholas Beris

As a Network Engineer, I spend a lot of time on, in, and around the terminal. Many of the systems that I work with are remote and taking the time to download a packet capture in the middle of an emergency call and waiting for Wireshark to get the necessary details is just too much of a hassle. (Plus, it makes me feel like I’m an operator in the Matrix with the scrolling code.) Now don’t get me wrong, Wireshark is a great tool and has many uses, but a lot of times it’s just not practical. Besides, are you really going to download the packets from a snort alert and pump them into Wireshark? This my friends is where Tcpdump comes into play and shines.

What is Tcpdump?

Tcpdump is the most commonly and widely used tool to analyze and intercept various types of Ethernet traffic. A network administrator, security auditor, or anyone else dealing in the end to end connectivity of their infrastructure will find this tool pre-installed most of time. Many times when working with third party vendors, sometimes you have to prove that its isn’t your network, firewall, or NAT causing the issue with the application and its just poor coding on their end.

First we will look at some simple traffic and in this case will be an apt install of 2 packages.

The following is the command that I used to to ‘capture’ or record this network traffic.

tcpdump -s 1500 -Avvvn -i wlan0 -w package.pcap host 208.100.4.53

Command Breakdown

tcpdump: Name of the application.

-s 1500: Snap length is how much of the packet to get. The default is 65535 bytes. Setting the snap length to 0 sets it to it’s default. (According to the man page for my version)

-A: Prints the packet in ASCII. Useful for plain text traffic and application troubleshooting.

-vvv: Very very verbose – Prints more information about the packet such as TTL and a lot more

-n: Won’t convert address to human names

-i: Which interface to listen and capture on

-w: Write the packet to said file name

host: The remote peer

Now that we have successfully written the packets to a file we can now analyze the traffic. In any type of troubleshooting situation you have to start at square one. Lets open this file and pipe it into something useful instead of filling the scroll back buffer and missing the first essential connection details.

Since the TCP/IP stack has retransmission as part of the protocol if the first few packets fail then the rest of the connection is doomed.

tcpdump -s 1500 -Avvvn -r package.pcap | less

The -r switch reads the file instead of writing it. Since we already filtered out any other traffic with the host argument we don’t need to be as detailed in our command. The | (pipe) means direct the standard output (console screen) to another application, in the case “less”. This give us the ability to scroll through the whole .pcap file.

The first the 3 packets represent the 3 way-handshake which every TCP connection must go through to set up the connection.

12:26:58.632628 IP (tos 0x0, ttl 64, id 20547, offset 0, flags [DF]
, proto TCP (6), length 60)
10.0.1.38.59181 > 208.100.4.53.80: Flags [S], cksum 0xb5e9 (cor
rect), seq 1526109302, win 5840, options [mss 1460,sackOK,TS val 68
00437 ecr 0,nop,wscale 7], length 0
E.. .
..&.d.5.-.PZ..v...................
.g.5........

12:26:58.663268 IP (tos 0x0, ttl 51, id 0, offset 0, flags [DF], pr
oto TCP (6), length 60)
208.100.4.53.80 > 10.0.1.38.59181: Flags [S.], cksum 0x7075 (co
rrect), seq 2554095472, ack 1526109303, win 5792, options [mss 1460
,sackOK,TS val 2580458519 ecr 6800437,nop,wscale 7], length 0
E..<..@.3.g..d.5
..&.P.-. .....g.5....

12:26:58.663321 IP (tos 0x0, ttl 64, id 20548, offset 0, flags [DF]
, proto TCP (6), length 52)
10.0.1.38.59181 > 208.100.4.53.80: Flags [.], cksum 0xb5ab (cor
rect), ack 1, win 46, options [nop,nop,TS val 6800445 ecr 258045851
9], length 0
E..4PD@.@.
.
..&.d.5.-.PZ..w. .g.=....

And now my host is sending its HTTP GET request to the remote HTTP server. Remember when I said in the beginning that I was going to install two packages? Well you can see two GET requests made to the server in the output below. Can you tell what I was installing?

12:26:58.663524 IP (tos 0x0, ttl 64, id 20549, offset 0, flags [DF]
, proto TCP (6), length 435)
10.0.1.38.59181 > 208.100.4.53.80: Flags [P.], cksum 0x0d68 (correct), seq 1:384, ack 1, win 46, options [nop,nop,TS val 6800445 ecr 2580458519], length 383
E...PE@.@. A
..&.d.5.-.PZ..w. .g.=....GET /debian/pool/main/a/awn-extras-applets/awn-applets-c-extras_0.4.0-3_amd64.deb HTTP/1.1
Host: mirror.steadfast.net
Connection: keep-alive
User-Agent: Debian APT-HTTP/1.3 (0.8.10.3)

GET /debian/pool/main/a/awn-extras-applets/awn-applets-python-extras_0.4.0-3_all.deb HTTP/1.1
Host: mirror.steadfast.net
Connection: keep-alive
User-Agent: Debian APT-HTTP/1.3 (0.8.10.3)

How do we know that the server even received our request? TCP will always send an ACK, or in the case of a corrupt packet, a reset (RST) the last packet. As you can see in the following output there is the acknowledge of the GET request and then the server’s HTTP 200 OK response.

12:26:58.693420 IP (tos 0x0, ttl 51, id 52036, offset 0, flags [DF]
, proto TCP (6), length 52)
208.100.4.53.80 > 10.0.1.38.59181: Flags [.], cksum 0xb406 (cor
rect), ack 384, win 54, options [nop,nop,TS val 2580458549 ecr 6800
445], length 0
E..4.D@.3....d.5
..&.P.-. ...5.g.=
12:26:58.750220 IP (tos 0x0, ttl 51, id 52037, offset 0, flags [DF]
, proto TCP (6), length 1500)
208.100.4.53.80 > 10.0.1.38.59181: Flags [.], seq 1:1449, ack 3
84, win 54, options [nop,nop,TS val 2580458605 ecr 6800445], length
1448
E....E@.3....d.5
..&.P.-. ...m.g.=HTTP/1.1 200 OK
Date: Sun, 18 Mar 2012 16:27:03 GMT
Server: Apache/2.2.3 (CentOS)
Last-Modified: Mon, 19 Jul 2010 07:02:03 GMT
ETag: "2ae23c67-1e17e-48bb8254eb0c0"
Accept-Ranges: bytes
Content-Length: 123262
Connection: close
Content-Type: text/plain; charset=UTF-8

And as they say “the rest is history”. Well, technically the rest is of the TCP stream for my packages, but if you are troubleshooting further than the initial connections you are going to need to roll up your sleeves and have a firm grasp of the TCP protocol. If you’re not as strong at reading packet captures or understanding how the whole TCP/IP stacks work, then this is the best way to learn with simple, easy to define and read traffic. In my next entry I plan on going more in depth with situational examples.

Read the original blog entry...

More Stories By Hurricane Labs

Christina O’Neill has been working in the information security field for 3 years. She is a board member for the Northern Ohio InfraGard Members Alliance and a committee member for the Information Security Summit, a conference held once a year for information security and physical security professionals.

@ThingsExpo Stories
IoT is still a vague buzzword for many people. In his session at @ThingsExpo, Mike Kavis, Vice President & Principal Cloud Architect at Cloud Technology Partners, discussed the business value of IoT that goes far beyond the general public's perception that IoT is all about wearables and home consumer services. He also discussed how IoT is perceived by investors and how venture capitalist access this space. Other topics discussed were barriers to success, what is new, what is old, and what the future may hold. Mike Kavis is Vice President & Principal Cloud Architect at Cloud Technology Pa...
Dale Kim is the Director of Industry Solutions at MapR. His background includes a variety of technical and management roles at information technology companies. While his experience includes work with relational databases, much of his career pertains to non-relational data in the areas of search, content management, and NoSQL, and includes senior roles in technical marketing, sales engineering, and support engineering. Dale holds an MBA from Santa Clara University, and a BA in Computer Science from the University of California, Berkeley.
The Internet of Things (IoT) is rapidly in the process of breaking from its heretofore relatively obscure enterprise applications (such as plant floor control and supply chain management) and going mainstream into the consumer space. More and more creative folks are interconnecting everyday products such as household items, mobile devices, appliances and cars, and unleashing new and imaginative scenarios. We are seeing a lot of excitement around applications in home automation, personal fitness, and in-car entertainment and this excitement will bleed into other areas. On the commercial side, m...
Almost everyone sees the potential of Internet of Things but how can businesses truly unlock that potential. The key will be in the ability to discover business insight in the midst of an ocean of Big Data generated from billions of embedded devices via Systems of Discover. Businesses will also need to ensure that they can sustain that insight by leveraging the cloud for global reach, scale and elasticity.
The 3rd International Internet of @ThingsExpo, co-located with the 16th International Cloud Expo - to be held June 9-11, 2015, at the Javits Center in New York City, NY - announces that its Call for Papers is now open. The Internet of Things (IoT) is the biggest idea since the creation of the Worldwide Web more than 20 years ago.
"People are a lot more knowledgeable about APIs now. There are two types of people who work with APIs - IT people who want to use APIs for something internal and the product managers who want to do something outside APIs for people to connect to them," explained Roberto Medrano, Executive Vice President at SOA Software, in this SYS-CON.tv interview at Cloud Expo, held Nov 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA.
Performance is the intersection of power, agility, control, and choice. If you value performance, and more specifically consistent performance, you need to look beyond simple virtualized compute. Many factors need to be considered to create a truly performant environment. In his General Session at 15th Cloud Expo, Harold Hannon, Sr. Software Architect at SoftLayer, discussed how to take advantage of a multitude of compute options and platform features to make cloud the cornerstone of your online presence.
SYS-CON Media announced that Splunk, a provider of the leading software platform for real-time Operational Intelligence, has launched an ad campaign on Big Data Journal. Splunk software and cloud services enable organizations to search, monitor, analyze and visualize machine-generated big data coming from websites, applications, servers, networks, sensors and mobile devices. The ads focus on delivering ROI - how improved uptime delivered $6M in annual ROI, improving customer operations by mining large volumes of unstructured data, and how data tracking delivers uptime when it matters most.
Cultural, regulatory, environmental, political and economic (CREPE) conditions over the past decade are creating cross-industry solution spaces that require processes and technologies from both the Internet of Things (IoT), and Data Management and Analytics (DMA). These solution spaces are evolving into Sensor Analytics Ecosystems (SAE) that represent significant new opportunities for organizations of all types. Public Utilities throughout the world, providing electricity, natural gas and water, are pursuing SmartGrid initiatives that represent one of the more mature examples of SAE. We have s...
Since 2008 and for the first time in history, more than half of humans live in urban areas, urging cities to become “smart.” Today, cities can leverage the wide availability of smartphones combined with new technologies such as Beacons or NFC to connect their urban furniture and environment to create citizen-first services that improve transportation, way-finding and information delivery. In her session at @ThingsExpo, Laetitia Gazel-Anthoine, CEO of Connecthings, will focus on successful use cases.
The Internet of Things will greatly expand the opportunities for data collection and new business models driven off of that data. In her session at @ThingsExpo, Esmeralda Swartz, CMO of MetraTech, discussed how for this to be effective you not only need to have infrastructure and operational models capable of utilizing this new phenomenon, but increasingly service providers will need to convince a skeptical public to participate. Get ready to show them the money!
DevOps Summit 2015 New York, co-located with the 16th International Cloud Expo - to be held June 9-11, 2015, at the Javits Center in New York City, NY - announces that it is now accepting Keynote Proposals. The widespread success of cloud computing is driving the DevOps revolution in enterprise IT. Now as never before, development teams must communicate and collaborate in a dynamic, 24/7/365 environment. There is no time to wait for long development cycles that produce software that is obsolete at launch. DevOps may be disruptive, but it is essential.
“With easy-to-use SDKs for Atmel’s platforms, IoT developers can now reap the benefits of realtime communication, and bypass the security pitfalls and configuration complexities that put IoT deployments at risk,” said Todd Greene, founder & CEO of PubNub. PubNub will team with Atmel at CES 2015 to launch full SDK support for Atmel’s MCU, MPU, and Wireless SoC platforms. Atmel developers now have access to PubNub’s secure Publish/Subscribe messaging with guaranteed ¼ second latencies across PubNub’s 14 global points-of-presence. PubNub delivers secure communication through firewalls, proxy ser...
The industrial software market has treated data with the mentality of “collect everything now, worry about how to use it later.” We now find ourselves buried in data, with the pervasive connectivity of the (Industrial) Internet of Things only piling on more numbers. There’s too much data and not enough information. In his session at @ThingsExpo, Bob Gates, Global Marketing Director, GE’s Intelligent Platforms business, to discuss how realizing the power of IoT, software developers are now focused on understanding how industrial data can create intelligence for industrial operations. Imagine ...
The true value of the Internet of Things (IoT) lies not just in the data, but through the services that protect the data, perform the analysis and present findings in a usable way. With many IoT elements rooted in traditional IT components, Big Data and IoT isn’t just a play for enterprise. In fact, the IoT presents SMBs with the prospect of launching entirely new activities and exploring innovative areas. CompTIA research identifies several areas where IoT is expected to have the greatest impact.
Connected devices and the Internet of Things are getting significant momentum in 2014. In his session at Internet of @ThingsExpo, Jim Hunter, Chief Scientist & Technology Evangelist at Greenwave Systems, examined three key elements that together will drive mass adoption of the IoT before the end of 2015. The first element is the recent advent of robust open source protocols (like AllJoyn and WebRTC) that facilitate M2M communication. The second is broad availability of flexible, cost-effective storage designed to handle the massive surge in back-end data in a world where timely analytics is e...
The 3rd International Internet of @ThingsExpo, co-located with the 16th International Cloud Expo - to be held June 9-11, 2015, at the Javits Center in New York City, NY - announces that its Call for Papers is now open. The Internet of Things (IoT) is the biggest idea since the creation of the Worldwide Web more than 20 years ago.
The Internet of Things promises to transform businesses (and lives), but navigating the business and technical path to success can be difficult to understand. In his session at @ThingsExpo, Sean Lorenz, Technical Product Manager for Xively at LogMeIn, demonstrated how to approach creating broadly successful connected customer solutions using real world business transformation studies including New England BioLabs and more.
"There is a natural synchronization between the business models, the IoT is there to support ,” explained Brendan O'Brien, Co-founder and Chief Architect of Aria Systems, in this SYS-CON.tv interview at the 15th International Cloud Expo®, held Nov 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA.
The Internet of Things will put IT to its ultimate test by creating infinite new opportunities to digitize products and services, generate and analyze new data to improve customer satisfaction, and discover new ways to gain a competitive advantage across nearly every industry. In order to help corporate business units to capitalize on the rapidly evolving IoT opportunities, IT must stand up to a new set of challenges. In his session at @ThingsExpo, Jeff Kaplan, Managing Director of THINKstrategies, will examine why IT must finally fulfill its role in support of its SBUs or face a new round of...