Welcome!

Linux Containers Authors: Mehdi Daoudi, Elizabeth White, Mano Marks, Pat Romanski, Carmen Gonzalez

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
WebRTC is bringing significant change to the communications landscape that will bridge the worlds of web and telephony, making the Internet the new standard for communications. Cloud9 took the road less traveled and used WebRTC to create a downloadable enterprise-grade communications platform that is changing the communication dynamic in the financial sector. In his session at @ThingsExpo, Leo Papadopoulos, CTO of Cloud9, discussed the importance of WebRTC and how it enables companies to focus o...
Big Data engines are powering a lot of service businesses right now. Data is collected from users from wearable technologies, web behaviors, purchase behavior as well as several arbitrary data points we’d never think of. The demand for faster and bigger engines to crunch and serve up the data to services is growing exponentially. You see a LOT of correlation between “Cloud” and “Big Data” but on Big Data and “Hybrid,” where hybrid hosting is the sanest approach to the Big Data Infrastructure pro...
In his General Session at 16th Cloud Expo, David Shacochis, host of The Hybrid IT Files podcast and Vice President at CenturyLink, investigated three key trends of the “gigabit economy" though the story of a Fortune 500 communications company in transformation. Narrating how multi-modal hybrid IT, service automation, and agile delivery all intersect, he will cover the role of storytelling and empathy in achieving strategic alignment between the enterprise and its information technology.
Buzzword alert: Microservices and IoT at a DevOps conference? What could possibly go wrong? In this Power Panel at DevOps Summit, moderated by Jason Bloomberg, the leading expert on architecting agility for the enterprise and president of Intellyx, panelists peeled away the buzz and discuss the important architectural principles behind implementing IoT solutions for the enterprise. As remote IoT devices and sensors become increasingly intelligent, they become part of our distributed cloud enviro...
With major technology companies and startups seriously embracing IoT strategies, now is the perfect time to attend @ThingsExpo 2016 in New York. Learn what is going on, contribute to the discussions, and ensure that your enterprise is as "IoT-Ready" as it can be! Internet of @ThingsExpo, taking place June 6-8, 2017, at the Javits Center in New York City, New York, is co-located with 20th Cloud Expo and will feature technical sessions from a rock star conference faculty and the leading industry p...
"LinearHub provides smart video conferencing, which is the Roundee service, and we archive all the video conferences and we also provide the transcript," stated Sunghyuk Kim, CEO of LinearHub, in this SYS-CON.tv interview at @ThingsExpo, held November 1-3, 2016, at the Santa Clara Convention Center in Santa Clara, CA.
Things are changing so quickly in IoT that it would take a wizard to predict which ecosystem will gain the most traction. In order for IoT to reach its potential, smart devices must be able to work together. Today, there are a slew of interoperability standards being promoted by big names to make this happen: HomeKit, Brillo and Alljoyn. In his session at @ThingsExpo, Adam Justice, vice president and general manager of Grid Connect, will review what happens when smart devices don’t work togethe...
"There's a growing demand from users for things to be faster. When you think about all the transactions or interactions users will have with your product and everything that is between those transactions and interactions - what drives us at Catchpoint Systems is the idea to measure that and to analyze it," explained Leo Vasiliou, Director of Web Performance Engineering at Catchpoint Systems, in this SYS-CON.tv interview at 18th Cloud Expo, held June 7-9, 2016, at the Javits Center in New York Ci...
The 20th International Cloud Expo has announced that its Call for Papers is open. Cloud Expo, to be held June 6-8, 2017, at the Javits Center in New York City, brings together Cloud Computing, Big Data, Internet of Things, DevOps, Containers, Microservices and WebRTC to one location. With cloud computing driving a higher percentage of enterprise IT budgets every year, it becomes increasingly important to plant your flag in this fast-expanding business opportunity. Submit your speaking proposal ...
Discover top technologies and tools all under one roof at April 24–28, 2017, at the Westin San Diego in San Diego, CA. Explore the Mobile Dev + Test and IoT Dev + Test Expo and enjoy all of these unique opportunities: The latest solutions, technologies, and tools in mobile or IoT software development and testing. Meet one-on-one with representatives from some of today's most innovative organizations
20th Cloud Expo, taking place June 6-8, 2017, at the Javits Center in New York City, NY, will feature technical sessions from a rock star conference faculty and the leading industry players in the world. Cloud computing is now being embraced by a majority of enterprises of all sizes. Yesterday's debate about public vs. private has transformed into the reality of hybrid cloud: a recent survey shows that 74% of enterprises have a hybrid cloud strategy.
SYS-CON Events announced today that Super Micro Computer, Inc., a global leader in Embedded and IoT solutions, will exhibit at SYS-CON's 20th International Cloud Expo®, which will take place on June 7-9, 2017, at the Javits Center in New York City, NY. Supermicro (NASDAQ: SMCI), the leading innovator in high-performance, high-efficiency server technology, is a premier provider of advanced server Building Block Solutions® for Data Center, Cloud Computing, Enterprise IT, Hadoop/Big Data, HPC and E...
SYS-CON Events announced today that Linux Academy, the foremost online Linux and cloud training platform and community, will exhibit at SYS-CON's 20th International Cloud Expo®, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY. Linux Academy was founded on the belief that providing high-quality, in-depth training should be available at an affordable price. Industry leaders in quality training, provided services, and student certification passes, its goal is to c...
WebRTC is the future of browser-to-browser communications, and continues to make inroads into the traditional, difficult, plug-in web communications world. The 6th WebRTC Summit continues our tradition of delivering the latest and greatest presentations within the world of WebRTC. Topics include voice calling, video chat, P2P file sharing, and use cases that have already leveraged the power and convenience of WebRTC.
"Matrix is an ambitious open standard and implementation that's set up to break down the fragmentation problems that exist in IP messaging and VoIP communication," explained John Woolf, Technical Evangelist at Matrix, in this SYS-CON.tv interview at @ThingsExpo, held Nov 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA.
WebRTC sits at the intersection between VoIP and the Web. As such, it poses some interesting challenges for those developing services on top of it, but also for those who need to test and monitor these services. In his session at WebRTC Summit, Tsahi Levent-Levi, co-founder of testRTC, reviewed the various challenges posed by WebRTC when it comes to testing and monitoring and on ways to overcome them.
"A lot of times people will come to us and have a very diverse set of requirements or very customized need and we'll help them to implement it in a fashion that you can't just buy off of the shelf," explained Nick Rose, CTO of Enzu, in this SYS-CON.tv interview at 18th Cloud Expo, held June 7-9, 2016, at the Javits Center in New York City, NY.
DevOps is being widely accepted (if not fully adopted) as essential in enterprise IT. But as Enterprise DevOps gains maturity, expands scope, and increases velocity, the need for data-driven decisions across teams becomes more acute. DevOps teams in any modern business must wrangle the ‘digital exhaust’ from the delivery toolchain, "pervasive" and "cognitive" computing, APIs and services, mobile devices and applications, the Internet of Things, and now even blockchain. In this power panel at @...
Every successful software product evolves from an idea to an enterprise system. Notably, the same way is passed by the product owner's company. In his session at 20th Cloud Expo, Oleg Lola, CEO of MobiDev, will provide a generalized overview of the evolution of a software product, the product owner, the needs that arise at various stages of this process, and the value brought by a software development partner to the product owner as a response to these needs.
WebRTC services have already permeated corporate communications in the form of videoconferencing solutions. However, WebRTC has the potential of going beyond and catalyzing a new class of services providing more than calls with capabilities such as mass-scale real-time media broadcasting, enriched and augmented video, person-to-machine and machine-to-machine communications. In his session at @ThingsExpo, Luis Lopez, CEO of Kurento, introduced the technologies required for implementing these idea...