Network Traffic Analysis Using tcpdump Introduction to tcpdump

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Network Traffic Analysis Using tcpdump Introduction to tcpdump

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The objectives of this course are to introduce you to the fundamentals and benefits of using tcpdump as a tool to analyze your network traffic. We’ll start with introducing concepts and output of tcpdump. One of the most important aspects of using tcpdump is being able to write tcpdump filters to look for specific traffic. Filter writing is fairly basic unless you want to examine fields in an IP datagram that don’t fall on byte boundaries. So, that is why an entire section is devoted to the art of writing filters....

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Nội dung Text: Network Traffic Analysis Using tcpdump Introduction to tcpdump

  1. Network Traffic Analysis Using tcpdump Introduction to tcpdump Judy Novak Johns Hopkins University Applied Physics Laboratory 1 All material Copyright  Novak, 2000, 2001. All rights reserved. 1
  2. Table of Contents Topics Introduction to tcpdump Writing tcpdump Filters Examination of Datagram Fields Beginning Analysis Real World Examples Step by Step Analysis References 2 2
  3. Course Objectives • Introduce the fundamentals of tcpdump • Explain how to write tcpdump filters • Examine fields in datagram for uses/misuses • Analyze traffic by placing it in categories • Demonstrate “real-world” analysis using tcpdump • Let you participate in the analysis process 3 The objectives of this course are to introduce you to the fundamentals and benefits of using tcpdump as a tool to analyze your network traffic. We’ll start with introducing concepts and output of tcpdump. One of the most important aspects of using tcpdump is being able to write tcpdump filters to look for specific traffic. Filter writing is fairly basic unless you want to examine fields in an IP datagram that don’t fall on byte boundaries. So, that is why an entire section is devoted to the art of writing filters. Before we start to use tcpdump to analyze traffic, we’ll examine many of the fields found in the IP datagram. This is done to familiarize you with those fields in theory and also how they might be used in practice. We’ll study how and why fields might be changed and for what purpose. Next, we’ll start the basic analysis process by looking at tcpdump output and categorizing the kind of traffic that you can see. Then, we’ll take a look at some real-world examples and of how tcpdump was used on monitored networks to discover what was happening. Next, the analysis process will be inspected step by step often with missteps to get you comfortable with it. As a note, all tcpdump output shown in this course is activity that actually occurred. Source and destination hosts/IP’s have been altered to obfuscate the true identities. 3
  4. Overview • Introduction to tcpdump • Writing tcpdump filters • Examination of Datagram Fields • Beginning Analysis • Real World Examples • Step by Step Analysis 4 This page intentionally left blank. 4
  5. Introduction to tcpdump • Introduction to tcpdump • Writing tcpdump Filters • Examination of Datagram Fields • Beginning Analysis • Real World Examples • Step by Step Analysis 5 This page intentionally left blank. 5
  6. Objectives • Examine the strengths/weaknesses of tcpdump • Organize collection/analysis process of tcpdump data via Shadow • Examine tcpdump output • Standard • Hexadecimal • Length fields and how to convert them to bytes • Application layer • Interpretation of payload/hex output 6 This page intentionally left blank. 6
  7. Introduction 7 This page intentionally left blank. 7
  8. Strengths • Provides audit trail/historical record of network activity • Provides absolute fidelity • Universally available and used A 8 One of the most important parts of an arsenal in your security infrastructure is at least one tool or software package that captures an audit trail or a historical record of the traffic that enters or leaves your network. There will be times when you will be required to examine activity or connections that occurred in your network – not just traffic that caused an alarm to sound. For instance, what if you suspect that your packet filtering router that acts as your perimeter defense was acting strangely after some major network changes were made. You would have to examine the traffic that was allowed into your network to assist in determining the problem. That is where tcpdump is invaluable. Also, many tools - even logs from firewalls will display suspicious traffic, yet only partial data is displayed. What if you get a log of rejected traffic, but it doesn’t display or keep TCP flags? You’ll never know what kind of connection was attempted. tcpdump allows the analyst to examine all the bits and fields that are collected. If nothing is “wrong” with the connection, examination at the bit level is unnecessary. Yet, if you suspect something “foul” with the traffic, you really need access to all the data down to the bit level. And tcpdump is a tool that is universally used and very portable. If you become familiar with this software or its Windows counterpart, windump, it can be used on just about any platform to assist you in analysis of traffic. 8
  9. Weaknesses • By default, doesn’t collect all the payload • Does not scale well on large networks • No idea of state • Limited operations • Do-it-yourself interpretations 9 tcpdump will capture 68 bytes of data from the network interface. Some of this data might be used for the link layer frame header. For Ethernet, 14 bytes of the data are used to capture fields like the source and destination MAC address, along with the type of embedded data. That leaves only 54 bytes to capture the IP header and embedded protocol header as well as any data. Most of the time this size will allow you to capture the IP header and embedded protocol header. But, sometimes protocol headers or data will be truncated. And, if you are interested in the data payload, tcpdump is really not the tool to use for this. tcpdump can collect a large volume of data for larger networks. This can be alleviated by not collecting all the data on the network – perhaps omit web traffic (port 80). Or, another way to deal with this is more disk space and faster processors to analyze all the collected data. But, at some point, the volume gets unwieldy. tcpdump blindly collects packet after packet. It has no idea of state or being able to know that a given packet is anomalous because it does not follow the flow of a normal connection. And while tcpdump has some primitive arithmetic operations or ways to manipulate bits, it cannot do complex operations for analyzing data. Finally, while it is an excellent way to collect data, tcpdump does not attempt to make interpretations of what it sees. It does have some integrity checking operations for certain data to make sure that the data is not irregular, but the analyst has to have the training and savvy to interpret the data. For the sophisticated analyst, this is a bonus because she or he can make the correct call. Compare this with a tool that is prone to false positives that gives no way of verifying the alarmed event. But, for an analyst who has little training, tcpdump can be daunting since it does not interpret events. 9
  10. tcpdump Versions • tcpdump: Unix version; official current version 3.4 • • • windump: Windows version • • • Collective effort; current version 3.5: • tcpdump-3.5.tar.gz • libpcap-0.5.tar.gz 10 tcpdump is officially supported by the Lawrence Berkeley Labs. The current version is 3.4. There is an effort to improve tcpdump and patch known problems with tcpdump and libpcap that appears to be a collective effort of anyone interested. The software for this effort can be found at Their current version is 3.5 For the Unix versions of tcpdump, you need to download software known as libpcap that implements a portable framework for capturing low-level network traffic. windump is a Windows variant of tcpdump. It also requires an application program interface to collect the traffic known as winpcap. The unofficial version of tcpdump has some nice enhancements. It decrypts more of the applications at the application layer and has a very nice capability of converting hexadecimal payload to character output. 10
  11. tcpdump in Action 0101001110 111010010011000 00100011011 packets Network tcpdump running on a host “sniffing” network packets tcpdump output 07:00:48.036746 > icmp: echo request (DF) 07:00:48.036776 > icmp: echo reply (DF) 07:02:12.622460 > udp 101 07:03:01.132414 > S 248631:248631(0) win 8760 11 We see on this slide, a host running tcpdump and gathering records from the network interface. We see the records that tcpdump has collected below. tcpdump has a default standard output based on the protocol (TCP, UDP, ICMP) of the record that is displayed. While each of the various protocols has a similar format to the other, they are also distinct in what is displayed. By default, tcpdump will collect and print, in a standard format, all the traffic passing on the network. There are command line options for tcpdump that will alter the default behavior, either by collecting specified records, printing in a more verbose mode, printing in hexadecimal or writing records as “raw packets” to a file instead of printing as standard output. 11
  12. Sample tcpdump Output Sample UDP Record 09:39:19.470000 > udp 56 timestamp source . port dest . port : protocol bytes Sample TCP Record beginning seq # data bytes 09:35:53.660000 > SF 136747297:136747297(0) win 1028 flags ending seq # 09:32:43.910000 > S 2490962508:2490962508(0) win 512 09:32:43.910000 > S 62697789:62697789(0) win 512 09:32:43.910000 > S 1360146849:1360146849(0) win 512 09:32:43.920000 > S 372884098:372884098(0) win 512 12 Since we’ll review a lot of tcpdump output in this course, here’s a chance to get more comfortable with it. This is sample output from what appears to be an nmap scan; a popular and informative scan. All records have a timestamp. The sensor host (Redhat Linux 5.2) that captured these records has the precision to capture hundredths of seconds although tcpdump allows places for up to millionths. Different protocols will have different representations in tcpdump output. One of the first challenges is to identify the protocol (TCP, UDP, ICMP). Most will be labeled and while TCP isn’t explicitly labeled, it is the only one with flag bits, sequence and acknowledgment numbers to name a few. Some protocols like DNS will be interpreted at the application layer. Because of this, you may not see the normal clues that you are used to. It may not be obvious if it is UDP or TCP so it is important to look for clues as to which it is. In general, tcpdump gives details about the source/host > destination/host. Note that the bytes (0) transferred on SYN packets is normally 0 since they do not carry a payload because this is just part of establishing the three-way handshake. 12
  13. Organizing tcpdump Using Shadow 13 This page intentionally left blank. 13
  14. How Does Shadow Help Organize? Shadow: • Collects tcpdump data in hourly files • Analyzes each hour’s data for anomalies • Formats anomalous data in html for browsing • Comes with scripts to assist in examining data 14 Shadow (Secondary Heuristics for Defensive Online Warfare) is an intrusion detection system available to all for free. It can be found at Shadow uses tcpdump as its underlying collection and processing tool. Shadow turns tcpdump from a packet collecting tool into an intrusion detection system. Shadow collects data from the network interface and stores it in hourly files in raw tcpdump compressed format. It analyzes each hour’s collected data after-the-fact and runs a series of tcpdump filters against it looking for anomalies and one-to-many source IP to destination IP traffic. Shadow will format into html all the events of interest detected by the tcpdump filters and processed by some perl programs. The analyst can examine the output with a browser and further investigate activity using some additional perl scripts to look through an hour’s or day’s worth of data. Using Shadow relieves the analyst from having to worry about the collection of tcpdump data; it automates this process. Further, it gives the analyst an automated way of examining activity. Still, the analyst has to interpret the output. As with any other intrusion detection system, it requires a savvy analyst to accurately interpret the output. However, since it is predicated upon tcpdump, the analyst has the ability to examine all the collected data down to the bit level. 14
  15. What is Shadow? • Intrusion detection system based on tcpdump • Unix-based • Performs traffic analysis • Primary focus on datagram headers • Pull-based architecture • Analyst reviews hourly events of interest via web browser • Requires a savvy analyst to interpret output • Freeware available from 15 Shadow is a Unix based intrusion detection system. It has a sensor and analysis component. The sensor component collects network traffic and the analysis component fetches that traffic and analyzes it. Both the sensor and analysis host process data in an hourly timeframe. The entire IP datagram is not captured because Shadow is mostly concerned with anomalies or events of interest found in the header portions of the datagram. The headers examined are the IP, TCP, UDP and ICMP headers. Much insight can be gained from examining these headers. By default, some payload or data is captured in the datagram. Shadow does not attempt to analyze this, but it is there in case you want to analyze it. Each hour the analysis host analyzes the previous hour’s traffic for events of interest. These events of interest are formatted in html for viewing by an analyst using a browser. This is known as a pull- based approach since the analyst is required to examine the records; the analyst is not informed or pushed alerts of anomalous events. Shadow was developed by the Shadow team at the Naval Surface Warfare Center. It is still maintained and upgraded by this team. Shadow can be downloaded at no cost from Click on the link for Current Shadow Software. 15
  16. Why Shadow? • $$$$ (free for all) • Tunable • Customize your own signatures • Change at will • Provides an audit trail of activity to/from network • Provides an intimate view of activity 16 While not the only reason to install and use Shadow, a very compelling reason is the price tag. In many cases, but not this one, you get what you pay for. Shadow is an excellent no-cost traffic analysis tool. Another benefit is that once you master Shadow, you can change it liberally at any time that you want. For instance, if you hear of a new exploit and can fashion a signature with a tcpdump filter, you can modify Shadow instantaneously. Compare this with some intrusion detection systems that do not offer the capability to change filters or signatures. You have to wait for the software company to update the filters when they get around to it and the updates may not include signatures that you would like to see. Also, since you get all the source code with Shadow, you can customize it for your whims and needs. This is highly unusual and allows you to make changes based on your proficiency of the software. Shadow uses tcpdump as its collection software. By default, you will collect most activity going into and out of your network. This can be very beneficial in providing an audit trail of activity in the network. If you ever find yourself in the midst of some kind of incident, this may be a very valuable attribute for an intrusion detection system to have. Finally, some of the more GUI kinds of intrusion detection systems do not allow the user to examine the actual traffic at the IP datagram level. Shadow, by virtue of tcpdump, will allow the user a very intimate view of the data collected. You will maintain fidelity of data and you can use all fields for interpretation and analysis. If the traffic you are analyzing is corrupted in some way, you want to be able to inspect the entire datagram. 16
  17. Shadow Architecture tcpdump filters secure copy hour 00 data hour 01 data hour 02 data DMZ sensor analysis html host output 17 The Shadow architecture is a two-host system. Typically, the sensor resides on the DMZ, but it can be placed anywhere on the network. It collects the traffic from the network interface and stores the data in hourly files which are in raw tcpdump compressed format. Each hour, the analysis host securely copies the files from the sensor. Using perl scripts it orchestrates the process of running the previous hour’s tcpdump data through a set of tcpdump filters that looks for anomalous activity. Another filter and perl script examine the data for signs of scans – one source IP attempting connections to multiple destination IP’s. All of this information is then formatted into html for viewing by the analyst. 17
  18. What is a Shadow Event of Interest? • The default filters will extract the following types of inbound traffic: • Traffic sent to broadcast address • Traffic from reserved private networks • Fragmentation • Initial SYN connections • Particular UDP ports • Specific ICMP traffic • Scans • Traffic to core infrastructure hosts A 18 Shadow comes with several tcpdump filters to examine each hour’s traffic. All records extracted by the filters are processed and formatted in html. Shadow’s focus is mainly external to internal traffic. Specifically, the default filters will look for traffic to the .255 or .0 addresses. Any IP address that is from the reserved private address spaces is displayed as well as any from the address. Also, fragments are examined – for all traffic other than ICMP, only the first fragment is displayed. Fragmentation can be normal, but it can be a sign of subversive activity trying to get around a stateless packet filtering device or elude notice of a stateless NID. For ICMP, all fragments are displayed except the last. ICMP messages should be small enough so as not to require fragmentation. For TCP records, the initial SYN connections are examined. This doesn’t necessarily mean that the connection was successful, it just indicates that the connection was attempted. Also, certain ports or hosts may have to be excluded so as not to false alarm. For UDP records, you have to maintain a list of UDP destination ports that are of interest to you. Shadow looks for signs of a one-to-many relationship of source IP to multiple destination hosts – often indicative of a scan. Finally, Shadow can be tuned to look at more granular activity to the core infrastructure hosts in your network. 18
  19. Sample Shadow Output 19 Shadow output is sorted tcpdump output. It is sorted by source IP and time to allow the analyst to group the activity by source IP. The above activity indicates a probe of port 3128 (squid proxy server port) by host A second host that is displayed because it was extracted by one of the tcpdump filters is host which appears to be probing for destination port 139 which is a NetBIOS port. Typically DNS servers do not have the NetBIOS ports open. The final set of activity appears to be a full-blown scan from source IP It is scanning the hosts on the 172.16.1 subnet for port 1243 which is a trojan known as SubSeven or BackDoorG. Having the output displayed in html for the analyst makes it easier for the analyst to examine the hour’s traffic. 19
  20. Examining tcpdump Output 20 This page intentionally left blank. 20


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