Cyber security based on computer networks traffic

Cyber security based on computer networks traffic – theoretical
Adrian Florin Badea*
Victor Croitoru**
Daniel Gheorghicӑ*
Rezumat. Articolul descrie o prezentare
generalӑ a vulnerabilitӑților și posibilitӑților
curente de exploatare ale rețelelor de
calculatoare, posibilitӑți de monitorizare a
traficului de rețea prin interceptare și
duplicare și modalitӑți de analizare a
traficului monitorizat.
Cuvinte cheie: securitate, securitate
ciberneticӑ, vulnerabilitӑți, exploatӑri,
monitorizarea traficului, analizarea traficului
de rețea
Abstract. The paper presents an overview of
the current computer networks
vulnerabilities and exploits, possibilities of
monitoring the network traffic, by
intercepting and duplicating it and ways of
analyzing the monitored traffic.
Keywords: security, cyber security,
vulnerabilities, exploits, monitoring traffic,
analyze network traffic
1. Introduction
network administrator to maintain a robust
Lately there was a very important
development of computer network security
systems. With the extraordinary expansion
of the internet and of all the devices that
connect to computer networks, the cyber
security became critical for protecting the
information and data hold and used in
virtual environments.
Computer networks security is, at this
moment, integral part of computer networks
and it involves protocols, technologies,
systems, instruments and techniques for
securing and blocking the malicious attacks.
Cyber-attacks increased considerably in the
last few years.
Monitoring computers, systems and services
that make up computer networks is achieved
by collecting traffic from systems.
Examination of the collected traffic allows a
Regular collection of network information
allows the creation of log files, records and
reports of systems performance, so that the
network can be monitored. With these data
obtained, performance and infrastructure of
the network can be optimized.
Vulnerabilities in computer networks are
weaknesses which allows attackers to bypass
the system's or network’s security. This
grants the attacker access to critical
information on victim’s computer. To exploit
a vulnerability, an attacker must have at least
one applicable tool or technique that allows
connection to a system weakness.
Vulnerability connection is the cyclical
practice of identifying, classifying,
remediating, and mitigating vulnerabilities.
* Kapsch S.R.L, Piaţa Montreal, nr. 10, World Trade Center, intrarea E, etaj 2, Sector 1, Bucureşti, mail:
** Universitatea Politehnica Bucureşti, Facultatea de Electronică, Telecomunicaţii şi Tehnologia
Informaţiei, Bd. Iuliu Maniu, nr. 1-3, Sector 6, Bucureşti, mail: 2. Computer network security
2.1 Network security
Cyber security is, at this moment, an integral
part of computer networks and involves
protocols, technologies, systems, tools and
techniques, to secure and stop malicious
attacks. Cyber attacks have increased
considerably in recent years.
Since the systems for collecting network
traffic can be maliciously used to spy users
or to obtain personal and confidential
information such as passwords, it is
important to maintain the possibility of
securing the computer networks.
A good design is considered if security is
implemented consistently, in the entire
network, in as many ways and places as
Controlled access - i.e. the idea of "Policy of
Least Privilege"- the policy of fewer
privileges is based on blocking anything and
allowing only what is truly necessary.
To design a secure computer network is
important to follow these principles [1]:
- specific security role – according to
which the decision about the access and
privileges is based on the role of the network
- user awareness – is essential for the
network members to understand the
importance of security. The vast majority of
security problems do not occur because of
malicious people, but due to
misunderstanding the purpose of the security
- monitoring - using systems to
collect network traffic, which ensure the
preservation of security and network
protection from any attack;
- patching/upgrading – is a
fundamental action in order to protect
against possible vulnerabilities.
The most strategies for internal protection
are based on policy implementation. Even a
small computer network has a number of
accounts and user groups with different
levels of rights and permissions. Every time
when a user has the right to access
resources, it is possible to create potential
network security breaches. To protect the
network insider attacks, the network
administrator must implement the
appropriate measures to control passwords,
user accounts, permissions and policies.
The passwords are the major key to protect
the computer network. An user account with
a valid password can penetrate any system. Even though, the user has limited
permissions, security breaches can still be
created. The users must choose strong
passwords with at least 6-8 characters,
including letters, numbers and special
characters. Also, users must change their
passwords at regular periods of time. Because most of the time users are using
very simple passwords which can be hacked
very easily or they simply forget to change
the passwords, there are two alternatives to
passwords: smart devices and biometric
The access to user accounts must be granted
only to the allocated persons, and these
accounts must have the permissions to
access only the resources they need and
nothing more. The strict control of user
accounts is critical to prevent unauthorized
access. One of the best methods to control
the accounts is the group. Rights and
permissions are granted to groups instead of
individual user accounts.
While the rights/permissions control the way
in which users have access to shared
resources, there are also a number of other
functions that must be under control. Using
policies, all network operating systems have
different ways to control hundreds of
security parameters (for example,
permissions for users to install software or
the specific users that may connect to
different systems etc.). For security reasons,
computer networks should be using different
levels of protection, as represented in
figure 1.
Fig. 1. Network using different levels of cyber security
2.2 Computer networks vulnerabilities
Computer users and network administrators
can protect computer systems from
vulnerabilities by keeping software security
patches up to date. These patches can
remedy flaws or security holes that were
found in the initial release. Computer and
network personnel should also stay informed
about current vulnerabilities in the software
they use and seek out ways to protect against
A typical day [2] at a company:
At every 1 minute a computer accesses a
page with malicious content.
At every 3 minutes a bot communicates data
to command and control center.
At every 9 minutes a highly risk application
is accessed.
At every 10 minutes a known virus is
At every 27 minutes an unknown virus is
At every 49 minutes confidential data is sent
outside the company.
At every 24 hours a computer is infested
with a bot.
The main current cyber security risks and
attacks [1] for which protection on networks
is needed are:
Blocking a system service (Denial of Service
- DoS) – represents an attack which tries to
put the target in a blocking state so that the
customers cannot use it anymore. There are
a lot of ways that can set a blocking state to
a target, like overwhelming the target with
continuous connecting attempts.
Distributed blocking of a system service
(Distributed denial of Service - DDoS) – is
an attack which uses a collection of
accomplices for attacking a target from
multiple locations at once, without their
SYN flood attack (Synchronization) - this
takes place when a network is overwhelmed
with SYN packets which initiates
incomplete connection requests, so 8
UDP flood attack - similar with ICMP flood
attack, UDP flood takes place when UDP
packets are sent in order to slow down the
system, so it cannot handle valid
Port scan attack – takes place when packets
are sent with different port numbers, in order
to scan the available services, hoping that
port will respond.
Ping of death - TCP/IP specifications
require a certain packet size for transmitting
data. Many implementations of ping
command allows the user to specify a bigger
size for the transmitted packet.
IP spoofing – takes place when an attacker
tries to bypass the security of a network
firewall, by using the IP address of a real
user, e-mail address or the ID of a user. This
is important when an attacker decides to
exploit trust relationships between several
Land attack – it is a combination of a SYN
flood attack and IP spoofing attack and it
takes place when an attacker sends fake
SYN packets that contain the victim’s IP
address as both destination and source. The
receiving system responds transmitting the
SYN-ACK packet to itself and as a result it
creates an empty session which lasts until
the waiting period is reached. Bombing a
system with empty sessions can overwhelm
the system and it can block the target’s
Tear Drop attack – exploits the reassembly
of fragmented IP packets. In the header of
IP, one of the options is the length value.
When the sum between the length and the
size of a fragmented packet differs from the
next fragmented packet, the packets overlap
and the server’s attempts to reassembly the
packets might block the system.
Ping Scanning - similar to port scanning
attack, an attack with ping scan takes place
when an attacker sends ICMP echo requests
(or ping commands) to different IP
addresses, hoping that it will receive a
response back, and consequently, it will
discover the IP address of a possible target.
Java/ActiveX/ZIP/EXE attack – Java or
ActiveX malicious components can be
hidden in web pages. When they are
downloaded, these mini-applications can
install a Trojan horse in the computer.
Similarly, Trojan horses can be hidden in
compressed files like .zip, .gzip and .tar and
in executable files (.exe). By validating this
function, it can block all the Java and
ActiveX mini-applications contained by web
pages and by .zip, .gzip, .tar and .exe e-mail
WinNuke attack – WinNuke is a hacker
application whose only intention is to block
any computer from the network that is
running Windows operating system.
WinNuke transmits data out of the band,
usually on port 139 NetBIOS, to a host with
an established connection, by introducing an
overlap of a NetBIOS fragment which
causes the computer to stop. This is the
reason why NetBIOS should not be
permitted in or outside the network.
Smurf attack – uses the ping command to
target devices using an intermediate system,
hiding the real attacking source.
Brute force attack – an attacker attempts to
guess passwords using techniques like
repeated attempts to open a work session on
an account or using a dictionary with
possible passwords.
Source routing – represents an option in the
header IP which defines the way of directing
the packets. When this option is activated on
several firewall systems, rules are not used,
granting an attacker access to the network.
For example, the information held by the IP
header, might contain different data to direct
to another source IP address, than the
original header.
ICMP flood attack – when ping commands
overload a system with so many echo
requests, so that the system uses all its
resources to respond, until it cannot process
valid network traffic.
Sniffing packets – using a device to intercept
network traffic represents a passive attack
which allows a network interface to be
configured in promiscuous mode.
3. Traffic monitoring and pattern
3.1 Monitoring the information in
computer networks
A sniffer is a device or a program that can
collect network traffic, in order to decode it
and to process it. The program will collect
data which is addressed to other machines,
saving it for future analyses.
The networks are using IP packets to send
all their information. The transfer unit of a
TCP/IP data packet is called IP packet. The
IP packet is made of a header (which
contains IP information) and the actual data
(the only part of the IP packet relevant for
higher levels).
In the easiest network example, when all the
computers are sharing the same Ethernet
cable, all the packets which are traveling
between computers are detected by every
single computer within the network. A hub
transmits every packet to every node or
machine from the network, and then each
computer’s filter remove all the packages
that are not addressed to itself. The sniffer
deactivates this filter to collect and to
analyze some or even all the packets that are
travelling the Ethernet cable; this depends
on how the sniffer is configured. If the user
of computer A sends an e-mail to the user of
computer B, the software installed on
computer C can passively intercept the
communication packets, without the
knowledge of the two users. This sniffer is
very hard to detect because it cannot
generate network traffic on its own.
A safer environment is represented by a
switched Ethernet network. Unlike a central
hub, that transmits all the network traffic to
all machines, a switch acts like a distribution
device: it receives packets directly from the
computer that generates them and it sends
them only to the computer which is
There are many possibilities of avoiding the
protocol used by a switch. A procedure,
called ARP poisoning, can make the switch
believe that the message is addressed to the
sniffer and not to the computer with the real
destination. After collecting the data, sniffer
can forward the packets to the real
destination. Another technique (MAC
flooding) seeks to send lots of physical
MAC addresses to the switch, in a short
period of time, so that the switch change to
fail-open mode. In this way, the switch starts
to act as a hub, by transmitting all the
packets to all the machines to assure that
traffic reaches the destination. Both
techniques – ARP poisoning and MAC
flooding – generate traffic signatures which
can be detected with the right software.
These programs might be used also on
internet to capture data transmitted between
different computers. The packets send to
internet must travel very long distances,
passing hundreds of routers. The sniffer can
be installed at any point along the
transmission path or on a server, which acts
like a gateway that collects personal
Sniffing programs existed for a long period
of time in two forms: commercial sniffer,
used to monitor and troubleshoot the
network and underground sniffer, used by
A sniffer is not only an instrument used by
hackers. It can be used to troubleshoot the
networks and also for many useful purposes.
On the wrong hands, this software can
capture personal and confidential data.
The flexibility of sniffers assumes:
- analyzing problems of a network;
- detecting the attempts to access the
- gathering and reporting the network
- obtain information in order to
perform a network intrusion;
- filtering suspicious network traffic
- troubleshooting client/server
Used maliciously, sniffer can spy the
network users in order to collect personal
and confidential information, like
Given that a sniffer can be used maliciously,
the most efficient way to protect a network
is by encryption. When an encryption
algorithm is used, no packets can be read /
decoded on other place than that indicated in
the destination address. The sniffer can
gather information, but their content is
indecipherable. This thing shows how
necessary it is to use only secure websites
when sending important information, like
names, addresses, passwords or credit card
• Monitoring internet traffic in
routed networks, using ARP-Spoofing
a. Sniffing in routed networks
This technique is using the ARP protocol
weaknesses. ARP protocol is used in LAN
to gather MAC addresses for their
corresponding IP addresses. When a
computer from the local network exchanges
data with another computer from the
internet, the local computer needs the
router’s MAC address, and the router in turn
needs the MAC address of the local
computer. In order to obtain the MAC
address, the router sends a broadcast
message, in the whole network, which
contains the IP address for which the MAC
address is needed. Being a broadcast
message, all the computers from that
network will receive the message. When
such a message is received, the computer’s
operating system compares the IP address
contained in the message with its own IP
address and if they match, the computer
sends its own MAC address to the router. To
minimize the number of ARP interrogations,
the operating systems are using cache
• Monitoring in promiscuous mode
To use this monitoring possibility, the
promiscuous operating mode must be
available on the network adapter. In this
mode, the network adapter arbitrarily
accepts all the packets passing through the
network segment. In a network that uses
hub, it is enough for the network adapter to
be configured in promiscuous mode in order
to have access to all the traffic generated by
the local network, because a hub is rather a
primitive device. When a packet has been
received on a specific port, the hub
retransmits the packet on all the other ports.
• Monitoring internet traffic in
routed networks
Most local networks are routed networks
that are using devices like switches and
routers. In such a network, when a packet
arrives on a particular port, this is
retransmitted on a different single port, to
the computer which the original packet was
intended. Switches maintain a MAC address
and port table associated to each of these
addresses. When a packet is received, switch
validates the destination MAC address of the
recipient from the table and selects the port
on which the packet can be routed. The
newest switches are using a port that can be
configured so that all packets to pass
through this specific port. This port is called
“port mirroring”, “Switched Port Analyzer
(SPAN)” or “Roving Analysis Port”.
ARP-Spoofing technique is based on the fact
that the ARP protocol does not use any type
of protection for possible frauds of the MAC
addresses, so that anyone can use the
router’s MAC address as its own MAC
address. Only the MAC address owner
should respond to broadcast messages, but
there is no method to prevent another
computer from the network to stop
answering to such interrogations.
b. Sniffing in wireless networks
A sniffer for wireless networks is a packet
analyzer. It can be software or hardware
specifically designed to intercept data
transmitted over the wireless network and to
decode data; it is made in a readable format.
Sniffers for wireless networks are able to
analyze the particular packets of the wireless
networks. These can monitor, intercept and
decode data for: troubleshooting network
problems, for monitoring activity and the
security of the network, for vulnerability
assessment, for traffic filtering and for
identifying various encountered problems.
3.2 Analyzing traffic in computer
Wireless sniffers can also be used to initiate
attacks over network. These can be used also
to steal data and passwords of user accounts.
The tools used to analyze traffic are usually
sniffer devices. These devices can be
hardware or software.
There are two possibilities for sniffing in
wireless networks: monitoring mode and
promiscuous mode. In monitoring mode, a
wireless sniffer is capable to collect and to
read data without transmitting data by itself.
In promiscuous mode, a wireless sniffer is
capable of reading all the data that pass
through an access point.
Many of the problems possible to occur in a
network can be caused by the TCP/IP
protocol or by specific applications.
Analyzing traffic [3] represents the process
of listening and investigating network
traffic. Analyzing the network provides an
understanding of network communications
in order to identify performance problems,
to locate security breaches and to observe
the behavior of applications.
Before analyzing traffic to identify
problems, the administrator needs to know
what is considered normal in a
communication of the network.
When a client communicates with a specific
server, the TCP/IP protocol uses a process of
resolution based on several steps highlighted
by figure 2.
Fig. 2. The necessary data communication between client and server
This process includes the following steps
(see figure 3):
- if the target is on the local network,
obtain the hardware address (MAC address);
- define the source and destination
ports used by the application;
- if the target is not on the local
network, identify the nearest router to obtain
the access path to the target;
- identifying name with an IP
- if the target is not on the local
network, identify MAC address of the
Fig. 3. The communication resolution process between client and server
When it is desired to identify the route of the
target which is in the local network, the user
first compares its own network address with
the network address of the target, such as to
determine whether the target is on the same
network or not.
If the destination is a local device, the client
should obtain the MAC address of the target.
First it checks the cache memory, to see if
the ARP information already exists. If not,
the client sends a broadcast ARP message to
search the MAC address of the target. After
receiving an ARP response, the client
updates its ARP cache. This may cause extra
traffic on the network.
If the target device is in another network, the
client must identify the route by setting the
nearest router. The client searches the local
routing tables to determine whether the input
is a computer or a network for target. If none
is available, the client looks for a default
gateway. This process cannot cause extra
traffic on the network.
In the last stage, the client must identify the
MAC address of the nearest router or of the
default gateway. The client first checks the
ARP cache. If the information is not in
cache, the client sends a broadcast ARP
message to find the MAC address of the
router and then updates its cache. This
process may cause extra traffic on the
Wireshark [4] is the most used system for
collecting and analyzing network traffic.
Available free of charge, Wireshark can be
used on many platforms. It has become a
standard for analyzing computer network
traffic. With the expansion of internet and
networks based on TCP/IP model,
Wireshark was used increasingly more for
collecting, troubleshooting, and helping
network administrators to understand the
problems they may face.
For port spanning the principle used is to
locate the device that needs to be monitored,
to connect another device to a switch and to
configure the port for the device being
Usually, “port spanning” technique is used
to configure the switch to submit a copy of
the traffic obtained from all the configured
ports to the monitored port. On the
monitoring port, a program like Wireshark is
used to collect, process and analyze data (as
shown in figure 4). This method can be used
only if the switch is hardware equipped with
a span port.
By this, an administrator can easily monitor:
LAN ports, WAN ports, server ports,
routers, or any other ports of devices that are
connected in the network.
There can be used tap or span ports to listen
to a VLAN traffic. The destination port must
coincide with the interface on the switch that
is monitored by Wireshark. In order to see
the VLAN tags, Wireshark interface does
not need to be configured on the switch as a
member on the same VLAN.
Fig. 4. Monitoring traffic in routed networks using TAP devices and SPAN port
Filters that can collect data are based on
tcpdump syntax. This syntax appears in the
libpcap/WinPcap library.
Tcpdump is used for capturing data packets
transferred over the network in the following
cases: to design networks/protocols, to
check if some network services are properly
running, to troubleshoot, to monitor and to
make statistics based on traffic. The
captured packets can be displayed
selectively in text mode, on the console
terminal or saved to a file. In addition, the
output of tcpdump application can be
viewed using Wireshark application.
Tcpdump acquires network packets captured
through the second OSI layer interface,
corresponding to a local network interface.
By default, package collection involves
intercepting all the packages, whether or not
they are intended for the host on which the
application is currently running.
For data collection, Wireshark uses Berkeley
Packet Filters. Filters are capturing packets
quite easily using host source and
destination. The engine that captures events
compares conditions with the actual MAC
address and it only considers the relevant
ones. For filtering hosts, when a hostname is
introduced, Wireshark will translate the
name into an IP address and will capture
packets that refer to that address.
Layer 4 protocols, TCP and UDP are
protocols that interconnect applications.
Node on a certain part (web client for
example) sends messages to the other side
(web server for example) to request
permission to connect to the server.
Processes codes that send the request and
also the processes that receive the request
are called port numbers. Both the TCP and
the UDP port numbers indicate the
application code that is used. Using packet
capture, packets can be filtered from/to
specific applications and also with certain
specific flags. An example of a traffic
capture using Wireshark program is shown
in figure 5.
Fig. 5. Traffic capture using Wireshark program
NetFlow is a Cisco proprietary network
protocol used for data collection and
monitoring network traffic generated by
routers and switches which support this
technology. NetFlow can analyze a large volume of
traffic to determine from where the traffic
came into the internal network, where it
goes and the overall generated traffic.
Before this innovation, in order to monitor
network performances, Simple Network
Management Protocol was used, which
although has the necessary facilities to plan
the capacity, and cannot characterize
network traffic used by applications.
NetFlow allowed visibility at the packet
level and even visibility at the byte level to
understand which IP addresses are the
sources for traffic and which applications
generated all the traffic. A traffic flow
(represented in figures 6, 7 and 8) will have
the following information about the traffic
Fig. 6. Flows per second for all the protocols collected (including TCP, UDP and ICMP) using
Fig. 7. Data flows (a), packets (b) and bits (c) per second measured in different weeks
session: network interface, source IP
address, destination IP address, IP protocol,
source port, destination port, TCP flags, the
total number of packets in the flow, the total
number of bytes in the flow, the number of
packets per second, the number of bits per
second, the average number of bits per
packet, the duration (milliseconds).
Fig. 8. Data flows per second in different days and hours
To take advantage of this protocol, the
collecting system needs a switch that is
configured to use the SPAN port. The
collection of the network traffic using
NetFlow is represented in figure 9.
Fig. 9. Collecting network traffic using NetFlow [5]
4. Conclusions
Cyber security became critical for protecting
the information and data used in virtual
environments. Vulnerabilities allow
attackers to bypass the system's or network’s
security. This grants the attacker access to
critical information on victim’s computer.
This is why monitoring computers, systems
and services that make up computer
networks is a challenging and complex task.
Monitoring is achieved by collecting traffic
from systems using sniffers on both routed
and wireless networks.
For a network administrator it is very
important to be able to analyze the network
traffic in order to identify performance
problems, to locate security breaches, to
assess the vulnerabilities and to observe the
behavior of applications. The analysis can
be made with both Wireshark software and
using the NetFlow protocol. By analyzing
network traffic, administrators should be
aware of the behavior of the users in order to
detect possible security issues.
[1] Thomas T., Network Security first-step,
Cisco Press, Indianapolis, USA, 2004
[2] Product documentation, Check Point
Security Report 2014, Check Point, San
Carlos, USA, March 2014
[3] Dean T., Network+ Guide to Networks,
Fifth edition, Course Technology, Boston,
USA, 2010
[4] Chappell L., Wireshark Network
Analysis, Second edition, Chappell
University, San Jose, USA, 2012
[5] Product documentation, Introduction to
Cisco IOS NetFlow, Cisco, San Jose, USA,
May 2012
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