Cyber warfare is unfortunately no longer found only in speculative fiction; it is with us today. Distributed
) attacks have been launched against the United States, South Korea,
Kyrgyzstan, Estonia, and Georgia in recent years, and military and government computer systems
around the world are assaulted by intruders daily. Some attacks come from nation-states, but others are
perpetrated by transnational and unaligned rogue groups. Those bent on inflicting harm on nations and
citizens not only use networks as an attack vector, but also for organizing, recruiting, and publicizing their
beliefs and activities.
On the other side of the fence are the good guys, the members of the cyber intelligence community
who aim to understand and track the terrorists, and ultimately stymie their plans. Due to the pervasive
use of networks by radical and criminal organizations in the modern world, a great deal can be learned
about terrorists by examining their use of the World Wide Web, and how the Internet is used as a vector
to attack both public and private systems. This field of study is called "terrorism informatics," which is
defined as "the application of advanced methodologies and information fusion and analysis techniques
to acquire, integrate, process, analyze, and manage the diversity of terrorism-related information for
national/international and homeland security-related applications" (Hsinchun Chen et al, eds., Terrorism
Informatics. New York: Springer, 2008, p. xv).
Terrorism informatics analyzes information from data-at-rest sources such as blogs, social media, and
databases. For other types of analyses, it is necessary to examine data in motion, in other words,
information as it travels on a network. Access to data-in-motion is often obtained by eavesdropping on the
network traffic using Span
ports in switches. This paper focuses specifically on the implications of using
ports in counter-terrorism monitoring applications. It shows that Span
ports are particularly ill-suited
to this use. Note also that the security vulnerabilities of Span
ports in counter-terrorism applications
apply equally when Span
ports are used for other monitoring needs such as performance or compliance
Span Ports Mirror Traffic for Monitoring
or mirror ports are a convenient and inexpensive way to access traffic flowing through a network
switch. Switches that support Span
ports-typically high-end switches-can be configured to mirror traffic
from selected ports or VLAN
s to the Span
port, where monitoring tools can be attached. At first glance,
it seems that a Span
port could be a good way to connect an intrusion detection
recorder, or other security monitoring device.
ports have several characteristics that can be troublesome and risky in a
counter-terrorism application. These characteristics include:
- The possibility of dropping packets
- The need for reconfiguring switches
- The vulnerability of Span ports to attack
- The fact that Span ports are not passive mechanisms
These issues are elaborated in the following sections.
Problem #1: Dropped Packets
The first issue with Span
ports in a counter-terrorism application is that the visibility of network traffic is
less than perfect. In counter-terrorism monitoring, a fundamental requirement is that the security device
must be able to see every single packet on the wire. An IDS
cannot detect a virus if it doesn’t see the
packets carrying it. Span
ports cannot meet this requirement because they drop packets. Span
ning is the
switch’s lowest priority task, and Span
traffic is the first thing to go when the switch gets busy. In fact, it
is allowable for any port on a switch to drop packets because network protocols are specifically designed
to be robust in spite of dropped packets, which are inevitable in a network. But it is not acceptable in a
counter-terrorism monitoring application.
Different switches may be more or less prone to drop Span
packets depending on their internal
architecture, which varies from switch to switch. However, it is unlikely that the performance of the Span
port was evaluated as an important criterion when the switching gear was selected. As a
counter-terrorism professional, you probably don’t want your security strategy to be dependent on a
procurement policy that you don’t control.
Nevertheless, suppose you do have switches with the best possible Span
ning performance. Dropped
packets may still be an issue depending on how much traffic you need to send through the Span
you need to see all of the traffic on a full-duplex
1 Gigabit link, a 1 Gigabit Span
port won’t do the job. Full
duplex link traffic exceeds the 1 Gigabit SPAN
port capacity when link utilization goes above 50 percent
in both directions. To see all the traffic, you need to dedicate a 10 Gigabit port for Span
ning, and now the
port doesn’t seem so inexpensive any more.
port visibility issues go beyond simply dropping packets. Being switch technology, Span
ports by their very nature are not transparent for layer 1 and layer 2 information: for example, they drop
undersized and oversized packets, and packets with CRC
errors. They usually remove VLAN
In addition, Span
ports do not preserve the packet timing of the original traffic, or in some cases even the
packet order. This type of information can be critical for detecting certain types of network attacks such as
network worms and viruses, and for some behavior-based packet classification algorithms. For example,
network consultant Betty DuBois observed, "[Regarding] losing the VLAN
tag information when Span
if there is an issue with ISL or 802.1q, how will I ever know with a Span
Problem #2: The Need for Switch Configuration
Another issue with using Span
ports in a counter-terrorism application is the very fact that the switch
needs to be configured to send specific traffic to the Span
port. This fact leads to a host of complications:
- The configuration may not be done correctly. “If the switch owner mistakenly or intentionally
configures the Span port to not show all the traffic it should, you may or may not discover the
misconfiguration. I have seen this happen countless times,” said Richard Bejtlich, the highly
respected author of The Tao of Network Security Monitoring.
- Sharing the Span port. A switch typically supports only one or two Span ports, and the network
administrator or someone else may need to use “your” Span port for one reason or another.
They may or may not tell you when the Span traffic profile is changed for their needs. IT Manager
Bob Huber recalled, “Span was a huge issue we dealt with on the IDS team where I used to work.
We had constant issues with the Span going up and down. When there are network issues to
deal with, the network engineers have priority to the limited number of Span ports available.
Hoping they remember to reconfigure your Span port was a waste of time.”
- Switch configuration may not be available when you need it. If you need to change the profile
of the traffic you are Spanning, or change it back after someone else used the port, it may not
be easy to get the switch owner’s time to do it. In larger organizations, you may also need to get
the change authorized through a Change Control Board, and then wait for a maintenance window
to get it implemented.
- Changes to the network switches for other reasons can impact the Span traffic. Networks
are constantly being reconfigured to optimize applications or support new requirements. If the
counter-terrorism monitoring solution depends on Span ports, it is vulnerable to changes (planned
or surprises) any time the network is reconfigured for any reason.
- Switch configuration itself is a security vulnerability. In any counter-terrorism activity, the
network’s security is of course paramount. Switches are a highly vulnerable network point, and
the ability to reconfigure them must be tightly controlled. Does it make sense to require switch
reconfiguration as part of the counter-terrorism monitoring solution, when reconfiguring a switch
can accidentally or deliberately expose or bring down the network?
If you have any doubt that Span
port misconfiguration can be an issue, take a look at this note in the
Cisco Catalyst 6500 Series documentation: “Connectivity issues because of the misconfiguration of Span
ports occur frequently in CatOS... Be very careful of the port that you choose as a Span
Problem #3: Vulnerability to Attack
ports are usually configured for uni-directional traffic, restricted to transmitting traffic to the
monitoring device. However, in some cases they can receive traffic as well (a feature Cisco calls ingress
traffic forwarding), in order to enable management of the monitoring device over the same switch
port and monitoring device NIC
as the mirror traffic. When this configuration is used, the Span
becomes an open ingress port to the switch, creating a serious security vulnerability. Therefore, this
configuration should be avoided as a best practice. If for some reason it becomes necessary to use this
configuration, you should at least lock the Span
port to the monitoring tool’s MAC address if possible, so
an unauthorized user won’t be able to plug a laptop into the connection and hack the switch.
Problem #4: Not Passive
A final important consideration when using Span
ports for counter-terrorism monitoring access is that
ports are not passive: They can affect the performance of the switch’s other ports. For example,
Gerald Combs, the father of Wireshark, warns, “Some switch families (e.g., the Cisco 3500 sercies) don’t
set a lower priority on Span
traffic, and will slow down the backplane in order to deliver packets to a
) This effect violates a primary
principal of security and especially forensic monitoring, that monitoring should not affect the traffic being
monitored. It may have legal as well as practical implications.
The Tap Alternative
Fully passive fiber network Tap with optical splitters
To avoid the problems that Span
ports bring to counter-terrorism monitoring applications, security
experts like Bejtlich recommend using traffic access ports (Taps) for access to the network traffic. Taps
are specifically designed to provide 100 percent traffic visibility without any impact on monitored traffic.
Optical Taps for fiber links use optical splitters to divert part of the light from the link to a monitor port,
creating a true copy of the link traffic all the way down to layer 1 and layer 2 errors. Taps for copper links
perform a similar function electronically. Optical Taps do not use any power at all, while copper Taps
include relays which ensure that link traffic continues to flow even when the Tap loses power.
Taps avoid all of the pitfalls of Span
ports in counter-terrorism applications:
- Taps send the monitoring tool an exact copy of the link traffic, including layer 1 and layer 2 errors
and malformed packets, no matter how busy the link is. They never drop packets.
- Taps require little or no configuration. Once a Tap is installed in a link, monitoring access to the
link traffic is always available, consistently and persistently.
- Taps are secure. They do not have an IP address so attackers cannot see them, and they cannot
inject traffic into the network under any circumstances. In fact, a Tap actually hides the
monitoring tool from the network as well, providing true “stealth” monitoring.
- Taps are completely passive. They cannot affect the link traffic, not even if they lose power.
Tap technology has evolved to offer a range of additional features as well, most of which are not
available with Span
ports. (Note that some of these features require a trade-off with the previously
- Regeneration Taps produce multiple copies of the link traffic so multiple tools and multiple users
can view the same traffic simultaneously. Your counter-terrorism monitoring device does not
need to give up access when the network administrator needs to put an additional protocol
analyzer onto the link.
- Aggregator Taps combine the traffic from both directions of full-duplex links and from multiple
links and sends it to a single NIC on the monitoring tool. No packets are dropped as long as the
aggregated traffic does not exceed the monitor port bandwidth.
- Active Response Taps permit monitoring tools to send response packets such as TCP resets,
ICMP messages, and ACL changes into the tapped link. This feature can be used by an IDS to
take action when certain types of intrusions are detected. (Active Response Taps are an
exception to the Tap “one direction only” traffic rule.)
- iTaps provide a remote management interface and basic monitoring data about link traffic, such
as packet counts and utilization levels. (Remote management interfaces require IP addresses,
but they are secured with passwords, SSH, HTTPS, and other measures.)
- Media Conversion refers to Taps that support different media types on their network and
monitor ports. Many Taps have pluggable SFP or XFP ports enabling different media types to be
accommodated simply by plugging in different transceiver types. Some Taps even perform 10
Gigabit to 1 Gigabit and 1 Gigabit to 10 Gigabit data rate conversion as well.
- Filter Taps enable mirrored traffic to be restricted to particular protocols, source and destination
IP addresses, VLANs, ports, and other criteria, making it easier to isolate or troubleshoot
issues, and relieving monitoring tools from spending valuable processing cycles on
pre-filtering traffic. For example, the Net Optics
Director Data Monitoring Switch supports filtering
as well as regeneration, aggregation, remote management, and media conversion, all in a single
- Bypass Switches create fail-safe access ports for in-line devices such as intrusion
systems and firewalls.
The wide range of Tap devices available today enable appropriate monitoring access to be built into all
parts of the network architecture, at the edges, distribution, LAN, and core. Such a Monitoring Access
Platform (MAP) does not depend on Span
ports for strategic information access, but in fact frees up the
ports for tactical monitoring access when special needs arise. Permanent and ongoing
counter-terrorism monitoring can rely on a Tap-based
for consistent, persistent, and secure
monitoring access, immune to the vagaries of day-to-day network administration and management.
Integrated Monitoring Access Platform Based On Tap Technology - Zum Vergrößern bitte anklicken
Monitoring is an essential building block of Bejtlich´s "defensible network architecture," the first of its
seven key characteristics: monitored, inventoried, controlled, claimed, minimized, assessed, and current.
) Utilizing Span
for counter-terrorism monitoring access is placing that building block on a weak foundation, subject to
packet loss, misconfiguration, and intrusion. A Monitoring Access Platform
, based on Tap technology and
integrated within the network architecture, is an alternate access approach that provides a solid base on
which to build your network’s security and counter-terrorism applications.