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This article provides in-depth analysis of DHCP Option 82 (DHCP Relay Agent) which is one of the +180 DHCP Options available to the DHCP protocol and used by the Bootstrap Protocol (BOOTP) used for allowing diskless client machines to discover and obtain their IP address. We’ll show you how DHCP Option 82 is used when implementing DHCP Snooping, the structure and content of DHCP Option 82, how and where it’s injected and removed from DHCP messages plus much more. You’ll can also download our DHCP/BOOTP Options Excel file and Wireshark packet captures of DHCP packets with Option 82 used in this article to help further understand all topics covered.
Let’s take a look at the list of topics covered in this article:
It’s highly recommend to read through our DHCP Snooping – DHCP Attack Mitigation article which is a foundation article.
The DHCP Options field is included inside every DHCP packet and is critical for the correct operation of the DHCP/BOOTP protocol. You’d be surprised to know that there are almost 200 different DHCP Options available and there are more added as new features are introduced in the protocol.
The material used in this article such as wireshark DHCP Options 82 packet captures and DHCP/BOOTP Options excel file are freely available to download from our Article Attachments section.
The diagram below shows the structure of a DHCP packet and highlights the position of the DHCP Options field.
It is important to understand that the above DHCP packet is the data payload within an Ethernet frame using UDP as the transport protocol.
The below screenshot was taken from a packet analyzer and shows an Ethernet frame with the DHCP data payload expanded:
We’ve highlighted sections of the DHCP protocol using the same colours as our previous diagram to help the correlation process. Every field shown in our diagram maps directly to the fields of the captured DHCP packet.
The area marked in green is the section where the DHCP Options field is located. In our captured packet there are a total of 8 DHCP Options used, among them is also Option 82 (Agent Information Option).
The DHCP Option 82, aka Agent Relay Information Option or Agent Information Option, was originally created by RFC 3046 to allow the DHCP relay agent (e.g switch, router, firewall or server) to identify itself and the DHCP client that sent the original DHCP message.
The DHCP Option 82 is inserted and removed by the DHCP Agent Relay (e.g switch) as shown in the diagram below:
While some DHCP servers might not support the Option 82 they are still required to copy the Option 82 value received from the DHCP client and include it in all replies back to the client. We’ll discuss the Option 82 insertion and removal process in the next section.
As we saw earlier, the DHCP Options field is positioned at the end of the DHCP packet and always contains multiple DHCP options. This of course means the DHCP Option field varies in length according to the number of options used:
In the rush to keep pace with the many challenges facing today’s organizations, all too often networking teams end up adopting practices and processes that are, shall we say, less than perfect. You probably have seen a few yourself in your own organization.
Management refusing to consider new vendors because, well, they’re new. Engineers wanting to do everything manually when automation would save them a ton of time. Overspending on capacity when there are more affordable alternatives. You get the picture.
Some practices are well known, others are less obvious. A great starting point for identifying the worst of the worst in your organization was a recent list compiled by Gartner. The list culls insight from several thousand client interactions. While the Gartner report requires payment, a free eBook from Cato Networks explains each networking practice and how they can be addressed with a cloud-based SD-WAN.
The practices fall into three categories — cultural, design and operational, and financial:
We have some exciting news for you today!
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This article covers popular Layer 2 & Layer 3 network attacks with a focus on DHCP Starvation Attacks, Man-in-the-Middle attacks, unintentional rogue DHCP servers and explains how security features like DHCP Snooping help protect networks from these attacks. We explain how DHCP Snooping works, cover DHCP Snooping terminology (trusted, untrusted ports/interfaces) and more. Finally we talk about the importance and purpose the DHCP Snooping Binding Database also used by Dynamic ARP Inspection to prevent ARP Poisoning and ARP Spoofing attacks.
Topics covered include:
DHCP Starvation attack is a common network attack that targets network DHCP servers. Its primary objective is to flood the organization’s DHCP server with DHCP REQUEST messages using spoofed source MAC addresses. The DHCP server will respond to all requests, not knowing this is a DHCP Starvation attack, and assign available IP addresses until its DHCP pool is depleted.
At this point the attacker has rendered the organization’s DHCP server useless and can now enable his own rogue DHCP server to serve network clients. DHCP Starvation is often accompanied by a Man-in-the-Middle attack as the rogue DHCP server distributes fake IP address parameters, including Gateway & DNS IP address, so that all client traffic passes through the attacker for inspection.
Typical Man-in-the-Middle attack. Client data streams flow through the attacker
Using packet capture and protocol analysis tools the attacker is able to fully reconstruct any data stream captured and export files from it. In fact the process so simple it only requires a basic level of understanding of these type of network tools.
In other cases the Man-in-the-Middle attack can be used as a reconnaissance attack with the objective to obtain information about the network infrastructure, services but also identify hosts of high interest such as financial or database servers.
It should be by now evident how a simple attack can become a major security threat for any organization. The above attacks are examples on how easy hackers can infiltrate the network and get access to valuable information by simply connecting an unauthorized/untrusted device to an available network port effectively bypassing firewalls and other levels of security.
Rogue DHCP servers are a common problem within enterprise organizations and are not always directly related with an attack. Rogue DHCP Servers tend to appear out of nowhere thanks to users who connect consumer-grade network devices to the network infrastructure unaware that they have connected an unauthorized device with a rogue DHCP server enabled.
The Rogue DHCP server then begins assigning IP addresses to hosts within the network therefore causing network connectivity problems and in many cases – major service disruptions. In a best case scenario DHCP clients are served with an invalid IP address disconnecting them from the rest of the network. Worst case scenario would be the clients been assigned an IP address used by network infrastructure devices e.g the VLAN interface on the Core switch or a firewall interface, causing serious network disruptions and conflicts.
While many organizations enforce security policies that do not allow 3rd party or unauthorized devices to be connected to their network, there are still incidents where users who do not understand (or care about) the security implications continue to connect these devices to the network infrastructure without consulting their IT Department.
Educating users and enforcing security policies can be extremely challenging which is why security mechanisms need to be in place to help mitigate these incidents and is where DHCP Snooping comes into the picture.
22nd May 2018: Acunetix, the pioneer in automated web application security software, has announced the release of version 12. This new version provides support for JavaScript ES7 to better analyse sites which rely heavily on JavaScript such as SPAs. This coupled with a new AcuSensor for Java web applications, sets Acunetix ahead of the curve in its ability to comprehensively and accurately scan all types of websites. With v12 also comes a brand new scanning engine, re-engineered and re-written from the ground up, making Acunetix the fastest scanning engine in the industry.
“Acunetix was always in the forefront when it came to accuracy and speed, however now with the re-engineered scanning engine and sensors that support the latest JavaScript and Java technologies, we are seeing websites scanned up to 2x faster without any compromise on accuracy.” announced Nicholas Sciberras, CTO.
A free trial version can be downloaded from: http://www.acunetix.com/vulnerability-scanner/download/
Acunetix DeepScan and the Acunetix Login Sequence Recorder have been updated to support ECMAScript version 6 (ES6) and ECMAScript version 7 (ES7). This allows Acunetix to better analyse JavaScript-rich sites which make use of the latest JavaScript features. The modularity of the new Acunetix architecture also makes it much easier now for the technology to stay ahead of the industry curve.
Acunetix version 12 includes a new AcuSensor for Java web applications. This improves the coverage of the web site and the detection of web vulnerabilities, decreases false positives and provides more information on the vulnerabilities identified. While already supporting PHP and ASP .NET, the introduction of Java support in AcuSensor means that Acunetix coverage for interactive gray box scanning of web applications is now possibly the widest in the industry.
Combining the fastest scanning engine with the ability to scan multiple sites at a time, in a multi-engine environment, allows users to scan thousands of sites in the least time possible. The Acunetix Multi-engine setup is suitable for Enterprise customers who need to scan more than 10 websites or web applications at the same time. This can be achieved by installing one Main Installation and multiple Scanning Engines, all managed from a central console.
Acunetix Version 12 allows the user to pause a Scan and Resume the scan at a later stage. Acunetix will proceed with the scan from where it had left off. There is no need to save any scan state files or similiar - the information about the paused scan is automatically retained in Acunetix.
A free trial version can be downloaded from: http://www.acunetix.com/vulnerability-scanner/download/