What constitutes the end-of-line character? That depends on whom you ask. However, so many incorrect implementations occur that most parsers will also accept a bare LF as the end-of-line indication. You might think of structured text formats as delimited text on steroids, and as such, rules must be in place for how values are represented and hierarchies constructed.
MIME messages separate the body parts by defining a common separator line prefixed with two dashes The message is terminated by following this separator with the same two dashes.
Listing shows an example of a text message combined with a binary version of the same message. MIME-Version: 1. Each type consists of the form of the data it represents, such as text or application, in the format of the data. In this case, plain is unencoded text and octet-stream is a series of bytes. Within these braces are zero or more member entries, each consisting of a key and a value. It has many similarities to HTML, but it aims to be stricter in its definition in order to simplify parsers and create fewer security issues.
Elements are the main structural values. They have a name and can contain child elements or text content. Only one root element is allowed in a single document. Attributes are additional name-value pairs that can be assigned to an element. Text content is just that, text. Text is a child of an element or the value component of an attribute. Listing shows a very simple XML document with elements, attributes, and text values. Hello World! Certain specifications, such as XML Schema, aim to remedy this type information deficiency but they are not required in order to process XML content.
The XML specification defines a list of well-formed criteria that can be used to determine whether an XML document meets a minimal level of structure. Encoding Binary Data In the early history of computer communication, 8-bit bytes were not the norm. Because most communication was text based and focused on English-speaking countries, it made economic sense to send only 7 bits per byte as required by the ASCII standard.
This allowed other bits to provide control for serial link protocols or to improve performance. But a 7-bit limitation presents a problem if you want to send that amusing picture to your friend via email or you want to write your mail in a non-English character set.
To overcome this limitation, developers devised a number of ways to encode binary data as text, each with varying degrees of efficiency or complexity. As it turns out, the ability to convert binary content into text still has its advantages. For example, if you wanted to send binary data in a structured text format, such as JSON or XML, you might need to ensure that delimiters were appropriately escaped.
Instead, you can choose an existing encoding format, such as Base64, to send the binary data and it will be easily understood on both sides. Hex Encoding One of the most naive encoding techniques for binary data is hex encoding. In hex encoding, each octet is split into two 4-bit values that are converted to two text characters denoting the hexadecimal representation.
The result is a simple representation of the binary in text form, as shown in Figure Figure Example hex encoding of binary data Although simple, hex encoding is not space efficient because all binary data automatically becomes percent larger than it was originally. But one advantage is that encoding and decoding operations are fast and simple and little can go wrong, which is definitely beneficial from a security perspective.
Base64 To counter the obvious inefficiencies in hex encoding, we can use Base64, an encoding scheme originally developed as part of the MIME specifications. The 64 in the name refers to the number of characters used to encode the data. The input binary is separated into individual 6-bit values, enough to represent 0 through This value is then used to look up a corresponding character in an encoding table, as shown in Figure To counter this problem, the input is taken in units of three octets, because dividing 24 bits by 6 bits produces 4 values.
Thus, Base64 encodes 3 bytes into 4, representing an increase of only 33 percent, which is significantly better than the increase produced by hex encoding. Figure shows an example of encoding a three-octet sequence into Base But yet another issue is apparent with this strategy.
What if you have only one or two octets to encode? Would that not cause the encoding to fail? If in the encoding process, no valid bits are available to use, the encoder will encode that value as the placeholder. Figure shows an example of only one octet being encoded. Note that it generates two placeholder characters. If two octets were encoded, Base64 would generate only one.
Figure Base64 encoding 3 bytes as 4 characters Figure Base64 encoding 1 byte as 3 characters To convert Base64 data back into binary, you simply follow the steps in reverse. But what happens when a non-Base64 character is encountered during the decoding? We can only hope that it makes a secure decision.
Final Words In this chapter, I defined many ways to represent data values in binary and text protocols and discussed how to represent numeric data, such as integers, in binary. Understanding how octets are transmitted in a protocol is crucial to successfully decoding values. Being able to quickly identify the structures is key to easily processing unknown protocols.
Most of the advanced traffic-capturing techniques discussed in this chapter use existing network infrastructure and protocols to redirect traffic. Rerouting Traffic IP is a routed protocol; that is, none of the nodes on the network need to know the exact location of any other nodes. A gateway is also commonly called a router, a device that routes traffic from one location to another.
For example, in Figure , the client It first sends traffic destined for the server to Router A. In turn, Router A sends the traffic to Router B, which has a direct connection to the target server; Router B passes the traffic on to its final destination. In this case, Routers A and B only know about the two networks they are directly connected to.
To get from the client to the server, the traffic must be routed. Figure An example of routed traffic Using Traceroute When tracing a route, you attempt to map the route that the IP traffic will take to a particular destination. Most operating systems have built-in tools to perform a trace, such as traceroute on most Unix-like platforms and tracert on Windows.
Listing shows the result of tracing the route to www. The output refers to a maximum number of hops. A single hop represents the network between each gateway in the entire route. If the maximum hop count is exceeded, the traceroute process will stop probing for more routers. The output also shows the round-trip time from the machine performing the traceroute and the discovered node. Routing Tables The OS uses routing tables to figure out which gateways to send traffic to. A routing table contains a list of destination networks and the gateway to route traffic to.
If a network is directly connected to the node sending the network traffic, no gateway is required, and the network traffic can be transmitted directly on the local network. Listing shows the output from Windows when you execute this command. This destination is a placeholder for the default gateway, which simplifies the management of the routing table.
Traffic sent to any destination that has no known matching route will be sent to the gateway registered for the 0.
How can you use routing to your advantage? You might not be able to influence the network configuration in an embedded system as you might not even have access to the underlying operating system, but if you can present your capturing device as a gateway between the system generating the traffic and its ultimate destination, you can capture the traffic on that system.
The following sections discuss ways to configure an OS to act as a gateway to facilitate traffic capture. Configuring a Router By default, most operating systems do not route traffic directly between network interfaces. This is mainly to prevent someone on one side of the route from communicating directly with the network addresses on the other side. The default configuration is very important for security: imagine the implications if the router controlling your connection to the internet routed traffic from the internet directly to your private network.
Therefore, to enable an OS to perform routing, you need to make some configuration changes as an administrator. Once routing is enabled, you can configure the network devices to forward traffic via your new router. Enabling Routing on Windows By default, Windows does not enable routing between network interfaces.
To enable routing on Windows, you need to modify the system registry. Incorrect changes could completely break Windows and prevent it from booting! Be sure to make a system backup using a utility like the built-in Windows backup tool before performing any dangerous changes. To enable routing on Linux for IPv4, enter the following command as root no need to reboot; the change is immediate : sysctl net.
NAT is used extensively to extend the limited IPv4 address space by hiding multiple devices behind a single public IP address. NAT can make network configuration and security easier, too. The differences between the two relate to which address is modified during the NAT processing of the network traffic. In the worst case, if the routing information is incorrect, traffic will flow in only one direction. SNAT addresses this potential problem by changing the source address of the traffic to an IP address that the destination node can route to—typically, the one assigned to the external interface of the router.
Thus, the destination node can send traffic back in the direction of the router. Figure shows a simple example of SNAT. Figure An example of SNAT from a client to a server When the client wants to send a packet to a server on a different network, it sends it to the router that has been configured with SNAT.
When the server receives this packet, it assumes the packet came from the router; so, when it wants to send a packet back, it sends the packet to 1. The router receives the packet, determines it came from an existing NAT connection based on destination address and port numbers , and reverts the address change, converting 1.
Finally, the packet can be forwarded back to the original client without the server needing to know about the client or how to route to its network. You can do so by using the ifconfig command. The outbound interface might be named something like eth0. Now you can configure the NAT rules using the iptables.
The iptables command is most likely already installed on your Linux distribution. But first, flush any existing NAT rules in iptables by entering the following command as the root user: iptables -t nat -F If the outbound network interface has a fixed address, run the following commands as root to enable SNAT. You can use DNAT to redirect specific traffic to a different destination, as shown in Figure , which illustrates traffic being redirected from both the router and the server to a proxy at Figure An example of DNAT to a proxy Figure shows a client application sending traffic through a router that is destined for domain.
When a packet is received at the router, that router would normally just forward the packet to the original destination. The proxy then establishes a new connection to the server and forwards any packets sent from the client to the server.
All traffic between the original client and the server can be captured and manipulated. Also, you have limited ability to change the network configuration the device is connected to. You need some way to reconfigure or trick the sending device into forwarding traffic through your gateway. When DHCP is used, the network configuration pushed to a node can include an IP address as well as the default gateway, routing tables, the default DNS servers, and even additional custom parameters.
If the device you want to test uses DHCP to configure its network interface, this flexibility makes it very easy to supply a custom configuration that will allow easy network traffic capture. Four types of DHCP packets are sent when negotiating the network configuration: Discover Sent to all nodes on the IP network to discover a DHCP server Offer Sent by the DHCP server to the node that sent the discovery packet to offer a network configuration Request Sent by the originating node to confirm its acceptance of the offer Acknowledgment Sent by the server to confirm completion of the configuration The interesting aspect of DHCP is that it uses an unauthenticated, connectionless protocol to perform configuration.
This is called DHCP spoofing. The dialog shown in Figure should prompt you to select the network interface you want to sniff on. Select the interface connected to the network you want to perform DHCP spoofing on. Figure Selecting the sniffing interface 4. Supply a range of IP addresses that you configured for the network interface that is capturing traffic. For example, in Figure , the IP Pool value is set to The packet is transmitted on the Ethernet network with a destination MAC address of ff:ff:ff:ff:ff:ff, which is the defined broadcast address.
Normally, an Ethernet device only processes packets with a destination address that matches its address, but if it receives a packet with the destination MAC address set to the broadcast address, it will process it, too. If one of the recipients of this broadcasted message has been assigned the target IP address, it can now return an ARP response, as shown in Figure This response is almost exactly the same as the request except the sender and target fields are reversed.
You can use Ettercap to spoof the packets, as shown in Figure Of course, if all you want is one side of the communication, you only need to poison one or the other node.
To start ARP poisoning, follow these steps: 1. Select the network interface to poison the one connected to the network with the nodes you want to poison. Configure a list of hosts to ARP poison. Depending on the size of the network, scanning can take from a few seconds to hours.
In this case, one is the client node that you want to capture, which is on IP address The other node is the gateway to the internet on IP address Choose your targets. Select one of the hosts from the list and click Add to Target 1; select the other host you want to poison and click Add to Target 2. Target 1 and Target 2 differentiate between the client and the gateway.
This should enable one-way ARP poisoning in which only data sent from Target 1 to Target 2 is rerouted. A dialog should appear. Accept the defaults and click OK. Ettercap should attempt to poison the ARP cache of your chosen targets.
If poisoning is successful, the client node should look similar to Figure For comparison, see the corresponding entry in Figure Now any traffic that is sent from the client to the router will instead be sent to the proxy shown by the MAC address of The proxy can forward the traffic to the correct destination after capturing or modifying it. One advantage that ARP poisoning has over DHCP spoofing is that you can redirect nodes on the local network to communicate with your gateway even if the destination is on the local network.
I began by describing how to configure your OS as an IP gateway, because if you can forward traffic through your own gateway, you have a number of techniques available to you. If you can determine which features a protocol supports, you can assess its security.
Analysis of an unknown protocol is typically incremental. You begin by capturing network traffic, and then analyze it to try to understand what each part of the traffic represents.
Our approach will include extracting structures and state information. NET version, and so on. For the sake of simplicity, all example command lines will use the Windows executable binaries.
When running files for. NET Core, prefix the command with the dotnet binary. The files for. NET will have a. Starting the Server Start the server by running ChatServer.
If successful, it should print some basic information, as shown in Listing This application has not been designed to be a secure chat system. Notice in Listing that the final line prints the port the server is running on in this case and whether the server has bound to all interfaces global. This is especially important on Windows. To bind to all interfaces, specify the --global parameter.
Starting Clients With the server running, we can start one or more clients. To start a client, run ChatClient. When you run the client, you should see output similar to that shown in Listing Connection from When you first start Wireshark on Windows, you should see a window similar to the one shown in Figure Figure The main Wireshark window on Windows The main window allows you to choose the interface to capture traffic from.
To ensure we capture only the traffic we want to analyze, we need to configure some options on the interface. Figure shows the options dialog that opens. In this case, we specify the filter ip host Change the IP address as appropriate for your configuration.
Click the Start button to begin capturing traffic. Generating Network Traffic and Capturing Packets The main approach to packet analysis is to generate as much traffic from the target application as possible to improve your chances of finding its various protocol structures. For example, Listing shows a single session with ChatClient for alice. Description ; service. Next, we reconfigure the ChatClient application to communicate with local port instead of the original port Review Chapters 2 and 4 for ideas on how to redirect an arbitrary application into your proxy.
Created Listener TCP PM - Tag 'Out' - Network ' Tag 'Out' - Network ' If your terminal supports bit color, as do most Linux, macOS, and even Windows 10 terminals, you can enable color support in Canape Core using the --color parameter when starting a proxy script.
The colors assigned to inbound packets are similar to those in Wireshark: pink for outbound and blue for inbound.
Protocol Analysis Using a Proxy With our proxy set up, we can begin the basic analysis of the protocol. The packets shown in Listing are simply the raw data, but we should ideally write code to parse the traffic as we did with the Python script we wrote for Wireshark. Layers; using System. ReadUInt32 ; Console. We can use this negotiation method to handle the magic value the protocol uses, but we could also use it for more complex tasks, such as enabling encryption if the protocol supports it.
The first task for the negotiation method is to read the magic value from the client and pass it to the server. Add the line load "parser. Using this loading feature allows you to write each component of your parser in a separate file to make the task of writing a complex proxy manageable.
Then add the line template. AddLayer ; just after template. This addition will instantiate a new instance of the Parser class in Listing with every connection so you can store any state you need as members of the class. By implementing this basic parsing and proxying of a client connection, all nonessential information, such as lengths and checksums, should be removed from the data.
As an added bonus, if you modify data inside the proxy, the sent packet will have the correct checksum and length to match your modifications. Add Listing to the Parser class to implement these changes and restart the proxy. ReadInt32 ; return reader. ReadBytes length. ToArray ; writer. WriteInt32 data. Length ; writer. WriteInt32 CalcChecksum data ; writer. It first reads a big endian bit integer, which is the length, then the bit checksum, and finally the data as bytes before calling a function to convert that byte array to a DataFrame.
A DataFrame is an object to contain network packets; you can convert a byte array or a string to a frame depending on what you need. It uses the ToArray method on the incoming DataFrame to convert the packet to bytes for writing. Once we have the byte array, we can recalculate the checksum and the length, and then write it all back to the DataWriter class.
Put together all the different scripts for network proxy and parsing and start a client connection through the proxy, and all nonessential information, such as lengths and checksums, should be removed from the data. Changing Protocol Behavior Protocols often include a number of optional components, such as encryption or compression.
For basic analysis, it would be nice to be able to simply remove the component. Also, if the encryption or compression is optional, the protocol will almost certainly indicate support for it while negotiating the initial connection. So, if we can modify the traffic, we might be able to change that support setting and disable that additional feature. Although this is a trivial example, it demonstrates the power of using a proxy instead of passive analysis with a tool like Wireshark.
We can modify the connection to make analysis easier. For example, consider the chat application. To enable this feature, you would pass the --xor parameter to the client. Listing compares the first couple of packets for the connection without the XOR parameter and then with the XOR parameter. My guess would be that this flag indicates that the client supports XOR encryption. My guess would be that this is a key for the XOR encryption.
Although the negotiation is valid traffic, if you now try to send a message with the client through the proxy, the connection will no longer work and may even be disconnected.
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Addressing the firewall capabilities of Linux, a handbook for security professionals describes the Netfilter infrastruction in the Linux kernel and explains how to use Netfilter as an intrusion detection system by integrating it with custom open source software and Snort rulesets, discussin such topics as Linux firewall log analysis and policies, passive network authentication and authorization, and more.
Skip to content. Attacking Network Protocols. Attacking Network Protocols Book Review:. Seven Deadliest Network Attacks. Network Attacks and Defenses. Network Attacks and Defenses Book Review:.
Serious Cryptography. Serious Cryptography Book Review:. Practical IoT Hacking. Web Technologies and Applications. Web Technologies and Applications Book Review:. Network Security Assessment.
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