IP Spoofing: An Introduction
IP Spoofing: An Introduction
Matthew Tanase 2003-03-11
IP Spoofing: An Introduction
by Matthew Tanase
last updated March 11, 2003
Criminals have long employed the tactic of masking their true identity, from disguises to aliases to caller-id blocking. It should come as no surprise then, that criminals who conduct their nefarious activities on networks and computers should employ such techniques. IP spoofing is one of the most common forms of on-line camouflage. In IP spoofing, an attacker gains unauthorized access to a computer or a network by making it appear that a malicious message has come from a trusted machine by “spoofing” the IP address of that machine. In this article, we will examine the concepts of IP spoofing: why it is possible, how it works, what it is used for and how to defend against it.
History
The concept of IP spoofing, was initially discussed in academic circles in the 1980's. While known about for sometime, it was primarily theoretical until Robert Morris, whose son wrote the first Internet Worm, discovered a security weakness in the TCP protocol known as sequence prediction. Stephen Bellovin discussed the problem in-depth in Security Problems in the TCP/IP Protocol Suite, a paper that addressed design problems with the TCP/IP protocol suite. Another infamous attack, Kevin Mitnick's Christmas Day crack of Tsutomu Shimomura's machine, employed the IP spoofing and TCP sequence prediction techniques. While the popularity of such cracks has decreased due to the demise of the services they exploited, spoofing can still be used and needs to be addressed by all security administrators.
Technical Discussion
To completely understand how these attacks can take place, one must examine the structure of the TCP/IP protocol suite. A basic understanding of these headers and network exchanges is crucial to the process.
Internet Protocol – IP
Internet protocol (IP) is a network protocol operating at layer 3 (network) of the OSI model. It is a connectionless model, meaning there is no information regarding transaction state, which is used to route packets on a network. Additionally, there is no method in place to ensure that a packet is properly delivered to the destination.
Examining the IP header, we can see that the first 12 bytes (or the top 3 rows of the header) contain various information about the packet. The next 8 bytes (the next 2 rows), however, contains the source and destination IP addresses. Using one of several tools, an attacker can easily modify these addresses – specifically the “source address” field. It's important to note that each datagram is sent independent of all others due to the stateless nature of IP. Keep this fact in mind as we examine TCP in the next section.
Transmission Control Protocol – TCP
IP can be thought of as a routing wrapper for layer 4 (transport), which contains the Transmission Control Protocol (TCP). Unlike IP, TCP uses a connection-oriented design. This means that the participants in a TCP session must first build a connection - via the 3-way handshake (SYN-SYN/ACK-ACK) - then update one another on progress - via sequences and acknowledgements. This “conversation”, ensures data reliability, since the sender receives an OK from the recipient after each packet exchange.
As you can see above, a TCP header is very different from an IP header. We are concerned with the first 12 bytes of the TCP packet, which contain port and sequencing information. Much like an IP datagram, TCP packets can be manipulated using software. The source and destination ports normally depend on the network application in use (for example, HTTP via port 80). What's important for our understanding of spoofing are the sequence and acknowledgement numbers. The data contained in these fields ensures packet delivery by determining whether or not a packet needs to be resent. The sequence number is the number of the first byte in the current packet, which is relevant to the data stream. The acknowledgement number, in turn, contains the value of the next expected sequence number in the stream. This relationship confirms, on both ends, that the proper packets were received. It’s quite different than IP, since transaction state is closely monitored.
Consequences of the TCP/IP Design
Now that we have an overview of the TCP/IP formats, let's examine the consequences. Obviously, it's very easy to mask a source address by manipulating an IP header. This technique is used for obvious reasons and is employed in several of the attacks discussed below. Another consequence, specific to TCP, is sequence number prediction, which can lead to session hijacking or host impersonating. This method builds on IP spoofing, since a session, albeit a false one, is built. We will examine the ramifications of this in the attacks discussed below.
Spoofing Attacks
There are a few variations on the types of attacks that successfully employ IP spoofing. Although some are relatively dated, others are very pertinent to current security concerns.
Non-Blind Spoofing
This type of attack takes place when the attacker is on the same subnet as the victim. The sequence and acknowledgement numbers can be sniffed, eliminating the potential difficulty of calculating them accurately. The biggest threat of spoofing in this instance would be session hijacking. This is accomplished by corrupting the datastream of an established connection, then re-establishing it based on correct sequence and acknowledgement numbers with the attack machine. Using this technique, an attacker could effectively bypass any authentication measures taken place to build the connection.
Blind Spoofing
This is a more sophisticated attack, because the sequence and acknowledgement numbers are unreachable. In order to circumvent this, several packets are sent to the target machine in order to sample sequence numbers. While not the case today, machines in the past used basic techniques for generating sequence numbers. It was relatively easy to discover the exact formula by studying packets and TCP sessions. Today, most OSs implement random sequence number generation, making it difficult to predict them accurately. If, however, the sequence number was compromised, data could be sent to the target. Several years ago, many machines used host-based authentication services (i.e. Rlogin). A properly crafted attack could add the requisite data to a system (i.e. a new user account), blindly, enabling full access for the attacker who was impersonating a trusted host.
Man In the Middle Attack
Both types of spoofing are forms of a common security violation known as a man in the middle (MITM) attack. In these attacks, a malicious party intercepts a legitimate communication between two friendly parties. The malicious host then controls the flow of communication and can eliminate or alter the information sent by one of the original participants without the knowledge of either the original sender or the recipient. In this way, an attacker can fool a victim into disclosing confidential information by “spoofing” the identity of the original sender, who is presumably trusted by the recipient.
Denial of Service Attack
IP spoofing is almost always used in what is currently one of the most difficult attacks to defend against – denial of service attacks, or DoS. Since crackers are concerned only with consuming bandwidth and resources, they need not worry about properly completing handshakes and transactions. Rather, they wish to flood the victim with as many packets as possible in a short amount of time. In order to prolong the effectiveness of the attack, they spoof source IP addresses to make tracing and stopping the DoS as difficult as possible. When multiple compromised hosts are participating in the attack, all sending spoofed traffic, it is very challenging to quickly block traffic.
Misconceptions of IP Spoofing
While some of the attacks described above are a bit outdated, such as session hijacking for host-based authentication services, IP spoofing is still prevalent in network scanning and probes, as well as denial of service floods. However, the technique does not allow for anonymous Internet access, which is a common misconception for those unfamiliar with the practice. Any sort of spoofing beyond simple floods is relatively advanced and used in very specific instances such as evasion and connection hijacking.
Defending Against Spoofing
There are a few precautions that can be taken to limit IP spoofing risks on your network, such as:
Filtering at the Router - Implementing ingress and egress filtering on your border routers is a great place to start your spoofing defense. You will need to implement an ACL (access control list) that blocks private IP addresses on your downstream interface. Additionally, this interface should not accept addresses with your internal range as the source, as this is a common spoofing technique used to circumvent firewalls. On the upstream interface, you should restrict source addresses outside of your valid range, which will prevent someone on your network from sending spoofed traffic to the Internet.
Encryption and Authentication - Implementing encryption and authentication will also reduce spoofing threats. Both of these features are included in Ipv6, which will eliminate current spoofing threats. Additionally, you should eliminate all host-based authentication measures, which are sometimes common for machines on the same subnet. Ensure that the proper authentication measures are in place and carried out over a secure (encrypted) channel.
Conclusion
IP Spoofing is a problem without an easy solution, since it’s inherent to the design of the TCP/IP suite. Understanding how and why spoofing attacks are used, combined with a few simple prevention methods, can help protect your network from these malicious cloaking and cracking techniques.
IP address spoofing
From Wikipedia, the free encyclopedia
(Redirected from Internet protocol spoofing)
Jump to: navigation, search
In computer networking, the term Internet Protocol address spoofing is the creation of IP packets with a forged (spoofed) source IP address. Since "IP address" is sometimes just referred to as an IP, IP spoofing is another name for this term.
Contents
[hide]
* 1 How spoofing is done
* 2 How to stop spoofing
o 2.1 Upper layers
* 3 Data compression
* 4 Satellite Internet access
* 5 Other definitions
* 6 See also
* 7 External links
[edit] How spoofing is done
The header of every IP packet contains its source address. This is normally the address that the packet was sent from. By forging the header, so it contains a different address, an attacker can make it appear that the packet was sent by a different machine. This can be a method of attack used by network intruders to defeat network security measures, such as authentication based on IP addresses.
This method of attack on a remote system can be extremely difficult, as it involves modifying thousands of packets at a time and cannot usually be done using a Microsoft Windows computer. IP spoofing involves modifying the packet header, which lists, among other things, the source IP, destination IP, a checksum value, and most importantly, the order value in which it was sent. As when a box sends packets into the Internet, packets sent may, and probably will arrive out of order, and must be put back together using the order sent value. IP spoofing involves solving the algorithm that is used to select the order sent values, and to modify them correctly. This poses a major problem because if one evaluates the algorithm in the wrong fashion, the IP spoof will be unsuccessful.
This type of attack is most effective where trust relationships exist between machines. For example, it is common on some corporate networks to have internal systems trust each other, so that a user can log in without a username or password provided they are connecting from another machine on the internal network (and so must already be logged in). By spoofing a connection from a trusted machine, an attacker may be able to access the target machine without authenticating.
[edit] How to stop spoofing
Packet filtering is one defense against IP spoofing attacks. The gateway to a network should perform ingress filtering; blocking of packets from outside the network with a source address inside the network. This prevents an outside attacker spoofing the address of an internal machine. Ideally outgoing packets should also be filtered, dropping packets from inside the network with a source address that is not inside (egress filtering); this prevents an attacker within the network performing filtering from launching IP spoofing attacks against external machines.
[edit] Upper layers
Some upper layer protocols provide their own defence against IP spoofing. For example, Transmission Control Protocol (TCP) uses sequence numbers negotiated with the remote machine to ensure that arriving packets are part of an established connection. The poor implementation of TCP sequence numbers in many older operating systems and network devices, however, means that spoofing is often still possible.
The attacker is limited to the fact that any reply packets that the destination may send are sent to the spoofed source address. However, an attacker can utilize this property of IP spoofing in techniques such as smurf attacks and SYN floods.
[edit] Data compression
Protocol spoofing is also used as a data compression technique, and was used as early as 1985 when the Hayes Smartmodem incorporated spoofing of portions of the UUCP protocol to improve throughput. In this context, protocol headers and trailers are trimmed down or removed entirely, and then reconstructed at the far end.
[edit] Satellite Internet access
Today, protocol spoofing is particularly important for satellite Internet access. Satellite Internet access has a relatively long latency when packets must travel between the ground and a satellite in geosynchronous orbit, and many network protocols do not handle this latency very well. Therefore, satellite broadband providers may spoof the protocol so that each end of a packet flow gets acknowledgments with a short latency. Some satellite Internet providers now also offer special VPN software that incorporates protocol spoofing, because VPN applications are particularly prone to problems with latency.
[edit] Other definitions
The term spoofing is also sometimes used to refer to header forgery, the insertion of false or misleading information in email or netnews headers. Falsified headers are used to mislead the recipient, or network applications, as to the origin of a message. This is a common technique of spammers and sporgers, who wish to conceal the origin of their messages to avoid being tracked down. Less fraudulently, some netnews users place obviously false email addresses in their headers to avoid spam or other unwanted responses.
Matthew Tanase 2003-03-11
IP Spoofing: An Introduction
by Matthew Tanase
last updated March 11, 2003
Criminals have long employed the tactic of masking their true identity, from disguises to aliases to caller-id blocking. It should come as no surprise then, that criminals who conduct their nefarious activities on networks and computers should employ such techniques. IP spoofing is one of the most common forms of on-line camouflage. In IP spoofing, an attacker gains unauthorized access to a computer or a network by making it appear that a malicious message has come from a trusted machine by “spoofing” the IP address of that machine. In this article, we will examine the concepts of IP spoofing: why it is possible, how it works, what it is used for and how to defend against it.
History
The concept of IP spoofing, was initially discussed in academic circles in the 1980's. While known about for sometime, it was primarily theoretical until Robert Morris, whose son wrote the first Internet Worm, discovered a security weakness in the TCP protocol known as sequence prediction. Stephen Bellovin discussed the problem in-depth in Security Problems in the TCP/IP Protocol Suite, a paper that addressed design problems with the TCP/IP protocol suite. Another infamous attack, Kevin Mitnick's Christmas Day crack of Tsutomu Shimomura's machine, employed the IP spoofing and TCP sequence prediction techniques. While the popularity of such cracks has decreased due to the demise of the services they exploited, spoofing can still be used and needs to be addressed by all security administrators.
Technical Discussion
To completely understand how these attacks can take place, one must examine the structure of the TCP/IP protocol suite. A basic understanding of these headers and network exchanges is crucial to the process.
Internet Protocol – IP
Internet protocol (IP) is a network protocol operating at layer 3 (network) of the OSI model. It is a connectionless model, meaning there is no information regarding transaction state, which is used to route packets on a network. Additionally, there is no method in place to ensure that a packet is properly delivered to the destination.
Examining the IP header, we can see that the first 12 bytes (or the top 3 rows of the header) contain various information about the packet. The next 8 bytes (the next 2 rows), however, contains the source and destination IP addresses. Using one of several tools, an attacker can easily modify these addresses – specifically the “source address” field. It's important to note that each datagram is sent independent of all others due to the stateless nature of IP. Keep this fact in mind as we examine TCP in the next section.
Transmission Control Protocol – TCP
IP can be thought of as a routing wrapper for layer 4 (transport), which contains the Transmission Control Protocol (TCP). Unlike IP, TCP uses a connection-oriented design. This means that the participants in a TCP session must first build a connection - via the 3-way handshake (SYN-SYN/ACK-ACK) - then update one another on progress - via sequences and acknowledgements. This “conversation”, ensures data reliability, since the sender receives an OK from the recipient after each packet exchange.
As you can see above, a TCP header is very different from an IP header. We are concerned with the first 12 bytes of the TCP packet, which contain port and sequencing information. Much like an IP datagram, TCP packets can be manipulated using software. The source and destination ports normally depend on the network application in use (for example, HTTP via port 80). What's important for our understanding of spoofing are the sequence and acknowledgement numbers. The data contained in these fields ensures packet delivery by determining whether or not a packet needs to be resent. The sequence number is the number of the first byte in the current packet, which is relevant to the data stream. The acknowledgement number, in turn, contains the value of the next expected sequence number in the stream. This relationship confirms, on both ends, that the proper packets were received. It’s quite different than IP, since transaction state is closely monitored.
Consequences of the TCP/IP Design
Now that we have an overview of the TCP/IP formats, let's examine the consequences. Obviously, it's very easy to mask a source address by manipulating an IP header. This technique is used for obvious reasons and is employed in several of the attacks discussed below. Another consequence, specific to TCP, is sequence number prediction, which can lead to session hijacking or host impersonating. This method builds on IP spoofing, since a session, albeit a false one, is built. We will examine the ramifications of this in the attacks discussed below.
Spoofing Attacks
There are a few variations on the types of attacks that successfully employ IP spoofing. Although some are relatively dated, others are very pertinent to current security concerns.
Non-Blind Spoofing
This type of attack takes place when the attacker is on the same subnet as the victim. The sequence and acknowledgement numbers can be sniffed, eliminating the potential difficulty of calculating them accurately. The biggest threat of spoofing in this instance would be session hijacking. This is accomplished by corrupting the datastream of an established connection, then re-establishing it based on correct sequence and acknowledgement numbers with the attack machine. Using this technique, an attacker could effectively bypass any authentication measures taken place to build the connection.
Blind Spoofing
This is a more sophisticated attack, because the sequence and acknowledgement numbers are unreachable. In order to circumvent this, several packets are sent to the target machine in order to sample sequence numbers. While not the case today, machines in the past used basic techniques for generating sequence numbers. It was relatively easy to discover the exact formula by studying packets and TCP sessions. Today, most OSs implement random sequence number generation, making it difficult to predict them accurately. If, however, the sequence number was compromised, data could be sent to the target. Several years ago, many machines used host-based authentication services (i.e. Rlogin). A properly crafted attack could add the requisite data to a system (i.e. a new user account), blindly, enabling full access for the attacker who was impersonating a trusted host.
Man In the Middle Attack
Both types of spoofing are forms of a common security violation known as a man in the middle (MITM) attack. In these attacks, a malicious party intercepts a legitimate communication between two friendly parties. The malicious host then controls the flow of communication and can eliminate or alter the information sent by one of the original participants without the knowledge of either the original sender or the recipient. In this way, an attacker can fool a victim into disclosing confidential information by “spoofing” the identity of the original sender, who is presumably trusted by the recipient.
Denial of Service Attack
IP spoofing is almost always used in what is currently one of the most difficult attacks to defend against – denial of service attacks, or DoS. Since crackers are concerned only with consuming bandwidth and resources, they need not worry about properly completing handshakes and transactions. Rather, they wish to flood the victim with as many packets as possible in a short amount of time. In order to prolong the effectiveness of the attack, they spoof source IP addresses to make tracing and stopping the DoS as difficult as possible. When multiple compromised hosts are participating in the attack, all sending spoofed traffic, it is very challenging to quickly block traffic.
Misconceptions of IP Spoofing
While some of the attacks described above are a bit outdated, such as session hijacking for host-based authentication services, IP spoofing is still prevalent in network scanning and probes, as well as denial of service floods. However, the technique does not allow for anonymous Internet access, which is a common misconception for those unfamiliar with the practice. Any sort of spoofing beyond simple floods is relatively advanced and used in very specific instances such as evasion and connection hijacking.
Defending Against Spoofing
There are a few precautions that can be taken to limit IP spoofing risks on your network, such as:
Filtering at the Router - Implementing ingress and egress filtering on your border routers is a great place to start your spoofing defense. You will need to implement an ACL (access control list) that blocks private IP addresses on your downstream interface. Additionally, this interface should not accept addresses with your internal range as the source, as this is a common spoofing technique used to circumvent firewalls. On the upstream interface, you should restrict source addresses outside of your valid range, which will prevent someone on your network from sending spoofed traffic to the Internet.
Encryption and Authentication - Implementing encryption and authentication will also reduce spoofing threats. Both of these features are included in Ipv6, which will eliminate current spoofing threats. Additionally, you should eliminate all host-based authentication measures, which are sometimes common for machines on the same subnet. Ensure that the proper authentication measures are in place and carried out over a secure (encrypted) channel.
Conclusion
IP Spoofing is a problem without an easy solution, since it’s inherent to the design of the TCP/IP suite. Understanding how and why spoofing attacks are used, combined with a few simple prevention methods, can help protect your network from these malicious cloaking and cracking techniques.
IP address spoofing
From Wikipedia, the free encyclopedia
(Redirected from Internet protocol spoofing)
Jump to: navigation, search
In computer networking, the term Internet Protocol address spoofing is the creation of IP packets with a forged (spoofed) source IP address. Since "IP address" is sometimes just referred to as an IP, IP spoofing is another name for this term.
Contents
[hide]
* 1 How spoofing is done
* 2 How to stop spoofing
o 2.1 Upper layers
* 3 Data compression
* 4 Satellite Internet access
* 5 Other definitions
* 6 See also
* 7 External links
[edit] How spoofing is done
The header of every IP packet contains its source address. This is normally the address that the packet was sent from. By forging the header, so it contains a different address, an attacker can make it appear that the packet was sent by a different machine. This can be a method of attack used by network intruders to defeat network security measures, such as authentication based on IP addresses.
This method of attack on a remote system can be extremely difficult, as it involves modifying thousands of packets at a time and cannot usually be done using a Microsoft Windows computer. IP spoofing involves modifying the packet header, which lists, among other things, the source IP, destination IP, a checksum value, and most importantly, the order value in which it was sent. As when a box sends packets into the Internet, packets sent may, and probably will arrive out of order, and must be put back together using the order sent value. IP spoofing involves solving the algorithm that is used to select the order sent values, and to modify them correctly. This poses a major problem because if one evaluates the algorithm in the wrong fashion, the IP spoof will be unsuccessful.
This type of attack is most effective where trust relationships exist between machines. For example, it is common on some corporate networks to have internal systems trust each other, so that a user can log in without a username or password provided they are connecting from another machine on the internal network (and so must already be logged in). By spoofing a connection from a trusted machine, an attacker may be able to access the target machine without authenticating.
[edit] How to stop spoofing
Packet filtering is one defense against IP spoofing attacks. The gateway to a network should perform ingress filtering; blocking of packets from outside the network with a source address inside the network. This prevents an outside attacker spoofing the address of an internal machine. Ideally outgoing packets should also be filtered, dropping packets from inside the network with a source address that is not inside (egress filtering); this prevents an attacker within the network performing filtering from launching IP spoofing attacks against external machines.
[edit] Upper layers
Some upper layer protocols provide their own defence against IP spoofing. For example, Transmission Control Protocol (TCP) uses sequence numbers negotiated with the remote machine to ensure that arriving packets are part of an established connection. The poor implementation of TCP sequence numbers in many older operating systems and network devices, however, means that spoofing is often still possible.
The attacker is limited to the fact that any reply packets that the destination may send are sent to the spoofed source address. However, an attacker can utilize this property of IP spoofing in techniques such as smurf attacks and SYN floods.
[edit] Data compression
Protocol spoofing is also used as a data compression technique, and was used as early as 1985 when the Hayes Smartmodem incorporated spoofing of portions of the UUCP protocol to improve throughput. In this context, protocol headers and trailers are trimmed down or removed entirely, and then reconstructed at the far end.
[edit] Satellite Internet access
Today, protocol spoofing is particularly important for satellite Internet access. Satellite Internet access has a relatively long latency when packets must travel between the ground and a satellite in geosynchronous orbit, and many network protocols do not handle this latency very well. Therefore, satellite broadband providers may spoof the protocol so that each end of a packet flow gets acknowledgments with a short latency. Some satellite Internet providers now also offer special VPN software that incorporates protocol spoofing, because VPN applications are particularly prone to problems with latency.
[edit] Other definitions
The term spoofing is also sometimes used to refer to header forgery, the insertion of false or misleading information in email or netnews headers. Falsified headers are used to mislead the recipient, or network applications, as to the origin of a message. This is a common technique of spammers and sporgers, who wish to conceal the origin of their messages to avoid being tracked down. Less fraudulently, some netnews users place obviously false email addresses in their headers to avoid spam or other unwanted responses.
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