TCP does not build in a layer of strong authentication or encryption; this task is left to the application layer. As a result, TCP sessions are vulnerable to a variety of attacks designed to inject data into a TCP stream, hijack a session, or force a session to close.
In order to inject data into an established TCP connection, the attacker must know (or guess) the current sequence number used to track data delivery, which depends on the initial sequence number that each side of the connection chose before any data was transmitted. Significant work has gone into some TCP stacks to ensure that initial sequence numbers are randomly chosen (the OpenBSD TCP stack is a great example of this), and the size of the sequence number field in the TCP header (32 bits) also provides some resistance to guessing when a TCP connection cannot be sniffed by an attacker. However, a rather famous example of guessing TCP sequence numbers in the context of tearing down BGP peering sessions in Cisco routers with RST packets was reported by Paul A. Watson in "Slipping in the Window: TCP Reset Attacks" (see http://osvdb.org/reference/SlippingInTheWindow_v1.0.doc for more information).
Whenever a network gateway is running iptables, one of the best ways to hinder someone on an internal network from using sequence-guessing attacks against external TCP sessions is to build in rules that drop spoofed packets that originate from the internal network. That is, for such attacks to be successful, an attacker must spoof packets past iptables and into the connection from either the external TCP client or server IP address. With iptables, it's easy to stop spoofed packets from being forwarded by dropping any packet that hits an internal interface with a source address that lies outside the internal network. (This is implemented by the default iptables policy discussed in Chapter 1.)
A SYN flood creates massive numbers of TCP SYN packets from spoofed source addresses and directs them toward a particular TCP server. The goal is to overwhelm the server by forcing the targeted TCP stack to commit all of its resources to sending out SYN/ACK packets and wait around for ACK packets that will never come. A SYN flood is purely a Denial of Service attack. Some protection from SYN floods is offered by iptables with the limit match:
[iptablesfw]# iptables -I FORWARD 1 -p tcp --syn -m limit --limit 1/s -j ACCEPT
Under certain conditions, the transport layer can issue responses to traffic. Firewalls or other filtering devices can implement filtering operations based on transport layer headers (see the iptables.sh script presented in Chapter 1), manufacture TCP RST or RST/ACK packets to tear down TCP connections, or throttle rates of incoming packets (such as the number of TCP SYN packets in a given period of time).
NOTE We will see more active response measures in Chapters 10 and 11, where we'll show how iptables is used to respond at both the network and transport layers upon detecting application layer attacks.
However, the application layer is where most of the interesting action is these days in terms of breaking into systems. The transport layer communications involved in delivering an application layer exploit to a targeted system are benign by themselves (an attacker wants the transport layer to work, after all). Responding to transport layer activities such as port scans and port sweeps is risky because of the ease with which port scans and port sweeps are sent from spoofed source IP addresses.
In the context of TCP, the transport layer has a built-in response mechanism for terminating a connection. This ability is implemented in the form of a TCP RST (Reset) or RST/ACK (Reset/Acknowledgment) packet. This packet informs the receiving TCP stack that no more data can be sent and that the connection is to be terminated, regardless of its current state. The RST flag is one of the elements in the 6-bit-wide control bits field in the TCP header. It is used whenever an untenable condition is encountered by either a TCP client or server, and either side of the connection may issue a RST.
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