Who Is at the Door: The SYN Denial of Service

How to survive the SYN attack on a TCP/IP protocol weakness.
The Transport Station

Network traffic arrives at a given host and accumulates at the IP-TCP boundary, but nothing happens until a user-level process performs a request for network service through the transport station (TCP or UDP).

Most user-level Internet applications use a “virtual circuit” model for communication with web browsers such as Netscape or Lynx, FTP clients and Telnet clients. Steps in creating a connection or virtual circuit require the remote computer to request a “connect” which puts an IP packet in the local computer's IP-TCP boundary buffers. The local computer program requests a “listen”, then an “accept”. It is during these listen-connect-accept phases that TCP employs the three-way handshake to establish a virtual circuit.

Let's say there are two hosts, A and B, which exist on a network. A wishes to connect to B and issues a connect request. There are six bits defined in the TCP datagram header, two of which are the “SYN” (synchronize) and “ACK” (acknowledge) bits. The connect request datagram has the SYN bit set and the ACK bit cleared. When the process on host B receives the datagram, it accepts the sequence number, builds a reply datagram with B's separate sequence number plus host A's sequence number incremented by one, and the datagram is sent to A with the SYN and ACK bits on. Host A now has confirmation that B has provisionally accepted the connection, and it sends out the first data using the incremented sequence from its first datagram and returning B's incremented sequence number as an acknowledgment. The datagram now has just the ACK bit set and when it is received by host B, the connection is established. (See Figure 2.)

Figure 2. Establishing a Connection Using 3-Way Handshake

The SYN Attack

Returning to the above analogy for a moment, we can see that the knock code is able to defeat SS's rambunctious neighbors, but what if they decide to knock once an hour or once every five minutes? What is our studious student to do? The knocks distract him from his homework, but if he ignores the knocks he misses any friends who come by. In other words, frequent knocks deny service to SS's friends.

The same is true at the IP-TCP boundary buffers. Once the host receives a SYN datagram and replies with an ACK datagram, how long does the host wait for the third part of the handshake? Unfortunately, current implementations wait forever.

Under normal circumstances, connections are established quickly, and so developers assumed that only a few buffers would be needed for all possible connections in the host. Under the 1.2.x Linux kernels, only 10 buffers are allocated.

To create a SYN attack, a program does not simply use the connect request; instead it opens a raw network connection directly and sends a burst of TCP SYN datagrams, ignoring any replies from the target host. The few buffers are now full and the target host is unable to establish any subsequent connections. Service has been denied to the target host. (See Figure 3.)

Figure 3. The SYN Attack

What makes this attack so insidious is that the attacker also inserts random IP source addresses in each datagram, thereby making it almost impossible for the remote host to trace the datagrams back to the real source.

A Case Study

An Internet Service Provider (ISP) closed a user's account because the user violated their acceptable use policy. This user now gets an account at a competing ISP and, armed with the latest issue of 2600, dials up the new ISP using his PC running Linux. The user compiles the sample program given in 2600, and runs it repeatedly against his old ISP's mail server and web server, filling up the connection queue on the ISP's servers. No one can receive mail or reach the ISP's web pages.

After restarting his web server several times, an administrator at the ISP runs netstat and notices a lot of the entries are flagged SYN_RECV. All of these entries are from random IP addresses. The administrator tries to ping several of the addresses, but they all fail to return any pings. The administrator then calls his network provider, a prominent National Service Provider (NSP), and requests help in tracking the attacks to the source. Unfortunately, the NSP is very busy maintaining its network, and doesn't have the resources to assist in such a search.

The ISP goes out of business.


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