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TCP SYN attack
Published: 1996-09-17 00:00:00
Updated: 1996-09-17 00:00:00


Defining Strategies to Protect Against TCP
SYN Denial of Service Attacks

     Cisco Systems Advises Industry On Security Against Possible Hacker Attacks 
     White Paper: Strategies to Protect Against UDP Diagnostic Port Denial of Service
     CERT Advisory CA-96.21: TCP SYN Flooding and IP Spoofing Attacks 
     SGI Security Advisory: TCP SYN Denial of Service Attack 

Note: Cisco IOS 11.3 software has a feature to actively prevent TCP denial of service attacks. This
feature is described in the document Configuring TCP Intercept (Prevent Denial-of-Service Attacks). 


There is a potential denial of service attack at internet service providers (ISPs) that targets network

     TCP SYN attack: A sender transmits a volume of connections that cannot be completed. This
     causes the connection queues to fill up, thereby denying service to legitimate TCP users. 

This paper contains a technical description of how the potential TCP SYN attack occurs and suggested
methods for using Cisco IOS software to defend against it. 


The TCP SYN attack:

When a normal TCP connection starts, a destination host receives a SYN (synchronize/start) packet
from a source host and sends back a SYN ACK (synchronize acknowledge). The destination host must
then hear an ACK (acknowledge) of the SYN ACK before the connection is established. This is
referred to as the "TCP three-way handshake." 

While waiting for the ACK to the SYN ACK, a connection queue of finite size on the destination host
keeps track of connections waiting to be completed. This queue typically empties quickly since the
ACK is expected to arrive a few milliseconds after the SYN ACK. 

The TCP SYN attack exploits this design by having an attacking source host generate TCP SYN
packets with random source addresses toward a victim host. The victim destination host sends a SYN
ACK back to the random source address and adds an entry to the connection queue. Since the SYN
ACK is destined for an incorrect or non-existent host, the last part of the "three-way handshake" is
never completed and the entry remains in the connection queue until a timer expires, typically for about
one minute. By generating phony TCP SYN packets from random IP addresses at a rapid rate, it is
possible to fill up the connection queue and deny TCP services (such as e-mail, file transfer, or WWW)
to legitimate users. 

There is no easy way to trace the originator of the attack because the IP address of the source is forged.

The external manifestations of the problem include inability to get e-mail, inability to accept connections
to WWW or FTP services, or a large number of TCP connections on your host in the state


Devices behind firewalls

The TCP SYN attack is characterized by an influx of SYN packets from random source IP addresses.
Any device behind a firewall that stops inbound SYN packets is already protected from this mode of
attack and no further action is needed. Examples of firewalls include a Cisco Private Internet Exchange
(PIX) Firewall or a Cisco router configured with access lists. For examples of how to set up access lists
on a Cisco router, please refer to the document Increasing Security On IP Networks (also available
from the Cisco Fax-on-Demand service at 415-596-4408, Doc ID# 116). 

Devices offering publicly available services (mail servers, public web servers)

Preventing SYN attacks on devices behind firewalls from random IP addresses is relatively simple since
you can use access lists to explicitly limit inbound access to a select few IP addresses. However, in the
case of a public web server or mail server facing the Internet, there is no way to determine which
incoming IP source addresses are friendly and which are unfriendly. Therefore, there is no clear cut
defense against an attack from a random IP address. Several options are available to hosts: 

     Increase the size of the connection queue (SYN ACK queue). 
     Decrease the time-out waiting for the three-way handshake. 
     Employ vendor software patches to detect and circumvent the problem (if available). 

You should contact your host vendor to see if they have created specific patches to address the TCP
SYN ACK attack. 

Note: Filtering IP addresses at the server is ineffective since an attacker can vary his IP address, and
the address may or may not be the same as that of a legitimate host. 


Since a primary mechanism of this denial of service attack is the generation of traffic sourced from
random IP addresses, we recommend filtering traffic destined for the Internet. The basic concept is to
throw away packets with invalid source IP addresses as they enter the Internet. This does not prevent a
denial of service attack on your network, but will help attacked parties rule out your location as the
source of the attacker. In addition, it makes your network less attractive as a base for this class of

Preventing Transmission of Invalid IP Addresses

By filtering packets on your routers that connect your network to the Internet, you can permit only
packets with valid source IP addresses to leave your network and get into the Internet. 

For example, if your network consists of network, and your router connects to your ISP
using a serial 0/1 interface, you can apply the access list as follows: 

access-list 111 permit ip any
access-list 111 deny ip any any log 

interface serial 0/1
ip access-group 111 out

Note: The last line of the access list determines if there is any traffic with an invalid source address
entering the Internet. It is not crucial to have this line, but it will help locate the source of the possible

Preventing Reception of Invalid IP Addresses

For ISPs who provide service to end networks, we highly recommend the validation of incoming
packets from your clients. This can be accomplished by the use of inbound packet filters on your border

For example, if your clients have the following network numbers connected to your router via a serial
interface named "serial 1/0", you can create the following access list: 

The network numbers are to, and

access-list 111 permit ip any
access-list 111 permit ip any
access-list 111 deny ip any any log

interface serial 1/0
ip access-group 111 in

Note: The last line of the access list determines if there is any traffic with invalid source addresses
entering the Internet. It is not crucial to have this line, but it will help locate the source of the possible


This topic has been discussed in some detail on the NANOG [North American Network Operator1s
Group] mailing list. The list archives are located at: 

For a detailed description of the TCP SYN denial of service attack and IP spoofing, see: 

Also, Steve Bellovin at AT&T has written two technical security papers which discuss similar attacks in
more detail: 

Posted: Tue Dec 1 07:55:22 PST 1998 

Copyright 1996  Cisco Systems Inc. All rights reserved.


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