by Mariusz Burdach
|Most people know how problematic protection against SYN denial of service attacks can be. Several methods, more or less effective, are usually used. In almost every case proper filtering of packets is a viable solution. In addition to creating packet filters, the modification of the TCP/IP stack of a given operating system can be performed by an administrator. This method, the tuning of the TCP/IP stack in various operating systems, will be described in depth in this article.
While SYN attacks may not be entirely preventable, tuning the TCP/IP stack will help reduce the impact of SYN attacks while still allowing legitimate client traffic through. It should be noted that some SYN attacks do not always attempt to upset servers, but instead try to consume all of the bandwidth of your Internet connection. This kind of flood is outside the scope of scope of this article, as is the filtering of packets which has been discussed elsewhere.
What can an administrator do when his servers are under a classic, non-bandwidth flooding SYN attack? One of most important steps is to enable the operating system's built-in protection mechanisms like SYN cookies or
Note that an attacker can simply send more packets with the SYN flag set and then the above tasks will not solve the problem. However, we can still increase the likelihood of creating a full connection with legitimate clients by performing the above operations.
We should remember that our modification of variables will change the behavior of the TCP/IP stack. In some cases the values can be too strict. So, after the modification we have to make sure that our server can properly communicate with other hosts. For example, the disabling of packet retransmissions in some environments with low bandwidth can cause a legitimate request to fail. In this article you will find a description of the TCP/IP variables for the fallowing operating systems: Microsoft Windows 2000, RedHat Linux 7.3, Sun Solaris 8 and HP-UX 11.00. These variables are similar or the same in current releases.
Definitions: SYN flooding and SYN spoofing
A SYN flood is a type of Denial of Service attack. We can say that a victim host is under a SYN flooding attack when an attacker tries to create a huge amount of connections in the SYN RECEIVED state until the backlog queue has overflowed. The SYN RECEIVED state is created when the victim host receives a connection request (a packet with SYN flag set) and allocates for it some memory resources. A SYN flood attack creates so many half-open connections that the system becomes overwhelmed and cannot handle incoming requests any more.
To increase an effectiveness of a SYN flood attack, an attacker spoofs source IP addresses of SYN packets. In this case the victim host cannot finish the initialization process in a short time because the source IP address can be unreachable. This malicious operation is called a SYN spoofing attack.
We need to know that the process of creating a full connection takes some time. Initially, after receiving a connection request (a packet with SYN flag set), a victim host puts this half-open connection to the backlog queue and sends out the first response (a packet with SYN and ACK flags set). When the victim does not receive a response from a remote host, it tries to retransmit this SYN+ACK packet until it times out, and then finally removes this half-open connection from the backlog queue. In some operating systems this process for a single SYN request can take about 3 minutes! In this document you will learn how to change this behavior. The other important information you need to know is that the operating system can handle only a defined amount of half-open connections in the backlog queue. This amount is controlled by the size of the backlog queue. For instance, the default backlog size is 256 for RedHat 7.3 and 100 for Windows 2000 Professional. When this size is reached, the system will no longer accept incoming connection requests.
How to detect a SYN attack
It is very simple to detect SYN attacks. The netstat command shows us how many connections are currently in the half-open state. The half-open state is described as SYN_RECEIVED in Windows and as SYN_RECV in Unix systems.
# netstat -n -p TCP tcp 0 0 10.100.0.200:21 18.104.22.168:25882 SYN_RECV - tcp 0 0 10.100.0.200:21 22.214.171.124:2577 SYN_RECV - tcp 0 0 10.100.0.200:21 127.160.6.129:51748 SYN_RECV - tcp 0 0 10.100.0.200:21 126.96.36.199:47393 SYN_RECV - tcp 0 0 10.100.0.200:21 188.8.131.52:60427 SYN_RECV - tcp 0 0 10.100.0.200:21 184.108.40.206:278 SYN_RECV - tcp 0 0 10.100.0.200:21 220.127.116.11:5122 SYN_RECV - tcp 0 0 10.100.0.200:21 18.104.22.168:49162 SYN_RECV - tcp 0 0 10.100.0.200:21 22.214.171.124:37899 SYN_RECV - ...
We can also count how many half-open connections are in the backlog queue at the moment. In the example below, 769 connections (for TELNET) in the SYN RECEIVED state are kept in the backlog queue.
# netstat -n -p TCP | grep SYN_RECV | grep :23 | wc -l 769
The other method for detecting SYN attacks is to print TCP statistics and look at the TCP parameters which count dropped connection requests. While under attack, the values of these parameters grow rapidly.
In this example we watch the value of the
# netstat -s -P tcp | grep tcpHalfOpenDrop tcpHalfOpenDrop = 473
It is important to note that every TCP port has its own backlog queue, but only one variable of the TCP/IP stack controls the size of backlog queues for all ports.
The backlog queue
The backlog queue is a large memory structure used to handle incoming packets with the SYN flag set until the moment the three-way handshake process is completed. An operating system allocates part of the system memory for every incoming connection. We know that every TCP port can handle a defined number of incoming requests. The backlog queue controls how many half-open connections can be handled by the operating system at the same time. When a maximum number of incoming connections is reached, subsequent requests are silently dropped by the operating system.
As mentioned before, when we detect a lot of connections in the SYN RECEIVED state, host is probably under a SYN flooding attack. Moreover, the source IP addresses of these incoming packets can be spoofed. To limit the effects of SYN attacks we should enable some built-in protection mechanisms. Additionally, we can sometimes use techniques such as increasing the backlog queue size and minimizing the total time where a pending connection in kept in allocated memory (in the backlog queue).
Built-in protection mechanisms
Operating system: Windows 2000
The most important parameter in Windows 2000 and also in Windows Server 2003 is
In general, when a SYN attack is detected the
When the value of
As we can see, by enabling the
The operating system enables protection against SYN attacks automatically when it detects that values of the following three parameters are exceeded. These parameters are
To change the values of these parameters, first we have to add them to the same registry key as we made for
Operating system: Linux RedHat
RedHat, like other Linux operating systems, has implemented a SYN cookies mechanism which can be enabled in the following way:
# echo 1 > /proc/sys/net/ipv4/tcp_syncookies
Note that to make this change permanent we need to create a startup file that sets this variable. We must do the same operation for other UNIX variables described in this paper because the values for these variables will return to default upon system reboot.
SYN cookies protection is especially useful when the system is under a SYN flood attack and source IP addresses of SYN packets are also forged (a SYN spoofing attack). This mechanism allows construction of a packet with the SYN and ACK flags set and which has a specially crafted initial sequence number (ISN), called a cookie. The value of the cookie is not a pseudo-random number generated by the system but instead is the result of a hash function. This hash result is generated from information like: source IP, source port, destination IP, destination port plus some secret values. During a SYN attack the system generates a response by sending back a packet with a cookie, instead of rejecting the connection when the SYN queue is full. When a server receives a packet with the ACK flag set (the last stage of the three-way handshake process) then it verifies the cookie. When its value is correct, it creates the connection, even though there is no corresponding entry in the SYN queue. Then we know that it is a legitimate connection and that the source IP address was not spoofed. It is important to note that the SYN cookie mechanism works by not using the backlog queue at all, so we don't need to change the backlog queue size. More information about SYN cookies can be found at http://cr.yp.to/syncookies.html.
Also note that the SYN cookies mechanism works only when the CONFIG_SYNCOOKIES option is set during kernel compilation.
The next section will describe other useful methods of protection against SYN attacks. I would like to emphasize that under heavy SYN attacks (like Distributed SYN flooding attack) these methods may help but still not solve the problem.
Increasing the backlog queue
Under a SYN attack, we can modify the backlog queue to support more connections in the half-open state without denying access to legitimate clients. In some operating systems, the value of the backlog queue is very low and vendors often recommend increasing the SYN queue when a system is under attack.
Increasing the backlog queue size requires that a system reserve additional memory resources for incoming requests. If a system has not enough memory for this operation, it will have an impact on system performance. We should also make sure that network applications like Apache or IIS can accept more connections.
Operating system: Windows 2000
Aside from described above
The table below shows the recommended values for the AFD.SYS driver:
Operating system: Linux
# sysctl -w net.ipv4.tcp_max_syn_backlog="2048"
Operating system: Sun Solaris
In Sun Solaris there are two parameters which control the maximum number of connections. The first parameter controls the total number of full connections. The second
# ndd -set /dev/tcp tcp_conn_req_max_q0 2048
Operating system: HP-UX
In HP-UX, a
# ndd -set /dev/tcp tcp_syn_rcvd_max 2048
Decreasing total time of handling connection request
As we know, SYN flooding/spoofing attacks are simply a series of SYN packets, mostly from forged IP addresses. In the last section we tried to increase the backlog queue. Now that our systems can handle more SYN requests, we should decrease the total time we keep half-open connections in the backlog queue. When a server receives a request, it immediately sends a response with the SYN and ACK flags set, puts this half-open connection into the backlog queue, and then waits for a packet with the ACK flag set from the client. When no response is received from the client, the server retransmits a response packet (with the SYN and ACK flags set) several times (depending on default value in each operating system) by giving the client a chance to send the ACK packet again. It is clear that when the source IP address of client was spoofed, the ACK packet will never arrive. After a few minutes the server removes this half-open connection. We can speed up this time of removing connections in the SYN RECEIVED state from the backlog queue by changing time of first retransmission and by changing the total number of retransmissions.
Another technique of protection against SYN attacks is switching off some TCP parameters that are always negotiated during the three-way handshake process. Some of these parameters are automatically turned off by mechanisms described in the first section (
Now, I will describe TCP/IP stack variables which allow a decrease in the time half-open connections are kept in the backlog queue.
Operating system: Windows 2000
In Windows 2000, the default time for a first retransmission is set to 3 seconds (3000 milliseconds) and can be changed by modifying the value of the TcpInitialRtt registry entry (for every interface). For example, to decrease time of a first retransmission to 2 seconds we have to set this registry value to 2000 milliseconds in decimal format. The number of retransmissions (packets with the SYN and ACK flags set) is controlled by a
The table below contains a few examples of values and corresponding times for keeping half-open connections in the backlog queue (the time of a first retransmission is set to 3 seconds).
We can set this registry value to 0, whereby Windows doesn't try to retransmit packets at all. In this case, the system sends only one response and cancels the half-open connection after 3 seconds. This setting is ignored when its value is equal or greater than 2 and when
Operating system: Linux RedHat
Operating system: Sun Solaris
In this operating system it is impossible to turn off retransmissions of packets directly using the ndd command. Moreover, in Sun Solaris there are parameters which are non-configurable by ndd and which control the number of retransmissions (at least 3) and total time of packet retransmissions (at least 3 minutes). More information about these parameters can be found in the "Solaris 2.x - Tuning Your TCP/IP stack and More" document.
Operating system: HP-UX
For HP-UX, the time spent handling half-open connections in the backlog queue is controlled by the
We can change the time of a first retransmission by modifying
The methods of hardening the TCP/IP stack that are presented in this article make servers more resistant to SYN flooding and SYN spoofing - Denial of Service attacks. A modification of your default TCP/IP stack settings is also recommended during the process of securing of the operating system.
Mariusz Burdach is a computer security consultant who specializes in vulnerability assessment, intrusion detection and computer forensics. During the last few years he has worked as a consultant in the European Network Security Institute where he conducted penetration tests, vulnerability assessments and security audits for Internet banks, government and financial institutions in Poland. He is co-author of the Solaris Security Administrator's Guide, a step-by-step guide to securing SUN's Solaris operating system. Comments on this article are appreciated, send them to M_Burdach@compfort.pl.
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