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Operating system: Sun Solaris<br>
<br>
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 tcp_conn_req_max_q0 parameter defines how many half-open connections are allowed without the dropping of incoming requests. In Sun Solaris 8, the default value is set to 1024. Using the ndd command we can modify this value.<br>
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<br>
# ndd -set /dev/tcp tcp_conn_req_max_q0 2048<br>
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Operating system: HP-UX<br>
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In HP-UX, a tcp_syn_rcvd_max TCP/IP stack variable is responsible for control of the maximum number of half-open connections in the SYN RECEIVE state. In HP-UX 11.00 this value is set to 500. We can change this value by using the ndd command, similar to the one used in a Sun Solaris system.<br>
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<br>
# ndd -set /dev/tcp tcp_syn_rcvd_max 2048<br>
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Decreasing total time of handling connection request<br>
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.<br>
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 (SynAttackProtect and Syncookies).<br>
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Now, I will describe TCP/IP stack variables which allow a decrease in the time half-open connections are kept in the backlog queue.<br>
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Operating system: Windows 2000<br>
<br>
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 TcpMaxConnectResponseRetransmissions registry parameter which has to be added to HKLMSYSTEMCurrentControlSetServicesTcpipParameters registry key.<br>
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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).<br>
<br>
Value Time of retransmission Total time to keep half-open connections in the backlog queue<br>
1 in 3rd second 9 seconds<br>
2 in 3rd and 9th second 21 seconds<br>
3 in 3rd , 9th and 21st second 45 seconds<br>
<br>
<br>
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 SynAttackProtect is enabled.<br>
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Operating system: Linux RedHat<br>
<br>
A tcp_synack_retries variable is responsible for controlling the number of retransmissions in Linux operating system. Its default value is set to 5 for most Linux operating systems, which causes the half-open connection to be removed after 3 minutes. In the below table there are calculations for other values.<br>
<br>
Value Time of retransmission Total time to keep half-open connections in the backlog queue<br>
1 in 3rd second 9 seconds<br>
2 in 3rd and 9th second 21 seconds<br>
3 in 3rd , 9th and 21st second 45 seconds<br>
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<br>
Operating system: Sun Solaris<br>
<br>
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.<br>
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Operating system: HP-UX<br>
<br>
For HP-UX, the time spent handling half-open connections in the backlog queue is controlled by the tcp_ip_abort_cinterval parameter. By using the ndd command we can define how long a HP-UX operating system will be waiting for the ACK packet. We can control how many retransmissions will be performed indirectly by changing this value. Have a look at the table below.<br>
<br>
Value Time of retransmission Total time to keep half-open connections in the backlog queue<br>
1000 - 1 second<br>
5000 in 2nd second 5 seconds<br>
10000 in 2nd and 5th second 10 seconds<br>
60000 In 2nd, 5th, 11th, 23rd and 47th second 1 minute<br>
<br>
<br>
We can change the time of a first retransmission by modifying tcp_rexmit_interval_initial. Intervals of subsequent retransmissions are controlled by two parameters: tcp_rexmit_interval and tcp_rexmit_interval_min. These three variables are the same as in a Sun Solaris operating system.<br>
<br>
Summary<br>
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. <br>
<br>
<br>
另外 每秒 最多3个 syn 封包 进入 表达为 ：<br>
<br>
/sbin/iptables -A synfoold -p tcp --syn -m limit --limit 1/s --limit-burst 3 -j RETURN<br>
<br>
cgweb 回复于：2003-09-15 11:52:58<br>
good<br>
<br>
webwanglei 回复于：2003-09-21 15:59:19<br>
假使我频道的PAGEVIEW是150万/天，有3台服务器顶。假使首页有10个对象，那么SYN就是150万÷3*10÷24÷3600=57<br>
<br>
就是说我每秒至少要保证有57个SYN，否则正常的都无法访问了。单纯设为3个每秒对我来说太小了。
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