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Post  Post subject: Quick & temporary tuning of FreeBSD under spoofed SYN flood attack  |  Posted: Tue Jan 22, 2013 9:31 am
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Quick & temporary tuning of FreeBSD under spoofed SYN flood attack

Syn flood is purposed to take up resources on specific network segment (firewall/router) or server by filling up CPU / memory resources (by filling syn backlog, memory for session table and so on).

In FreeBSD, TCP SYN_RCVD state timer can be dropped down by limiting the nunmber of SYN+ACK retries. This is done by modifying kernel parameter:

Code:
# sysctl net.inet.tcp.syncache.rexmtlimit
net.inet.tcp.syncache.rexmtlimit: 10


This means after the first SYN+ACK packet in response to a spoofed SYN packet, TCP will send another 10 such retries, keeping the TCP Control Block in SYN_RECEIVED state as well as a session open in the firewall (unless the firewall has a very small TCP initial timeout).

By setting this value to a smaller value, at least during the attack, can make the life of the attacker a little harder.





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admin
Post  Post subject: Re: Quick & temporary tuning of FreeBSD under spoofed SYN flood attack  |  Posted: Tue Jan 22, 2013 9:43 am
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Joined: Mon Aug 03, 2009 8:43 am
Posts: 104

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For Linux, this is controled with:
Code:
# sysctl net.ipv4.tcp_synack_retries
net.ipv4.tcp_synack_retries = 5


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