SYN Flood Options

You can set the following parameters for proxying uncompleted TCP connection requests:

• Attack Threshold: This option allows you to set the number of SYN segments (that is, TCP segments with the SYN flag set) to the same destination address and port number per second required to activate the SYN proxying mechanism. Although you can set the threshold to any number, you need to know the normal traffic patterns at your site to set an appropriate threshold for it. For example, if it is an e-business site that normally gets 20,000 SYN segments per second, you might want to set the threshold to 30,000 per second. If a smaller site normally gets 20 SYN segments per second, you might consider setting the threshold to 40.

  user@host# set security screen zone-syn-flood tcp syn-flood attack-threshold number

  user@host# set security zones security-zone zone screen zone-syn-flood

• Alarm Threshold: This option allows you to set the number of proxied, half-complete TCP connection requests per second after which JUNOS software enters an alarm in the event log. The value you set for an alarm threshold triggers an alarm when the number of proxied, half-completed connection requests to the same destination address and port number per second exceeds that value. For example, if you set the SYN attack threshold at 2000 SYN segments per second and the alarm at 1000, then a total of 3001 SYN segments to the same destination address and port number per second is required to trigger an alarm entry in the log. More precisely:
  1. The firewall passes the first 2000 SYN segments per second that meet policy requirements.
  2. The firewall proxies the next 1000 SYN segments in the same second.
  3. The 1001st proxied connection request (or 3001st connection request in that second) triggers the alarm.
     user@host# set security screen zone-syn-flood tcp syn-flood alarm-threshold <number>
user@host# set security zones security-zone zone screen zone-syn-flood

      

     For each SYN segment to the same destination address and port number in excess of the alarm threshold, the attack detection module generates a message. At the end of the second, the logging module compresses all similar messages into a single log entry that indicates how many SYN segments to the same destination address and port number arrived after exceeding the alarm threshold. If the attack persists beyond the first second, the event log enters an alarm every second until the attack stops.

• Source Threshold: This option allows you to specify the number of SYN segments received per second from a single source IP address—regardless of the destination IP address and port number—before JUNOS software begins dropping connection requests from that source.
  user@host# set security screen zone-syn-flood tcp syn-flood source-threshold <number>
user@host# set security zones security-zone zone screen zone-syn-flood

  Tracking a SYN flood by source address uses different detection parameters from tracking a SYN flood by destination address and destination port number. When you set a SYN attack threshold and a source threshold, you put both the basic SYN flood protection mechanism and the source-based SYN flood tracking mechanism in effect.

• Destination Threshold: This option allows you to specify the number of SYN segments received per second for a single destination IP address before JUNOS software begins dropping connection requests to that destination. If a protected host runs multiple services, you might want to set a threshold based on destination IP address only—regardless of the destination port number.
  user@host# set security screen zone-syn-flood tcp syn-flood destination-threshold <number>
user@host# set security zones security-zone zone screen zone-syn-flood

  When you set a SYN attack threshold and a destination threshold, you put both the basic SYN flood protection mechanism and the destination-based SYN flood tracking mechanism in effect.

  Tracking a SYN flood by destination address uses different detection parameters from tracking a SYN flood by destination address and destination port number. Consider the following case where JUNOS software has policies permitting FTP requests (port 21) and HTTP requests (port 80) to the same server. If the SYN flood attack threshold is 1000 packets per second (pps) and an attacker sends 999 FTP packets and 999 HTTP packets per second, neither set of packets (where a set is defined as having the same destination address and port number) activates the SYN proxying mechanism. The basic SYN flood attack mechanism tracks destination address and port number, and neither set exceeds the attack threshold of 1000 pps. However, if the destination threshold is 1000 pps, JUNOS software treats both FTP and HTTP packets with the same destination address as members of a single set and rejects the 1001st packet—FTP or HTTP—to that destination.

• Timeout: This option allows you to set the maximum length of time before a half-completed connection is dropped from the queue. The default is 20 seconds, and you can set the timeout from 0–50 seconds. You might try decreasing the timeout value to a shorter length until you begin to see any dropped connections during normal traffic conditions. Twenty seconds is a very conservative timeout for a three-way handshake ACK response.
  user@host# set security screen zone-syn-flood tcp syn-flood timeout <number>
user@host# set security zones security-zone zone screen zone-syn-flood

   

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