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Example: SYN Flood Protection

In this example, you protect four Web servers in the DMZ zone from SYN flood attacks originating in the external zone by enabling the SYN flood protection SCREEN option for the external zone.

Before You Begin
For background information, read Understanding SYN Flood Attacks.

Note: We recommend that you augment the SYN flood protection that JUNOS software provides with device-level SYN flood protection on each of the Web servers. In this example, the Web servers are running UNIX, which also provides some SYN flood defenses, such as adjusting the length of the connection request queue and changing the timeout period for incomplete connection requests.

Figure 32: Device-Level SYN Flood Protection

Image ex_SYN_flood.gif

To configure the SYN flood protection parameters with appropriate values for your network, you must first establish a baseline of typical traffic flows. For one week, you run a sniffer on ethernet3—the interface bound to zone_external—to monitor the number of new TCP connection requests arriving every second for the four Web servers in the DMZ zone. Your analysis of the data accumulated from one week of monitoring produces the following statistics:

Average number of new connection requests per server: 250 per second

Average peak number of new connection requests per server: 500 per second

Note: A sniffer is a network-analyzing device that captures packets on the network segment to which you attach it. Most sniffers allow you to define filters to collect only the type of traffic that interests you. Later, you can view and evaluate the accumulated information. In this example, you want the sniffer to collect all TCP packets with the SYN flag set arriving at ethernet3 and destined for one of the four Web servers in the DMZ.You might want to continue running the sniffer at regular intervals to see if there are traffic patterns based on the time of day, days of the week, the time of month, or the season of the year. For example, in some organizations, traffic might increase dramatically during a critical event. Significant changes probably warrant adjusting the various thresholds.

Based on this information, you set the following SYN flood protection parameters for zone_external, as shown in Table 37.

Table 37: SYN Flood Protection Parameters

Parameter	Value	Reason for Each Value
Attack Threshold	625 packets per second (pps)	This is 25% higher than the average peak number of new connection requests per second per server, which is unusual for this network environment. When the number of SYN packets per second for any one of the four Web servers exceeds this number, the device begins proxying new connection requests to that server. (In other words, beginning with the 626th SYN packet to the same destination address and port number in one second, the device begins proxying connection requests to that address and port number.)
Alarm Threshold	250 pps	When the device proxies 251 new connection requests in one second, it makes an alarm entry in the event log. By setting the alarm threshold somewhat higher than the attack threshold, you can avoid alarm entries for traffic spikes that only slightly exceed the attack threshold.
Source Threshold	25 pps	When you set a source threshold, the device tracks the source IP address of SYN packets, regardless of the destination address and port number. (Note that this source-based tracking is separate from the tracking of SYN packets based on destination address and destination port number that constitutes the basic SYN flood protection mechanism.) In the one week of monitoring activity, you observed that no more than 1/25 of new connection requests for all servers came from any one source within a one-second interval. Therefore, connection requests exceeding this threshold are unusual and provide sufficient cause for the device to execute its proxying mechanism. (25 pps is 1/25 of the attack threshold, which is 625 pps.) If the device tracks 25 SYN packets from the same source IP address, beginning with the 26th packet, it rejects all further SYN packets from that source for the remainder of that second and the next second as well.
Destination Threshold	0 pps	When you set a destination threshold, the device runs a separate tracking of only the destination IP address, regardless of the destination port number. Because the four Web servers only receive HTTP traffic (destination port 80)—no traffic to any other destination port number reaches them—setting a separate destination threshold offers no additional advantage.
Timeout	20 seconds	The default value of 20 seconds is a reasonable length of time to hold incomplete connection requests.
Parameter	Value	Reason for Each Value
Attack Threshold	625 packets per second (pps)	This is 25% higher than the average peak number of new connection requests per second per server, which is unusual for this network environment. When the number of SYN packets per second for any one of the four Web servers exceeds this number, the device begins proxying new connection requests to that server. (In other words, beginning with the 626th SYN packet to the same destination address and port number in one second, the device begins proxying connection requests to that address and port number.)
* Half-completed connection requests are incomplete three-way handshakes. A three-way handshake is the initial phase of a TCP connection. It consists of a TCP segment with the SYN flag set, a response with the SYN and ACK flags set, and a response to that with the ACK flag set.

You can use either J-Web or the CLI configuration editor to configure SYN flood protection parameters.

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