Packet Buffer Protection
Protect the firewall’s packet buffers from single-session DoS attacks that attempt to take down the firewall.
Packet Buffer Protection defends your firewall and network from single session DoS attacks that can overwhelm the firewall’s packet buffer and cause legitimate traffic to drop. Although you don’t configure Packet Buffer Protection in a Zone Protection profile or in a DoS Protection profile or policy rule, Packet Buffer Protection defends ingress zones. While zone and DoS protection apply to new sessions (connections) and are granular, Packet Buffer Protection applies to existing sessions and is global.
You Configure Packet Buffer Protection globally to protect the entire firewall and also enable Packet Buffer Protection on each zone to protect zones:
- Global Packet Buffer Protection—The firewall monitors sessions from all zones (regardless of whether Packet Buffer Protection is enabled in a zone) and how those sessions utilize the packet buffer. You must configure Packet Buffer Protection globally (DeviceSetupSession Settings) to protect the firewall and to enable it on individual zones. When packet buffer consumption reaches the configured Activate percentage, the firewall used Random Early Drop (RED) to drop packets from the offending sessions (the firewall doesn’t drop complete sessions at the global level).
- Per-Zone Packet Buffer Protection—Enable Packet Buffer Protection on each zone (NetworkZones) to layer in a second level of protection. When packet buffer consumption crosses the Activate threshold and global protection begins to apply RED to session traffic, that starts the Block Hold Time timer. The Block Hold Time is the amount of time in seconds that the offending session can continue before the firewall blocks the entire session. The offending session remains blocked until the Block Duration time expires.If you don’t enable Packet Buffer Protection globally, it won’t be active in zones until you enable it globally.
Take baseline measurements of firewall packet buffer utilization over a period of time—at least one business week, but a longer measurement period provides a better baseline—to understand typical usage. One method of taking baseline measurements is to use a script to automate packet buffer monitoring, which also provides a good method for continuous monitoring and helps you understand when anomalous events occur. Your Palo Alto Networks account team can provide a sample script you can use to develop your own script.
If baseline measurements consistently show abnormally high packet buffer utilization, then the firewall’s capacity may be undersized for typical traffic loads. In this case, consider resizing the firewall deployment. Otherwise, you need to tune the Packet Buffer Protection thresholds carefully to prevent impacted buffers from overflowing (and to prevent dropping legitimate traffic). When firewall sizing is correct for the deployment, only an attack should cause a large spike in buffer usage.
Overrunning the firewall packet buffer negatively impacts the firewall’s packet forwarding capabilities. When the buffers are full, no packets can enter the firewall on any interface, not just the interface that experienced the attack.
The best practices for setting the thresholds are:
- Alert and Activate—Start with the default threshold values (50% in both cases), monitor packet buffer utilization, and adjust the thresholds as necessary. If the firewall is sized correctly, buffer utilization should be well below 50%. If the packet buffer utilization crosses Alert threshold, the firewall creates an alert entry in the System log.
- Block Hold Time—When packet buffer utilization triggers the Activate threshold, the Block Hold Time sets the amount of time the offending session can continue before the firewall blocks the session. During the Block Hold Time, the firewall continues to apply RED to the packets of offending sessions. Start with the default Block Hold Time threshold value (60 seconds), monitor packet buffer utilization, and adjust the threshold as necessary. If the packet buffer utilization percentage falls below the Activate threshold before the Block Hold Time expires, the timer resets and doesn’t start until the Activate threshold is crossed again. Increasing the Block Hold Time imposes a greater penalty on offending sessions and reducing it imposes a lesser penalty on offending sessions.
- Block Duration—When the Block Hold Time expires, the firewall blocks the offending session for the period of time defined by the Block Duration. Start with the default threshold value (3600 seconds), monitor packet buffer utilization, and adjust the threshold as necessary. When you enable Packet Buffer Protection on a zone, Block Duration affects every session from the IP address even if only one session from an IP address overutilizes the packet buffer. If you believe that blocking an IP address for one hour (3600 seconds) is too great a penalty, reduce the Block Duration to an acceptable value.
In addition to monitoring the buffer utilization of individual sessions, Packet Buffer Protection can also block an IP address if certain criteria are met. While the firewall monitors the packet buffers, if it detects a source IP address rapidly creating sessions that would not individually be seen as an attack, it blocks that IP address for the configured Block Duration.
Network Address Translation (NAT) (an external source that has translated its internet-bound traffic using source NAT) can give the appearance of greater packet buffer utilization because of IP address translation activity. If this occurs, adjust the thresholds in a way that penalizes individual sessions but doesn’t penalize the underlying IP addresses (so other sessions from the same IP address aren’t affected). To do this, reduce the Block Hold Time so the firewall blocks individual sessions that overutilize the buffers faster, and reduce the Block Duration so that the underlying IP address is not unduly penalized.
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