Flood Protection

Protect the entire zone against SYN, UDP, ICMP, ICMPv6, and Other IP flood attacks.
A Zone Protection profile with flood protection configured defends an entire ingress zone against SYN, ICMP, ICMPv6, UDP, and other IP flood attacks. The firewall measures the aggregate amount of each flood type entering the zone in new connections-per-second (CPS) and compares the totals to the thresholds you configure in the Zone Protection profile. (You protect critical individual devices within a zone with DoS Protection profiles and policy rules.)
Measure and monitor firewall dataplane CPU consumption to ensure that each firewall is properly sized to support DoS and Zone Protection and any other features that consume CPU cycles, such as decryption. If you use Panorama to manage your firewalls, Device Monitoring (
Managed Devices
All Devices
) shows you the CPU and memory consumption of each managed firewall. It can also show you a 90-day trend line of CPU average and peak use to help you understand the typical available capacity of each firewall.
For each flood type, you set three thresholds for new CPS entering the zone, and you can set a drop
for SYN floods. If you know the baseline CPS rates for the zone, use these guidelines to set the initial thresholds, and then monitor and adjust the thresholds as necessary.
  • Alarm Rate
    —The new CPS threshold to trigger an alarm. Target setting the
    Alarm Rate
    to 15-20% above the average CPS rate for the zone so that normal fluctuations don’t cause alerts.
  • Activate
    —The new CPS threshold to activate the flood protection mechanism and begin dropping new connections. For ICMP, ICMPv6, UDP, and other IP floods, the protection mechanism is Random Early Drop (RED, also known as Random Early Detection). For SYN floods only, you can set the drop
    to SYN Cookies or RED. Target setting the
    rate to just above the peak CPS rate for the zone to begin mitigating potential floods.
  • Maximum
    —The number of connections-per-second to drop incoming packets when RED is the protection mechanism. Target setting the
    rate to approximately 80-90% of firewall capacity, taking into account other features that consume firewall resources.
If you don’t know the baseline CPS rates for the zone, start by setting the
CPS rate to approximately 80-90% of firewall capacity and use it to derive reasonable flood mitigation alarm and activation rates. Set the
Alarm Rate
rate based on the Maximum rate. For example, you could set the
Alarm Rate
to half the
rate and adjust it depending on how many alarms you receive and the firewall resources being consumed. Be careful setting the
Activate Rate
since it begins to drop connections. Because normal traffic loads experience some fluctuation, it’s best not to drop connections too aggressively. Err on the high side and adjust the rate if firewall resources are impacted.
SYN Flood Protection is the only type for which you set the drop
. Start by setting the
SYN Cookies
. SYN Cookies treats legitimate traffic fairly and only drops traffic that fails the SYN handshake, while using Random Early Drop drops traffic randomly, so RED may affect legitimate traffic. However, SYN Cookies is more resource-intensive because the firewall acts as a proxy for the target server and handles the three-way handshake for the server. The tradeoff is not dropping legitimate traffic (SYN Cookies) versus preserving firewall resources (RED). Monitor the firewall, and if SYN Cookies consumes too many resources, switch to RED. If you don’t have a dedicated DDoS prevention device in front of the firewall, always use RED as the drop mechanism.
SYN Cookies
is activated, the firewall does not honor the TCP options that the server sends because it does not know these values at the time that it proxies the SYN/ACK. Therefore, values such as the TCP server’s window size and MSS values cannot be negotiated during the TCP handshake and the firewall will use its own default values. In the scenario where the MSS of the path to the server is smaller than the firewall’s default MSS value, the packet will need to be fragmented.
The default threshold values are high so that activating a Zone Protection profile doesn’t unexpectedly drop legitimate traffic. Adjust the thresholds to values appropriate for your network’s traffic. The best method for understanding how to set reasonable flood thresholds is to take baseline measurements of average and peak CPS for each flood type to determine the normal traffic conditions for each zone and to understand the capacity of the firewall, including the impact of other resource-consuming features such as decryption. Monitor and adjust the flood thresholds as needed and as your network evolves.
Firewalls with multiple dataplane processors (DPs) distribute connections across DPs. In general, the firewall divides the CPS threshold settings equally across its DPs. For example, if a firewall has five DPs and you set the
Alarm Rate
to 20,000 CPS, each DP has an
Alarm Rate
of 4,000 CPS (20,000 / 5 = 4,000), so if the new sessions on a DP exceeds 4,000, it triggers the
Alarm Rate
threshold for that DP.

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