DoS Protection Profiles
Protect groups of devices and critical individual devices
from flood attacks, and limit the maximum concurrent sessions for
resources.
DoS Protection profiles set thresholds that
protect against new session IP
flood attacks and provide resource protection (maximum concurrent
session limits for specified endpoints and resources). DoS Protection
profiles protect specific devices (classified profiles) and groups
of devices (aggregate profiles) against SYN, UDP, ICMP, ICMPv6,
and Other IP flood attacks. Configuring Flood Protection thresholds
in a DoS Protection profile is similar to configuring
Flood Protection in a Zone
Protection profile, but Zone Protection profiles protect entire
ingress zones, while DoS protection profiles and policy rules are granular
and targeted, and can even be classified to a single device (IP
address). The firewall measures the aggregate number of connections-per-second
(CPS) to a group of devices (aggregate profile) or measures the
CPS to individual devices (classified profile).
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 ()
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 to
a group of devices (aggregate) or to individual devices (classified) and
a Block Duration, and you can set a drop Action for
SYN floods:
Alarm Rate—When new CPS exceeds
this threshold, the firewall generates a DoS alarm. For classified
profiles, set the rate to 15-20% above the device’s average CPS
rate so that normal fluctuations don’t cause alerts. For aggregate
profiles, set the rate to 15-20% above the group’s average CPS rate.
Activate Rate—When new CPS exceeds
this threshold, the firewall begins to drop new connections to mitigate
the flood until the CPS rate drops below the threshold. For classified
profiles, the Max Rate should be an acceptable
CPS rate for the device(s) you’re protecting (the Max
Rate won’t flood the critical device(s)). You can set
the Activate Rate to the same threshold as
the Max Rate so that the firewall doesn’t
use RED or SYN Cookies to begin dropping traffic before it reaches
the Max Rate. Set the Activate
Rate lower than the Max Rate only
if you want to drop traffic before it reaches the Max
Rate. For aggregate profiles, set the threshold just
above the average peak CPS rate for the group to begin mitigating
floods using RED (or SYN Cookies for SYN floods).
Max Rate—When new CPS exceeds this
threshold, the firewall blocks (drops) all new connections from
the offending IP address for the specified Block Duration time
period. For classified profiles, base the Max Rate threshold
on the capacity of the device(s) you’re protecting so that the CPS
rate can’t flood them. For aggregate profiles, set to 80-90% of
the group’s capacity.
Block Duration—When new CPS exceeds
the Max Rate, the firewall blocks new connections
from the offending IP address. The Block Duration specifies
the amount of time the firewall continues to block the IP address’s
new connections. While the firewall blocks new connections, it doesn’t
count incoming connections and doesn’t increment the threshold counters.
For classified and aggregate profiles, use the default value (300 seconds)
to block the attacking session without penalizing legitimate sessions
from the source for too long a period of time.
SYN Flood Protection is the only type for which you set
the drop Action. Start by setting the Action to 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.
The default threshold values are high so that DoS Protection
profiles don’t unexpectedly drop legitimate traffic. Monitor connection
traffic and adjust the thresholds to values appropriate for your
network. Start by taking baseline measurements of average and peak
CPS for each flood type to determine the normal traffic conditions
for the critical devices you want to protect. Because normal traffic
loads experience some fluctuation, it’s best not to drop connections
too aggressively. Monitor and adjust the flood thresholds as needed
and as your network evolves.
Another method of setting flood thresholds is to use the baseline
measurements to set the maximum CPS you want to allow and work back
from there to derive reasonable flood mitigation alarm and activation
rates.
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.
In addition to setting IP flood thresholds, you can also use
DoS Protection profiles to detect and prevent session exhaustion attacks
in which a large number of hosts (bots) establish as many sessions
as possible to consume a target’s resources. On the profile’s Resources
Protection tab, you can set the maximum number of concurrent
sessions that the device(s) defined in the DoS Protection policy
rule to which you apply the profile can receive. When the number
of concurrent sessions reaches its maximum limit, new sessions are
dropped.
The maximum number of concurrent sessions to set depends on your
network context. Understand the number of concurrent sessions that
the resources you are protecting (defined in the DoS Protection
policy rule to which you attach the profile) can handle. Set the
threshold to approximately 80% of the resources’ capacity, then
monitor and adjust the threshold as needed.
For aggregate profiles, the Resources Protection threshold
applies to all traffic of the devices defined in the policy rule
(source and destination). For classified profiles, the Resources
Protection threshold applies to the traffic based on
whether the classified policy rule applies to the source IP only,
to the destination IP only, or to both the source and destination
IPs.
