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.
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 () 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 () 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.
You must enable Packet
Buffer Protection globally in order for it to be active in zones.
There are two types of packet buffer protection:
Packet Buffer
Protection Based on Buffer Utilization
Packet Buffer Protection based on buffer utilization is enabled by default. Packet Buffer
Protection based on buffer utilization is enabled by default. Take baseline
measurements of firewall packet buffer utilization over a period of time until
you’re comfortable that you understand typical usage. Take measurements for at least
one business week; however, a longer measurement period provides a better baseline.
To see packet buffer utilization for a specified period of time, use the operational
CLI command:
admin1138@thxvm1>show running resource-monitor [day | hour | minute | second | week]
The
CLI command provides a snapshot of buffer utilization for the specified period of
time, but is neither automated nor continuous. To automate continuous packet buffer
utilization measurements so you can monitor changes in behavior and anomalous
events, use a script. Your Palo Alto Networks account team can provide a sample
script that you can modify to develop your own script; however, the script is not
officially supported and there is no technical support available for script usage or
modification.
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, monitor packet buffer utilization, and
adjust the thresholds as necessary. The Alert threshold
defaults to 50%; when packet buffer utilization exceeds the threshold for
more than 10 seconds, the firewall creates an alert entry in the
System log every minute. The Activate threshold
defaults to 80%; when the threshold is reached, the firewall begins
to mitigate the most abusive sessions. If the firewall is sized
correctly, buffer utilization should be well below 50%.
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.
Packet Buffer
Protection Based on Latency
As an alternative to packet
buffer protection based on utilization, you can trigger
packet buffer protection
based on packet latency caused by dataplane packet buffering,
which indicates congestion on the firewall. Such packet buffer protection
mitigates head-of-line blocking by alerting you to the congestion
and performing random early drop (RED) on packets. Packet buffer
protection based on latency can trigger the protection before latency-sensitive protocols
or applications are affected.
If your traffic includes protocols
or applications that are latency-sensitive, then packet buffer protection
based on latency will be more helpful than packet buffer protection
based on buffer utilization.
Packet buffer protection based
on latency includes setting a Latency Alert threshold
(in milliseconds), above which the firewall starts generating an
Alert log event. The Latency Activate threshold
indicates when the firewall activates RED on incoming packets and starts
generating an Activate log. The Latency Max Tolerate threshold
indicates when the firewall uses with RED with almost 100% drop
probability.
The Block Hold Time and Block
Duration settings function for packet buffer protection
based on latency in the same way they do for packet buffer protection
based on utilization.
