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Configuring the IPsec anti-replay function
The IPsec anti-replay function protects networks against anti-replay attacks by using a sliding window
mechanism called anti-replay window. This function checks the sequence number of each received IPsec
packet against the current IPsec packet sequence number range of the sliding window. If the sequence
number is not in the current sequence number range, the packet is considered a replayed packet and is
discarded.
IPsec packet de-encapsulation involves complicated calculation. De-encapsulation of replayed packets
not only makes no sense, but also consumes large amounts of resources and degrades performance,
resulting in DoS. IPsec anti-replay checking, when enabled, is performed before the de-encapsulation
process, reducing resource waste.
In some cases, however, the sequence numbers of some normal service data packets may be out of the
current sequence number range, and the IPsec anti-replay function may drop them as well, affecting the
normal communications. If this happens, disable IPsec anti-replay checking or adjust the size of the
anti-replay window as required.
To configure IPsec anti-replay checking:
Ste
Command
Remarks
1. Enter system view.
system-view N/A
2. Enable IPsec anti-replay
checking.
ipsec anti-replay check
Optional.
Enabled by default.
3. Set the size of the IPsec
anti-replay window.
ipsec anti-replay window width
Optional.
32 by default.
CAUTION:
• IPsec anti-replay checking is enabled by default. Do not disable it unless it needs to be disabled.
• A wider anti-replay window results in higher resource cost and more system performance de
radation,
which is against the original intention of the IPsec anti-replay function. Specify an anti-replay window
size that is as small as possible.
NOTE:
IPsec anti-replay checking does not affect manually created IPsec SAs. According to the IPsec protocol,
only IPsec SAs negotiated by IKE support anti-replay checking.
Configuring packet information pre-extraction
If you apply both an IPsec policy and QoS policy to an interface, by default, the interface first uses IPsec
and then QoS to process IP packets, and QoS classifies packets by the headers of IPsec-encapsulated
packets. If you want QoS to classify packets by the headers of the original IP packets, enable the packet
information pre-extraction feature.
For more information about QoS policy and classification, see ACL and QoS Configuration Guide.
To configure packet information pre-extraction:
Ste
Command
Remarks
1. Enter system view. system-view N/A
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