R2511-HP MSR Router Series Security Configuration Guide(V5)

158
Typically, IKE uses UDP port 500 for communication, and AH and ESP use the protocol numbers 51 and
50 respectively. Make sure that flows of these protocols are not denied on the interfaces with IKE or IPsec
configured.
Configuring an ACL
ACLs can be used to identify traffic. They are widely used in scenarios where traffic identification is
desired, such as QoS and IPsec.
Keywords in ACL rules
IPsec uses ACLs to identify data flows. An ACL is a collection of ACL rules. Each ACL rule is a deny or
permit statement. A permit statement identifies a data flow protected by IPsec, and a deny statement
identifies a data flow that is not protected by IPsec. With IPsec, a packet is matched against the
referenced ACL rules and processed according to the first rule that it matches:
• Each ACL rule matches both the outbound traffic and the returned inbound traffic. Suppose there is
a rule rule 0 permit ip source 1.1.1.0 0.0.0.255 destination 2.2.2.0 0.0.0.255. This rule matches
both traffic from 1.1.1.0 to 2.2.2.0 and traffic from 2.2.2.0 to 1.1.1.0.
• In the outbound direction, if a permit statement is matched, IPsec considers that the packet requires
protection and continues to process it. If a deny statement is matched or no match is found, IPsec
considers that the packet does not require protection and delivers it to the next function module.
• In the inbound direction:
{ Non-IPsec packets that match a permit statement are dropped.
{ IPsec packets that match a permit statement and are destined for the device itself are
de-encapsulated and matched against the rule again. Only those that match a permit statement
are processed by IPsec.
When defining ACL rules for IPsec, follow these guidelines:
• Permit only data flows that need to be protected and use the any keyword with caution. With the
any keyword specified in a permit statement, all outbound traffic matching the permit statement will
be protected by IPsec and all inbound IPsec packets matching the permit statement will be received
and processed, but all inbound non-IPsec packets will be dropped. This will cause the inbound
traffic that does not need IPsec protection to be all dropped.
• Avoid statement conflicts in the scope of IPsec policy groups. When creating a deny statement, be
careful with its matching scope and matching order relative to permit statements. The policies in an
IPsec policy group have different match priorities. ACL rule conflicts between them are prone to
cause mistreatment of packets. For example, when configuring a permit statement for an IPsec
policy to protect an outbound traffic flow, you must avoid the situation that the traffic flow matches
a deny statement in a higher priority IPsec policy. Otherwise, the packets will be sent out as normal
packets. If they match a permit statement at the receiving end, they will be dropped by IPsec.
The following configuration example shows how an improper statement causes unexpected packet
dropping. Only the ACL-related configurations are presented.
Router A connects the segment 1.1.2.0/24 and Router B connects the segment 3.3.3.0/24. On Router A,
apply the IPsec policy group test to the outbound interface of Router A. The IPsec policy group contains
two policies, test 1 and test 2. The ACLs referenced by the two policies each contain a rule that matches
traffic from 1.1.2.0/24 to 3.3.3.0/24. The one referenced in policy test 1 is a deny statement and the one
referenced in policy test 2 is a permit statement. Because test 1 is matched prior to test 2, traffic from
1.1.2.0/24 to 3.3.3.0/24 will match the deny statement and sent as normal traffic. When the traffic
arrives at Router B, it will be dropped if it matches a permit statement in the ACL referenced in the applied
IPsec policy.