The Cisco multicast Rendezvous Point (RP) switch must be configured to limit the multicast forwarding cache so that its resources are not saturated by managing an overwhelming number of Protocol Independent Multicast (PIM) and Multicast Source Discovery Protocol (MSDP) source-active entries.

Overview

Finding ID	Version	Rule ID	IA Controls	Severity
V-221135	CISC-RT-000820	SV-221135r622190_rule		Low

Description
MSDP peering between networks enables sharing of multicast source information. Enclaves with an existing multicast topology using PIM-SM can configure their RP switches to peer with MSDP switches. As a first step of defense against a denial-of-service (DoS) attack, all RP switches must limit the multicast forwarding cache to ensure that switch resources are not saturated managing an overwhelming number of PIM and MSDP source-active entries.

Description

MSDP peering between networks enables sharing of multicast source information. Enclaves with an existing multicast topology using PIM-SM can configure their RP switches to peer with MSDP switches. As a first step of defense against a denial-of-service (DoS) attack, all RP switches must limit the multicast forwarding cache to ensure that switch resources are not saturated managing an overwhelming number of PIM and MSDP source-active entries.

STIG	Date
Cisco NX-OS Switch RTR Security Technical Implementation Guide	2021-03-29

Details

Check Text ( C-22850r409894_chk )
The Cisco switch does not have a mechanism to limit the multicast forwarding cache. However, the risk associated with this requirement can be fully mitigated by configuring the switch to filter PIM register messages and accept MSDP packets only from known MSDP peers. Step 1: Verify that the RP is configured to filter PIM register messages for any undesirable multicast groups and sources. The example below will deny any multicast streams for groups 239.5.0.0/16 and allow from only sources x.1.2.6 and x.1.2.7. ip pim register-policy PIM_REGISTER_FILTER … … … route-map PIM_REGISTER_FILTER deny 10 match ip multicast group 239.5.0.0/16 route-map PIM_REGISTER_FILTER permit 20 match ip multicast source x.1.2.6/32 route-map PIM_REGISTER_FILTER permit 30 match ip multicast source x.1.2.7/32 route-map PIM_REGISTER_FILTER permit 40 match ip multicast group-range 232.0.0.0 to 233.255.255.255 route-map PIM_REGISTER_FILTER deny 50 match ip multicast source 0.0.0.0/0 Step 2: Review the switch configuration to determine if there is a receive path or interface filter to only accept MSDP packets from known MSDP peers as shown in the example below. Step 2a: Verify that interfaces used for MSDP peering have an inbound ACL as shown in the example. interface Ethernet2/3 no switchport ip access-group EXTERNAL_ACL_INBOUND in ip address x.1.28.8/24 ip pim sparse-mode Step 2b: Verify that the ACL restricts MSDP peering to only known sources. ip access-list EXTERNAL_ACL_INBOUND 10 permit tcp any any established 20 permit tcp x.1.28.2/32 x.1.28.8/32 eq 639 30 deny tcp any x.1.28.8/32 eq 639 log 40 permit tcp x.1.28.2/32 10.x.28.8/32 eq bgp 50 permit tcp x.1.28.2/32 eq bgp x.1.28.8/32 60 permit pim x.1.28.2/32 x.1.28.8/32 … … … 120 deny ip any any log Note: MSDP connections is via TCP port 639. If the RP switch is not configured to filter PIM register messages and accept MSDP packets only from known MSDP peers, this is a finding.

Check Text ( C-22850r409894_chk )

The Cisco switch does not have a mechanism to limit the multicast forwarding cache. However, the risk associated with this requirement can be fully mitigated by configuring the switch to filter PIM register messages and accept MSDP packets only from known MSDP peers.

Step 1: Verify that the RP is configured to filter PIM register messages for any undesirable multicast groups and sources. The example below will deny any multicast streams for groups 239.5.0.0/16 and allow from only sources x.1.2.6 and x.1.2.7.

ip pim register-policy PIM_REGISTER_FILTER
…
…
…
route-map PIM_REGISTER_FILTER deny 10
match ip multicast group 239.5.0.0/16
route-map PIM_REGISTER_FILTER permit 20
match ip multicast source x.1.2.6/32
route-map PIM_REGISTER_FILTER permit 30
match ip multicast source x.1.2.7/32
route-map PIM_REGISTER_FILTER permit 40
match ip multicast group-range 232.0.0.0 to 233.255.255.255
route-map PIM_REGISTER_FILTER deny 50
match ip multicast source 0.0.0.0/0

Step 2: Review the switch configuration to determine if there is a receive path or interface filter to only accept MSDP packets from known MSDP peers as shown in the example below.

Step 2a: Verify that interfaces used for MSDP peering have an inbound ACL as shown in the example.

interface Ethernet2/3
no switchport
ip access-group EXTERNAL_ACL_INBOUND in
ip address x.1.28.8/24
ip pim sparse-mode

Step 2b: Verify that the ACL restricts MSDP peering to only known sources.

ip access-list EXTERNAL_ACL_INBOUND
10 permit tcp any any established
20 permit tcp x.1.28.2/32 x.1.28.8/32 eq 639
30 deny tcp any x.1.28.8/32 eq 639 log
40 permit tcp x.1.28.2/32 10.x.28.8/32 eq bgp
50 permit tcp x.1.28.2/32 eq bgp x.1.28.8/32
60 permit pim x.1.28.2/32 x.1.28.8/32
…
…
…
120 deny ip any any log

Note: MSDP connections is via TCP port 639.

If the RP switch is not configured to filter PIM register messages and accept MSDP packets only from known MSDP peers, this is a finding.

Fix Text (F-22839r409895_fix)
The risk associated with this requirement can be fully mitigated by configuring the switch to filter PIM register messages, rate limiting the number of PIM register messages, and accept MSDP packets only from known MSDP peers. Step 1: Configure the switch to filter PIM register messages received from a multicast DR for any undesirable multicast groups and sources. The example below will deny any multicast streams for groups 239.5.0.0/16 and allow from only sources x.1.2.6 and x.1.2.7. Step 1a: Configure a route map to filter multicast groups and sources as shown in the example below: SW1(config)# route-map PIM_REGISTER_FILTER deny 10 SW1(config-route-map)# match ip multicast group 239.5.0.0/16 SW1(config-route-map)# route-map PIM_REGISTER_FILTER permit 20 SW1(config-route-map)# match ip multicast source x.1.2.6/32 SW1(config-route-map)# route-map PIM_REGISTER_FILTER permit 30 SW1(config-route-map)# match ip multicast source x.1.2.7/32 SW1(config-route-map)# route-map PIM_REGISTER_FILTER permit 40 SW1(config-route-map)# match ip multicast group-range 232.0.0.0 to 233.255.255.255 SW1(config-route-map)# route-map PIM_REGISTER_FILTER deny 50 SW1(config-route-map)# match ip multicast source 0.0.0.0/0 SW1(config-route-map)# exit Step 1b: Configure a multicast register policy referencing the configured route map. SW1(config)# ip pim register-policy PIM_REGISTER_FILTER SW1(config)# end Step 2: Configure the receive path or interface ACLs to only accept MSDP packets from known MSDP peers. SW1(config)# ip access-list EXTERNAL_ACL_INBOUND SW1(config-acl) # permit tcp any any established SW1(config-acl) # permit tcp host x.1.28.2 host x.1.28.8 eq 639 SW1(config-acl) # deny tcp any host x1.28.8 eq 639 SW1(config-acl) # permit tcp host x.1.28.2 host x.1.28.8 eq bgp SW1(config-acl) # permit tcp host x.1.28.2 eq bgp host x.1.28.8 SW1(config-acl) # permit pim host x.1.28.2 host x.1.28.8 … … … SW1(config-acl)# deny ip any any

Fix Text (F-22839r409895_fix)

The risk associated with this requirement can be fully mitigated by configuring the switch to filter PIM register messages, rate limiting the number of PIM register messages, and accept MSDP packets only from known MSDP peers.

Step 1: Configure the switch to filter PIM register messages received from a multicast DR for any undesirable multicast groups and sources. The example below will deny any multicast streams for groups 239.5.0.0/16 and allow from only sources x.1.2.6 and x.1.2.7.

Step 1a: Configure a route map to filter multicast groups and sources as shown in the example below:

SW1(config)# route-map PIM_REGISTER_FILTER deny 10
SW1(config-route-map)# match ip multicast group 239.5.0.0/16
SW1(config-route-map)# route-map PIM_REGISTER_FILTER permit 20
SW1(config-route-map)# match ip multicast source x.1.2.6/32
SW1(config-route-map)# route-map PIM_REGISTER_FILTER permit 30
SW1(config-route-map)# match ip multicast source x.1.2.7/32
SW1(config-route-map)# route-map PIM_REGISTER_FILTER permit 40
SW1(config-route-map)# match ip multicast group-range 232.0.0.0 to 233.255.255.255
SW1(config-route-map)# route-map PIM_REGISTER_FILTER deny 50
SW1(config-route-map)# match ip multicast source 0.0.0.0/0
SW1(config-route-map)# exit

Step 1b: Configure a multicast register policy referencing the configured route map.

SW1(config)# ip pim register-policy PIM_REGISTER_FILTER
SW1(config)# end

Step 2: Configure the receive path or interface ACLs to only accept MSDP packets from known MSDP peers.

SW1(config)# ip access-list EXTERNAL_ACL_INBOUND
SW1(config-acl) # permit tcp any any established
SW1(config-acl) # permit tcp host x.1.28.2 host x.1.28.8 eq 639
SW1(config-acl) # deny tcp any host x1.28.8 eq 639
SW1(config-acl) # permit tcp host x.1.28.2 host x.1.28.8 eq bgp
SW1(config-acl) # permit tcp host x.1.28.2 eq bgp host x.1.28.8
SW1(config-acl) # permit pim host x.1.28.2 host x.1.28.8
…
…
…
SW1(config-acl)# deny ip any any