Specific Recommendations: Access Lists

1. Always start an access-list definition with the privileged command no access-list nnn to clear out any previous versions of access list number nnn.

fumtek(config)# no access-list 51

fumtek(config)# access-list 51 permit host 14.2.9.6

fumtek(config)# access-list 51 deny any log

2. Log access list port messages properly. To ensure that logs contain correct port number information, use the port range arguments shown below at the end of an access list.

access-list 106 deny udp any range 1 65535 any range 1 65535 log

access-list 106 deny tcp any range 1 65535 any range 1 65535 log

access-list 106 deny ip any any log

The last line is necessary to ensure that rejected packets of protocols other than TCP and UDP are properly logged.

3. Enforce traffic address restrictions using access lists. On a border router, allow only internal addresses to enter the router from the internal interfaces, and allow only traffic destined for internal addresses to enter the router from the outside (external interfaces).

Block illegal addresses at the outgoing interfaces. Besides preventing an attacker from using the router to attack other sites, it helps identify poorly configured internal hosts or networks. This approach may not be feasible for complicated networks. [RFC 2827]

fumtek(config)# no access-list 101

fumtek(config)# access-list 101 permit ip 14.2.6.0 0.0.0.255 any

fumtek(config)# access-list 101 deny ip any any log

fumtek(config)# no access-list 102

fumtek(config)# access-list 102 permit ip any 14.2.6.0 0.0.0.255

fumtek(config)# access-list 102 deny ip any any log

fumtek(config)# interface eth 1

fumtek(config-if)# ip access-group 101 in

fumtek(config-if)# exit

fumtek(config)# interface eth 0

fumtek(config-if)# ip access-group 101 out

fumtek(config-if)# ip access-group 102 in

4. Block packets coming from the outside (untrusted network) that are obviously fake or have source or destination addresses that are reserved, for example networks 0.0.0.0/8, 10.0.0.0/8, 169.254.0.0/16, 172.16.0.0/20, 192.168.0.0/16. This protection should be part of the overall traffic filtering at the interface attached to the external,

untrusted network. [RFC 1918]

5. Block incoming packets that claim to have a source address of any internal (trusted) networks. This impedes TCP sequence number guessing and other attacks. Incorporate this protection into the access lists applied to interfaces facing any untrusted networks.

6. Drop incoming packets with loopback addresses, network 127.0.0.0/8. These packets cannot be real.

7. If the network doesn’t need IP multicast, then block multicast packets.

8. Block broadcast packets. (Note that this may block DHCP and BOOTP services, but these services should not be used on external interfaces and certainly shouldn’t cross border routers.)

9. A number of remote probes and attacks use ICMP echo, redirect, and mask request messages, block them. (A superior but more difficult approach is to permit only necessary ICMP packet types.)

The example below shows one way to implement these

recommendations.

North(config)# no access-list 107

North(config)# ! block our internal addresses

North(config)# access-list 107 deny ip 14.2.0.0 0.0.255.255 any log

North(config)# access-list 107 deny ip 14.1.0.0 0.0.255.255 any log

North(config)# ! block special/reserved addresses

North(config)# access-list 107 deny ip 127.0.0.0 0.255.255.255 any log

North(config)# access-list 107 deny ip 0.0.0.0 0.255.255.255 any log

North(config)# access-list 107 deny ip 10.0.0.0 0.255.255.255 any log

North(config)# access-list 107 deny ip 169.254.0.0 0.0.255.255 any log

North(config)# access-list 107 deny ip 172.16.0.0 0.15.255.255 any log

North(config)# access-list 107 deny ip 192.168.0.0 0.0.255.255 any log

North(config)# ! block multicast (if not used)

North(config)# access-list 107 deny ip 224.0.0.0 15.255.255.255 any

North(config)# ! block some ICMP message types

North(config)# access-list 107 deny icmp any any redirect log

North(config)# access-list 107 deny icmp any any echo log

North(config)# access-list 107 deny icmp any any mask-request log

North(config)# access-list 107 permit ip any 14.2.0.0 0.0.255.255

North(config)# access-list 107 permit ip any 14.1.0.0 0.0.255.255

North(config)# interface Eth 0/0

North(config-if)# description External interface

North(config-if)# ip access-group 107 in

10. Block incoming packets that claim to have the same destination and source address (i.e. a ‘Land’ attack on the router itself). Incorporate this protection into the access list used to restrict incoming traffic into each interface, using a rule like the one shown below.

access-list 102 deny ip host 14.1.1.250

host 14.1.1.250 log

interface Eth 0/1

ip address 14.1.1.250 255.255.0.0

ip access-group 102 in

11. Configure an access list for the virtual terminal lines to control Telnet access. See example commands below.

South(config)# no access-list 92

South(config)# access-list 92 permit 14.2.10.1

South(config)# access-list 92 permit 14.2.9.1

South(config)# line vty 0 4

South(config-line)# access-class 92 in

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