3.6 What are some reasonable filtering rules for a Cisco?The example in figure 4 shows one possible configuration for using the Cisco as filtering router. It is a sample that shows the implementation of as specific policy. Your policy will undoubtedly vary.
In this example, a company has Class C network address 184.108.40.206. Company network is connected to Internet via IP Service Provider. Company policy is to allow everybody access to Internet services, so all outgoing connections are accepted. All incoming connections go through ``mailhost''. Mail and DNS are only incoming services.
Only incoming packets from Internet are checked in this configuration. Rules are tested in order and stop when the first match is found. There is an implicit deny rule at the end of an access list that denies everything. This IP access list assumes that you are running Cisco IOS v. 10.3 or later.
no ip source-route ! interface ethernet 0 ip address 220.127.116.11 no ip directed-broadcast ! interface serial 0 no ip directed-broadcast ip access-group 101 in ! access-list 101 deny ip 127.0.0.0 0.255.255.255 any access-list 101 deny ip 10.0.0.0 0.255.255.255 any access-list 101 deny ip 172.16.0.0 0.15.255.255 any access-list 101 deny ip 192.168.0.0 0.0.255.255 any access-list 101 deny ip any 0.0.0.255 255.255.255.0 access-list 101 deny ip any 0.0.0.0 255.255.255.0 ! access-list 101 deny ip 18.104.22.168 0.0.0.255 access-list 101 permit tcp any any established ! access-list 101 permit tcp any host 22.214.171.124 eq smtp access-list 101 permit tcp any host 126.96.36.199 eq dns access-list 101 permit udp any host 188.8.131.52 eq dns ! access-list 101 deny tcp any any range 6000 6003 access-list 101 deny tcp any any range 2000 2003 access-list 101 deny tcp any any eq 2049 access-list 101 deny udp any any eq 2049 ! access-list 101 permit tcp any 20 any gt 1024 ! access-list 101 permit icmp any any ! snmp-server community FOOBAR RO 2 line vty 0 4 access-class 2 in access-list 2 permit 184.108.40.206 0.0.0.255
Use at least Cisco version 9.21 so you can filter incoming packets and check for address spoofing. It's still better to use 10.3, where you get some extra features (like filtering on source port) and some improvements on filter syntax.
You have still a few ways to make your setup stronger. Block all incoming TCP-connections and tell users to use passive-FTP clients. You can also block outgoing ICMP echo-reply and destination-unreachable messages to hide your network and to prevent use of network scanners. Cisco.com use to have an archive of examples for building firewalls using Cisco routers, but it doesn't seem to be online anymore. There are some notes on Cisco access control lists, at least, at ftp://ftp.cisco.com/pub/mibs/app_notes/access-lists .
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