Automatic Bogon Firewalling Using uRPF and Team Cymru

A "bogon" is a prefix (IP subnet) that shouldn't be on the internet. These include RFC1918 private IP addresses, as well as public IP blocks that haven't been assigned to companies by their local RIR. The folks over at Team Cymru have a great list of all the IPv4 and IPv6 bogons. It's updated frequently, and is available as static text files or a BGP feed.

Routers have an option called a uRPF check, short for Unicast Reverse Path Forwarding, which looks at an incoming packet and compares the source address against the routing table. If the packet was received on the same interface that the router would use for the return traffic, the packet is allowed though. Otherwise, it gets discarded. Combine this with the bogon routes you get from Team Cymru, and you've got automatic bogon filtering.

Why might you want it?

• DDoS attacks may have spoofed source IPs, which could include bogon IPs.
• Some nefarious people may be cyber squatting on some unassigned IPs, and using them for any number of less-than-good purposes.
• You've set up a dark net. Instead of dropping packets from bogons, you could redirect them to a collector.
• You want to be the Alpha Nerd.

It doesn't even need to be using Team Cymru's list. If you have a bunch of routers, and just want some centralized firewall control, you can use this method.

ip bgp-community new-format

ip community-list expanded CYMRU-BOGONS permit 65000:888

ip route 192.0.2.1 255.255.255.255 Null0 name NULL_ROUTE_FOR_CYMRU_BOGONS

route-map CYMRU-BOGONS permit 10
 description BOGONS LIST FOR URPF FILTERING
 match community CYMRU-BOGONS
 set ip next-hop 192.0.2.1

ip prefix-list CYMRU-OUT seq 10 deny 0.0.0.0/0 le 32

router bgp 65504
 bgp log-neighbor-changes
 neighbor CYMRU peer-group
 neighbor CYMRU remote-as 65000
 neighbor CYMRU description CYMRU BOGONS LIST
 neighbor CYMRU ebgp-multihop 255
 neighbor CYMRU password 7 1234567890
 neighbor CYMRU update-source Loopback6
 neighbor 2000:B00:B00:10::2 peer-group CYMRU
 neighbor 2000:C00:C00:10::2 peer-group CYMRU

 address-family ipv4
  neighbor CYMRU soft-reconfiguration inbound
  neighbor CYMRU prefix-list CYMRU-OUT out
  neighbor CYMRU route-map CYMRU-BOGONS in
  neighbor 2000:B00:B00:10::2 activate
  neighbor 2000:C00:C00:10::2 activate
  no auto-summary
  no synchronization
 exit-address-family

access-list 100 remark ---- ACL FOR URPF ----
access-list 100 permit udp any eq bootps any eq bootpc
access-list 100 deny   ip any any

interface FastEthernet0
 description EXTERNAL
 ip verify unicast reverse-path 100

ip prefix-list CYMRU_NAT-T_FIX seq 10 permit 192.168.0.0/16

route-map CYMRU-BOGONS deny 5
 description TO FIX NAT-T
 match ip address prefix-list CYMRU_NAT-T_FIX
 match community CYMRU-BOGONS