– Chapter 4 – Secure Routing Build security into the design of routing router authentication route authentication control directed broadcast black hole filtering URPF Path integrity 2 Case studies Network Security

Design issues of secure routing Route filtering When designing a private network, it is important to ensure that ‘route filtering’ is used to filter out any bogus or undesired routes coming into the private net. Examples: special addresses (p.82) It is equally important to ensure that the only networks advertised by the private network are those desired. To ensure that IP address blocks belonging to a private network are not allowed to be advertised back into the network from outside. ‘net police filtering’ (aka. ‘prefix filtering’) – next Network Security

Design issues of secure routing Prefix Filtering No routes with prefixes more specific than /20 (or up to /24) are allowed to come in. To ensure that an attack cannot be staged on a large ISP’s router by increasing the size of its routing tables Routes more specific than /20 are often not needed by large ISPs, so those routes can be filtered out to keep its routing table from getting out of control. Example: p.93 (incoming route filtering in a BGP router) Another example: next Network Security

Prefix Filtering Example http://www. netkit Network Security

Prefix Filtering Example http://www. netkit ! only 195.11.14.0/24 is announced to neighbor 193.10.11.2 ! all, with the exception of 200.1.1.0/24, is accepted from 193.10.11.2 router bgp 1 network 195.11.14.0/24 network 195.11.15.0/24 neighbor 193.10.11.2 remote-as 2 neighbor 193.10.11.2 description Router 2 of AS2 neighbor 193.10.11.2 prefix-list partialOut out neighbor 193.10.11.2 prefix-list partialIn in ! ip prefix-list partialOut permit 195.11.14.0/24 ip prefix-list partialIn deny 200.1.1.0/24 ip prefix-list partialIn permit any Network Security

Design issues of secure routing network convergence depends on many factors complexity of the net architecture redundancy in the network route calculation algorithms and configuration loops in the network Fast convergence is desirable. Problems with a a slow-converging network can mean a considerable loss of revenue and/or productivity may be subject to DoS attacks, because it takes longer to recover from network-disrupting attacks and thus aggravates problems Network Security

Design issues of secure routing static routes discussed earlier (example 3-1) can be used to hard code information in the routing tables such that this info is unaffected by a network attack or propagated impact from other parts of the network Disadvantage? scalability Network Security

Authentication of Router and Routes Rationale of authenticating routers and routes: As part of an attack, the attacker may configure his machine or router to share incorrect routing information with the attacked router (AR). Impacts? Incorrect routing, disabled router, traffic redirection Flood of routing talbe e.g., A rogue router may act as a BGP speaker and neighbor, and advertises lots of specific routes into a core router’s routing table. slow or disabled router Network Security

Authentication of Router and Routes Solutions? Router authentication: Routers must authenticate each other before sharing information. Password-based authentication - Drawback? MD5-HMAC - Implications? Route authentication: Integrity of the exchanged routing information must be verified. Hashing-based methods, such as MD5-HMAC, can be used to authenticate routes. Figure 4-1 Examples 4-1, 4-2, 4-3 Network Security

Control/disable directed broadcast ‘Directed broadcast’ allows packets to be broadcast to all the machines on the subnet directly attached to a router. May be used by attackers to start attacks e.g., smurf attack A type of DoS attack Figure 21-3 An attacker sends a ping echo request to the broadcast address on a network, causing all the machines in that segment to send echo replies to the attacked router.  impact: packet flood Network Security

Black Hole Filtering Purpose: to filter out undesired traffic, by directing specific routes to a null interface An alternative to ACL Advantage: no access list processing  save processing time Disadvantage: Null routing is based on the packets’ destination IP addresses only, while ACL can work on source address, destination address, and layer 4 info as well. A weaker form of route filtering Example 4-5: interface null0 Network Security

URPF Unicast Reverse Path Forwarding Purpose: to thwart attempts to send packets with spoofed source IP addresses A mechanism configured on a router to disable outgoing packets with source IP addresses not in the range belonging to its site Advantage: A more efficient and effective outgoing packets filtering mechanism than ACL Requirement: CEF (Cisco Express Forwarding) must be enabled on that router, because URPF looks at the FIB (forwarding information base) rather than the the routing table. Example: Figure 4-2 Network Security

URPF (cont.) Constraint: can not be deployed on a router that has asymmetric routes set up. In asymmetric routing, more than one interface is used (by a router or firewall) to route packets of a private network.  The interface through which the router sends return traffic for a packet may not be the same interface on which the original packet was received. In general, URPF is deployed on the edge of a network.  allowing the antispoofing capabilities to be effective to the entire network Example 4-6: ip verify unicast reverse-path Network Security

Path Integrity Rule of thumb: Routing should be performed based on the optimum paths calculated by the underlying routing protocols.  However, the routing protocols may be affected by ICMP redirects and IP source routing when making such calculations. ICMP redirects allows a router to inform another router on its local segment not to use certain hop in its path to certain host.  because including the hop will result in paths that’s not optimal ICMP redirects is the default setting on Cisco routers. Should be disabled unless absolutely necessary IP source routing: next Network Security

Path Integrity (cont.) IP source routing: an IP feature allowing a user to set a field in the IP packet to specify the desired path May be used by attackers to subvert the workings of normal routing protocols Example: An attacker can specify a router (A) that is attached to both a private and the public network as an intermediate point in the source path to reach a private address (e.g., 10.1.1.1). All intermediate routers, with IP source routing enabled, will forward the packet to router A.  causing DoS attack Advice: disable IP source routing on the router Network Security

Case study 1 Securing the BGP Routing Protocol an exterior gateway protocol Example techniques: Enable BGP peer authentication Filter incoming routes Filter outgoing routes Use the network statement to advertise the network block Disable BGP multihop feature (that is, do not allow peering between routers not directly connected to each other) Control TCP port 179  using the firewall or ACLs to do the filtering Disable BGP version negotiation (instead, hard-code the version info) Use police filters and null routes Set up route dampening values  to prevent flapping routes Use the maximum-prefix command Logging changes in neighbor status Network Security

Case Study 2 Securing the OSPF routing protocols an interior gateway protocol Example techniques: Router authentication Nonbroadcast neighbor configuration Using stub areas Using loopback interfaces as the router Ids Tweaking SPF timers Route filtering … Network Security

Summary Security of routers and routes is critical for the security of the whole network. The net administrator should configure his routers and routes, not only to protect the private network, but also to help to protect the whole Internet. Next: security of LAN switching Network Security