1. – 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
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2. 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
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3. 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
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4. Prefix Filtering Example http://www. netkit
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5. Prefix Filtering Example http://www. netkit
! only /24 is announced to neighbor
! all, with the exception of /24, is accepted from
router bgp 1
network /24
network /24
neighbor remote-as 2
neighbor description Router 2 of AS2
neighbor prefix-list partialOut out
neighbor prefix-list partialIn in !
ip prefix-list partialOut permit /24
ip prefix-list partialIn deny /24
ip prefix-list partialIn permit any
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6. 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
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7. 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
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8. 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
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9. 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
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10. 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
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11. 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
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12. 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
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13. 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
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14. 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
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15. 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., ).
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
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16. 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
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17. 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 …
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18. 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
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