Transcript – Chapter 4 – Secure Routing

– Chapter 4 –
Secure Routing
• Build security into the design of routing
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router authentication
route authentication
control directed broadcast
black hole filtering
URPF
Path integrity
2 Case studies
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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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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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Prefix Filtering Example
http://www.netkit.org/software/netkit_labs/bgp/lab-bgp-3-prefix-filtering/netkit-lab-bgp-3-prefixfiltering.pdf
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Prefix Filtering Example
http://www.netkit.org/software/netkit_labs/bgp/lab-bgp-3-prefix-filtering/netkit-lab-bgp-3-prefixfiltering.pdf
! 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
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ip prefix-list partialOut permit 195.11.14.0/24
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ip prefix-list partialIn deny 200.1.1.0/24
ip prefix-list partialIn permit any
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Design issues of secure routing
1. network convergence
– depends on many factors
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complexity of the net architecture
redundancy in the network
route calculation algorithms and configuration
loops in the network
– Fast convergence is desirable.
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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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Design issues of secure routing
2. 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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Authentication of Router and Routes
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Rationale of authenticating routers and routes:
1. 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
2. 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.
Impacts?
slow or disabled router
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Authentication of Router and Routes
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Solutions?
1. Router authentication: Routers must authenticate
each other before sharing information.
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Password-based authentication
- Drawback?
MD5-HMAC
- Implications?
2. Route authentication: Integrity of the exchanged
routing information must be verified.
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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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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
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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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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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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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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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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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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
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Case study 1
Securing the BGP Routing Protocol
• an exterior gateway protocol
• Example techniques:
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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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Case Study 2
Securing the OSPF routing protocols
• an interior gateway protocol
• Example techniques:
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Router authentication
Nonbroadcast neighbor configuration
Using stub areas
Using loopback interfaces as the router Ids
Tweaking SPF timers
Route filtering
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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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