1. – Chapter 3 – Device Security (B)
Security of major devices:
How to protect the device against attacks aimed at compromising the device itself
Routers
Firewalls
Switches
Authentication servers
wireless access points …
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2. Steps to secure a router:
Backup of configuration files and the router software
Controlling access to the router (tty, vty ports)
Securing access to the router (via SSH)
Password management
Logging events on the router
Disabling unnecessary services
Using loopback interfaces
Controlling SNMP as a management protocol
Controlling HTTP as a management protocol
Using CEF as a switching mechanism
Setting up the scheduler from a security perspective
Using the Network Time Protocol (NTP)
Login banners
Capturing core dumps
Using service nagle to improve Telnet access during high CPU events
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3. 2. Controlling access to the router
A tty port is physically connected to a terminal or workstation for local administrative access to the router.
An aux ports, similar to a tty port, is connected to a modem for remote out-of-band administrative access to the router.
A vty (virtual tty) port is used to allow remote in-band connection sessions, via telnet, ssh, or rlogin.
See for Q&A on “out-of-band” network management.
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4. Vulnerabilities of tty or aux ports
A tty or aux port may suffer reverse telnet attack, where the terminal server connected to the tty port or the modem connected to the aux port of the router is used by the attacker (as a remote client) to access the router.
Reverse Telnet (as defined in Wikipedia)
Check out this link to see an illustration of using ‘reverse telnet’ to remotely access a router. (diagrams below)
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5. Normal telnet
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6. Reverse Telnet
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7. Reverse Telnet (cont.) Another example:
What is Reverse Telnet and how do I configure it?
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8. Vulnerabilities of tty or aux ports
Solution?
Disable the console port
Line con0
transport input none
Allow only SSH access to a router’s console port (a feature added to IOS v12.2 or higher)
login authentication default
rotary 1
transport input ssh
ip ssh port 2001 rotary 1
Requirement: The router must be set up as a SSH server.
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9. Controlling vty access
Restricted access: Only allow the protocols that will be used by the network admin
Since Cisco IOS v11.1, the default is none.
Example: To allow only telnet and ssh connections
line vty 0 4
transport input telnet ssh
Only addresses in the ‘access list’ are allowed to connect: access-class, access-list (See example 3-6)
Short timeouts:
The default timeout value is 10 minutes. To set it to 5 min. 30 sec.
exec-timeout 5 30
Authentication for vty access: either local or RADIUS authentication (preferred).
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10. 3. Securing access to the router using encryption
IPsec VPN client (preferred; more details in Ch 13)
Two cases:
The VPN client access a back-end LAN (the destination) by building a tunnel between itself and a router (the IPsec gateway), behind which the LAN is located.
The VPN client is used to remotely administer the router, which is both the gateway and the destination.
SSH: Only SSH v1 is supported by Cisco IOS
Example 3-11
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11. 4. Password Management
Passwords stored on the router should be properly encrypted.
The default password-encryption is either type 0 (clear text passwords) or type 7 (weak encryption).
Use the enable secret command to activate MD5 when encrypting passwords.
Example 3-12
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12. 5. Logging events Advantages: Allows auditing and tracking
 forensics (in case of an attack)
performance tuning (maintenance)
Requirement:
good time stamping  using NTP
Example: 3-13
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13. 6. Disable unnecessary services
If a service is not being actively used on a device, it should be disabled.
Otherwise it may be used as a back door for the attacker to gain access to the device.
Sample services to be disabled: Table 3-1
TCP small servers, UDP small servers, Finger server, …
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14. 7. Using loopback interfaces
Advantages: Enable a block of IP addresses to be assigned to be used by loopback.
All routers can be forced to use these loopback IP addresses as source addresses when accessing the servers.
The servers can then also be locked down to allow access only from this block of IP addresses.
Accesses from addresses outside this block are denied.
Example 3-14
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15. 8. Controlling SNMP (as a management protocol)
SNMP can be used in read-only and ‘read and write’ modes
Unless necessary, use read-only mode on routers.
The ‘read and write’ mode allows the admin to modify the router’s configurations via SNMP.
Access into the network via SNMP should be blocked at the network’s boundary.
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16. 8. Controlling SNMP (as a management protocol)
Security of SNMP:
v1 and v2 use ‘community strings’ as the only authentication mechanism. (Not secure)
v3 is more secure by providing MD5 or SHA for authentication, and DES for encryption.
SNMP v3: threats vs protections (p.65)
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17. 9. Controlling HTTP (as a management protocol)
Unless necessary, HTTP access to the router should be disabled.
Admin access to the router via HTTP should be secured, by activating authentication.
Example: 3-19
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18. 10. Using CEF as a switching mechanism
Cisco Express Forwarding
Routers using the traditional switching mechanisms need to update routing caches when packets destined for new addresses arrive.
SYN floods and DDoS attacks use a large number of random or pseudo-random IP addresses as ultimate targets.
CEF replaces the normal routing cache with a data structure that mirrors the entire routing tables.
It does away with the need to update the cache each time a new IP address needs to be routed to.
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19. 11. Using the scheduler scheduler allocate scheduler interval
To prevent the router from becoming too busy responding to the interrupts on its interfaces due to the large number of packets arriving  large-scale network attack, esp. a DDoS attack
Example 3-21
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20. 12. Using NTP Network Time Protocol
Critical for services requiring good time stamping: logging, AAA, Kerberos, …
Challenge: authentication between devices exchanging NTP information
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21. 13. Login banners Sequence: Example: 3-25 Login banner login session
MOTD banner
EXEC banner (or incoming banner)
Example: 3-25
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22. 14. Capturing core dumps
In the event of system crash, the core dump may provide useful info for tracking the attack(s).
Example: 3-26
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23. 15. Service nagle
Nagle is an algorithm that can be enabled as a service on a Cisco router, to allow the router to pace the TCP connection for Telnet in a way that reduces the burden on the CPU and generally improves the performance of the Telnet session.
service nagle (Example 3-27)
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24. Security of other devices
Firewalls, switches, …
Similar procedure
Check the default settings
‘Harden’ the device before placing it into use in the production network.
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