Network Security

1. Security Concerns in LANs
Enterprise Networking, Security, and Automation v7.0
(ENSA)

2. Switch Vulnerabilities
 Switches are vulnerable to a variety of attacks including:
 Password attacks
 DoS attacks
 CDP attacks
 MAC address flooding
 DHCP attacks
 To mitigate against these attacks:
 Disable unused ports
 Disable CDP
 Configure Port Security
 Configure DHCP snooping
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3. Disable Unused Ports and Assign to an
Unused (Garbage) VLAN
S1(config)#int range fa0/20 – 24
S1(config-if-range)# switchport access vlan 100
S1(config-if-range)# shutdown
%LINK-5-CHANGED: Interface FastEthernet0/20, changed state to
administratively down
%LINK-5-CHANGED: Interface FastEthernet0/21, changed state to
administratively down
%LINK-5-CHANGED: Interface FastEthernet0/22, changed state to
administratively down
%LINK-5-CHANGED: Interface FastEthernet0/23, changed state to
administratively down
%LINK-5-CHANGED: Interface FastEthernet0/24, changed state to
administratively down
S1(config-if-range)#
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4. Leveraging the Cisco Discovery Protocol
 The Cisco Discovery Protocol is a Layer 2 Cisco proprietary protocol
used to discover other directly connected Cisco devices.
 It is designed to allow the devices to autoconfigure their
connections.
 If an attacker is listening to Cisco Discovery Protocol messages, it
could learn important information, such as the device model or the
running software version.
5

5. Leveraging the Cisco Discovery Protocol
 Cisco recommends disabling CDP when it is not in use.
Cisco IOS Software, C2960 Software (C2960-LANBASEK9-M), Version
12.2(44)SE, RELEASE SOFTWARE (fc1)…
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6. Disabling CDP
S1(config)# no cdp run
S1(config)#
S1(config)# interface range fa0/1 – 24
S1(config-if-range)# no cdp enable
S1(config-if-range)#exit
S1(config)#
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7. Layer 2 Switching
Packet Tracer Topology
In this scenario, the
switch has just
rebooted.
Verify the content of
the MAC address
table.
192.168.1.0 /24
.10 .11
.12 .13
000a.f38e.74b3 00d0.ba07.8499
0090.0c23.ceca
0001.9717.22e0
F0/1
F0/3
F0/2
F0/4
Sw1# show mac-address-table
Mac Address Table
-------------------------------------------
Vlan Mac Address Type Ports
---- ----------- -------- -----
Sw1#
8

8. Layer 2 Switching
PC-A pings PC-B.
192.168.1.0 /24
.12 .13
0090.0c23.ceca
0001.9717.22e0
F0/1
F0/3
F0/2
F0/4
PC-A> ping 192.168.1.11
Pinging 192.168.1.11 with 32 bytes of data:
Reply from 192.168.1.11: bytes=32 time=62ms TTL=128
Reply from 192.168.1.11: bytes=32 time=62ms TTL=128
Reply from 192.168.1.11: bytes=32 time=63ms TTL=128
Reply from 192.168.1.11: bytes=32 time=63ms TTL=128
Ping statistics for 192.168.1.11:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 62ms, Maximum = 63ms, Average = 62ms
PC-A> 9
.10 .11
00d0.ba07.8499
000a.f38e.74b3

9. Layer 2 Switching
Sw1# show mac-address-table
Mac Address Table
-------------------------------------------
Vlan Mac Address Type Ports
---- ----------- -------- -----
1 000a.f38e.74b3 DYNAMIC Fa0/1
1 00d0.ba07.8499 DYNAMIC Fa0/2
Sw1#
10
192.168.1.0 /24
.12 .13
0090.0c23.ceca
0001.9717.22e0
F0/1
F0/3
F0/2
F0/4
.10 .11
00d0.ba07.8499
000a.f38e.74b3

10. Unicast Flooding
11
Unicast
Mac Address Table
Port MAC Address
Mac Address Table
1.Learn – Examine Source MAC
address
In table: Reset 5 min timer
Not in table: Add Source MAC
address and port # to table
2.Forward – Examine
Destination MAC address
In table: Forward out that port.
Not in table: Flood out all ports
except incoming port.
AAAA BBBB
AAAA
BBBB
Not in table
Unknown Unicast
1 2

11. 12
MAC Flood Attack
 If the attack is launched before the
beginning of the day, the CAM table would
be full as the majority of devices are
powered on.
 If the initial, malicious flood of invalid CAM
table entries is a one-time event:
 Can generate 155,000 MAC entries
per minute
 “Typical” switch can store 4,000 to
8,000 MAC entries
 Eventually, the switch will age out
older, invalid CAM table entries
 New, legitimate devices will be able to
create an entry in the CAM
 Traffic flooding will cease
 Intruder may never be detected
(network seems normal).

12. Unicast Flooding
13
Unicast
Mac Address Table
Port MAC Address
Mac Address Table
1.Learn – Examine Source MAC
address
In table: Reset 5 min timer
Not in table: Add Source MAC
address and port # to table
2.Forward – Examine
Destination MAC address
In table: Forward out that port.
Not in table: Flood out all ports
except incoming port.
AAAA BBBB
AAAA
BBBB
Not in table or table is full
Unknown Unicast
1 2

13. Configure Port Security
 Port security allows an administrator to limit the number of MAC addresses learned on a
port.
 If this is exceeded, a switch action can be configured.
 Configure each access port to accept 1 MAC address only or a small group of MAC
addresses.
 Frames from any other MAC addresses are not forwarded.
 By default, the port will shut down if the wrong device connects.
 It has to be brought up again manually.
1 1 1 1
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14. Configuring Port Security
 Use the switchport port-security interface command to
enable port security on a port.
 It is used to:
 Set a maximum number of MAC addresses.
 Define violation actions.
 MAC address(es) can be learned dynamically, entered manually,
or learned and retained dynamically.
 Set the aging time for dynamic and static secure address entries.
 To verify port security status: show port-security
switchport port-security [max value] [violation {protect |
restrict | shutdown}] [mac-address mac-address [sticky]]
[aging time value]
Switch(config-if)#
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15. Port Security: Secure MAC Addresses
 The switch supports these types of secure MAC addresses:
 Static
 Configured using switchport port-security mac-address
mac-address
 Stored in the address table
 Added to running configuration.
 Dynamic
 These are dynamically configured
 Stored only in the address table
 Removed when the switch restarts
 Sticky
 These are dynamically configured
 Stored in the address table
 Added to the running configuration.
 If running-config saved to startup-config, when the switch restarts, the
interface does not need to dynamically reconfigure them.
 Note: When you enter this command, the interface converts all the
dynamic secure MAC addresses, including those that were dynamically
learned before sticky learning was enabled, to sticky secure MAC
addresses. The interface adds all the sticky secure MAC addresses to
the running configuration.
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16. Port Security: Steps
17

17. Port Security Defaults
 Secure MAC addresses can be configured as follows:
 Dynamically (learned but not retained after a reboot)
 Statically (prone to errors)
 Sticky (learned dynamically and retained)
Feature Default setting
Port Security Disabled on a port
Maximum # of Secure MAC
Addresses
1
Violation
Shutdown
• The port shuts down when the maximum number of secure MAC
addresses is exceeded, and an SNMP trap notification is sent.
Sticky Address Learning Disabled
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18. Dynamic Secure MAC address
 Learned dynamically
 S1(config-if)# switchport mode access
 S1(config-if)# switchport port-security
 By default, only 1 address is learned.
 Put in MAC address table
 Not shown in running configuration
 It is not saved or in the configuration when switch restarts.
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19. Static Secure MAC address
 Static secure MAC address is manually configured in interface
config mode
S1(config-if)# switchport mode access
S1(config-if)# switchport port-security mac-address
000c.7259.0a63
 MAC address is stored in MAC address table
 Shows in the running configuration
 Can be saved with the configuration. 20

20. Sticky Secure MAC address
 Dynamically learned and can be retained.
 S1(config-if)# switchport mode access
 S1(config-if)# switchport port-security mac-address sticky
 You can choose how many can be learned (default 1).
 Added to the running configuration
 Saved only if you save running configuration.
 Note:
 When you enter this command, the interface converts all the
dynamic secure MAC addresses, including those that were
dynamically learned before sticky learning was enabled, to sticky
secure MAC addresses.
 The interface adds all the sticky secure MAC addresses to the
running configuration. 21

21. 22
interface FastEthernet0/2
switchport mode access
 Sets the interface mode as access; an interface in the default mode (dynamic
desirable) cannot be configured as a secure port.
switchport port-security
 Enables port security on the interface
switchport port-security maximum 6
 (Optional) Sets the maximum number of secure MAC addresses for the interface. The
range is 1 to 132; the default is 1.
switchport port-security aging time 5
 Learned addresses are not aged out by default but can be with this command. Value
from 1 to 1024 in minutes.
switchport port-security mac-address 0000.0000.000b
 (Optional) Enter a static secure MAC address for the interface, repeating the
command as many times as necessary. You can use this command to enter the
maximum number of secure MAC addresses. If you configure fewer secure MAC
addresses than the maximum, the remaining MAC addresses are dynamically
learned.
switchport port-security mac-address sticky
 (Optional) Enable stick learning on the interface.
switchport port-security violation shutdown
 (Optional) Set the violation mode, the action to be taken when a security violation is
detected. (Next)
NOTE: switchport host command will disable channeling, and enable access/portfast
Switch(config-if)# switchport host
switchport mode will be set to access
spanning-tree portfast will be enabled
channel group will be disabled

22. Port Security: Steps
23

23. Port Security: Maximum of 1
 The secure MAC addresses are stored in an address table.
 Setting a maximum number of addresses to 1 and configuring the
MAC address of an attached device ensures that the device has the
full bandwidth of the port.
Switch(config-if)# switchport port-security maximum 1
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24. Port Security: Static
Addresses
 Restricts input to an interface by limiting and identifying MAC
addresses of the stations allowed to access the port.
 The port does not forward packets with source addresses outside the
group of defined addresses.
Switch(config)# interface fa 0/1
Switch(config-if)# switchport mode access
Switch(config-if)# switchport port-security
Switch(config-if)# switchport port-security maximum 3
Switch(config-if)# switchport port-security mac-address 0000.0000.000a
Switch(config-if)# switchport port-security mac-address 0000.0000.000b
Switch(config-if)# switchport port-security mac-address 0000.0000.000c
X
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25. 26
Port Security: Violation
 Station attempting to access the
port is different from any of the
identified secure MAC addresses,
a security violation occurs.

26. Port Security: Violation
 By default, if the maximum number of connections is achieved and a
new MAC address attempts to access the port, the switch must take
one of the following actions:
 Protect: Frames from the nonallowed address are dropped, but there
is no log of the violation.
 Restrict: Frames from the nonallowed address are dropped, a log
message is created and Simple Network Management Protocol
(SNMP) trap sent.
 Shut down: If any frames are seen from a nonallowed address, the
interface is errdisabled, a log entry is made, SNMP trap sent and
manual intervention (no shutdown) or errdisable recovery must be
used to make the interface usable.
Switch(config-if)#switchport port-security violation
{protect | restrict | shutdown}
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27. DHCP Attacks
 DHCP is a network protocol used to automatically assign IP
information.
 Two types of DHCP attacks are:
 DHCP spoofing: A fake DHCP server is placed in the network to
issue DHCP addresses to clients.
 DHCP starvation: DHCP starvation is often used before a
DHCP spoofing attack to deny service to the legitimate DHCP
server.
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28. DHCP Review
29

29. DHCP Spoof Attacks
“I need an IP
address/mask, default
gateway, and DNS
server.”
“Here you go, I
might be first!”
(Rouge)
“Here you go.”
(Legitimate)
“Got it, thanks!”
“Already got the info.”
All default gateway
frames and DNS
requests sent to
Rogue.
“I can now
forward these on
to my leader.”
(Rouge)
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30. Solution:
Configure
DHCP
Snooping
 DHCP snooping is a Cisco Catalyst feature that determines which
switch ports can respond to DHCP requests.
 Ports are identified as trusted and untrusted.
 Trusted ports: Host a DHCP server or can be an uplink toward
the DHCP server and can source all DHCP messages, including
DHCP offer and DHCP acknowledgement packets
 Untrusted ports: Can source requests only. 31

31. DHCP Snooping
S1(config)# ip dhcp snooping
S1(config)# ip dhcp snooping vlan 10,20
S1(config)# interface gig 0/1
S1(config-if)# ip dhcp snooping trust
S1(config)# interface fa 0/0
S1(config-if)# ip dhcp snooping limit rate 100
Fa0/0
Gig0/1
By default all interfaces are untrusted.
Fa0/0
S1
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32. DHCP Snooping
“I need an IP
address/mask,
default gateway,
and DNS server.”
“Here you go, I
might be first!”
(Rouge)
“Here you go.”
(Legitimate)
Switch: This is an
untrusted port, I will
block this DHCP Offer”
“Thanks, got it.”
Switch: This is a trusted port, I
will allow this DHCP Offer”
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