How to Secure SNMP in Cisco

How to Secure SNMP in Cisco Switches and Routers

This article focus on the ways how we can secure SNMP access in Cisco Switches and Routers

Simple Network Management Protocol (SNMP) uses the default UDP port 161 for general SNMP messages and UDP port 162 for SNMP trap messages.

SNMP is a service used to perform network management functions using a data structure called a Management Information Base (MIB). Unfortunately, SNMP version 1 is widely implemented but not very secure, using only clear-text community strings for access to information on the switch, including its configuration file.

If SNMP is not being used, then executing the following commands will disable the service.

Switch(config)# no snmp-server community

Switch(config)# no snmp-server enable traps

Switch(config)# no snmp-server system-shutdown

Switch(config)# no snmp-server

If SNMP is required for a switch or router configure the switch or router for SNMP version 3. This version is more secure than SNMP version 1 because version 3 can use cryptographic hashes for authentication to protect the community string. The above commands for disabling SNMP are recommended for use before deploying SNMP version 3 to remove any possible default community strings. The following commands show an example User Security Model for SNMP version 3 for the switch. The model begins with creating a standard access-list (e.g., 12) that allows only those systems that manage the switch. Next, define a group (e.g., admins) with read and write MIB views (e.g., adminview). Then each user (e.g., root) is added to the group with a password (e.g., 5ecret-5TR1N) that can be hashed (e.g., using md5) before being sent across the network. Also, the standard access-list (e.g., 12) is applied to the user. Finally, the MIB view (e.g., adminview) is defined by one or more statements to include or to exclude portions of the MIB. The MIB view in the following example gives access to the Internet branch of the MIB except the branches that display IP addresses and IP routing information.

Switch(config)# no access-list 12

Switch(config)# access-list 12 permit 10.0.0.2

Switch(config)# access-list 12 permit 10.0.0.4

Switch(config)# snmp-server group admins v3 auth read adminview write adminview

Switch(config)# snmp-server user root admins v3 auth md5 5ecret-5TR1N access 12

Switch(config)# snmp-server view adminview internet included

Switch(config)# snmp-server view adminview ipAddrEntry excluded

Switch(config)# snmp-server view adminview ipRouteEntry excluded

If SNMP is required for a switch and only SNMP version 1 is available, then the following commands show an example of how to configure the switch with a community string (e.g., g00d-5tr1n9) that has read-only permissions and a standard access-list (e.g., 12) applied to it.

Switch(config)# no access-list 12

Switch(config)# access-list 12 permit 10.0.0.2

Switch(config)# access-list 12 permit 10.0.0.4

Switch(config)# snmp-server community g00d-5tr1n9 ro 12

In addition to the configuration of the SNMP service, SNMP Trap information can be sent to the systems that manage the switches. The following commands show an example of this configuration.

Switch(config)# snmp-server host 10.0.0.2 traps g00d-5tr1n9-2

Switch(config)# snmp-server host 10.0.0.4 traps g00d-5tr1n9-2

Switch(config)# snmp-server trap-source Loopback0

Switch(config)# snmp-server enable traps

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Netflow

How to configure ASA/PIX firewall to collect Net flow data from an external router to the netflow collector located in Inside Network.

This article provides an example of Net flow configurations in a Cisco Router, ASA/PIX firewall to collect the Net flow data in the internal network.

Components Uses
The information in this document is based on following hardware and software versions
• Cisco Router 3745 – IOS version 12.3(17b. (Network 192.168.10.0)
• PIX 525 7.0.3 ( ASA can also be used) (Internal 10.0.0.2)
• Manage Engine Net flow Analyzer 6 ( Any net flow collector can be used)(

In this example let’s start by configuring Net flow in a Cisco Router

Cisco Router Configuration
Here the IP address for the interface is 192.168.10.1

Enabling Net flow in an Interface
Enter global configuration mode on the router and issue the following commands for each interface on which you want to enable Net Flow:

interface {interface} {interface_number}
ip route-cache flow
bandwidth
exit

After applying the commands the example will be as follows

router3745#configure terminal
router-3745(config)#interface FastEthernet 0/1
router-3745(config)#ip address 192.198.10.1 255.255.255.240
router-3745(config-if)#ip route-cache flow
router-3745(config-if)Bandwidth 1000
router-3745(config-if)#exit

Exporting NetFlow Data

Issue the following commands to export Net Flow data to the server on which NetFlow Analyzer is running:

ip flow-export destination {hostname|ip_address} 9996 ( Exports the NetFlow cache entries to the specified IP address. Use the IP address of the NetFlow Analyzer server and the configured NetFlow listener port. The default port is 9996. )

ip flow-export source {interface} {interface_number} (Sets the source IP address of the NetFlow exports sent by the device to the specified IP address. NetFlow Analyzer will make SNMP requests of the device on this address.)

ip flow-export version 5 [peer-as | origin-as] (Sets the NetFlow export version to version 5. Version 5,7 & 9 are available)

ip flow-cache timeout active 1 (Breaks up long-lived flows into 1-minute fragments. You can choose any number of minutes between 1 and 60. If you leave it at the default of 30 minutes your traffic reports will have spikes.It is important to set this value to 1 minute in order to generate alerts and view troubleshooting data.)

ip flow-cache timeout inactive 15 (Ensures that flows that have finished are periodically exported. The default value is 15 seconds. You can choose any number of seconds between 10 and 600. )

snmp-server ifindex persist (Enables ifIndex persistence (interface names) globally. This ensures that the ifIndex values are persisted during device reboots.)

The following example shows the above mentioned commands

router-3745(config)#ip flow-export destination 192.168.10.5 9996
router-3745(config)#ip flow-export source FastEthernet 0/1
router-3745(config)#ip flow-export version 5
router-3745(config)#ip flow-cache timeout active 1
router-3745(config)#ip flow-cache timeout inactive 15
router-3745(config)#snmp-server ifindex persist
router-3745(config)#^Z

Issue the following commands in normal (not configuration) mode to verify whether NetFlow export has been configured correctly:

show ip flow export (Shows the current NetFlow configuration)
show ip cache flow (These commands summarize the active flows and give an indication of how much NetFlow data the device is exporting

router-3745#show ip flow export
router-3745#show ip cache flow

The next step is make a Natting in ASA/PIX

pix-525# configure t
pix-525# (config)# static (inside,outside) 192.168.10.5 10.0.0.6 netmask 255.255.255.255 dns

In order to export to the netflow statistics to the netflow analyzer located in the internal network we have configure the following access-list and apply it to outside interface to allow the Netflow traffic

pix-525# configure t
pix-525# (config)#access-list NETFLOW extended permit udp any host 192.168.10.5 eq 9996
pix-525# (config)#access-list NETFLOW extended permit tcp any any

Apply the created access-list to the outside interface

pix-525# (config)#access-group NETFLOW in interface outside

Now install the Netflow Analyzer software and configure it to recieve the netflow statists from the external router.

Troubleshooting tips

Verify Netflow is working in Cisco Router

router-3745#sho ip cache flow

IP packet size distribution (78841980 total packets):
1-32 64 96 128 160 192 224 256 288 320 352 384 416 448 480
.003 .453 .023 .012 .008 .010 .004 .003 .003 .003 .004 .003 .003 .003 .004

512 544 576 1024 1536 2048 2560 3072 3584 4096 4608
.003 .005 .022 .021 .401 .000 .000 .000 .000 .000 .000

IP Flow Switching Cache, 278544 bytes
548 active, 3548 inactive, 4045717 added
84147818 ager polls, 0 flow alloc failures
Active flows timeout in 1 minutes
Inactive flows timeout in 15 seconds
IP Sub Flow Cache, 33416 bytes
548 active, 1500 inactive, 4045717 added, 4045717 added to flow
0 alloc failures, 0 force free
2 chunks, 14 chunks added
last clearing of statistics never
Protocol Total Flows Packets Bytes Packets Active(Sec) Idle(Sec)
——– Flows /Sec /Flow /Pkt /Sec /Flow /Flow
TCP-Telnet 143 0.0 2 52 0.0 0.4 12.7
TCP-FTP 255 0.0 6 100 0.0 9.0 7.2
TCP-FTPD 15010 0.0 1 63 0.0 0.6 15.4
TCP-WWW 1100665 2.5 14 607 37.7 8.2 6.9
TCP-SMTP 171448 0.3 69 633 27.3 35.8 6.2
TCP-X 723 0.0 2 245 0.0 0.4 13.0
TCP-other 1966270 4.5 21 656 95.4 11.7 6.6
UDP-DNS 56825 0.1 12 66 1.5 20.5 11.6
UDP-NTP 8 0.0 1 76 0.0 0.0 15.5
UDP-Frag 1 0.0 1 1476 0.0 0.0 15.0
UDP-other 684203 1.5 11 319 17.9 4.8 14.9
ICMP 48198 0.1 1 78 0.2 1.6 15.4
GRE 1358 0.0 183 182 0.5 50.0 4.2
IP-other 62 0.0 83 108 0.0 53.4 3.2
Total: 4045169 9.2 19 601 180.9 10.6 8.3

SrcIf SrcIPaddress DstIf DstIPaddress Pr SrcP DstP Pkts
Fa0/1 192.168.10.5 Tu0 69.26.190.118 11 1705 0D96 8
Fa0/1 192.168.10.5 Tu0 65.55.111.92 06 0019 10EC 32
Fa0/1 192.168.10.5 Tu0 206.190.48.113 06 0019 714B 29

Check Nating is working in the Firewall

pix-525# show xlate
2in use, 417 most used
Global 192.168.10.5 Local 10.0.0.6

Check access -list is forwading the netflow traffic

pix-525# sho access-list NETFLOW
access-list NETFLOW; 2 elements
access-list NETFLOW line 1 extended permit udp any host 192.168.10.5 eq 9996 (hitcnt=7)
access-list NETFLOW line 2 extended permit ip any any (hitcnt=140861)

To know more about Netflow Analyzer and its configuration click this link Netflow

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