Introduction
This document describes how to secure your Simple Network Management Protocol (SNMP).
Prerequisites
Requirements
There are no specific requirements for this document.
Components Used
The information in this document is based on these software and hardware versions:
The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, ensure that you understand the potential impact of any command.
Conventions
For more information on document conventions, refer to the Cisco Technical Tips Conventions.
Background Information
It is important to secure your SNMP especially when the vulnerabilities of SNMP can be repeatedly exploited to produce a denial of service (DoS).
Strategies to Secure SNMP
Choose a Good SNMP Community String
It is not a good practice to use public as read-only and private as read-write community strings.
Setup SNMP View
The Setup SNMP view command can block the user with only access to limited Management Information Base (MIB). By default, there is no SNMP view entry exists . This command is configured at the global configuration mode and first introduced in Cisco IOS Software version 10.3. It works similar to access-list in that if you have any SNMP View on certain MIB trees, every other tree is denied inexplicably. However, the sequence is not important and it goes through the entire list for a match before it stops.
To create or update a view entry, use the snmp-server view global configuration command. To remove the specified SNMP server view entry, use the no form of this command.
Syntax:
snmp-server view view-name oid-tree {included | excluded}
no snmp-server view view-name
Syntax Description:
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view-name— Label for the view record that you update or create. The name is used to reference the record.
-
oid-tree — Object identifier of the Abstract Syntax Notation One (ASN.1) subtree to be included or excluded from the view. To identify the subtree, specify a text string that consists of numbers, such as 1.3.6.2.4, or a word, such as system. Replace a single sub-identifier with the asterisk (*) wildcard to specify a subtree family; for example 1.3.*.4.
-
included | excluded— Type of view. You must specify either included or excluded.
Two standard predefined views can be used when a view is required instead of a view that must be defined. One is everything, which indicates that the user can see all objects. The other is restricted , which indicates that the user can see three groups: system, snmpStats, and snmpParties. The predefined views are described in RFC 1447.
Note: The first snmp-server command that you enter enables both versions of SNMP.
This example creates a view that includes all objects in the MIB-II system group except for sysServices (System 7) and all objects for interface 1 in the MIB-II interfaces group:
snmp-server view agon system included
snmp-server view agon system.7 excluded
snmp-server view agon ifEntry.*.1 included
This is a complete example for how to apply the MIB with community string and the output of the snmpwalk with view in place. This configuration defines a view that denies the SNMP access for the Address Resolution Protocol (ARP) table ( atEntry ) and allows it for MIB-II and Cisco private MIB:
snmp-server view myview mib-2 included
snmp-server view myview atEntry excluded
snmp-server view myview cisco included
snmp-server community public view myview RO 11
snmp-server community private view myview RW 11
snmp-server contact pvanderv@cisco.com
This is the command and output for the MIB-II System group:
NMSPrompt 82 % snmpwalk cough system
system.sysDescr.0 : DISPLAY STRING- (ascii):Cisco Internetwork Operating System Software
Cisco IOS (tm) 2500 Software (C2500-JS-L), Version 12.0(1)T,RELEASE SOFTWARE (fc2)
Copyright (c) 1986-1998 by cisco Systems, Inc.
Compiled Wed 04-Nov-98 20:37 by dschwart
system.sysObjectID.0 : OBJECT IDENTIFIER:
.iso.org.dod.internet.private.enterprises.cisco.ciscoProducts.cisco2520
system.sysUpTime.0 : Timeticks: (306588588) 35 days, 11:38:05.88
system.sysContact.0 : DISPLAY STRING- (ASCII):pvanderv@cisco.com
system.sysName.0 : DISPLAY STRING- (ASCII):cough
system.sysLocation.0 : DISPLAY STRING- (ASCII):
system.sysServices.0 : INTEGER: 78
system.sysORLastChange.0 : Timeticks: (0) 0:00:00.00
NMSPrompt 83 %
This is the command and output for the local Cisco System group:
NMSPrompt 83 % snmpwalk cough lsystem
cisco.local.lsystem.romId.0 : DISPLAY STRING- (ASCII):
System Bootstrap, Version 11.0(10c), SOFTWARE
Copyright (c) 1986-1996 by cisco Systems
cisco.local.lsystem.whyReload.0 : DISPLAY STRING- (ASCII):power-on
cisco.local.lsystem.hostName.0 : DISPLAY STRING- (ASCII):cough
This is the command and output for the MIB-II ARP table:
NMSPrompt 84 % snmpwalk cough atTable
no MIB objects contained under subtree.
NMSPrompt 85 %
Setup SNMP Community with Access-list
The best current practices recommend that you apply Access Control Lists (ACLs) to community strings and ensure that the requests community strings are not identical to notifications community strings. Access lists provide further protection when used in combination with other protective measures.
This example sets up ACL to community string:
access-list 1 permit 10.1.1.1
snmp-server community string1 ro 1
When you use different community strings for requests and trap messages, it reduces the likelihood of further attacks or compromises if the community string is discovered by an attacker. Otherwise, an attacker could compromise a remote device or sniff a trap message from the network without authorization.
Once you enable trap with a community string, the string can be enabled for SNMP access in some Cisco IOS software. You must explicitly disable this community. For example:
access-list 10 deny any
snmp-server host 10.1.1.1 mystring1
snmp-server community mystring1 RO 10
Setup SNMP Version 3
Do these steps to configure SNMP version 3:
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Assign an Engine ID for the SNMP Entity (Optional).
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Define a user, userone that belongs to the group groupone and apply noAuthentication (no password) and noPrivacy (no encryption) to this user.
-
Define a user, usertwo  ;that belongs the group grouptwo and apply noAuthentication (no password) and noPrivacy (no encryption) to this user.
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Define a user, userthree that belongs the group groupthree and apply Authentication (password is user3passwd) and noPrivacy (no encryption) to this user.
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Define a user, userfour , that belongs to the group groupfour and apply Authentication (password is user4passwd) and Privacy (des56 encryption) to this user.
-
Define a group, groupone , by means of User Security Model (USM) V3 and enable read access on the v1default view (the default).
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Define a group, grouptwo , by means of USM V3 and enable read access on the view myview .
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Define a group, groupthree , by means of USM V3, and enable read access on the v1default view (the default), by means of authentication .
-
Define a group, groupfour , by means of USM V3, and enable read access on the v1default view (the default), by means of Authentication and Privacy .
-
Define a view, myview , that provides read access on the MIB-II and denies read access on the private Cisco MIB.
The show running output gives additional lines for the group public, due to the fact that there is a community string Read-Only public that has been defined.
The show running output does not show the userthree.
Example:
snmp-server engineID local 111100000000000000000000
snmp-server user userone groupone v3
snmp-server user usertwo grouptwo v3
snmp-server user userthree groupthree v3 auth md5 user3passwd
snmp-server user userfour groupfour v3 auth md5 user4passwd priv des56
user4priv
snmp-server group groupone v3 noauth
snmp-server group grouptwo v3 noauth read myview
snmp-server group groupthree v3 auth
snmp-server group groupfour v3 priv
snmp-server view myview mib-2 included
snmp-server view myview cisco excluded
snmp-server community public RO
This is the command and output for the MIB-II System group with user userone:
NMSPrompt 94 % snmpwalk -v3 -n "" -u userone -l noAuthNoPriv clumsy system
Module SNMPV2-TC not found
system.sysDescr.0 = Cisco Internetwork Operating System Software
Cisco IOS (TM) 4500 Software (C4500-IS-M), Version 12.0(3)T,RELEASE SOFTWARE (fc1)
Copyright (c) 1986-1999 by cisco Systems, Inc.
Compiled Tue 23-Feb-99 03:59 by ccai
system.sysObjectID.0 = OID: enterprises.9.1.14
system.sysUpTime.0 = Timeticks: (28208096) 3 days, 6:21:20.96
system.sysContact.0 =
system.sysName.0 = clumsy.cisco.com
system.sysLocation.0 =
system.sysServices.0 = 78
system.sysORLastChange.0 = Timeticks: (0) 0:00:00.00
NMSPrompt 95 %
This is the command and output for the MIB-II System group with user usertwo:
NMSPrompt 95 % snmpwalk -v3 -n "" -u usertwo -l noAuthNoPriv clumsy system
Module SNMPV2-TC not found
system.sysDescr.0 = Cisco Internetwork Operating System Software
Cisco IOS (TM) 4500 Software (C4500-IS-M), Version 12.0(3)T,RELEASE SOFTWARE (fc1)
Copyright (c) 1986-1999 by cisco Systems, Inc.
Compiled Tue 23-Feb-99 03:59 by ccai
system.sysObjectID.0 = OID: enterprises.9.1.14
system.sysUpTime.0 = Timeticks: (28214761) 3 days, 6:22:27.61
system.sysContact.0 =
system.sysName.0 = clumsy.cisco.com
system.sysLocation.0 =
system.sysServices.0 = 78
system.sysORLastChange.0 = Timeticks: (0) 0:00:00.00
This is the command and output for the Cisco Local System group with user userone:
NMSPrompt 98 % snmpwalk -v3 -n "" -u userone -l noAuthNoPriv clumsy .1.3.6.1.4.1.9.2.1
Module SNMPV2-TC not found
enterprises.9.2.1.1.0 = "..System Bootstrap, Version 5.2(7b) [mkamson 7b],
RELEASE SOFTWARE (fc1)..Copyright (c) 1995 by cisco Systems,
Inc..."
enterprises.9.2.1.2.0 = "reload"
enterprises.9.2.1.3.0 = "clumsy"
enterprises.9.2.1.4.0 = "cisco.com"
This is the command and output that shows you cannot get the Cisco Local System group with user usertwo:
NMSPrompt 99 % snmpwalk -v3 -n "" -u usertwo -l noAuthNoPriv clumsy .1.3.6.1.4.1.9.2.1
Module SNMPV2-TC not found
enterprises.9.2.1 = No more variables left in this MIB View
NMSPrompt 100 %
This command and the output result is for a customized tcpdump (patch for SNMP version 3 support and addendum of printf):
NMSPrompt 102 % snmpget -v3 -n "" -u userone -l noAuthNoPriv clumsy system.sysName.0
Module SNMPV2-TC not found
system.sysName.0 = clumsy.cisco.com
Setup ACL on Interfaces
The ACL feature provides security measures that prevent attacks such as IP spoofing. The ACL can be applied on incoming or outgoing interfaces on routers.
On platforms that do not have the option to use receive ACLs (rACLs), it is possible to permit User Datagram Protocol (UDP) traffic to the router from trusted IP addresses with interface ACLs.
The next extended access list can be adapted to your network. This example assumes that the router has IP addresses 192.168.10.1 and 172.16.1.1 configured on its interfaces, that all SNMP access is to be restricted to a management station with the IP address of 10.1.1.1, and that the management station need only communicate with IP address 192.168.10.1:
access-list 101 permit udp host 10.1.1.1 host 192.168.10.1
The access-list must then be applied to all interfaces with these configuration commands:
interface ethernet 0/0
ip access-group 101 in
All devices that communicate directly with the router on UDP ports need to be specifically listed in the previous access list. Cisco IOS software uses ports in the range 49152 to 65535 as the source port for outbound sessions such as Domain Name System (DNS) queries.
For devices that have many IP addresses configured, or many hosts that need to communicate with the router, this is not always a scalable solution.
rACLs
For distributed platforms, rACLs can be an option that starts in Cisco IOS Software Release 12.0(21)S2 for the Cisco 12000 Series Gigabit Switch Router (GSR) and Release 12.0(24)S for the Cisco 7500 Series. The receive access lists protect the device from harmful traffic before the traffic can impact the route processor. Receive path ACLs also are considered a network security best practice, and must be considered as a long-term addition to good network security, as well as a workaround for this specific vulnerability. The CPU load is distributed to the line card processors and helps mitigate load on the main route processor. The white paper entitled GSR: Receive Access Control Lists helps to identify legitimate traffic. Use that white paper to understand how to send legitimate traffic to your device and also deny all unwanted packets..
Infrastructure ACLs
Although it is often difficult to block traffic that transits your network, it is possible to identify traffic that must never be allowed to target your infrastructure devices and block that traffic at the border of your network. Infrastructure ACLs (iACLs) are considered a network security best practice and must be considered as a long-term addition to good network security as well as a workaround for this specific vulnerability. The white paper, Protecting Your Core: Infrastructure Protection Access Control Lists ,presents guidelines and recommended deployment techniques for iACLs..
Cisco Catalyst LAN Switch Security Feature
The IP Permit List feature restricts inbound Telnet and SNMP access to the switch from unauthorized source IP addresses. Syslog messages and SNMP traps are supported to notify a management system when a violation or unauthorized access occurs.
A combination of the Cisco IOS software security features can be used to manage routers and Cisco Catalyst switches. A security policy needs to be established that limits the number of management stations that can access the switches and routers.
For more information on how to increase security on IP networks, refer to Increasing Security on IP Networks .
How to Check SNMP Errors
Configure the SNMP community ACLs with the log keyword. Monitor syslog for failed attempts, as show below.
access-list 10 deny any log
snmp-server community public RO 10
When someone tries to access the router with the community public, you see a syslog similar to this:
%SEC-6-IPACCESSLOGS: list 10 denied 172.16.1.15packet
This output means that access-list 10 has denied five SNMP packets from the host 172.16.1.1.
Periodically check SNMP for errors with the show snmp command, as shown here:
router#show snmp Chassis: 21350479 17005 SNMP packets input
37 Bad SNMP version errors**
15420 Unknown community name**
0 Illegal operation for community name supplied
1548 Encoding errors**
0 Number of requested variables
0 Number of altered variables
0 Get-request PDUs
0 Get-next PDUs
0 Set-request PDUs 0 SNMP packets output
0 Too big errors (Maximum packet size 1500)
0 No such name errors
0 Bad values errors
0 General errors
0 Response PDUs
0 Trap PDUs
Watch the counters marked ** for unexpected increases in error rates that can indicate attempted exploitation of these vulnerabilities. To report any security issue, refer to Cisco Product Security Incident Response.
Related Information
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