How to configure SNMPv3 securely in CentOS/RHEL/SL

May 4, 2014

Looking through the Internet I found more often than not insecure SNMP setups explained in Howtos, maybe the authors where happy to got running in the first place – don’t know. The Simple Network Management Protocol (SNMP) is used to monitor (and configure in the case of network equipment) systems via the network in a standardized way. There are 3 versions of it in the wild:

Version 1

The first version, it is really old and only uses community strings to “protect” access. If you look with Wireshark what the packet for the request contains you’ll see following:

snmpv1

Yes, thats the community in clear text …. so don’t use it, if you must not. If you must (old devices which supports only this) use it on a secure network and make sure that the device only accepts quests from a given source IP address.

Version 2c

Not better in any security related way. The e.g. traffic counters are just bigger, as on fast interfaces it was possible with v1 that the counter rotated more than ones within the interval you queried so you didn’t know how many bytes really got transfered over the link.

Version 3

Now we’re talking. This versions supports authentication passwords, that get not transfered in the clear and it also supports encryption passwords so a attacker does not see what is transfered. I’ll show you now how you configure your CentOS/RHEL/SL to let it be queried via SNMPv3 in a secure way.

  1. We need to install the package we need:
    yum install net-snmp

  2. We create our user. SHA1 is used for the authentication and AES for the encryption of the traffic. And for security it is important to use two separated passwords.
    service snmpd stop
    net-snmp-create-v3-user -ro -A snmpv3authPass -a SHA -X snmpv3encPass -x AES snmpv3user
    service snmpd start

    This will print something like this

    adding the following line to /var/lib/net-snmp/snmpd.conf:
    createUser snmpv3user SHA "snmpv3authPass" AES snmpv3encPass
    adding the following line to /etc/snmp/snmpd.conf:
    rouser snmpv3user

  3. Secure the SNMP daemon with a host firewall and only allow requests from source IP addresses you know and which need to query the system.
  4. Verify that everything works with following command:
    snmpwalk -u snmpv3user -A snmpv3authPass -a SHA -X snmpv3encPass -x AES -l authPriv 127.0.0.1 -v3

So as you see it is quite easy, so there is now reason to use anything less secure!

If you want to delete the user, just edit the two files mentioned above. In /etc/snmp/snmpd.conf delete the line rouser snmpv3user and in /var/lib/net-snmp/snmpd.conf look for a line like this and remove it:

usmUser 1 3 0x80001f88801e15113fbf17bf5100000000 0x736e6d7076337573657200 0x736e6d7076337573657200 NULL .1.3.6.1.6.3.10.1.1.3 0xaaa3bc2aa941d932a581bbb11956699a7d807238 .1.3.6.1.6.3.10.1.2.4 0xcd6eed84f9d1aa903e92162f0e3fcb26 ""

restart the daemon with service snmpd restart and the user is gone.

7 Comments »

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  1. I take it one is required to ignore the embedded text in /var/lib/net-snmp/snmpd.conf which says (right above the usmUser line:
    STOP STOP STOP STOP etc

    **** DO NOT EDIT THIS FILE ****

    STOP STOP etc.

    Comment by John Black — May 9, 2016 #

  2. Hello,
    I could use the above example to do a snmpwalk against local system,
    what changes or addition should i make to run against a remote host
    I am getting the following message when running snmpwalk against remote host:
    No log handling enabled – turning on stderr logging
    snmpwalk: Timeout

    Comment by jenson — September 22, 2016 #

  3. Centos/RHEL has a firewall active by default, you need to let udp 161 traffic in

    Comment by robert — September 26, 2016 #

  4. halu…

    nice post you have here.

    could you share also how to allowing only snmp v3 for all request? it would be great.

    thanks.

    Comment by adhitya christiawan nurprasetyo — January 10, 2017 #

  5. If you don’t add an SNMPv1/2 community in the config, SNMPv1/2 will be disabled.

    Comment by robert — January 11, 2017 #

  6. Thank you! I have been on the fence on having a blog, but your site and the reason why you made this blog is making me think it may be worth it. I could provide linux howtos and tutorials to enhance my own memory and knowledge by teaching things to others like you. 🙂

    Comment by Kyle Hamilton — April 19, 2017 #

  7. Hi
    please could you help me with configuration snmpv3 on CentOS7-Linux
    i can not do snmpwalk v3 ,,may i get the standard format commands
    thank you for your time

    halaaedi

    Comment by halaaedi — July 12, 2017 #

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