This version of net-snmp supports an experimental SNMPv3 security model

using Kerberos 5 for authentication.  The protocol is described in an

up-and-coming IETF Internet-Draft.

This document describes a brief overview of the Kerberos Security Model

and how to use it.

DESCRIPTION:

The Kerberos Security Model does not use USM; it is completely seperate

and is not tied to USM in any way.  It works by placing the following

ASN.1 sequence inside of the SNMPv3 msgSecurityParameters:

ksmSecurityParameters ::= SEQUENCE {

-- The Kerberos 5 checksum type used to checksum this message

    ksmChecksumType		INTEGER(0..2147483647),

-- The actual keyed checksum data returned by Kerberos

    ksmChecksum			OCTET STRING,

-- The Kerberos 5 message (either an AP_REQ or AP_REP)

    ksmKerberosMsg		OCTET STRING,

-- The cached ticket identifier

    ksmCachedTicket		INTEGER(0..2147483647)

}

Note that the whole SEQUENCE is BER encoded as an OCTET STRING.

ksmChecksumType is an integer which corresponded to the checksum algorithm

used to secure this message as defined by Kerberos (see section 8.3 of

RFC1510).

ksmChecksum is the output of the checksum algoritm defined by ksmChecksumtype

(with all NULs in the space for the checksum).

ksmKerberosMsg is a Kerberos 5 AP_REQ or AP_REP message, depending on

whether or not it is a request or a response (AP_REQ for requests, AP_REP

for responses).

ksmCachedTicket is a integer which uniquely identifies a ticked already

cached on the agent to save the overhead of transferring a whole AP_REQ/AP_REP.

If there is no such cached ticket, it is left at zero.

An agent, upon receiving a message using the KSM, will decode the AP_REQ

contained within the security parameters and thus validate the client's

identity.  Using the subkey contained within the AP_REQ, the agent will

validate the checksum (after first clearing the checksum bytes to zero),

and issue a response, encoding the appropriate AP_REP message in the

ksmSecurityParameters.

If the securityLevel of the message is set to AuthPriv, the scopedPdu

payload will be encrypted using the encryption key and algorithm of the

AP_REQ subkey.  Note that in this case, the msgData will be a BER-encoded

OCTET STRING corresponding to the "cipher" element of the EncryptedData

sequence defined in RFC 1510, section 6.1.

Since this security model is experimental, the number assigned to this

security model is taken from the recommendations of RFC 2271, section 5,

which specify enterprise-specific Security Models of the form:

	SnmpSecurityModel = enterpriseID * 256 + security model number

						 in that enterprise ID;

In the case of KSM this gives us:

	SnmpSecurityModel = 8072 * 256 + 0 = 2066432

USAGE:

To actually USE the Kerberos Security Model, do the following:

0) Install Kerberos

   Let it be stated up front - Installing Kerberos completely "cold", without

   any Kerberos experience at all, can be daunting (to say the least).  If you

   already have a Kerberos infrastructure at your site, then all of the hard

   work has been done.  If you do NOT, but you still want to tackle it,

   you might be interested in the Kerberos FAQ, which can be found at:

   http://www.nrl.navy.mil/CCS/people/kenh/kerberos-faq.html

   Currently the code in net-snmp only supports using MIT Kerberos

   libraries to link against (you should be able to use any kind of Kerberos

   server, however).

1) Compile net-snmp with Kerberos.

   This assumes that you already have Kerberos libraries in place.

   Configure net-snmp to include the Kerberos Security Model (ksm) and

   use --with-cflags and --with-ldflags to specify the location and names

   of Kerberos header files and libraries.  For example, on my system I

   run:

   ./configure --with-cflags='-I/usr/krb5/include' \

      --with-ldflags='-L/usr/krb5/lib -lkrb5 -lcrypto -lcom_err -R/usr/krb5/lib'

   Note that this is on Solaris, and that -R is required to set the correct

   shared library path.  If you have a newer version of Kerberos, you might

   instead have to use:

   -lkrb5 -lk5crypto -lcom_err

   as the libraries to link against.  If you get errors (for example, you

   get a message that says the compiler isn't working) you can check

   config.log for the output of the compiler.

2) Configure Kerberos and SNMP

   Currently, net-snmp uses the "host" principal assigned to a host.  This

   may change in the future.  You will want to create host principals of

   the form:

   host/f.q.d.n@YOUR.REALM

   For example:

   host/mydesktop.example.org@EXAMPLE.ORG

   and place the encryption keys for these principals on every machine you

   wish to run a SNMP agent (you place each key on it's corresponding machine).

   Your Kerberos documentation should explain how to do this (in the case

   of MIT Kerberos, you want to look at the "ktadd" command inside of

   kadmin).

   If you have a Kerberos infrastructure, you likely already have these

   principals in place on your systems.

   If you're installing Kerberos for the first time as well, you also

   need to create client principals corresponding to your userid.  See

   your Kerberos documentation.

   On the SNMP _agent_ side, you'll want to place in your snmpd.conf file

   (the one that lives in /usr/local/share/snmp/snmpd.conf, or whereever

   you have configured on your system):

   rwuser -s ksm userid@YOUR.REALM

   to allow the Kerberos principal 'userid@YOUR.REALM' read/write access to

   the MIB tree.

3) Run the agent and client applications

   Note that before you do any of this, you will have to have valid Kerberos

   credentials (generally acquired with the "kinit" program).

   The agent should run without any additional flags.

   You should run the client apps with the following flags:

   -Y defSecurityModel=ksm

   -v 3

   -u username

   -l authNoPriv

   for example:

   snmpget -v 3 -Y defSecurityModel=ksm -u myname -l authNoPriv testhost \

						system.sysDescr.0

   If you wish to encrypt the payload, change the -l argument to "authPriv".

   If you run into problems, you can add the -Dksm flag to both the manager

   applications and the agent to get more detailed Kerberos error messages.

   Note that this setup assumes a working Kerberos infrastructure; if you

   run into problems, check to make sure Kerberos is working for you.