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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.
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