← Back
Editing: README.krb5
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.
Save File
Cancel