Using DTLS

SNMP over DTLS over UDP is supported in Net-SNMP 5.5 and beyond. This page describes what is necessary to configure the software to use it.

Compiling Net-SNMP with DTLS Support

Just ensure you have OpenSSL installed as well as run configure with the following two options in addition to your normal options:

# ./configure --with-security-modules=tsm --with-transports=DTLSUDP

Generating X.509 Certificates

DTLS uses X.509 certificates to authenticate both the client and server sides of DTLS connections. This means that both the SNMP server and client need to have certificates generated and installed in order to make use of DTLS. The client will need to verify the servers certificate, to make sure it's talking to the server it thinks it is. The server needs to verify the clients certificate, and possibly extract user-name information from it, in order to verify the client is who they say they are and assign appropriate access control settings.

Note: At the time of this writing, the DTLS code present in Net-SNMP 5.5 only accepts SNMPv3 user-names from the CommonName component of the X.509 certificate.

You need to generate certificates using the openssl command (I know you have OpenSSL installed or else you wouldn't have been able to even compile in DTLS support). Certificates have both a private and public key halves. Guard the private key well, and give the public key out freely.

Generating a Self-signed Certificate

Generally you'll want to generate a master CA certificate that is used as a trust point for all you software. IE, you can configure snmpd to trust any certificate that has been signed by this single CA certificate. That doesn't mean they'll get access, however, because they'll still need to pass the VACM checks before they can get or send any data to the server.

Generating a Self-signed Certificate

If you don't want to generate a CA to sign everything, you can also simply generate self-signed certificates and use them instead by using the -x509 flag. The resulting server *.cert files should be all concatenated together for the clients to use as authorized servers, and the user certificates should be concatenated together for the servers to refer to (see below for configuration options).

# openssl req -newkey rsa:2048 -x509 -nodes -out myhost.example.com.cert -keyout myhost.example.com.priv

It'll ask you a bunch of questions, which you should answer. See the req(1) manual page for further reading about certificate generation.

You'll need to generate certificates for each of your servers and each of your users (for the Common Name, use the host name for servers and the user's name for users).

todo: define how to set the subjectAltName extension

Examining Certificates

If you want to examine the results after it is done, use openssl to read it back in:

# openssl -in myhost.example.com.cert -text

Configuring Servers

Configuring Certificates

The tokens for specifying which X.509 certificates are configured in the snmp.conf file. Note: NOT the snmpd.conf file. The tokens are specific to all applications and thus belong in the snmp.conf file (without the "d"). If you wish to put them in the snmpd.conf file you can do so by prefixing the lines with [snmp], as described in the snmp_config manual page.

Setting the Server's Certificate

The server needs to be configured with both it's private and public keys. To do this, use the following two snmp.conf tokens to configure the server:

 defX509ServerPub  /path/to/servercert.cert
 defX509ServerPriv /path/to/servercert.priv

Recognizing Client Certificates

Additionally, the client's incoming certificates need to be cryptographically checked and verified. To define the root CA certificates, or self-signed certificates, they all need to be put together in a single file containing a concatenated set of acceptable certificates:

 defX509ClientCerts /path/to/for-users.cert

Setting up Access Control

SNMP over DTLS is a mode of SNMPv3, so access control settings are done using the standard VACM configuration tokens. The security model used should normally be TSM (further discussed below in the example usage section). Here are some example snmpd.conf configuration settings for incoming users with a X.509 CommonName field of "Wes Hardaker":

 rwuser -s tsm "Wes Hardaker"

Opening and Listening on a Port for DTLS traffic

snmpd and snmptrapd can both be configured to accept and process connections sent over DTLS. This is done on the command line using the dtls: addressing specifier. E.G. this:

 # snmpd dtls:9161

tells snmpd to open port 9161 and listen for incoming SNMP over DTLS connections on it.

Note: at the time of this writing this is not yet a standardized port over which SNMP over DTLS should be run. There will probably be a standard port after the IETF's ISMS working group finishes with the specification and it becomes an RFC.

Configuring the Applications

Tools like snmpget, snmpwalk or anything that uses the core session structures within the main Net-Snmp library like the perl and python modules can make use of DTLS using the procedures described below.

Setting the Clients's Certificate

The client needs to be configured with both it's private and public keys. To do this, use the following two snmp.conf tokens to configure the client:

 defX509ClientPub  /path/to/user-cert.cert
 defX509ClientPriv /path/to/user-cert.priv

Recognizing Server Certificates

Additionally, the certificates for the servers that the client should connect to need to be cryptographically checked and verified. To define the server's root CA certificates, or self-signed certificates, they all need to be put together in a single file containing a concatenated set of acceptable certificates:

 defX509ServerCerts /path/to/servers.cert

Running and Testing

Start the server:

 # snmpd dtlsudp:9161

And try to get results from it:

 # ./snmpget --defSecurityModel=tsm dtlsudp:localhost:9161 sysContact.0

Debugging

For debugging in the server, run it in the foreground and turn on debugging of dtls and tsm:

# snmpd -f -Le -Dtsm,dtls dtlsudp:9161

Same for the clients:

# snmpget -Dtsm,dtls ...