Extending mini-snmpd

mini-snmpd does not pass SNMP requests through to other processes, and it does not load plugins. To extend it with your own MIB variables you have to build them in, as described below.

Obtain a vendor OID

The default vendor OID is only a placeholder; insert the PEN (private enterprise number) of your organization. If you do not have one, IANA assigns them free of charge, see the PEN application page at https://pen.iana.org/.

Set the vendor OID at build time with configure --with-vendor=OID, or at run time with the -V option.

Write the MIB file

Not needed by mini-snmpd itself, but you should be prepared to hand your users the MIB description in text form. The net-snmp package has plenty of examples to start from.

Add a define and the code for your extension

Make the extension a compile-time option and keep its code in #ifdef/#endif blocks, so built-in and added code stay easy to tell apart.

The demo extension, two random integers, shows how (CONFIG_ENABLE_DEMO in the sources). It uses PEN 99999, which IANA has not assigned — and one day might — so never enable the demo in a release build.

Things to consider

• The MIB table has a fixed maximum length, MAX_NR_VALUES. A runtime table-overflow error when creating an entry means you need to raise it.
• mib_update() runs on every request, so a handler can decide whether to refresh its variables every time (e.g. system uptime) or only on the interval set with -t (e.g. disk info). Make slow or expensive updates depend on the timeout counter to save CPU.
• For an octet string, call mib_build_entry() with a string of the maximum length the variable can reach at run time; that is how it sizes the data buffer for the entry.
• If the value you read is OS dependent, put the code in linux.c and/or freebsd.c, not utils.c, which is for OS-independent helpers.
• For debug output use the logit() macro, not a raw printf() or syslog().

Robert Ernst robert.ernst@aon.at