One of the best way to add SNMP support to an application is the use of AgentX protocol: an application can act as a subagent and handle requests from a master agent using AgentX protocol. The most widespread implementation for such a task is the one from Net-SNMP.

TL;DR: An application asynchronously leveraging Net-SNMP AgentX implementation has many opportunities to get stuck when the master agent becomes temporarily unavailable. A mostly efficient workaround is to dedicate a master agent for AgentX and delegate MIB handling to a subagent.

Pinging the master agent§

Net-SNMP uses an event-based model. It also provides some synchronous functions but they are built on top of the asynchronous ones. It is shipped with its own event loop. Therefore, Net-SNMP AgentX protocol implementation seems a fine choice to add SNMP support to an event-based program, for example in Keepalived, a VRRP daemon and a monitoring daemon for LVS clusters.

Unfortunately, an AgentX subagent is expected to check, at a regular interval, if the master agent is alive . This is handled automatically with Net-SNMP implementation as long as you call run_alarms() function which will call agentx_check_session():

 * check a session validity for connectivity to the master agent.  If
 * not functioning, close and start attempts to reopen the session 
agentx_check_session(unsigned int clientreg, void *clientarg)
    /* [...] */
    DEBUGMSGTL(("agentx/subagent", "checking status of session %p\n", ss));

    if (!agentx_send_ping(ss)) {
                 "AgentX master agent failed to respond to ping.  Attempting to re-register.\n");
         * master agent disappeared?  Try and re-register.
         * close first, just to be sure .
        agentx_close_session(ss, AGENTX_CLOSE_TIMEOUT);
        /* [...] */
        if (main_session != NULL) {
            /* [...] */
            main_session = NULL;
            agentx_reopen_session(0, NULL);
        else {
            main_session = NULL;
    } else {
        DEBUGMSGTL(("agentx/subagent", "session %p responded to ping\n",

This piece of code will end up calling (directly or indirectly) the following functions1 which are using agentx_synch_response():

  • agentx_send_ping()
  • agentx_open_session()
  • agentx_reopen_session()
  • agentx_close_session()
  • agentx_register()
  • agentx_unregister()
  • agentx_register_index()
  • agentx_unregister_index()
  • agentx_add_agentcaps()
  • agentx_remove_agentcaps()

agentx_synch_response() will synchronously wait for an answer from the master agent. Your event-based program will just sit here doing nothing while this happens. For example, keepalived may be unable to send VRRP probes resulting in some serious havoc in your cluster.

Functions prefixed by agentx_ are not part of Net-SNMP API and could therefore be rewritten to do their work asynchronously. Unfortunately, talk is cheap. Moreover, there exists other deep paths that will block a subagent. For example, the low-level connection to the master agent (using TCP or a Unix socket, but not with UDP) will use a blocking connect() call. It seems quite difficult to fix this.

Here are some (partial) workarounds:

  • Call agentx_check_session() less often. The default interval is 15 seconds. Disabling it completely or using a very high value is not advised because this function is the only way to reestablish a connection to the master agent when it is, for example, restarted. Here is how to change the delay to 120 seconds:

                       NETSNMP_DS_AGENT_AGENTX_PING_INTERVAL, 120);
  • Don’t call agentx_check_session() if we know the master agent is alive. For example, if it just sent us a new request, no need to ping it. I have written a patch implementing such a workaround. The ABI is left untouched, therefore, you only need to recompile Net-SNMP libraries.

  • Use a lower timeout and no retry. By default, the timeout for synchronous requests is 1 second and a failed request will be retried 5 times. Because several synchronous functions are used in a row, you may be trapped for as long as 30 seconds with those settings. Here is how2 to disable the retry mechanism and to set the timeout to a very low value:

    static int
    snmp_setup_session_cb(int majorID, int minorID,
                  void *serverarg, void *clientarg)
        netsnmp_session *sess = serverarg;
        sess->timeout = ONE_SEC / 3;
        sess->retries = 0;
        return 0;
    void some_init_function()
        /* [...] */
           snmp_setup_session_cb, NULL);
        /* [...] */

Unresponsive master agent§

Why is the master agent unresponsive? snmpd is also an event-based program and, trust me, there are a lot of places where it will become unresponsive. Here are two examples:

  • When using the pass or the pass_persist directive, Net-SNMP will block until the delegated command outputs a complete line.

    /* var_extensible_pass() in agent/mibgroup/ucd-snmp/pass.c */
     * valid call.  Exec and get output 
    if ((fd = get_exec_output(passthru)) != -1) {
        file = fdopen(fd, "r");
        if (fgets(buf, sizeof(buf), file) == NULL) {
            /* [...] */
        /* [...] */
        /* [...] */
  • DISMAN-PING-MIB implementation will just hang while pinging the remote host. If the host is down and you have requested 5 probes with a timeout of 1 second, snmpd will be unresponsive for 5 seconds. A similar problem exists with DISMAN-TRACEROUTE-MIB.

Minimal master agent§

More and more code is running in the main agent. You cannot expect it to not block. But we could add a minimal master agent to the mix: it will handle only a few MIB modules by itself while most MIB modules will be delagated to a complete agent running as a subagent. Your own subagent will connect to the minimal master agent.

Schema showing the use of a minimal master agent

It is quite easy to turn snmpd into a subagent: just use -X flag. We should also disable unwanted MIB modules for each agent with -I flag. For the minimal master agent, we only enable some modules. All other modules3 will be served by snmpd acting as a subagent.

$ MODS="snmp_mib,sysORTable,usmConf,usmStats,usmUser,vacm_conf,vacm_context,vacm_vars"
$ snmpd -Lsd -Lf /dev/null -u snmp -g snmp \
>   -C -c /etc/snmp/snmpd.master.conf \
>   -p /var/run/ \
>   -I $MODS
$ snmpd -Lsd -Lf /dev/null -u snmp -g snmp \
>   -p /var/run/ -X \
>   -I -$MODS

Access control configuration should be placed in /etc/snmp/snmpd.master.conf, as well as master agentx directive. Other directives (like pass, pass_persist, load, sysname, …) in /etc/snmp/snmpd.conf.

The minimal master agent is very unlikely to block and while the ground cause is still here, it is now very much mitigated. You may extend this solution by splicing snmpd into several subagents: one handling safe modules, one handling pass and pass_persist stuff and a last one for unsafe modules (like DISMAN-PING-MIB).

  1. agentx_reopen_session() will call agentx_close_session() and agentx_open_session(). agentx_register() and the following functions will be called by those two functions. 

  2. NETSNMP_DS_AGENT_AGENTX_TIMEOUT and NETSNMP_DS_AGENT_AGENTX_RETRIES variables are only used by the master agent, not by a subagent. 

  3. Use snmpd -Dmib_init -H to get the list of modules. You may want to add smux module to the master agent.