When trying to set up Dell OpenManage on some servers, I found that SMUX was not working. SMUX is a protocol that allows agents to connect to a SNMP daemon and provide answers to SNMP queries in a portion of the tree.
When working, SNMP should generate this log message in /var/log/daemon.log:
May 11 17:14:18 serverx snmpd[29678]: accepted smux peer: oid SNMPv2-SMI::enterprises.674.10892.1, descr Systems Management SNMP MIB Plug-in Manager
In my case, I saw this instead:
May 11 17:06:59 serverx snmpd[29471]: /etc/snmp/snmpd.conf: line 370: Warning: Unknown token: smuxpeer.
After a long time fussing with SNMP and Dell OpenManage, it turned out that the problem was that the SMUX subsystem was being disabled at daemon startup by an option set in /etc/default/snmpd. Using the -I option will turn on (or off) a particular module used by snmpd. In this case, the line looked like this:
SNMPDOPTS='-Lsd -Lf /dev/null -u snmp -g snmp -I -smux -p /var/run/snmpd.pid'
With this configuration, the SMUX module is disabled. For snmpd to support SMUX, the line should look like this instead (removing the -I option and its argument):
SNMPDOPTS='-Lsd -Lf /dev/null -u snmp -g snmp -p /var/run/snmpd.pid'
After making the change, restart the daemon:
service snmpd restart
This should then fix problems in using Dell OpenManage (or other SMUX agents). You don’t have to restart Dell OpenManage to make this work, but it should have SNMP enabled (which provides the smuxpeer line in snmpd.conf).
To enable SNMP for Dell OpenManage, use this:
service dataeng enablesnmp
To restart the Dell OpenManage services, don’t use the usual services; use this command instead (which takes care of all Dell OpenManage services):
srvadmin-services.sh restart
Why SMUX should be disabled I couldn’t say. The system is running Ubuntu 10.04.2 LTS with snmpd 5.4.2.1-dfsg0ubuntu1-0ubuntu2.1.
UNIX Administratosphere 
Thanks for sharing. This is not a problem on 12.04 though 🙂