{"id":3859,"date":"2019-09-08T10:40:34","date_gmt":"2019-09-08T15:40:34","guid":{"rendered":"https:\/\/dev.iachieved.it\/iachievedit\/?p=3859"},"modified":"2019-09-08T10:40:34","modified_gmt":"2019-09-08T15:40:34","slug":"openwrt-snmp-interface-descriptions","status":"publish","type":"post","link":"https:\/\/dev.iachieved.it\/iachievedit\/openwrt-snmp-interface-descriptions\/","title":{"rendered":"OpenWrt SNMP Interface Descriptions"},"content":{"rendered":"

If you’re familiar with configuring network gear, you know that a very<\/i> useful best practice is providing “plain English” descriptions of your device’s ports. For example, on my Cisco SF500-48MP switch<\/a> port 24 is the “uplink port” to the gateway router. I make this clear in the port’s description:<\/p>\n

[code lang=text]
\nsw01#show interfaces description fa1\/2\/24
\nPort Description
\n——- ———–
\nfa1\/2\/24 Uplink to Internet Gateway
\n[\/code]<\/p>\n

By doing so, the ifAlias<\/code> OID for this interface is set:<\/p>\n

[code lang=text]
\nsnmpget -c public -v2c sw01.iachieved.it IF-MIB::ifAlias.24
\nIF-MIB::ifAlias.24 = STRING: Uplink to Internet Gateway
\n[\/code]<\/p>\n

What is particularly nice about this is that a network monitoring tool such as Observium<\/a> will display the ifAlias<\/code> string as a part of the description of the port. Like I said, this becomes very<\/i> useful, particularly when trying to track down where ports lead to.<\/p>\n

\"\"<\/a><\/p>\n

In the previous post<\/a> we installed SNMP on an OpenWrt router and surfaced it in Observium. By default the snmpd<\/a> package doesn’t present any information for ifAlias<\/code>, but we can fix that with snmpset<\/code><\/a>.<\/p>\n

Permitting snmpset Access<\/h2>\n

snmpset<\/code> will make use of the SNMP private<\/code> community on our OpenWrt (note: if you were working in a production environment you might consider using SNMP v3 with authentication or at the very least changing your community strings). By default the OpenWrt SNMP configuration only permits use of the private<\/code> community from localhost<\/code> (i.e., the router itself). We’ll change that to permit access from our private subnet:<\/p>\n

Find this section in your \/etc\/config\/snmpd<\/code> file<\/p>\n

[code lang=text]
\nconfig com2sec private
\n option secname rw
\n option source localhost
\n option community private
\n[\/code]<\/p>\n

and change the option source<\/code> like this:<\/p>\n

[code lang=text]
\n option source 192.168.77.0\/24
\n[\/code]<\/p>\n

Obviously you’ll use the appropriate subnet in your configuration.<\/p>\n

Restart snmpd<\/code> on the router with \/etc\/init.d\/snmpd restart<\/code>.<\/p>\n

Updating ifAlias<\/h2>\n

To update the appropriate ifAlias<\/code> entries we need to see the ifDescr<\/code> list. This can be obtained by walking ifDescr<\/code> with snmpwalk<\/code>:<\/p>\n

[code lang=text]
\nsnmpwalk -c public -v2c gw.gw01.chcgil01.iachieved.it ifDescr
\nIF-MIB::ifDescr.1 = STRING: lo
\nIF-MIB::ifDescr.2 = STRING: eth1
\nIF-MIB::ifDescr.3 = STRING: eth0
\nIF-MIB::ifDescr.5 = STRING: wlan0
\nIF-MIB::ifDescr.6 = STRING: wlan1
\nIF-MIB::ifDescr.7 = STRING: br-lan
\nIF-MIB::ifDescr.8 = STRING: eth0.1
\nIF-MIB::ifDescr.9 = STRING: eth1.2
\nIF-MIB::ifDescr.10 = STRING: eth0.100
\nIF-MIB::ifDescr.11 = STRING: eth1.3
\nIF-MIB::ifDescr.12 = STRING: eth1.4
\n[\/code]<\/p>\n

In our Chicago router example let’s label the three interfaces that are OSPF links to other routers:<\/p>\n