SNMP Environmental Monitoring using ESP8266-based Sensors

(click here to see index of all ESP8266 posts)

This is a somewhat large project, combining two ‘experiments’ I wanted to try: providing environmental monitoring with ESP8266-based sensors and using NET-SNMP’s extend facility to interface external data to SNMP.

Long ago, I managed a large international network with hundreds of routers. SNMP was used heavily to monitor many aspects of the network. Then I ended up managing a data center. The building monitoring systems were a hodge-podge so I figured out how to convert each system to an SNMP-based monitoring system. That allowed all building systems to be monitored from the same SNMP console that I knew exhaustively.

I would have liked to have placed temperature sensors in every rack in the data center, but it was cost prohibitive. The devices we were using at the time were hundreds of dollars each so there weren’t many of them (a quick check shows the cheapest current models to cost $200). So I have long wanted to come up with a system where I could place several DS18B20 temp sensors in each rack and tie perhaps a few racks together with an MCU and a network connections.

That desire is the basis for this project. Here is a diagram of the concept I’m implementing in this post.


While the broad idea would be to support multiple ESP8266’s with multiple temperature sensors on each ESP8266, for this experiment, I will implement one ESP8266 with one temperature sensor.

The flow of data is as follows:

  • ESP8266 reads the DS18B20 temperature sensor every 10 seconds
  • That temperature is transmitted, along with the ESP8266’s MAC address to the Raspberry Pi
  • The Raspberry Pi receives the temperature update and writes it to a file. The file is named after the MAC address (allowing for multiple ESP8266’s).
  • The SNMP server will then use the contents of that file if an SNMP request is made for the temperature.


Here are some of the resources I used when creating this project.

The ESP8266 code is based on the accumulation of projects I’ve done so far

Extending SNMP is described here

Install SNMP on the Raspberry Pi

For the Raspberry Pi to act as a SNMP server between the ESP8266 and the SNMP console, the SNMP service must be installed on the Raspberry Pi. I covered this some time ago here:

Installing SNMP onto a Raspberry Pi

After snmpd is installed and running, snmpwalk should work much like this:

rpi/snmp:snmpwalk -v 1 -c public localhost system
SNMPv2-MIB::sysDescr.0 = STRING: Linux rpi 3.18.7+ #755 PREEMPT Thu Feb 12 17:14:31 GMT 2015 armv6l
SNMPv2-MIB::sysObjectID.0 = OID: NET-SNMP-MIB::netSnmpAgentOIDs.10
DISMAN-EVENT-MIB::sysUpTimeInstance = Timeticks: (852799) 2:22:07.99
SNMPv2-MIB::sysContact.0 = STRING: Me <>
SNMPv2-MIB::sysName.0 = STRING: rpi
SNMPv2-MIB::sysLocation.0 = STRING: Sitting on the Dock of the Bay
SNMPv2-MIB::sysServices.0 = INTEGER: 72

Testing Extended SNMP

The next step is to make sure Extended SNMP is working. The snmpd.conf file that was installed onto my RPI already has some test Extended SNMP calls:

snmpwalk -v 1 -c public localhost NET-SNMP-EXTEND-MIB::nsExtendOutput1Line
NET-SNMP-EXTEND-MIB::nsExtendOutput1Line."test1" = STRING: Hello, world!
NET-SNMP-EXTEND-MIB::nsExtendOutput1Line."test2" = STRING: Hello, world!

snmpwalk should give you 2 test OID (test1 and test2) and both will have the value ‘Hello World’.

If not, edit your snmpd.conf file and add these two lines:

extend    test1   /bin/echo  Hello, world!
extend-sh test2   echo Hello, world! ; echo Hi there ; exit 35

Restart the snmpd service:

service snmpd restart

and the snmpwalk above should properly return the ‘hello world’ lines. If not, you need to troubleshoot until you resolve the problem, as the succeeding steps require extended SNMP.

Write ESP8266/nodeMCU Lua Code to Transmit the Temperature

This part of the project is based fairly closely on my prior blog:

ESP8266 and DS18B20: Transmitting Temperature Data

Every 10 seconds, I will read the temperature from the DS18B20, then transmit that and the ESP8266’s MAC address via UDP (port 9999) to the RPI.

Note the temperature transmitted is in Celsius * 10000 to get rid of the decimal point.  The Raspberry Pi can handle floating point and will convert it  to floating point Fahrenheit.

The ESP8266 program consists of the file getTemp.lua and init.lua.


function getTemp()

    local addr      = nil
    local count     = 0
    local data      = nil
    local pin       = 4             -- pin connected to DS18B20
    local s         = ''

    -- setup gpio pin for oneWire access

    -- do search until addr is returned
        count   = count + 1
        addr    = ow.reset_search(pin)
        addr    =
        until((addr ~= nil) or (count > 100))

    -- if addr was never returned, abort
    if (addr == nil) then
        print('DS18B20 not found')
        return -999999
    -- validate addr checksum
    crc = ow.crc8(string.sub(addr,1,7))
    if (crc ~= addr:byte(8)) then
        print('DS18B20 Addr CRC failed');
        return -999999

    if not((addr:byte(1) == 0x10) or (addr:byte(1) == 0x28)) then
        print('DS18B20 not found')
        return -999999
    ow.reset(pin)               -- reset onewire interface, addr)        -- select DS18B20
    ow.write(pin, 0x44, 1)      -- store temp in scratchpad
    tmr.delay(1000000)          -- wait 1 sec
    present = ow.reset(pin)     -- returns 1 if dev present
    if present ~= 1 then
        print('DS18B20 not present')
        return -999999
        end, addr)        -- select DS18B20 again
    ow.write(pin,0xBE,1)        -- read scratchpad

    -- rx data from DS18B20
    data = nil
    data = string.char(
    for i = 1, 8 do
        data = data .. string.char(
    -- validate data checksum
    crc = ow.crc8(string.sub(data,1,8))
    if (crc ~= data:byte(9)) then
        print('DS18B20 data CRC failed')
        return -9999

    -- compute and return temp as 99V9999 (V is implied decimal-a little COBOL there)
    return (data:byte(1) + data:byte(2) * 256) * 625
    end -- getTemp

function xmitTemp()
    local temp = 0

    temp = getTemp()
    if temp == -999999 then

    cu:send(wifi.sta.getmac() .. ':' .. tostring(temp))

    end -- xmitTemp

function initUDP()

    -- setup UDP port
    end -- initUDP
function initWIFI()
    print("Setting up WIFI...")


    tmr.alarm(1, 1000, 1,   
            if wifi.sta.getip()== nil then 
                print("IP unavailable, Waiting...") 
                print("Config done, IP is "..wifi.sta.getip())
            end -- function
    end -- initWIFI

tmr.alarm(0, 5000, 1, xmitTemp)


function startup()
    if abort == true then
        print('startup aborted')
    print('Starting xmitTemp')

abort = false
print('Startup in 5 seconds')

Once the code is installed onto the ESP8266, I run wireshark on the Raspberry Pi to verify I’m getting UDP packets to port 9999 AND that the data within the packet contains the MAC address and a reasonable temperature in Celsius:


Receiving the Data on the Raspberry Pi

If you don’t already have Lua setup on your Raspberry Pi, here are instructions:

Installing LUA on Raspberry Pi and Getting it Running

Again, I’m going to use another post as the basis for this code:

ESP8266 UDP to/from Raspberry Pi running LUA

I am going to modify that program slightly to receive the data packet from the ESP8266, split the MAC address from the temperature, convert the temperature to fahrenheit, and finally write the temperature to a file named after the MAC address.

Here is the code:


-- Setup UDP socket. Bind to localhost, port 9999.
local socket = require "socket"
local udp = socket.udp()
udp:settimeout(0) -- indicates not to wait. If no data, return immediately
udp:setsockname('*', 9999)

local data              = ''
local mac               = ''
local msg_or_ip
local port_or_nil
local tempC             = ''
local tempF             = 0.0

print 'Beginning server loop'

while true do
    data, msg_or_ip, port_or_nil = udp:receivefrom() -- receive UDP packet
    if data then
        mac, tempC = string.match(data, '(.+):(.+)')
        tempF = (tonumber(tempC)/10000)*2 + 30
        print('temp: ' .. tempF .. 'F')
        os.execute('echo '..tempF..'>/tmp/'..mac)
    elseif msg_or_ip ~= 'timeout' then
        error("Unknown network error: "..tostring(msg))
    socket.sleep(0.01) -- sleep .01 secs

When you run this program on the RPI, it should see the data from the ESP8266 and display the current temp:


and if you look in the /tmp dir, you should see a file being updated:

rpi/tmp:cd /tmp
total 376K
-rw-r--r-- 1 danh danh    7 Apr 28 17:21 18-FE-34-A0-52-62
-rw------- 1 danh danh    0 Apr 28 13:39 hist8497
-rw------- 1 danh danh    0 Apr 28 13:41 hist8523
-rw------- 1 root root    0 Apr 28 14:21 hist8591
drwx------ 2 danh danh 4.0K Apr 27 14:42 ssh-JFZoo8p8BQDL/
-rw------- 1 danh danh 263K Apr 28 17:21 wireshark_eth0_20150428133554_hwJwTi
-rw-r--r-- 1 danh danh  97K Apr 27 17:12 xx.txt
rpi/tmp:cat 18-FE-34-A0-52-62

Modifying SNMP to Read the File Data

Now that the temperature is being recorded properly into a file, we merely need to get SNMP to recognize this data. This is really quite easy to do.

Edit the /etc/snmp/snmpd.conf file and add the following line (I am using the file name based on my ESP8266’s MAC address. You will need to change that to your own MAC address):

extend-sh tempSensor01  cat /tmp/18-FE-34-A0-52-62; exit 35

The name of this OID will be ‘tempSensor01’. When that OID is retrieved, it will execute the shell command ‘cat /tmp/18-FE-34-A0-52-62; exit 35‘. The output of that command is sent back to the SNMP console.

Once you are done editing snmpd.conf, save it and restart snmpd:

service snmpd restart

Now do this snmpwalk command:

snmpwalk -v 1 -c public localhost NET-SNMP-EXTEND-MIB::nsExtendOutput1Line
NET-SNMP-EXTEND-MIB::nsExtendOutput1Line."test1" = STRING: Hello, world!
NET-SNMP-EXTEND-MIB::nsExtendOutput1Line."test2" = STRING: Hello, world!
NET-SNMP-EXTEND-MIB::nsExtendOutput1Line."tempSensor01" = STRING: 85.125

or to get just the tempSensor01 OID:

snmpget -v 1 -c public localhost 'NET-SNMP-EXTEND-MIB::nsExtendOutput1Line."tempSensor01"'
NET-SNMP-EXTEND-MIB::nsExtendOutput1Line."tempSensor01" = STRING: 84.75


Putting together these various tools works quite well and the goal to allow an SNMP console the ability to read ESP8266-based is (very) roughly achieved. This is a long ways from a usable project, but I at least proved the concept to myself.

The one glaring issue that must be addressed to make this usable is the fact that the temperature  is being returned as a string and not an integer. That makes it hard to apply tests and set alarms (for example, if the temp were > 100 I might want to set an alarm on the management console).

From what I’ve seen of the Net-SNMP EXTEND facility so far, it appears this is doable, but would require writing an actual MIB. Not hard, but beyond the scope of this little ‘experiment’.



This entry was posted in c-esp8266, c-rpi. Bookmark the permalink.

8 Responses to SNMP Environmental Monitoring using ESP8266-based Sensors

  1. Ambro says:

    Very good site.
    I would like to read more than 5 sensors DS18b20 and send data to my mobile phone.
    How could I modify Your program ?
    Any help please ?

    • Dan TheMan says:

      It should be pretty easy to find sites discussing using multiple DS18B20s such as

      As far as being able to see it from a mobile phone, that is a major divergence from what I did here. If I were trying to do the same thing, I would either have the ESP8266 serve up a web page, or if it doesn’t have enough resources, have the Raspberry Pi do so. Then you just go to that device’s website using your phone. SNMP is great for data centers. Not so useful for individuals 🙂

  2. Javadgo says:

    Nice job. I am doing the same project as yours.
    The problem is my linux distortion is Openwrt for some reasons! And it don’t have net-snmp module in standard repository (instead it have snmpd)
    the question here is
    1. Is snmpd the same as net-snmp?
    2. I did not get how you find OID. Can you describe more please
    3. I heard we should make a MIB file for our device(sensor for example) in SNMP communication, but you did not. Is it correct?!

    Thanks you.

  3. Dan TheMan says:

    1. The daemon, snmpd, is just part of the entire snmp package. Just looking around briefly it looks like it may be called snmp-static for openwrt. See

    2. If you want the OID for a specific datum, the easiest way to find it is to walk the entire mib table looking for what you want. google ‘walking mib’. Here is one example

    3. I did not create my own MIB because I used the ‘extend’ option of net-snmp (look at I’ve written a couple of MIBs, and while not overly difficult, it isn’t simple either – especially the first time. The extended MIB lets you get to device data without having to write your own MIB. For quick and dirty it’s a great option. For production work, or stuff you want to share with others (or if an instructor requires it), then write your own MIB.

    • Javadgo says:

      Thanks Dan, it really works for me.
      But as I see, you return temp value via snmp and the data type is string (look at this in your result ==> “tempSensor01” = STRING: 85.125)
      the question is do all network management station (NMS) applications like cacti or nagios can understand numbers in string. I want to draw some chart in time, by sensor data in these application.
      I tried coding in shell and c and … all of them return values in “string” or by some trick in “int” data type.
      do you think we can some how return value in float data type?

      Thanks again

  4. Javadgo says:

    Hi Dan
    Long time no see! Almost a year since the last time lol
    Happy New Years to you.

    I have one more question about SNMP.
    Does the solution you have described (I mean extend snmp) work for snmp trap too?
    or in other words, can we get trap from an executable file that is defined as extend snmp?
    (again there is no MiB)


    • Dan TheMan says:

      I don’t know how to do this. I’m fairly certain the extend extension won’t support traps.

      There is an SNMP protocol called AGENTX that might be able to, but it isn’t clear to me how.

      If you are to the level of complexity of wanting to support traps, you may be better off writing and compiling a MIB for the traps. I believe you could then use snmptraps utility to transmit the traps to the net management station.

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