This is second in series of posts on monitoring your wireless router. In this post we will setup a script to extract the NVRAM usage information and use node exporter to expose it to prometheus using text file collector. Check here for instructions on how to setup node exporter on your router.

Caution: There is very little chance that these instructions will be portable as-is to any firmware other than Asus WRT Merlin.

NVRAM (short for Non-Volatile Random Access Memory) is a memory on our router which is used to persist data across power loss and reboots. Router uses NVRAM to store all the configurations. In most cases this memory is not larger than 64KB. Insufficient free space on this device can cause instability on your router or degraded performance.

I have an Asus RT-AC68U with AsusWRT-Merlin firmware installed on it. This allows me to setup cron jobs and custom scripts on the router which is required to setup the monitoring. The instructions here should work on other Asus routers with Meriln firmware but are unlikely to work on other firmwares.

For pulling and persisting the metrics I am be using Prometheus which is an open-source monitoring solution. I have it installed on a home server which is up 24x7 and connected to the router. A static IP is configured for the server which makes things convenient but the setup will also work with a dynamic IP address assignment.


You need to be able to SSH into the router and have node_exporter installed on it. Check here for instructions on how to setup node exporter on your router. You need to enable Custom JFFS scripts from Administration > System in your WebUI.

For the sake of this post I am assuming that we have an external USB stick connected to the router which is mounted at /mnt/<disk> and we have following directory structure created:

|- bin
|- etc
  |- services
|- var
  |- log
  |- run
  |- lib
    |- node_exporter
|- home
  |- user
|- tmp

You can create the directory structure using following command. (Change <disk> to the name of your disk.)

mkdir -p /mnt/<disk>/{bin,etc/services,home/user,var/{log,run,lib/node_exporter},tmp}

Setting up the script

We will be storing the data extraction script on the external drive. Create a file /mnt/<disk>/bin/nvram-free with following content in it. Here we use nvram show command to print the nvram details and grep the used and free from the output.


# Note the start time of the script.
START="$(date +%s)"

nvram_used=`nvram show 2>&1 >/dev/null | grep -Eo '[0-9]+' | head -1`
nvram_free=`nvram show 2>&1 >/dev/null | grep -Eo '[0-9]+' | tail -1`

# Adjust as needed.

# Write out metrics to a temporary file.
END="$(date +%s)"
cat << EOF > "$TEXTFILE_COLLECTOR_DIR/nvram.prom.$$"
router_nvram_duration_seconds $(($END - $START))
router_nvram_last_run_seconds $END
router_nvram_free_bytes $nvram_free
router_nvram_used_bytes $nvram_used

# Rename the temporary file atomically.
# This avoids the node exporter seeing half a file.
mv "$TEXTFILE_COLLECTOR_DIR/nvram.prom.$$" \

The script will report following 4 metrics:

  1. router_nvram_duration_seconds: runtime of nvram_free script in seconds
  2. router_nvram_last_run_seconds: unix timestamp of last run
  3. router_nvram_free_bytes: free NVRAM in bytes
  4. router_nvram_used_bytes: used NVRAM in bytes

Mark this script as executable.

chmod 755 /mnt/<disk>/bin/nvram-free

Run the script once and check if it creates the text file with metrics in /mnt/<disk>/var/lib/node_exporter/nvram.prom. Following is a sample content from file.

router_nvram_duration_seconds 0
router_nvram_last_run_seconds 1596200962
router_nvram_free_bytes  6158
router_nvram_used_bytes 59378

Next we setup cron job to run this script every minute using the cru command.

# Usage: cru a <unique_id> '<cron_expression> <command>'
cru a nvr '* * * * * /mnt/<disk>/bin/nvram-free'

The cron job entry will be gone after reboot. To persist it we will need to add the command to user script which is called on boot. Since we are storing the nvram-free command on external disk we also need to ensure that the disk is mounted before we add the cron job. We will be using the post-mount script. Create a file /jffs/scripts/post-mount if it not already exists. Add following content to the file. The post-mount script is passed the mountpoint as the first argument.



if [[ $MOUNTPOINT == "/tmp/mnt/<disk>" ]]; then
    echo "$1 Mounted" >> "$1/var/log/syslog"
    cru a nvr '* * * * * /mnt/<disk>/bin/nvram-free'

Now we have a script which periodically checks the NVRAM usage and reports the data in prometheus format in a file. The only thing remaining is to tell our node exporter to pickup the file on every scrape. To do this change the startup command for node exporter by adding parameter. If you followed the first post to setup the node exporter following is the updated service script.



pidof node_exporter

if [[ $? -ne 0 ]] ; then
    $CMD --web.listen-address=":9101"<disk>/var/lib/node_exporter >> "$LOG" 2>&1 &
    echo "Already running"

To restart the node exporter you can use following commands.

pidof node_exporter 
# Note the PID printed here

kill -15 <pid_of_node_exporter>

Thats it! You can visit http://<your_router_ip>:9101/metrics and check if router_nvram_ metrics are being reported.