LTE modem DNS config

Product support
1

Hopefully a network genius can help answer this question. I’ve noticed some spotty cell performance (really just high latencies) and decided to do some snooping.
nmcli is showing that wwan0 is pointed to a google’s DNS’s whereas my other interfaces seem to point locally (which I assume is a Balena DNS).

Is that an issue? How is that controlled? I don’t recall ever setting that DNS to google myself.

DNS configuration:
        servers: 192.168.1.1
        interface: eth0

        servers: 8.8.8.8 8.8.4.4
        interface: wwan0

        servers: 192.168.1.1
        interface: wlan0

I appreciate the help!

3

Those are most probably set by the mobile provider through DHCP when the modem connects to the mobile network. The Google DNS servers are quite fast in general, so I doubt this is the causing the high latencies.

You may check signal strength with mmcli -m 0. What type of antenna the modem is using? If the device is encased you may check how it performs without the case.

For more detailed diagnostic you may run tcpdump in a container that runs in privileged mode and with host networking and capture traffic on top of the wwan0 interface. Save the packet capture in a file and then examine it with Wireshark offline from your computer.

I do that with the following easy steps from a dashboard host OS terminal:

# Run an Alpine container:
source /etc/os-release
balena run --rm -ti -v /mnt/data:/data --privileged --network host balenalib/${SLUG}-alpine /bin/bash

# From the Alpine container install tcpdump:
cd /data
apk add --no-cache tcpdump

# Capture all traffic going through the wwan0 interface
tcpdump -w container.pcap -i wwan0
exit

# Upload the captured file to file.io from the host OS:
cd /mnt/data/
curl -F "file=@container.pcap" https://file.io

# Download the file locally from the upload link ^^

Please let me know if you have any questions.

1 Like
6

I think you’re right. Turns out the latency issue was temporary (hopefully not to return). I was experimenting with some SIMs from 1nce and hadn’t configured anything out of the box.

Thanks for your help! Also, thanks for the tcpdump tip, very useful.