Zerotier/Tailscale Combo on Odroid M1 causes excessive data usage

I have two SBCs in a remote location to run IP cameras for wildlife research, a RPi 400 and an Odroid M1. I’m still evaluating Tailscale and Zerotier and had them both running on each SBC. The SBCs are on physically separate networks and separate 4G mobile plans. Both SBCs run Tailscale Linux 1.26.1. The RPi has 32 bit Raspian Buster and the Odroid has Hardkernel’s version of Ubuntu 20.04 LTS and Kernel 4.19.219. Soon after turning both machines on, but more or less idle, I observed 3-4 GB/hr of data traffic on both 4G networks, which is a short path to bankruptcy here in New Zealand. The upload data on the Odroid’s network was about the same as the download data on the RPi’s network, and vice-versa. I deactivated Zerotier on the Odroid only and data use immediately dropped back to low MB/hr on both 4G Networks. I read an older post in this forum entitled Excessive Data Usage and tagged iOS that Zerotier and Tailscale did not play well when installed together. It looked like a fix had been done, but it does not appear to extend to the Odroid M1. Anyone having a similar issue? Thank you.

In order to be precise about the problem we’d need to look at some packet captures. That said, a common problem for this kind of configuration (putting an L2 tunnel into another tunnel like Tailscale) is that broadcast packets are being forwarded, and further amplified in size by being double packed. If you filter broadcast packets in your ZeroTier configuration this may address the bulk of the bandwidth issue.

Thank you Raggi. I’m not savvy enough to fully understand what you are saying and I’m sure I would not be able to get the mentioned diagnostic data, but I thought it was highly interesting that the excess data use was confined to the Odroid SBC and was propagated to the network which contained the RPi, installed the same way but without creating its own data storm. I guess this could be one for the Tailscale folks to pick up on as they work through all issues with various types of hardware to ensure that “it just works”. My workaround will be to make a final decision re Tailscale or Zerotier, and delete the one that loses out.