Is there a way to limit the transfer amount?
Is there a way to limit the total monthly transfer amount to the monthly quota/pool?
I'm using my node as a private VPN-server so it doesn't matter if it "goes down" in the middle of the month. I'd like to prevent overages.
This is a great question and I've passed this along as a feature request to see if this is something our Dev team can possibly implement in the future.
With that out of the way - I did some digging and found a lot of solutions for monitoring your usage, but I wasn't able to find much along the lines of a full service or application that can accomplish what you're looking for. I did, however, come across a post on Github from a user that created a Python script that might work for you.
Disclaimer: I have not tested this myself.
Essentially, a cron is utilized to run a vnstat command at scheduled intervals. The script will then compare the output of the vnstat to a limit that you assign ahead of time. If the total amount of transfer that is used is higher than the assigned limit, then a shutdown command will be issued.
There is a caveat here that I'd like to point out. While each Linode has its own transfer limit, all transfer on an account is pooled together and is treated as a single value -- 2 Nanodes have a total of 2 TB worth of transfer to use between them. You aren't limited to one of the Nanodes using only 1 TB. What this means is you'll need to make adjustments to the limit in the script based on what Linodes are on your account and what your needs are.
You can view all of the transfer limits for each of our plans on our Pricing page if you're unsure of how much transfer each of you Linodes may be allotted.
I'd also recommend checking out other monitoring solutions to see if there are any other applications that better meet your needs.