[tor-relays] How to reduce tor CPU load on a single bridge?

[Roman Mamedov]

On Fri, 28 Jan 2022 19:58:49 -0700
David Fifield <david at bamsoftware.com> wrote:

> > On the matter of onion key rotation, I had the idea of making the onion key files read-only. Roger did some source code investigation and said that it might work to prevent onion key rotation, with some minor side effects. I plan to give the idea a try on a different bridge. The possible side effects are that tor will continue trying and failing to rotate the onion key every hour, and "force a router descriptor rebuild, so it will try to publish a new descriptor each hour."
> 
> Making secret_onion_key and secret_onion_key_ntor read-only does not quite work, because tor first renames them to secret_onion_key.old and secret_onion_key_ntor.old before writing new files. (Making the *.old files read-only does not work either, because the `tor_rename` function first unlinks the destination.)
> https://gitweb.torproject.org/tor.git/tree/src/feature/relay/router.c?h=tor-0.4.6.9#n497
> 
> But a slight variation does work: make secret_onion_key.old and secret_onion_key_ntor.old *directories*, so that tor_rename cannot rename a file over them. It does result in an hourly `BUG` stack trace, but otherwise it seems effective.
> 
> I did a test with two tor instances. The rot1 instance had the directory hack to prevent onion key rotation. The rot2 had nothing to prevent onion key rotation.

I did not follow the thread closely, but if you want a file or directory
contents unchangeable, and not allowed to rename/delete even by root, there's
the "immutable" attribute (chattr +i).

-- 
With respect,
Roman