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

[David Fifield]

The DNS record for the Snowflake bridge was switched to a temporary staging server, running the load balancing setup, at 2022-01-25 17:41:00. We were debugging some initial problems until 2022-01-25 18:47:00. You can read about it here:

https://bugs.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/40095#note_2772325

Snowflake sessions are now using the staging bridge, except for those that started before the change happened and haven't finished yet, and perhaps some proxies that still have the IP address of the production bridge in their DNS cache. I am not sure yet what will happen with metrics, but we'll see after a few days.

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."

https://gitweb.torproject.org/tor.git/tree/src/feature/relay/router.c?h=tor-0.4.6.9#n523
```
  if (curve25519_keypair_write_to_file(&new_curve25519_keypair, fname,
                                       "onion") < 0) {
    log_err(LD_FS,"Couldn't write curve25519 onion key to \"%s\".",fname);
    goto error;
  }
  // ...
 error:
  log_warn(LD_GENERAL, "Couldn't rotate onion key.");
  if (prkey)
    crypto_pk_free(prkey);
```