## Describe your changes
The relay client's reconnect backoff was constructed without a
`RandomizationFactor`
([guard.go:156-165](https://github.com/netbirdio/netbird/blob/main/shared/relay/client/guard.go#L156-L165)),
so it kept the zero value: every client that lost the same relay server
retried on the identical 2/4/8/16/32/60, potentially in waves.
It was the only exponential backoff in the codebase without a
randomization factor.
Use `backoff.DefaultRandomizationFactor`, as random factor.
## Issue ticket number and link
No public issue. Found while reviewing the relay reconnect path for the
client-metrics review:
22k relay reconnection events in 24h, and the shared transport's own
retry schedule was
identical across all clients.
## Stack
<!-- branch-stack -->
### Checklist
- [x] Is it a bug fix
- [ ] Is a typo/documentation fix
- [ ] Is a feature enhancement
- [ ] It is a refactor
- [x] Created tests that fail without the change (if possible)
> By submitting this pull request, you confirm that you have read and
agree to the terms of the [Contributor License
Agreement](https://github.com/netbirdio/netbird/blob/main/CONTRIBUTOR_LICENSE_AGREEMENT.md).
## Documentation
Select exactly one:
- [ ] I added/updated documentation for this change
- [x] Documentation is **not needed** for this change (explain why)
Internal retry-timing change with no user-visible surface: no CLI flag,
configuration option
or API field is added or altered, and the mean reconnect delay is
unchanged.
### Docs PR URL (required if "docs added" is checked)
Paste the PR link from https://github.com/netbirdio/docs here:
N/A
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
* **Bug Fixes**
* Improved reconnect timing to distribute repeated connection attempts
more evenly and reduce synchronized retry spikes.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
* [relay] evict foreign client cache on disconnect
When a foreign relay's TCP connection drops, the manager's
onServerDisconnected handler only triggered reconnect logic for the
home server; the disconnected foreign entry stayed in the relayClients
cache. Subsequent OpenConn calls reused the closed client until the
60-second cleanup tick evicted it, breaking peer connectivity through
that relay for up to a minute.
Evict the foreign entry from the cache on disconnect so the next
OpenConn dials a fresh client.
Also:
- Make the reconnect backoff cap configurable via WithMaxBackoffInterval
ManagerOption; the previous hard-coded 60s constant forced
TestAutoReconnect to sleep ~61s. Test now polls Ready() and finishes
in ~2s.
- Add NB_HOME_RELAY_SERVERS env var that overrides the relay URL list
received from management, so a peer can be pinned to a specific home
relay (used by the netbird-conn-lab Edge 4 reproducer).
* [client] treat empty NB_HOME_RELAY_SERVERS as unset
Returning (urls=[], ok=true) when the env var contained only separators or
whitespace caused callers to wipe the mgmt-provided relay list, leaving the
peer with no relays. Treat a parsed-empty result the same as an unset env.
