[bug]: protocol: duplicate custom feature bit prevents startup

[jjnazareth]

Pre-Submission Checklist

• I have searched the existing issues and believe this is a new bug.
• I am not asking a question about how to use lnd, but reporting a bug (otherwise open a discussion).

LND Version

lnd version 0.21.0-beta commit=v0.21.0-beta

LND Configuration

lnd.conf:

[protocol]
protocol.custom-message=513
protocol.custom-nodeann=39
protocol.custom-init=39

Backend Version

Bitcoin Knots daemon version v29.3.knots20260508

Backend Configuration

...

Tor settings

create a Tor hidden service, listenonion=1 is default

listenonion=1
proxy=127.0.0.1:9050
bind=127.0.0.1:8334=onion

OS/Distribution

Linux unode 6.8.0-111-generic #111-Ubuntu SMP PREEMPT_DYNAMIC Sat Apr 11 23:16:02 UTC 2026 x86_64 x86_64 x86_64 GNU/Linux

Bug Details & Steps to Reproduce

The system crashes on startup with the message "unable to create server: feature bit: 39 already set" when lnd.conf has the following lines

protocol.custom-message=513
protocol.custom-nodeann=39
protocol.custom-init=39_

but works correctly if these lines are commented out

Expected Behavior

Should start correctly with the protocol lines in lnd.conf

Debug Information

No response

Environment

No response

[ziggie1984]

lnd currently fails during startup if a feature bit configured via protocol.custom-init or protocol.custom-nodeann is already advertised by the default feature set.

In this case, bit 39 is OnionMessagesOptional, which lnd already advertises by default in the init and node announcement feature sets. Adding:

protocol.custom-nodeann=39
protocol.custom-init=39

does not create a different effective feature vector, but startup fails with:

  unable to create server: feature bit: 39 already set

A reasonable fix may be to treat exact duplicate custom feature bits as idempotent and ignore them, possibly with a startup warning. We should still reject invalid pair conflicts, for example trying to configure bit 38 when bit 39 is already set, since that would advertise both required and optional variants of the same feature.

[kravens]

I also hit this issue on Bitcoin Core v31. Had to disable those protocol.xxx settings in lnd.conf in order to start LND without crashing.