The default onion bind uses -port + 1. With -port=65535 this wraps to 0, so the node binds 127.0.0.1 to a random port and the automatic onion service points there.
Now startup fails with a clear error when that port is needed. An explicit -bind=<addr>:<port>=onion still works, and so does -listen=0.
Added cases to feature_port.py.
The default onion bind uses -port + 1. For -port=65535 that wraps to 0,
so the node binds 127.0.0.1 to a random port and points the automatic
Tor onion service at it.
Return an init error instead when that port is actually needed (no
explicit onion bind port and listening is enabled). An explicit
-bind=<addr>:<port>=onion keeps working.
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Possible places where named args for integral literals may be used (e.g. func(x, /*named_arg=*/0) in C++, and func(x, named_arg=0) in Python):
src/init.cpp: Lookup(truncated_bind_arg, default_bind_port_onion.value_or(0), false)<sup>2026-10-31 14:34:29</sup>