d417efc got this backwards, in both the code comment and the installer summary.
It claimed authlib appends /session/minecraft/hasJoined itself, so the property
should be given the base URL only. Velocity does not work that way, and a real
login says so: pointed at http://127.0.0.1:8081, a Mojang login arrives at nano
as
GET /?username=FLYEMOJ1&serverId=-23ae0b50...
with no path at all. Velocity appends the query string to the property verbatim
and issues the request itself; authlib is not in the loop. nano has no route on
/, so it answers 404 and Velocity kicks the player with authservers_down.
Velocity's own default for the property is the full URL,
https://sessionserver.mojang.com/session/minecraft/hasJoined, which is the same
thing said another way.
With the full endpoint URL the same account logs straight in, so both the
nano.go header and summary_nano now print
-Dmojang.sessionserver=http://127.0.0.1:8081/session/minecraft/hasJoined
and note that the flag belongs between `java` and `-jar`.
Verified against Velocity 3.5.1 + Paper 26.2 on the deploy host: a Mojang login
reaches the backend with its real Mojang UUID unchanged, and a LittleSkin login
under the same username reaches it as UUIDv3(felisAuthNS, "littleskin:"+id) --
two different players on the backend, which is the point.
flyemoji
b323975ddb