siege use locahost as address reports error

Question

Localhost has nginx, which redirects that request to another host for example, 172.21.9.10.

curl -X POST http://localhost:9091/a/b responds correctly.

However: use nginx address, siege reports error like this:

siege -H "Content-Type:application/json" -c 1 -r 1 'http://localhost:9091/identity/compare POST < identity.json'
** SIEGE 4.1.6
** Preparing 1 concurrent users for battle.
The server is now under siege...
[error] socket: unable to connect sock.c:282: Connection refused

if using host address instead of nginx address, the same siege command works fine.