apr_socket_recv: Connection refused (61)

abでみかけたエラーです。

$ ab -n 1 -c 1 'http://localhost:8000/book'
This is ApacheBench, Version 2.3 <$Revision: 655654 $>
Copyright 1996 Adam Twiss, Zeus Technology Ltd, http://www.zeustech.net/
Licensed to The Apache Software Foundation, http://www.apache.org/

Benchmarking localhost (be patient)...apr_socket_recv: Connection refused (61)

stackoverflowで解決策を見つけました。
python - ab is erroring out with apr_socket_recv: Connection refused (61) - Stack Overflow

localhostの代わりに127.0.0.1にすれば普通に動きます。

$ ab -n 1 -c 1 'http://127.0.0.1:8000/book'