```
# http://127.0.0.1:8080/subdomain/a/b 404

server
{
listen 8080;
charset utf-8;

location /subdomain/ {
proxy_set_header Host $http_host;
proxy_pass http://127.0.0.1:8081/;
}
}

# http://127.0.0.1:8081/a/b 正常

server
{
listen 127.0.0.1:8081;
charset utf-8;

location /a/ {
alias /nginx/html/a/;
}
}
```