Internal redirect to another domain with proxy_pass and Nginx

gistfile1.nginxconf

server
{
  listen 80; 
  server_name a.com b.com c.com;

  location ~* ^/comment/(.*) {
    proxy_set_header HOST shared.com;
    # $1 - stores capture from the location on top
    # $is_args will return ? if there are query params
    # $args stores query params
    proxy_pass http://comment/$1$is_args$args;
  }

}

server {
  listen 80;
  server shared.com;

  location / {
    # Proxy to some app handler
  }
}

upstream comment {
  server localhost; # or any other host essentially
}