在 proxy_set_header REMOTE-HOST $remote_addr;后加入一行proxy_ssl_server_name on;即可

location ^~ /
{
proxy_pass https://a.com;
proxy_set_header Host  a.com;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header REMOTE-HOST $remote_addr;
proxy_ssl_server_name on; # 加入这行即可
}