My VPN client needs to pass through a NTLM proxy so as to continue connecting to the VPN server.

Previously I config the VPN client in PC or tomato. Only one command needed:
http-proxy <proxy ip> <proxy port> /etc/auth.txt ntlm

Then create a new file /etc/auth.txt and put my domain user name and password in this file. It works prefectly.

But in OpenWRT, I found that format of the configuration is totally different. Especially for the proxy. I tried to config:
option 'http_proxy' '192.168.1.100 80 /etc/auth.txt ntlm'

But it doesn't work. So I changed it to:
option 'http_proxy' '192.168.1.100 80'
option 'auth_user_pass' '/etc/auth.txt'
option 'http_proxy_option' 'ntlm'

After the modification, OpenVPN couldn't work at all. If I delete option 'http_proxy_option' 'ntlm', OpenVPN can be started. But the log is:
HTTP proxy returned: 'HTTP/1.0 407 Proxy Authentication Required.
Proxy requires authentication
HTTP proxy: no support for proxy authentication method
TCP/UDP: Closing socket

So I just wonder, how to config the ntlm proxy in OpenVPN on OpenWRT.