Sophos UTM + squid + MS L2TP/IPsec

1. Purpose

To connect to the internal NIC for Web Admin of the UTM
under VPN (SSL and/or L2TP/IPsec) (through squid proxy)

2. Procedure 
2-1. The proxy config of MS L2TP/IPsec client didn't work

  Once configured as shown in the picture,
  it always goes thru proxy.

2-2. squid config

# vi /etc/squid/squid.conf

acl to_utm201712 dst a.b.c.d/32    # a.b.c.d = IP of Web Admin
    : ... snip ...
acl SSL_ports port 443
    : ... snip ...
acl CONNECT method CONNECT
    : ... snip ...
http_access allow CONNECT to_utm201712
http_access deny CONNECT !SSL_ports

# /etc/init.d/squid restart

2-3. Sophos UTM Home Edition

Management -> WebAdmin Settings
  Allowed Networks : hopefully limited to internal and VPN Pool IPs