In an Ubuntu 16.04 machine I have set up a proxy relay configuration with squid 3.5.12 like this:

External_proxy <-> Internal_proxy <-> Host

At this moment, from both, external and internal proxy I can access to http and https websites, but from Host I can only access to http websites.

My current squid.conf in internal_proxy is:

http_port 3128
logformat lmc %ts %la %>a %ul %<A %rm %ru %mt %>Hs %Ss "%{Referer}>h" "%{User-Agent}>h" %>st %<st %tr %<pt %<tt
access_log syslog:local1.info
access_log syslog:local0.info lmc
icp_port 0
cache_peer <external_proxy_IP> parent 8080 0 proxy-only originserver
acl SSL_ports port 443 
acl Safe_ports port 80
acl Safe_ports port 443
acl local_subnet src
http_access allow CONNECT local_subnet
http_access allow local_subnet Safe_ports
http_access allow local_subnet SSL_ports
http_access deny all
connect_timeout 510 minutes
request_timeout 500 minutes
cache_dir ufs /var/cache/squid 8192 16 256
shutdown_lifetime 2 seconds
hierarchy_stoplist cgi-bin ?
refresh_pattern -i (/cgi-bin/|\?) 0     0%      0
refresh_pattern .               0       20%     4320
ftp_epsv off
via off
never_direct allow all
visible_hostname "internal_proxy_hostname"

When I try to connect to some https site I receive the following:

wget https://google.com
--2019-07-08 13:08:23--  https://google.com/
Connecting to internal_proxy:3128... connected.
Proxy tunneling failed: Service UnavailableUnable to establish SSL connection.

And in internal_proxy logs:

TAG_NONE/503 0 CONNECT google.com:443 - HIER_NONE/- -
CONNECT google.com:443 - 503 TAG_NONE "-"

I have tried this procedure: but the result is the same.

NOTE: I have no access to the external_proxy config


After changing the line in squid.conf:

cache_peer <external_proxy_IP> parent 8080 0 proxy-only originserver


cache_peer <external_proxy_IP> parent 8080 0

The problem was fixed.

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