Hai,
Squid 3.5 does provide the option to use gnutls. Which should fix : https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=641944 https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=180886 if squid can be updated also to 3.5.22 that would be great. Then its up2date before the freeze of Stretch. Greetz, Louis