Вот кусок

(-enable-external-acl-helpers=wbinfo_group)

auth_param ntlm program /usr/bin/ntlm_auth --helper-protocol=squid-2.5-ntlmssp
auth_param ntlm children 20
auth_param ntlm max_challenge_reuses 0
auth_param ntlm max_challenge_lifetime 2 minutes
auth_param basic program /usr/bin/ntlm_auth --helper-protocol=squid-2.5-basic
auth_param basic children 20
auth_param basic realm Squid proxy-caching web server
auth_param basic credentialsttl 2 hours

ntlm_auth это не от сквида а от самбы

external_acl_type NT_GROUP %LOGIN /usr/lib/squid/wbinfo_group.pl

acl Inet_ports port 80        # http
acl Inet_ports port 21        # ftp
acl Inet_ports port 443 563    # https, snews
acl Inet_ports port 70        # gopher
acl Inet_ports port 210        # wais
acl Inet_ports port 280        # http-mgmt
acl Inet_ports port 488        # gss-http
acl Inet_ports port 591        # filemaker
acl Inet_ports port 777        # multiling http
acl Inet_port port 28805    # WinXP Games

acl admins external NT_GROUP DOMAIN\ISA-UNLIMITED
acl isa-internet external NT_GROUP DOMAIN\ISA-INTERNET

http_access allow admins
http_access allow Inet_ports isa-internet
http_access deny all