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FreeBSD 5.4-RELEASE
Squid Cache: Version 2.5.STABLE10
configure options:  --with-pthreads --enable-icmp --enable-large-cache-files '--enable-auth=basic digest ntlm' '--enable-basic-auth-helpers=NCSA PAM SMB' --enable-digest-auth-helpers=password '--enable-ntlm-auth-helpers=winbind SMB'

squid.conf:
# ACCESS CONTROLS
# ------------------------------------------------------------------------
auth_param basic program /usr/local/squid/libexec/ncsa_auth /usr/local/squid/etc/password
auth_param basic children 5
auth_param basic realm Squid proxy-caching web server
auth_param basic credentialsttl 2 hours

#Recommended minimum configuration:
acl all src 0.0.0.0/0.0.0.0
acl manager proto cache_object
acl localhost src 127.0.0.1/255.255.255.255
acl to_localhost dst 127.0.0.0/8
acl SSL_ports port 443 563
acl Safe_ports port 80          # http
acl Safe_ports port 88
acl Safe_ports port 21          # ftp
acl Safe_ports port 1025-65535  # unregistered ports
acl CONNECT method CONNECT

acl users proxy_auth REQUIRED
http_access allow users
http_access deny all

authenticate_ttl 60 seconds

http_reply_access allow all

icp_access deny all

miss_access allow all

cache.log:

2005/11/29 11:42:04| aclCheck: checking 'http_access allow users'
2005/11/29 11:42:04| aclMatchAclList: checking users
2005/11/29 11:42:04| aclMatchAcl: checking 'acl users proxy_auth REQUIRED'
2005/11/29 11:42:04| authenticateValidateUser: Validating Auth_user request '0x0'.
2005/11/29 11:42:04| authenticateValidateUser: Auth_user_request was NULL!
2005/11/29 11:42:04| authenticateAuthenticate: broken auth or no proxy_auth header. Requesting auth header.
2005/11/29 11:42:04| aclMatchAcl: returning 0 sending authentication challenge.
2005/11/29 11:42:04| aclMatchAclList: no match, returning 0