This article is meant to help set up LDAP authentication with a Squid proxy. Often administrators are using Squid as the central proxy for a company. For security reasons users need to enter their username and password before they are allowed to surf the internet. Firewalls usually deny accesses to the internet that would bypass the proxys. This way – if a user it doing dangerous things – you know who it was.
(Please be fair to your users and don’t lurk around what people are doing. In many countries this is forbidden anyway. If you don’t want them to surf private web sites during business-hours then use a decent blocking software instead of lurking on them.)
What is LDAP?
LDAP is short for "lightweight directory access protocol". It is not a directory service (which is often confused). Instead it is a way to access a directory through a common interface. You may already have a user directory based on OpenLDAP, Novell Directory Service (NDS) or Microsoft Active Directory. Any client that speaks LDAP can fetch information from those services.
LDAP provides a tree structure. Just an example:
+-- Company | +-- IT-Services | | | +-- George | | | +-- Tim | +-- Sales | | | +-- Joe | +-- Management | +-- Tina | +-- George
There are different types of nodes like "o" for organizations or "ou" for organizational units. To complicate matters a bit there are some limitations on how these nodes can be connected. In the above example you start your tree with the main node which is your company. This node is addressed as
To refer to the sales department you would write the path to the node:
This path is called the "distinguished name" (dn). The dn is always unique in your directory.
And there may be different types of leafs like people or printers. You have probably noticed that I have two "George"s in the tree that are two different persons. George in the IT department has another distinguished name than George in the Management (although he probably would like to get the same salary). His distinguished name is:
I hope that you now have a basic understanding of how LDAP structures look like. This article does not cover how to set up your own directory. If you like to play around with LDAP without spending money you may want to take a look at OpenLDAP (see http://www.openldap.org).
Making Squid use LDAP for authenticating your users
As you probably already know you can make Squid ask your users for an authentication before they are allowed to surf the web. These are the parts of your configuration that set up how LDAP authentication is supposed to work:
auth_param basic program /usr/lib/squid/ldap_auth -b o=Company -h ldapserver -D cn=Tim,ou=IT-Services,o=Company -w timspassword -f (&(objectclass=person)(cn=%s)) auth_param basic children 50 auth_param basic realm Web-Proxy auth_param basic credentialsttl 1 minute
The option -b defines the context (branch of the tree) where you want to search for that users. In this example you are searching everywhere in the whole company tree. -h defines the hostname of the LDAP server to use. Next I need a user account to search the tree for the user (Novell requires that as it does not seem to allow anonymous searching). This account is provided using the -D option. -w lists the according password. And last I use a filter to search for all persons that have the name we look for (%s is the name this authenticator gets from Squid).
(Hint: this line is used in production in a network using a Novell Directory Service. There are other ways to call the ldap_auth authenticator which may be easier. But we don’t know where the user is located in the LDAP tree so we do a full search. Tim is a user account that is used to do that search.)
You also need to use ACLs to force the users to authenticate. Example:
acl ldap-auth proxy_auth REQUIRED http_access deny !ldap-auth
This would deny everybody access who is not authenticated (thus the "!").
By the way: an authenticator is just any program that reads lines from STDIN (like the console) that look like this
and return either OK (if the credentials are okay) or ERR if something went wrong.
Using LDAP groups
In addition to simple authentication you may also want to grant different privileges to different users. The easiest way is to use LDAP groups. A group is just a list of dinstiguished names. Example group:
Assume that these two users are allowed to use Google where all other users are not allowed. First you need to define the LDAP group mapping:
external_acl_type ldapgroup %LOGIN /usr/lib/squid/squid_ldap_group -b o=Company -f (&(objectclass=person)(cn=%v)(groupMembership=cn=%a,ou=Proxygroups,o=Company)) -D cn=Tim,ou=IT-Services,o=Company -w timspassword -h ldapserver
The filter expression looks a little complicated. It just means that we are looking for any user (person) with the name we are looking for (%v) who is a member of the group we are looking for (groupMembership). You can now just create any number of groups in the tree like
Then this pile of ACLs should block off everyone from google.com who is not a member of that group:
acl ldapgroup-googleallowed external ldapgroup googleallowed acl google dstdomain google.com http_access deny google !ldapgroup-googleallowed
You may be curious how such a group authorizator works. Simple. It just reads line after line from STDIN (like the console) where each line looks like
and returns either OK or ERR depending on whether the user is a member of that group or not.