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23. Authentication

23.1 How does Proxy Authentication work in Squid?

Note: The information here is current for version 2.5.

Users will be authenticated if squid is configured to use proxy_auth ACLs (see next question).

Browsers send the user's authentication credentials in the Authorization request header.

If Squid gets a request and the http_access rule list gets to a proxy_auth ACL, Squid looks for the Authorization header. If the header is present, Squid decodes it and extracts a username and password.

If the header is missing, Squid returns an HTTP reply with status 407 (Proxy Authentication Required). The user agent (browser) receives the 407 reply and then prompts the user to enter a name and password. The name and password are encoded, and sent in the Authorization header for subsequent requests to the proxy.

NOTE: The name and password are encoded using ``base64'' (See section 11.1 of RFC 2616). However, base64 is a binary-to-text encoding only, it does NOT encrypt the information it encodes. This means that the username and password are essentially ``cleartext'' between the browser and the proxy. Therefore, you probably should not use the same username and password that you would use for your account login.

Authentication is actually performed outside of main Squid process. When Squid starts, it spawns a number of authentication subprocesses. These processes read usernames and passwords on stdin, and reply with "OK" or "ERR" on stdout. This technique allows you to use a number of different authentication schemes, although currently you can only use one scheme at a time.

The Squid source code comes with a few authentcation processes for Basic authentication. These include:

In addition Squid also supports the NTLM and Digest authentication schemes which both provide more secure authentication methods where the password is not exchanged in plain text. Each scheme have their own set of helpers and auth_param settings. You can not mix helpers between the different authentication schemes. For information on how to set up NTLM authentication see winbind .

In order to authenticate users, you need to compile and install one of the supplied authentication modules found in the helpers/basic_auth/ directory, one of the others, or supply your own.

You tell Squid which authentication program to use with the auth_param option in squid.conf. You specify the name of the program, plus any command line options if necessary. For example:

auth_param basic program /usr/local/squid/bin/ncsa_auth /usr/local/squid/etc/passwd

23.2 How do I use authentication in access controls?

Make sure that your authentication program is installed and working correctly. You can test it by hand.

Add some proxy_auth ACL entries to your squid configuration. For example:

acl foo proxy_auth REQUIRED
acl all src 0/0
http_access allow foo
http_access deny all
The REQURIED term means that any authenticated user will match the ACL named foo.

Squid allows you to provide fine-grained controls by specifying individual user names. For example:

acl foo proxy_auth REQUIRED
acl bar proxy_auth lisa sarah frank joe
acl daytime time 08:00-17:00
acl all src 0/0
http_access allow bar
http_access allow foo daytime
http_access deny all
In this example, users named lisa, sarah, joe, and frank are allowed to use the proxy at all times. Other users are allowed only during daytime hours.

23.3 Does Squid cache authentication lookups?

Yes. Successful authentication lookups are cached for one hour by default. That means (in the worst case) its possible for someone to keep using your cache up to an hour after he has been removed from the authentication database.

You can control the expiration time with the auth_param option.

Note: This has nothing to do with how often the user needs to re-authenticate himself. It is the browser who maintains the session, and re-authentication is a business between the user and his browser, not the browser and Squid. The browser authenticates on behalf of the user on every request sent to Squid. What this parameter controls is only how often Squid will ask the defined helper if the password is still valid.

23.4 Are passwords stored in clear text or encrypted?

Squid stores cleartext passwords in itsmemory cache.

Squid writes cleartext usernames and passwords when talking to the external authentication processes. Note, however, that this interprocess communication occors over TCP connections bound to the loopback interface or private UNIX pipes. Thus, its not possile for processes on other comuters or local users without root privileges to "snoop" on the authentication traffic.

Each authentication program must select its own scheme for persistent storage of passwords and usernames.

23.5 How do I use the Winbind authenticators?

by Jerry Murdock

Winbind is a recent addition to Samba providing some impressive capabilities for NT based user accounts. From Squid's perspective winbind provides a robust and efficient engine for both basic and NTLM challenge/response authentication against an NT domain controller.

The winbind authenticators have been used successfully under Linux, FreeBSD and Solaris.

Supported Samba Releases

Samba-3.X is supported natively using the ntlm_auth helper shipped as part of Samba. No Squid specific winbind helpers need to be compiled (and even if compiled they won't work with Samba-3.X)

Samba-2.2.X is supported using the winbind helpers shipped with Squid, and uses an internal Samba interface to communicate with the winbindd daemon. It is therefore sensitive to any changes the Samba team may make to the interface.

The winbind helpers shipped with Squid-2.5.STABLE2 supports Samba-2.2.6 to Samba-2.2.7a and hopefully later Samba-2.X versions. To use Squid-2.5.STABLE2 with Samba versions 2.2.5 or ealier the new --with-samba-sources=... configure option is required. This may also be the case with Samba-2.2.X versions later than 2.2.7a or if you have applied any winbind related patches to your Samba tree.

Squid-2.5.STABLE1 supported Samba 2.2.4 or 2.2.5 only. Use of Squid-2.5.STABLE2 or later recommended with current Samba-2.X releases.

For Samba-3.X the winbind helpers shipped with Squid should not be used (and won't work if your attempt to do so), instead the ntlm_auth helper shipped as part of the Samba-3 distribution should be used. This helper supports all versions of Squid and both the ntlm and basic authentication schemes. For details on how to use this Samba helper see the Samba documentation. For group membership lookups the wbinfo_group helper shipped with Squid can be used (this is just a wrapper around the samba wbinfo program and works with all versions of Samba)

Configure Samba

For full details on how to configure Samba and joining a domain please see the Samba documentation. The Samba team has quite extensive documentation both on how to join a NT domain and how to join a Active Directory tree.

Build/Install Samba-3.X

Samba must be built with these configure options:

        --with-winbind

Then follow the Samba installation instructions. But please note that neither nsswitch or the pam modules needs to be installed for Squid to function, these are only needed if you want your OS to integrate with the domain for UNIX accounts.

Build/Install Samba-2.2.X

Samba must be built with these configure options:

        --with-winbind
        --with-winbind-auth-challenge

Optionally, if building Samba 2.2.5, apply the smbpasswd.diff patch. See SMBD and Machine Trust Accounts below to determine if the patch is worthwhile.

Test Samba's winbindd

  1. Edit smb.conf for winbindd functionality. The following entries in the [global] section of smbd.conf may be used as a template.
    workgroup = mydomain
    password server = myPDC
    security = domain
    winbind uid = 10000-20000
    winbind gid = 10000-20000
    winbind use default domain = yes 
    
  2. Join the NT domain as outlined in the winbindd man page for your version of samba.
  3. Start nmbd (required to insure proper operation).
  4. Start winbindd.
  5. Test basic winbindd functionality "wbinfo -t":
    # wbinfo -t
    Secret is good
    
  6. Test winbindd user authentication:
    # wbinfo -a mydomain\\myuser%mypasswd
    plaintext password authentication succeeded
    error code was NT_STATUS_OK (0x0)
    challenge/response password authentication succeeded
    error code was NT_STATUS_OK (0x0)
    
    NOTE: both plaintext and challenge/response should return "succeeded." If there is no "challenge/response" status returned then Samba was not built with "--with-winbind-auth-challenge" and cannot support ntlm authentication.

SMBD and Machine Trust Accounts

Samba 3.x

The Samba team has incorporated functionality to change the machine trust account password in the new "net" command. A simple daily cron job scheduling "net rpc changetrustpw" is all that is needed, if anything at all.

Samba 2.2.x

Samba's smbd daemon, while not strictly required by winbindd may be needed to manage the machine's trust account.

Well behaved domain members change the account password on a regular basis. Windows and Samba servers default to changing this password every seven days.

The Samba component responsible for managing the trust account password is smbd. Smbd needs to receive requests to trigger the password change. If the machine will be used for file and print services, then just running smbd to serve routine requests should keep everything happy.

However, in cases where Squid's winbind helpers are the only reason Samba components are running, smbd may sit idle. Indeed, there may be no other reason to run smbd at all.

There are two sample options to change the trust account. Either may be scheduled daily via a cron job to change the trust password.

UglySolution.pl is a sample perl script to load smbd, connect to a Samba share using smbclient, and generate enough dummy activity to trigger smbd's machine trust account password change code.

smbpasswd.diff is a patch to Samba 2.2.5's smbpasswd utility to allow changing the machine account password at will. It is a minimal patch simply exposing a command line interface to an existing Samba function.

Note: This patch has been included in Samba as of 2.2.6pre2.

Once patched, the smbpasswd syntax to change the password is:

        smbpasswd -t DOMAIN -r PDC

winbind privileged pipe permissions (Samba-3.X)

ntlm_auth requires access to the privileged winbind pipe in order to function properly. You enable this access by changing group of the winbind_privileged directory to the group you run Squid as (cache_effective_group setting in squid.conf).

chgrp squid /path/to/winbind_privileged

Configure Squid

Build/Install Squid

Samba-3.X

As Samba-3.x has it's own authentication helper there is no need to build any of the Squid authentication helpers for use with Samba-3.x (and the helpers provided by Squid won't work if you do). You do however need to enable support for the ntlm scheme if you plan on using this. Also you may want to use the wbinfo_group helper for group lookups

        --enable-auth="ntlm,basic"
        --enable-external-acl-helpers="wbinfo_group"

Samba-2.X<

Squid must be built with the configure options:

        --enable-auth="ntlm,basic"
        --enable-basic-auth-helpers="winbind"
        --enable-ntlm-auth-helpers="winbind"
        --enable-external-acl-helpers="wb_group"

Test Squid without auth

Before going further, test basic Squid functionality. Make sure squid is functioning without requiring authorization.

Test the Samba-3.x helper

Testing the winbind ntlm helper is not really possible from the command line, but the winbind basic authenticator can be tested like any other basic helper. Make sure to run the test as your cache_effective_user

        # /usr/local/bin/ntlm_auth --helper-protocol=squid-2.5-basic
        mydomain+myuser mypasswd                   
        OK
The helper should return "OK" if given a valid username/password. + is the domain separator set in your smb.conf

Test the Samba-2.2.X helper

Testing the winbind ntlm helper is not really possible from the command line, but the winbind basic authenticator can be tested like any other basic helper:

        # /usr/local/squid/libexec/wb_auth -d
        /wb_auth[65180](wb_basic_auth.c:136): basic winbindd auth helper ...
        mydomain\myuser mypasswd                   
        /wb_auth[65180](wb_basic_auth.c:107): Got 'mydomain\myuser mypasswd' from squid (length: 24).
        /wb_auth[65180](wb_basic_auth.c:54): winbindd result: 0
        /wb_auth[65180](wb_basic_auth.c:57): sending 'OK' to squid
        OK
The helper should return "OK" if given a valid username/password.

Edit squid.conf

Test Squid with auth

If no usernames appear in access.log and/or no password dialogs appear in either browser, then the acl/http_access portions of squid.conf are not correct.

Note that when using NTLM authentication, you will see two "TCP_DENIED/407" entries in access.log for every request. This is due to the challenge-response process of NTLM.

References

Samba Winbind Overview

Joining a Domain in Samba 2.2.x

winbindd man page

wbinfo man page

nmbd man page

smbd man page

smb.conf man page

smbclient man page

ntlm_auth man page

23.6 Authentication in accelerator mode

Authentication is by default disabled in acceleartor mode in Squid-2.X due to conflicts with transparent interception. To enable this feature, at the top of acl.c add the following line:


#define AUTH_ON_ACCELERATION 1

Then "make install".

This feature is somewhat hidden because

The whole concept of "acceleration" in Squid is currently being reworked for the Squid-3.0 release to fix this and a number of other issues.


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