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среда, 1 февраля 2012 г.

Squid: еще об аутентификации


Proxy Authentication


There are four major flavours of authentication available in the HTTP world at this moment:
  • Basic - been around since the very beginning
  • NTLM - Microsoft's first attempt at single-sign-on for LAN environments
  • Digest - w3c's attempt at having a secure authentication system
  • Negotiate (aka SPNEGO) - Microsoft's second attempt at single-sign-on.
Squid-2.6 and later support Basic, NTLM (SMB LM, v1 and v2), Digest, and Negotiate.

How does Proxy Authentication work in Squid?

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 HTTP Authorization: request header.
If Squid gets a request and the http_access rule list gets to a proxy_auth ACL or an external ACL (external_acl_type) with %LOGIN parameter, Squid looks for the Authorization: header. If the header is present, Squid decodes it and extracts a user credentials.
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 attempts to locate the users credentials. Sometimes this means a background lookup, sometimes a popup prompt for 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 user credentials on stdin, and reply with "OK" or "ERR" on stdout. This technique allows you to use a number of different authentication protocols (named "schemes" in this context). When multiple authentication schemes are offered by the server (Squid in this case), it is up to the User-Agent to choose one and authenticate using it. By RFC it should choose the safest one it can handle; in practice usually Microsoft Internet Explorer chooses the first one it's been offered that it can handle, and Mozilla browsers are bug-compatible with the Microsoft system in this field.
In addition to the well known Basic authentication Squid also supports the NTLM, Negotiate and Digest authentication schemes which provide more secure authentication methods, in that where the password is not exchanged in plain text over the wire. Each scheme have their own set of helpers and auth_param settings. Notice that helpers for different authentication schemes use different protocols to talk with squid, so they can't be mixed.
For information on how to set up NTLM authentication see NTLM config examples.
The Squid source code bundles with a few authentication backends ("helpers") for authentication. These include:
  • DB: Uses a SQL database.
  • getpwam: Uses the old-fashioned Unix password file.
  • LDAP: Uses the Lightweight Directory Access Protocol.
  • MSNT: Uses a Windows NT authentication domain.
  • MSNT-multi-domain: Allows login to one of multiple Windows NT domains.
  • NCSA: Uses an NCSA-style username and password file.
  • NIS (or YP): Uses the NIS database
  • PAM: Uses the Unix Pluggable Authentication Modules scheme.
  • POP3: Uses an email server to validate credentials. Useful for single-signon to proxy and email.
  • RADIUS: Uses a RADIUS server for login validation.
  • SASL: Uses SASL libraries.
  • SMB: Uses a SMB server like Windows NT or Samba.
  • SSPI: Windows native authenticator
Documentation for each of these helpers can be found at http://www.squid-cache.org/Doc/man/. Due to its simplicity Basic authentication has by far the most helpers, but the other schemes also have several helpers available.
In order to authenticate users, you need to compile and install one of the supplied authentication helpers, one of the others, or supply your own.
You tell Squid which authentication helper program to use with the auth_param directive in squid.conf. 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

(full configuration details for the specific helper you choose can be found in the manual pages linked above).

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
http_access allow foo
http_access deny all

The REQUIRED term means that any already authenticated user will match the ACL named foo.
  • /!\ Not that allow will NOT trigger the 407 authentication denial to fetch new auth details if the user is not correctly logged in already. Some browsers will send anonymous auth details by default.
A slightly better way to do this and ensure the browser auth gets validated is:
acl foo proxy_auth REQUIRED
http_access deny !foo
http_access allow localnet
http_access deny all

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
http_access allow foo daytime
http_access allow bar
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.
The ConfigExamples area contains some detailed examples:

How do I ask for authentication of an already authenticated user?

If a user is authenticated at the proxy you cannot "log out" and re-authenticate. The user usually has to close and re-open the browser windows to be able to re-login at the proxy. A simple configuration will probably look like this:
acl my_auth proxy_auth REQUIRED
http_access allow my_auth
http_access deny !my_auth
http_access deny all

But there is a trick which can force the user to authenticate with a different account in certain situations. This happens if you deny access with an authentication related ACL last in the http_access deny statement. Example configuration:
acl my_auth proxy_auth REQUIRED
acl google_users proxy_auth user1 user2 user3
acl google dstdomain .google.com
http_access deny google !google_users
http_access allow my_auth
http_access deny all

In this case if the user requests www.google.com then first second http_access line matches and triggers re-authentication unless the user is one of the listed users. Remember: it's always the last ACL on a http_access line that "matches". If the matching ACL deals with authentication a re-authentication is triggered. If you didn't want that you would need to switch the order of ACLs so that you get http_access deny !google_users google.
You might also run into an authentication loop if you are not careful. Assume that you use LDAP group lookups and want to deny access based on an LDAP group (e.g. only members of a certain LDAP group are allowed to reach certain web sites). In this case you may trigger re-authentication although you don't intend to. This config is likely wrong for you:
acl ldapgroup-allowed external LDAP_group PROXY_ALLOWED

http_access deny !ldapgroup-allowed
http_access allow all

The second http_access line would force the user to re-authenticate time and again if he/she is not member of the PROXY_ALLOWED group. This is perhaps not what you want. You rather wanted to deny access to non-members. So you need to rewrite this http_access line so that an ACL matches that has nothing to do with authentication. This is the correct example:
acl ldapgroup-allowed external LDAP_group PROXY_ALLOWED

http_access deny !ldapgroup-allowed all
http_access allow all

This way the http_access line still matches. But it's the all ACL which is now last in the line. Since all is a static ACL (that always matches) and has nothing to do with authentication you will find that the access is just denied.

How do I prevent Login Popups?

The login dialog box which pops up asking for username and password is a feature of your web browser. It only happens when the web browser has no working credentials it can hand to Squid when challenged for login.
Squid will only challenge for credentials when they are not sent and required:
acl mustLogin proxy_auth REQUIRED

this might cause a login popup. However modern browsers have a built-in password manager or access to the operating system credentials where they can locate a first attempt. This is commonly called single-sign-on.
If the browser is unable to find any initial details you WILL get the login popup. Regardless of what we do in Squid.
To prevent incorrect login details being re-challenged after sign-on has failed all you have to do is prevent the login ACL being the last on the authentication line.
For example, this normal configuration will cause a login re-challenge until working details are presented:
http_access deny mustLogin

This all hack will present a plain access denied page without challenging for different credentials:
http_access deny mustLogin all

Does Squid cache authentication lookups?

It depends on the authentication scheme; Squid does some caching when it can. Successful Basic 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 basic credentialsttl configuration 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.

Are passwords stored in clear text or encrypted?

In the basic scheme passwords is exchanged in plain text. In the other schemes only cryptographic hashes of the password is exchanges.
Squid stores cleartext passwords in its basic authentication memory cache.
Squid writes cleartext usernames and passwords when talking to the external basic authentication processes. Note, however, that this interprocess communication occurs over TCP connections bound to the loopback interface or private UNIX pipes. Thus, its not possible for processes on other computers 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.
For the digest scheme Squid never sees the actual password, but the backend helper needs either plaintext passwords or Digest specific hashes of the same.
In the NTLM or Negotiate schemes Squid also never sees the actual password. Usually this is connected to a Windows realm or Kerberos realm and how these authentication services stores the password is outside of this document but usually it's not in plain text.
In side-band authentication, using the external_acl_type directive. There is a password= value which is possibly transfered to Squid from the helper. This value is entirely optional and may in fact have no relation to a real password so we cannot be certain what risks are actually involved. When received it is generally treated by Squid as a cleartext Basic authentication password and it may be passed a such to peer proxies or services.

Can I use different authentication mechanisms together?

Yes, with limitations.
Commonly deployed user-agents support at least one and up to four different authentication protocols (also called schemes).
Those schemes are explained in detail elsewhere (see Features/NegotiateAuthentication and SquidFaq/TroubleShooting). You can enable more than one at any given moment, just configure the relevant auth_param sections for each different scheme you want to offer to the browsers.
Due to a bug in common User-Agents (most notably Microsoft Internet Explorer) the order the auth-schemes are configured is relevant. RFC 2617, chapter 4.6, states: A user agent MUST choose to use the strongest auth-scheme it understands. Microsoft Internet Explorer instead chooses the first authe-scheme (in the order they are offered) it understands

In other words, you SHOULD use this order for the auth_params directives:
  1. negotiate
  2. ntlm
  3. digest
  4. basic
omitting those you do not plan to offer.
Once the admin decides to offer multiple auth-schemes to the clients, Squid can not force the clients to choose one over the other.

Can I use more than one user-database?

Generally speaking the answer is no, at least not from within Squid.
Unix's PAM authentication method is quite flexible and can authenticate in an either/or/both fashion from multiple authentication sources.
You can configure two different authentication schemes with different user database. However since there is no control over which the browser chooses to use. This is an unreliable option, if it works for you great, if not there is nothing we can do to help.
The web server Basic authentication scheme provides another approach, where you can cook a proxy script which relays the requests to different authenticators and applies an 'OR' type of logic. For all other auth-schemes this cannot be done; this is not a limitation in squid, but it's a feature of the authentication protocols themselves: allowing multiple user-databases would open the door for replay attacks to the protocols.


Authentication in interception and transparent modes

Simply said, it's not possible to authenticate users using proxy authentication schemes when running in interception or transparent modes. See SquidFaq/InterceptionProxy for details on why.

Can I write my own authenticator?

Squid has a large range of versatile helpers to integrate with a very large number of popular authentication backends. Including custom-built corporate databases. Take a look through the bundled helpers manuals and online search engines. You will likely find someone has already done the hard work.
However, you may still find the need to write your own one for some system which has not been dreamed of yet. The protocol(s) Squid uses to communicate with its authentication helpers are very simple and documented in detail on the Features/AddonHelpers page.

Other Resources

Back to the SquidFaq
Features/Authentication (last edited 2011-12-29 10:09:37 by Amos Jeffries)

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Пожалуйста, воздержитесь от грубостей и персональных нападок.
Я не против матерщины, но она должна быть уместной и использоваться для выражения эмоций, а не в качестве основного средства выражения мыслей.