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    1    Link: Chapter 1. The Courier IMAP/POP3 proxy (start)
    2    Link: Requirements (next)
    4 Chapter 1. The Courier IMAP/POP3 proxy
    6    Table of Contents
    8    Requirements
   10    Configuration
   12                 Using the same configuration files on all servers
   14                 Alternative configurations
   16                 Homogenous environments
   18                 Heterogenous environments
   20    The Courier IMAP and POP3 servers now include a built-in proxy aggregator.
   21    With a proxy aggregator, the mail accounts are split between multiple,
   22    independent servers, with an IMAP/POP3 server running on each individual
   23    server. A separate, proxy server sits in front and accepts ordinary IMAP
   24    and POP3 connections. It reads the login ID, determines which server the
   25    account is located on, connects to the server, and logs in. Then, for the
   26    lifetime on the login session the front-end server takes itself out of the
   27    loop, and forwards all session traffic between the IMAP/POP3 client, and
   28    the back-end server.
   30 Requirements
   32    The Courier mail server can operate in IMAP/POP3 proxy mode only when the
   33    Courier Authentication Library uses the userdb, LDAP, MySQL, or the
   34    PostgreSQL authentication module. Challenge-Response (CRAM) authentication
   35    will also work with the LDAP, MySQL, or the PostgreSQL authentication
   36    module. Yes, CRAM authentication will work (except for userdb).
   38 Configuration
   40    Follow the regular installation instructions to set up The Courier mail
   41    server with the actual mail accounts. The proxy feature uses the “account
   42    options” feature of the Courier Authentication Library, specifically an
   43    option called “mailhost”. Account option configuration process depends on
   44    the authentication module. With userdb, account options are specified by
   45    the “options” userdb attribute:
   47  userdb user@example.com set options=mailhost=servera.example.com
   49    Instructions for setting up account options with LDAP, MySQL, or
   50    PostgreSQL, may be found in the appropriate configuration file. Briefly:
   52      * In authldaprc, put “LDAP_AUXOPTIONS<TAB>mailhost=mailhost”, then
   53        populate the “mailhost” LDAP attribute (this may entail modifications
   54        of the LDAP schema).
   56      * In authmysqlrc, put
   57        “MYSQL_AUXOPTIONS<TAB>CONCAT("mailhost=",mailhost)” (or modify the
   58        existing MYSQL_AUXOPTIONS setting accordingly), then create a
   59        “mailhost” column in the account table.
   61      * In authpgsqlrc, put “PGSQL_AUXOPTIONS<TAB>'mailhost=' || mailhost” (or
   62        append ",mailhost=" || mailhost to an existing setting), then create a
   63        “mailhost” column in the account table.
   65    The “mailhost” option for each account should be the name of the server
   66    where that account is located. If possible, this should match, exactly,
   67    the PROXY_HOSTNAME environment variable or the value returned by the
   68    “gethostname” on the server.
   70    The final step is to set “IMAP_PROXY” and/or “POP3_PROXY” to “1” in the
   71    imapd and/or the pop3d configuration file, in the Courier configuration
   72    file directory on the proxy server.
   74   Using the same configuration files on all servers
   76    It is possible to have both the proxy server, and the back-end servers
   77    with the actual accounts, read the same configuration file that enables
   78    proxying. Ordinarily, if the back-end server also has the proxy setting
   79    turned on, it will also attempt to establish a proxy connection (to
   80    itself; lather, rinse, repeat until the server runs out of sockets).
   82    However, if the “mailhost” option matches the server's hostname, as
   83    returned by “gethostname”, no proxying takes place. Therefore, if specific
   84    attention and care is made, when setting up the server names and account
   85    options, all servers can boot off the same configuration file.
   87   Alternative configurations
   89    If the server names are set up properly, it's possible to set things up
   90    without a dedicated front-end proxy aggregator server. All mail accounts
   91    are divided between a pool of servers, who are just one, big, happy
   92    family. IMAP and POP3 clients can connect to any server, at random. If
   93    they try to log into an account that happens to reside on the same box,
   94    then everything will be ready to go. If not, the server automatically
   95    opens a proxy connection to the right box, and everything will be ready to
   96    go as well.
   98   Homogenous environments
  100    Both servers involved in a proxy connections should be running the same
  101    version of the Courier IMAP/POP3 server. The proxy code included in the
  102    Courier-IMAP package tarball will talk to the server from the Courier-MTA
  103    package tarball that includes the same build of the IMAP daemon, and
  104    vice-versa. Run “imapd --version” to determine the build of the IMAP
  105    daemon.
  107    All servers MUST use the same identical imapd and pop3d configuration
  108    files (with the possible exception of the proxy flag). The next section
  109    explains why.
  111   Heterogenous environments
  113    It should generally be possible to have the The Courier IMAP/POP3 server
  114    establish a proxy connection to some other third party, non-Courier, IMAP
  115    or POP3 server. Of course, the Courier Authentication Library running on
  116    the proxy server must have the same understanding of the account names and
  117    passwords as the other IMAP/POP3 server. The main issue is the different
  118    levels of protocol implementations.
  120    Both the IMAP and POP3 protocols have optional features that different
  121    servers may or may not implement. Some servers will implement certain
  122    optional features of the IMAP or POP3 protocol; other servers will
  123    implement different features parts.
  125    When the IMAP/POP3 client connects to the server, the client typically
  126    obtains the list of available optional features. After logging in, the
  127    client will have no reason to expect that it's now talking to a different
  128    server with a different set of protocol features. Therefore, it may not be
  129    possible to use a Courier proxy with some other IMAP/POP3 server that
  130    implements a widely different set of features. This may work with some
  131    clients, that don't make use of optional features; while other clients
  132    will report strange, or unpredictable errors.
  134    In some cases, setting the IMAP_PROXY_FOREIGN flag, in the imapd
  135    configuration file, may help. This command will send a message to the IMAP
  136    client explicitly informing the client that the list of available protocol
  137    features has changed; however some clients may ignore or not implement
  138    this particular message. There is no equivalent POP3 command.
  140   Note
  142    As previously mentioned the IMAP/POP3 clients may use any supported
  143    authentication method, including CRAM authentication (in most cases), with
  144    or without encryption, to log in. However, Courier will always use plain
  145    userid/password authentication, without encryption, to establish proxy
  146    connections. When using a different server, that server must be configured
  147    to allow plain userid/password authentication.
  149    Note that the default configuration of the UW-IMAP server requires
  150    encryption, and refuses non-encrypted connections. Proxy connections are
  151    presumably carried over a private network, and there is no reason to use
  152    encryption. Therefore, the UW-IMAP server will have to be re-configured to
  153    allow non-encrypted connections, if it's to be used with Courier in proxy
  154    mode.
  156 References
  158    Visible links
  159    . file:///home/mrsam/src/courier.git/courier/libs/imap/README.proxy.html#proxy
  160    . file:///home/mrsam/src/courier.git/courier/libs/imap/README.proxy.html#requirements
  161    . file:///home/mrsam/src/courier.git/courier/libs/imap/README.proxy.html#requirements
  162    . file:///home/mrsam/src/courier.git/courier/libs/imap/README.proxy.html#configuration
  163    . file:///home/mrsam/src/courier.git/courier/libs/imap/README.proxy.html#idm44912407468848
  164    . file:///home/mrsam/src/courier.git/courier/libs/imap/README.proxy.html#idm44912407530944
  165    . file:///home/mrsam/src/courier.git/courier/libs/imap/README.proxy.html#idm44912407529744
  166    . file:///home/mrsam/src/courier.git/courier/libs/imap/README.proxy.html#idm44912407524720