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5. ezmlm support for SQL databases.

5.1 Why use an SQL database with ezmlm?

MySQL support is experimental. The main advantages are that you are using an address database system that can easily be accessed from any number of other programs via ODBC, perl, java, PHP, ... You can easily hook up ezmlm with your customer database, etc. An even bigger advantage is for use with distributed lists, which can be easily managed, changed, and monitored via a central database. ezmlm programs compiled with MySQL support (and when available also those compiled with support for other SQL servers) are entirely backwards compatible. You can mix SQL dbs with normal ezmlm dbs, and convert lists between them.

5.2 Why not to use an SQL database with ezmlm.

The main disadvantages of the MySQL version are that you need to be familiar with MySQL, the binaries are quite a bit larger, and you are trusting your addresses to a large database program, rather than a small and easily audited set of ezmlm programs. Also, the SQL server becomes a single point of failure.

MySQL is very robust and is used in many mission-critical situations. Also, ezmlm with MySQL support continues to rely on qmail stability. If connection fails, ezmlm aborts with a temporary error causing redelivery at a later time point.

If you create list clusters with main list and sublists, security for normal ezmlm sublists scales well: compromise of one sublist may lead to loss of that sublist. Security for the SQL sublists does not scale well: compromise of one sublist may lead to the loss of the entire list. This is of minimal importance if you want to split the load of a list onto a few local hosts (such as when your main list host becomes overloaded due to the popularity of your lists). Here, all host are under your control and equally secure.

There are other ways to do this: First, we could send the addresses with the message. This would use a single communication channel. I would prefer this model with a compressed and PGP-encrypted address block. However, it is hard to do within the framework of ezmlm (backwards compatibility) and encryption support is made complicated by ITAR. The dual channel system has advantages: addresses are harder to capture and logging is more reliable. Also, you can easy to tunnel MySQL and SMTP through ssh, if you desire encrypted communication.

In future, QMTP might become more available and able to transfer messages with multiple recipient addresses. This would allow outgoing load splitting from an ezmlm list host. Our implementation with ezmlm has more overhead, with the advantage that it is available now and that it also distributes the load of bounce handling.

Most parts of ezmlm-idx have been thoroughly tested through several versions. MySQL support is new for this version of ezmlm-idx. It is not as well tested as other parts of ezmlm/idx, it is not as well documented, and it may change in the future. This means that you can help to improve it. If this ``newness'' bothers you, compile your binaries without MySQL support.

5.3 Tables used for (My)SQL support.

The basic philosophy is that the database can be on any host (if you use SENDER restrictions, connectivity to the main host is more important than to the sublists), and you choose the database and ``table root'' names. The default database is ``ezmlm'' and the default table root is ``list''. Each list has a separate table root. Any number of lists can share a database.

The main list address table is named with the table root only, others have that name with various suffixes. In the following ``list'' is used as the table root.

Address tables.


List subscriber addresses.


Digest list subscriber addresses.


List subscriber alias addresses. Used only if SENDER restrictions are used for the list. This is configured in the default SQL list setup, but a local (ezmlm-style non-SQL) database could also be used.


List deny addresses. This table is created, but the default configuration, if it uses the ``deny'' addresses at all, will do so with a local database.


Moderator addresses. Created for completeness, but not used in the default configuration. If moderators are used, the addresses are stored in a local database.

Subscriber log tables.

For each of the above tables, there is a ``*_slog'' table that contains one row per transaction against the corresponding address table. The entries contain a time stamp, the subscription address; a direction indicator (``-'' for removals, ``+'' for additions); a type indicator (blank for ezmlm-manage, ``m'' for ``manual'', ``p'' for ``probe, i.e. bounce handling; and the subscriber ``From:'' line contents (only additions and only when made by ezmlm-manage or by ``ezmlm-sub(1) -n'').

Message logging tables.

For both the list and the digest list, there are a pair of tables that log messages:


The main list stores the message number and a pseudo-random cookie in this table when it processes the message. The cookie is derived from the secret DIR/key, the message sender and the message number. Thus, it is non-repeating and virtually impossible to guess beforehand. Sublists will check that the cookie sent with the message is the same as the one received with the message.

The digest list is created similarly, except that it is ezmlm-get(1) that originates the message and creates the cookie. This is done in ``list_digest_cookie''.


Both the main list and the sublists make entries in this table. Each entry consists of a time stamp, a message number, a list number, and a code. The code is 0 for message arrival, 1 for ``finished processing'', 2 for ``receipt received'' and -1 for bounce. The lists will refuse to process messages that do not have the correct cookie, or if the message already has an entry with a code of greater than 0. To inject a message at the sublist, an attacker would have to inject a message with the correct code before the list has processed the ``real'' message, or subvert the SQL server. In practice, this is very hard to do, unless the attacker has broken security at the database server or a sublist. This authentication mechanism is intended to make it safe to sublist moderated lists. It also blocks any message duplication between main list and sublist from being propagated to the subscribers.

The codes 2 for ``receipt received'' and -1 for bounce are entered by ezmlm-receipt(1) at the main list. This program is configured instead of ezmlm-return(1) if the main list was set up with ``ezmlm-make -w6''. ezmlm-receipt(1) checks the cookie of messages addresses to mainlocal-return-receipt@mainhost and if correct enters the ``receipt received'' code. This address is normally in the subscriber database with a hash of 98, so that each list sends a message to the address after all subscriber addresses.

Bounces of sublist messages should not lead to removal of the sublist from the database. ezmlm-receipt(1) will instead log the bounce to the ``list_mlog'' table. It will also store up to 50 bounces in the bounce directory. This helps error detection and diagnosis. After the first 50 bounces, no more bounces are stored, until you manually remove the old ones. This is to prevent filling up your hard disk in case a configuration error causes a deluge of bounces.

The digest list is treated in the same manner. Here, the tables is ``list_digest_mlog'' and the feedback address is mainlocal-digest-return-receipt@mainhost.

Sublist split tables.

Two tables are responsible for defining which sublist is responsible for which addresses. These tables are used only for distributed lists.


This is the sublist split table for the main list. It contains the list number (listno) as an auto_increment column, the list address (name), a flag to make the entry not active (notuse), and a range of message numbers for which the entry is active (msgnum_lo, msgnum_hi). The latter are useful when updating running lists, but can be ignored for now.

There is a ``domain'' column which contains the first 3 letters of the last part of the address domain that this list services. Thus, it would be ``jp'' for *@*.jp, ``edu'' for *@*.edu>, and ``hom'' for the hypothetical domain ``home''. The default is that domain is empty.

In addition, there is a hash range (hash_lo, hash_hi). The entry will apply only to addresses with a hash in that range. Since the hash of addresses is approximately evenly distributed in the available range of 0-52, the range corresponds to the fraction of addresses covered.


This works as the main list split table, but for the digest list.

5.4 How addresses are distributed among sublists.

To determine which addresses are handled by which sublists, the inactive sublist split table entries (notuse != 0 or current message number not in the msgnum_lo to msgnum_hi range) are first eliminated. Next, each sublist with a defined domain gets in that domain the addresses that fall within the hash range. Entries with a blank domain handle addresses with a blank domain as well as those not handled by other entries. The addresses handled by a list are the union of addresses handled by all entries for the particular list.

This means that the administrator needs to assure that there are lists with a blank domain covering the entire hash range 0-52. For each defined domain, the administrator must likewise assure that the entire hash range is covered. However, there is no corresponding requirement to cover the ``domain range'' since domains not specifically covered are handled by the entries with blank domains. This arrangement makes the database queries a little more complex, but makes administration considerable easier than other arrangements, while still allowing on-the-fly reconfiguration. See setting up a distributed ezmlm list with SQL support for a detailed example.

For logging, the listno is determined as the lowest listno of a ``list_name'' entry that is active for the current message.

5.5 How to set up a simple list with SQL support.

To use SQL database support, you have to compile the programs with SQL support. Currently, only MySQL support is available. See INSTALL.idx in the package on how to do this.

The programs with SQL support will work exactly like the normal programs for standard lists. However, if the file sql exists in the basedir, it turns on the SQL mode and it is expected to contain SQL server connect info in the format

Here, ``Host'' is the SQL database server host, ``port'' can be left blank to use the default port, ``user'' and ``password'' are connection credentials for a user you need to define and grant access to the database. ``Table'' is the name of the address table (``list'' in the examples above and ``list_digest'' for the corresponding digest list). For list clusters, ``:sublist'' is suffixed to this info and it is the name/address of the sublist.

For each address database, you also need to create the address table as well as the ``*_slog'' subscription log table. In addition, you should create a ``*_cookie'' and ``*_mlog'' table for message logging. This is all it takes to start using an SQL database.

Helper programs for SQL-enabled lists.

Two programs are supplied in the distribution to make it easier to create the database user and tables. Also, ezmlm-make(1) has support for setting up SQL-enabled lists.

Creating the tables

ezmlm-mktab(1) will create the necessary tables:

        % ezmlm-mktab -d table
Pipe this into the SQL client with the appropriate administrator credentials needed to create tables (see MySQL documentation, e.g.

For most lists, the only addresses that are stored in the SQL database are the subscribers of list and digest, and the ``allow'' aliases. It is NOT normally advisable to store moderator addresses there, since they are needed only at the main list and secrecy is more important. ``Deny'' addresses are few and again only needed at the main list. ``Allow'' are put in the SQL database when using the default ezmlmrc file only to make all relevant addresses manipulatable via the SQL server. The other tables are created, in case they are wanted (the cost for having them as empty table is zero). The basedir/sql file is the decision point. If it exists, an SQL table is used; if not a local ezmlm db is used.

Creating a user entry

ezmlm-grant(1) helps you to create a list user:

        % ezmlm-grant -d database -t table host user password
This will grant minimal access to ``user'' with ``password'' to ``database'' on ``host'' with table ``table, ``table_digest'', etc, conceivably needed. Just pipe this into the SQL client, e.g. mysql with the appropriate administrator credentials needed to modify grant tables (see MySQL documentation, e.g.

Creating the list

ezmlm-make(1) supports SQL-enabled lists with the ``-6'' switch:

        % ezmlm-make other_switches -6 'host:port:user:pw:db:table' \
                dir dot local host
Will create an SQL-enabled list that uses the SQL server for the main list subscribers, digest list subscribers (if configured) and ``allow'' poster alias addresses (if configured).

5.6 Manually manipulating the subscribers of a SQL-enabled list.

ezmlm-sub(1), ezmlm-unsub(1), and ezmlm-list(1) work as you would expect also with a SQL-enabled list. ezmlm-list(1) may be minimally slower (depending on network speed) if the SQL server is not local. ezmlm-sub(1) and ezmlm-unsub(1) will be faster, but this is noticeable only with very large subscriber lists and addition/removal of large numbers of addresses (more than several thousands).

5.7 Converting to and from and SQL database.

Just like other programs, ezmlm-list(1), ezmlm-sub(1), and ezmlm-unsub(1) will work with normal address databases in the absence of DIR/sql. However, they also have a ``-M'' switch to force this behavior even in the presence of DIR/sql. This is used to convert an address database from the standard type to the SQL type:

        % ezmlm-list -M dir | xargs ezmlm-sub dir
or from the SQL version to the standard type:
        % ezmlm-list dir | xargs ezmlm-sub -M dir
To synchronize the two, remove one and then update it with ezmlm-sub(1) from the other. Alternatively, sort the ezmlm-list(1) output for both, use diff and sed/awk to get separate files of the differences, and use ezmlm-sub(1) and ezmlm-unsub(1) to apply the differences to the appropriate database.

This type of conversion can serve as a convenient means to convert a list from one type to another, to back up databases, and to move subscriber addresses from a standard list to a SQL table for other purposes, or from a SQL database to a standard mailing list (you may need to use addresses from a SQL table, without wanting your lists to be dependent on an SQL server for day to day operation).

Note: This inter-conversion requires the DIR/sql file. If you do not run the list against an SQL server, you need to disable deliveries before you temporarily create this file. Otherwise, the list will run against the SQL database during the time DIR/sql exists.

5.8 Implementation of sublisting with SQL-enabled lists.

For more info on the SQL databases, see sql . In essence, we use one address table for the list and one for the digest list. For each address, a hash and domain is also stored. A ``*_name'' table has entries for sublists with sublist name, domain and hash range they serve. Multiple entries per list are allowed. In addition, there is a message number range and a ``notuse'' flag. These are provided to make it easier to make a new configuration take effect at exactly the same time as the old one ceases to be active. A sublist with an empty domain handles all domains not handled by another list. At least one such sublist is required. If you create an entry for a specific domain, it is your responsibility to make sure that the entire hash range is covered for that domain.

There are similar address databases for ``allow'', ``deny'', and ``mod''. Normally, none of these or only ``allow'' is used. ezmlm programs fall back on normal ezmlm databases, so it is quite possible (and advisable) to use a local ``mod'' database for a moderated list with an SQL table for subscriber addresses.

For each address database there is a ``*_slog'' table that contains one row per transaction, just like DIR/Log.

SQL sublists also use a ``*_cookie'' database. For each message processed by the main list, a pseudo-random ``cookie'' is generated and stored in this table. A header ``X-ezauth:'' with this cookie is added to the message. Sublists will explode a message only if the cookie matches the message number in the table, and if that sublist has not already processed that message. This makes it very hard (not impossible) for an attacker to inject messages at the sublist level.

To distinguish sublist addresses from normal addresses, sublists are added with a hash of 99. Thus, the main list only sends messages to addresses with this hash. The only way to make or remove such an entry is from the command line with:

        % ezmlm-sub -s dir sublist@subhost
This makes it impossible for users to remotely subscribe sublists to each other, or to unsubscribe sublists from the main list.

A hash of 98 also receives special treatment. Both the main list and all sublists send a message to such addresses, after all other addresses. Thus, receipt of such a message (at a remote address) means that all remote subscriber addresses have been tried at least once for this message. Normally, you subscribe a feedback address ``mainlocal-return-receipt@mainhost'' with this hash:

        % ezmlm-sub -r dir mainlocal-return-receipt@mainhost
This address is manned by ezmlm-receipt(1) which logs to the SQL database. A monitoring tool (e.g. supplied in the package) can be used to keep track of sublist function and efficiency.

Other hashed not in the normal (0-52) range can also be used. Messages are never sent to them. Since they cannot be remotely manipulated, their presence prevents normal subscription of the address. Here, you should add addresses that you wish to prevent from subscribing, such as possible aliases of sublist and mainlist. Even if done, the ``X-ezauth'' mechanism will thwart most attempts that we can think of.

5.9 Interpreting the message log.

This is easy. For each combination (listno,msgnum,done) there can be only one entry. Each entry has a time stamp in addition to this information. The first entry will remain and subsequent entries will fail. For any list, ``listno'' is the lowest list number for an entry in the ``*_name'' table which is active for the respective message. When the message arrives at the main list, a cookie is calculated for it, entered into the ``*_cookie'' table, and added to the message. An entry is made for message arrival (done=0) in the message log. When the list has finished processing the message and qmail has returned success, and entry for processed (done=1) is made. If the message processing fails, the message is redelivered, but the ``arrival'' entry first made stands. Thus, this is the time the message first arrived at the list/sublist.

At the end of all subscriber addresses, the return-receipt address is sent to qmail. If this is remote (usually) it will be tried after all (remote) subscriber addresses. Thus, sending it means that all subscriber addresses have been tried at least once. When the return-receipt is received at the main list by ezmlm-receipt(1), this program makes an entry for ``receceived'' (done=2) if the cookie is present and correct. If the message to the sublist bounces, ezmlm-receipt(1) makes an entry for ``bounce'' (done=-1) and saves the bounce in DIR/bounce.

Sublists will bounce messages for which there is already an entry with done of ``processed'' or ``receipt received'' (done > 0). If there is duplication of the message, or on rare occasions when the sublist operating systems misbehaves. Thus, you may have an entry both for ``receipt'' and ``bounce''. In this case, check your setup, but don't worry. The message was delivered ok.

If there is a long delay between ``processed'' on the main list and ``arrived'' on the sublist(s), there is an outgoing qmail problem at the main list, a network problem between the host, an incoming qmail problem at the sublist or possibly a list configuration problem. If there is a long delay between ``arrived'' and ``processed'' the sublist ezmlm-send(1) may be failing due to misconfiguration, memory shortage, etc. Alternatively, the sublist has trouble connecting to the SQL server. A long delay between ``processed'' and ``receipt'' suggests an outgoing qmail failure at the sublist, a network problem between sublist and main list, or an incoming failure at the main list. As ezmlm-receipt(1) at the main list needs to access the SQL server to log receipt, it is also possible that there is a problem in that communication. All this said, problems other than those caused by network outages and list server load are all that we've seen in testing, despite several network hardware failures, memory shortage, etc.

Since bouces can be ignored in the light of other entries, the best way to find out about the status of a message is to look for the entry on the particular sublist and message number with the highest ``done'' code. utils/ in the distribution interprets status codes and presents an overview over the last 10 messages (default). Most table entries are links to message and list statistics and explanatory texts. Depending on your particular lists, you may want to adapt the specific breakpoints in time chosen in the program.

5.10 Optimizing MySQL for ezmlm.

Address SELECTs.

If you run a simple list without sublists, address selection from the SQL table is (almost) a simple dump of all records. This is very fast and does not require any optimization of MySQL irrespective of subscriber number.

For list clusters with sublists, each sublist runs a more complicated SELECT against the database. This uses keys, but does not require sorting. A table JOIN is used, but the second table (``list_name'') is very small, so tuning of join and sort buffers should never be required.

MySQL is extremely efficient as a database for large ezmlm lists. The address table structure is optimized to make it even faster. On a 586-100 Mhz/32 Mb RAM IDE machine (Linux 2.0.34) with a key_buffer of 4M, 12 sublists, 19 ``list_name'' rows, and 200,000 subscribers it takes 100 s for all 12 address queries to complete. This can be improved by a factor of 10 by use of a faster processor, e.g. a Pentium II-266 MHz. More memory will also improve performance as will (if you have it) multiple processors. Assuming that each sublist host can deliver at a sustained rate on 1.5 M deliveries per day, the query time should be compared to the 16 min delivery would take. If you have lists with 100,000-1,000,000 subscribers, you might be able to afford a faster SQL server than the one used by us. Query time seems to scale about linearly with number of subscribers between 100,000 and 400,000 subscribers (not tested beyond, YMMV).

The format chosen where a (set of) list(s) handles all domains not handled by any other list is a trade-off. It leads to more complicated address queries, but without it administration becomes too difficult.

Address addition/removal.

INSERTS/subscribes run at about 400/s when the table is empty, down to 200/s above 150,000 subscribers staying at that speed at least up to 400,000 addresses (PII-266 MHz, 112 Mb RAM/IDE, key_buffer=4M). On the same machine adding addresses to a standard ezmlm database took 42 min for the first 18,000 scaling poorly above 3000 or so. Of course, with normal list use this is not a concern since subscriber address transactions are rare compared with message processing.

The speed can be increased by removing indices, then recreating them after the addition/removal of addresses. However, the address index itself (``a'') is required in the process. Removing the ``d'' and ``h'' indices on our test system increased the speed to 900/s. Indices can then be added in a minute or two (the domain and hash indices are essential for fast SELECTS). See the MySQL documentation ( for more info.

Even if you do not use sublists, a hash and domain are calculated for each address. This overhead is minimal. Addition/removal of addresses also requires updating of 3 indices (address, domain, hash) in the address table which takes some time. The latter 2 are used only for sublisting. Removal of these indices will speed up the process. If you frequently bulk add/remove large numbers of addresses (more than 10,000 or so) it might be worth to delete those indices. If the list does not use sublists, there is no need to recreate these indices.

Tunable MySQL parameters.


No change necessary unless you use the sublist features in combination with very large lists. Lists under 100,000 subscribers should do fine with 4MB, and then increasing by 4MB/100,000 subscribers should yield near maximal performance.


With up to 400,000 subscribers, we have failed to see any impact of increases in join_buffer, record_buffer, etc. However, this may be due to the excellent caching of linux. If you have information on how to improve performance, please E-mail details to

5.11 Maintenance of the MySQL database.

Weekly to monthly error checks on MySQL tables is recommended. Best is to use:

        # isamchk -s -O readbuffer=2M */*.ISM
Other options allow automatic correction of errors, but are dangerous if tables are accessed while isamchk is running.

Other isamchk options allow recovery of space after frequent insert/delete of addresses (can also be done with ``OPTIMIZE TABLE''), key optimization, etc. See the MySQL documentation ( for more info.

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