Cyrus Murder: Installation and Administration

Architecture

Overall structure of a Cyrus Murder.

The Cyrus Murder provides the ability to horizontally scale your Cyrus IMAP environment across multiple servers. It is similar in nature to various proxy solutions such as nginx or perdition with the difference being that Murder offers a uniform namespace. Those not currently using shared mailboxes and who don’t intend to use shared mailboxes in the future, should probably just consider using a simple proxy solution.

Before beginning Cyrus Murder configuration, we strongly recommended a thorough review of the Cyrus Murder Concepts guide.

Terminology

Master
In this document, Master always means the MUPDATE master server.

Installation

Cyrus IMAPd must be built with the --enable-murder configure option. This builds the proxyds and the associated utilities.

Note

Those using their distribution’s packages may need to install a separate package for aggregation support. For example, on Debian and derived distros, install the cyrus-murder package.

Requirements

  • At least one Cyrus IMAP server instance. If there are more than one, their namespaces must not conflict. That is, all the mailbox names must be unique (or in different namespaces)
  • At least one machine that will become the first Frontend Server.
  • One machine to become the MUPDATE Master server. This can be the same as one of the frontend servers.

Configuring the MUPDATE Master

The MUPDATE Master server needs to be running the mupdate service in master mode. The MUPDATE master may be one of the cluster’s frontend instances, in which case no slave mupdate process should be run on this instance.

On the mupdate master cyrus.conf(5) must include a line similar to the following in the SERVICES section:

mupdate       cmd="/usr/cyrus/bin/mupdate -m" listen=3905 prefork=1

Note the -m option to tell mupdate that it should start in master mode.

The MUPDATE Master will also need at least a skeleton imapd.conf(5) that defines the config directory, a bogus partition-default and the admins who can authenticate to the server. Slave mupdate servers as well as the back end servers will need to be able to authenticate as admins on the master.

Here is a very simple imapd.conf(5) for a master server:

configdirectory: /imap/conf
partition-default: /tmp

admins: mupdateslave1 backend1

SASL must also be configured as needed to properly allow authentication.

Setting up the backends to push changes to the MUPDATE Master

On the backends, configuration to be a part of a murder is easy. Simply set the mupdate_server option in imapd.conf(5) and add an entry to cyrus.conf(5) to push the mailboxes list to the MUPDATE Master.

Depending on the authentication mechanisms used, some or all of the following settings in imapd.conf(5) may be required:

  • mupdate_username
  • mupdate_authname
  • mupdate_realm
  • mupdate_password
  • servername

Once these settings are made, any mailbox operation on the backend will be sent to the mupdate master for confirmation and entry into the mupdate database.

At least one user/group must be configured using the proxyservers imapd.conf(5) option. This user should not be an administrator, as that would give anyone compromising this credential full administrative control on all back ends.

Note

For lmtp to work in a murder, the proxyservers entries must also appear in the lmtp_admins entry.

Example of the imapd.conf(5) settings discussed thus far:

# How this server identifies itself within the murder
servername: mailbox.example.com
# Who's permitted to authenticate for which purposes
admins: cyrus
proxyservers: mailproxy
lmtp_admins: mailproxy
# Auth credentials for MUPDATE Master
mupdate_server: postman.example.com
mupdate_username: postman
mupdate_authname: postman
mupdate_password: <secret>

All proxy user(s) must exist within the authentication domain of both the MUPDATE Master and the back end, as well.

Note

Do not set proxyservers on frontends.

Exporting the database from the backend

The existing mailboxes database must be exported to the MUPDATE Master. Use the ctl_mboxlist(8) command to do so. For the first synchronization, change to the cyrus user, and run ctl_mboxlist -m.

Important

One should use ctl_mboxlist -mw (dry run) first to be sure of understanding all the operations that this command will perform, since it does require that all mailboxes are unique in the murder namespace and could lead to deletions of conflicting mailboxes on other back ends already in the murder.

If everything is configured properly, the mailbox database of the current host will upload to the mupdate master. If there are problems, the most likely cause is a misconfiguration of the authentication settings, or mupdate(8) might not be running on the master. Using mupdatetest(1) may be helpful in this case (it establishes an authenticated connection to the mupdate server, if it can).

It is also useful to have the backends automatically resync the state of their local mailboxes database with the master on start up. This is configured by adding the following to the START section of cyrus.conf(5) on the backends:

mupdatepush   cmd="ctl_mboxlist -m"

This will perform synchronization with the mupdate master each time the backend restarts, bringing the mupdate database up to date with the contents of the backend (and performing ACTIVATE and DELETES as needed to do so).

Warning

If somehow a mailbox exists on two (or more) backend servers, each time one of them synchronizes its database that backend server will become authoritative. Though this should not happen during normal operation of the murder (because of the consistency guarantees of the MUPDATE protocol, and the fact that mailbox operations are denied if the mupdate master is down), it is possible when first creating the mupdate database or when bringing a new backend server into the murder.

Configuring the front ends

Configuring the front ends is a two step process. First, define mupdate_server (and friends) as done for the backends above. However, as the frontends only talk to the mupdate master via a slave running on the local machine, also set up a slave on the same machine, in the SERVICES section of cyrus.conf(5), like so:

# mupdate database service - must prefork at least 1
mupdate       cmd="mupdate" listen=3905 prefork=1

As this is a threaded service, prefork at least 1 so that the database synchronizes at startup. Otherwise, the service will not start running until after receiving a mupdate client connection to the slave (which is not a recommended configuration at this point).

The front end SERVICES section should now look like this:

mupdate       cmd="mupdate" listen=3905 prefork=1

imap          cmd="imap" listen="imap" prefork=5
imaps         cmd="imap -s" listen="imaps" prefork=1
pop3          cmd="pop3d" listen="pop3" prefork=0
pop3s         cmd="pop3d -s" listen="pop3s" prefork=0
kpop          cmd="pop3d -k" listen="kpop" prefork=0
nntp          cmd="nntpd" listen="nntp" prefork=0
nntps         cmd="nntpd -s" listen="nntps" prefork=0
http          cmd="httpd" listen="http" prefork=0
https         cmd="httpd -s" listen="https" prefork=0
sieve         cmd="timsieved" listen="sieve" prefork=0
lmtp          cmd="lmtpd" listen="/var/imap/socket/lmtp" prefork=0

Note that timsieved does not need a proxy daemon, the managesieve protocol deals with the murder with referrals to the backends internally.

Additionally, entries in imapd.conf(5) are required to indicate the proxy auth name and passwords (if using a SASL mechanism that requires them) to the backends.

For example, if the backends are mail1.andrew.cmu.edu and mail2.andrew.cmu.edu with passwords of foo and bar, and an auth name of mailproxy:

mail1_password: foo
mail2_password: bar
proxy_authname: mailproxy

For SASL mechanisms not using authnames or passwords (e.g. KERBEROS_V4), the password options are not required. Note the use of the same authname as configured in the proxyservers line of the backend’s imapd.conf(5) above.

Upon starting master(8) on the frontend, the local mailboxes database should automatically synchronize with the contents of the MUPDATE master, and it’s ready to go. Clients should connect to the frontends, and the frontends will proxy or refer as applicable to the backend servers.

Additional backend configuration

Authentication system expecting usernames, passwords, etc, to authenticate, will also need to specify proxy_authname (and friends) in the backend imapd.confs. This is so the backends can authenticate to each other to facilitate mailbox moves. (Backend machines will need to be full admins).

Delivering mail

To deliver mail to a Murder, configure MTAs just as before, but instead of connecting directly to lmtpd on a back end, they should connect to lmtpproxyd on any front end. Remote MTAs may connect to the lmtpproxyd running on any front end machine (listening on a TCP socket), or install master and lmtpproxyd on your SMTP servers to connect via Unix domain socket.

Administration

Keeping the database synced

Consistency in the database is maintained by pushing the current status of the backends to the master, and having the frontends stay up to date with the master’s database. Since the frontends resync themselves entirely when they startup, downtime should not be a problem. (While they are up they should be continuously receiving database updates, as well when they lose connection to the master, they will try to reconnect and resync their database upon reconnection)

Provided that the namespace of the backend servers is kept discrete (with no mailboxes existing on the same server), it is not a big deal to resync the mupdate master using ctl_mboxlist -m. If two servers do have the same mailbox, this will need to be resolved before database consistency can be guaranteed.

Moving Mailboxes between backends

There is currently no 100% foolproof way to do this, however, if you issue a rename command to a frontend (as you would to move a mailbox between partitions), and replace the partition name with the name of the new backend, it will move the mailbox to the indicated backend. You can also use the format backend.domain.com!partition to move to a specific partition (otherwise the default partition will be used).

In cyradm, this looks like:

cyrus.andrew.cmu.edu> rename user.bcyrus user.bcyrus mail2.andrew.cmu.edu!u2

Note that since seen state is stored per-user, it is possible that when moving a shared mailbox users will have strange effects. The general rule is that moving an INBOX will move the entire user (including all sub-mailboxes to the INBOX, and seen state, and subscriptions, and sieve scripts, etc). The seen state is merged with the seen state on the new backend, so that no data is lost (seen state is also the only part left behind on the source backend). In the case of any other mailbox, however, only that individual mailbox is moved. If it is a quota root, the new quota root is instantiated on the new server, but otherwise quotas can appear to be violated, since each backend only takes care of its own quota.

In general, it’s better to leave trees of mailboxes on the same server, and not move submailboxes of inboxes between servers.

Adding additional backend servers

This is very easy to do, simply configure an empty backend server and set its mupdate_server parameter to point at the mupdate master. Then, issue mailbox creates to it as you would any other backend server.

Distributing Mailboxes between Back Ends

Several options exist within imapd.conf(5) to aid in the distribution of new users and mailboxes within a murder; across servers and partitions. We recommend exploring these:

partition_select_mode: freespace-most

Partition selection mode.

random
(pseudo-)random selection
freespace-most
partition with the most free space (KiB)
freespace-percent-most
partition with the most free space (%)
freespace-percent-weighted
each partition is weighted according to its free space (%); the more free space the partition has, the more chances it has to be selected
freespace-percent-weighted-delta

each partition is weighted according to its difference of free space (%) compared to the most used partition; the more the partition is lagging behind the most used partition, the more chances it has to be selected

Note that actually even the most used partition has a few chances to be selected, and those chances increase when other partitions get closer

Allowed values: random, freespace-most, freespace-percent-most, freespace-percent-weighted, freespace-percent-weighted-delta

partition_select_exclude: <none>

List of partitions to exclude from selection mode.

partition_select_usage_reinit: 0

For a given session, number of operations (e.g. partition selection) for which partitions usage data are cached.

partition_select_soft_usage_limit: 0

Limit of partition usage (%): if a partition is over that limit, it is automatically excluded from selection mode.

If all partitions are over that limit, this feature is not used anymore.

serverlist: <none>

Whitespace separated list of backend server names. Used for finding server with the most available free space for proxying CREATE.

serverlist_select_mode: freespace-most

Server selection mode.

random
(pseudo-)random selection
freespace-most
backend with the most (total) free space (KiB)
freespace-percent-most
backend whose partition has the most free space (%)
freespace-percent-weighted
same as for partition selection, comparing the free space (%) of the least used partition of each backend
freespace-percent-weighted-delta

same as for partition selection, comparing the free space (%) of the least used partition of each backend.

Allowed values: random, freespace-most, freespace-percent-most, freespace-percent-weighted, freespace-percent-weighted-delta

serverlist_select_usage_reinit: 0

For a given session, number of operations (e.g. backend selection) for which backend usage data are cached.

serverlist_select_soft_usage_limit: 0

Limit of backend usage (%): if a backend is over that limit, it is automatically excluded from selection mode.

If all backends are over that limit, this feature is not used anymore.

Backups

Gotchas

Clients dealing with a pool of frontend servers
Some clients may not be terribly efficient caching connections to a pool of imap servers, this isn’t a problem, as such, but it may mean that you will see many more authentications than you are used to.
Kerberos issues
If you are using kerberos authentication, you will want to ensure that all your machines are keyed properly, as we have seen problems with different clients trying to authenticate to different services (e.g. imap.imap-pool instead of imap.pool-frontend-1), so test the clients in use in your environment and be sure that they work with whatever keying scheme you use.
Clients dealing with referrals
Some clients (we’ve had particular trouble with pine, though most of these issues have now been resolved and new versions should be OK (that is, pine > 4.44), but as referrals have not been extensively used by any IMAP server until now, referrals are very likely to not work correctly or have surprising effects.
Clients dealing with getting a NO on LSUB commands
Some clients (Outlook, for example) may behave poorly if an LSUB command returns a NO, which may be the case if the backend server with the user’s inbox is down. We have, for example, seen this result in the deletion of the disconnected message cache.
Behavior of cyradm / some mailbox operations
The behaviour of some administrative commands might be slightly unexpected. For example, you can only issue a SETQUOTA to a frontend server if the entire mailbox tree underneath where you are setting the quota exists on the same backend server, otherwise you will need to connect directly to the backend servers to perform the needed changes. Similarly, mailboxes will be created on the same backend server that their parent is in. In order to create them on a different server (or to create a new top level mailbox) you will need to connect directly to the desired backend server.
Subscriptions
If users want subscribe to a mailbox other than on their backend home server, they won’t be able to, unless you set allowallsubscribe: t in the backend imapd.confs. This essentially lets any string be subscribed to successfully.
Restarting the mupdate master
Because ctl_cyrusdb -r clears reservations on mailbox, if you restart the mupdate master (and run recovery), then this could (we suspect, very rarely) lead to inconsistencies in the mupdate database.

Troubleshooting

Mailbox operations are being denied
This is an indication that the mupdate master may be down. Restart it.
Mailbox operations are not being seen by one or more frontends
This indicates that the mupdate process on a slave may have died, you may need to restart master. Alternatively, mupdate will retry connections every 20 seconds or so for about 20 attempts if the master does go down.
A frontend’s mailboxes.db is corrupt or out of sync
Restart master on the frontend, and have the mupdate process resynch the local database. You may need to remove the local mailboxes database if the corruption is extreme.
A mailbox’s location keeps switching between two (or more) backend hosts.
It probably actually exists on both hosts. Delete the mailbox from all but one of the hosts, and run a ctl_mboxlist -m on the one where you want it to actually live.
Databases are never created on the frontends/slaves
Check to ensure that the mupdate slave process is started, (is prefork=1)