When Postfix reports a MySQL connection failure or a fatal “table lookup problem,” the most common cause is not SQL syntax: the Postfix process is chrooted under /var/spool/postfix while MySQL’s Unix socket is outside that jail. A second common trap is hosts = localhost, which makes the mysql_table client use a Unix socket rather than TCP. Test the map with postmap, then either force TCP with 127.0.0.1, place a socket inside the chroot, or disable chroot for the affected Postfix service.
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Start with the lookup, not the mail queue
Postfix usually reaches MySQL through a mysql_table map. The map definition is a small .cf file, commonly stored under /etc/postfix/, that contains the database host, database name, credentials and query. A failure in that map can appear in the mail log as a MySQL connection error followed by a fatal table lookup problem.
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postmap -q <key> mysql:/etc/postfix/<file>.cfThe command prints the first matching value. Add -v when you need verbose connection and query-attempt information:
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postmap -v -q <key> mysql:/etc/postfix/<file>.cfRun the command for every MySQL map used by Postfix, not only the map named in the first error. A domain lookup, mailbox lookup and alias lookup may each use a different .cf file. If a map fails from the shell, fix that map first. If it succeeds from the shell but the daemon still reports a connection failure, the difference in execution environment—most notably chroot isolation—is the leading suspect.
Why the connection fails
Postfix cannot see a socket outside its chroot
Many Postfix services run chrooted beneath /var/spool/postfix. A chroot changes the process’s apparent filesystem root: a path such as /var/run/mysqld/mysqld.sock is looked up inside the jail, not in the host’s ordinary root filesystem. MySQL commonly creates its socket at a location such as /var/run/mysqld/mysqld.sock or /var/lib/mysql/mysql.sock. If that socket is outside the jail, the map client has no file to open, even though MySQL is running and a shell command outside the jail can connect.
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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsThis explains the particularly confusing pattern in which postmap -q works when run by an administrator, while delivery or SMTP handling still fails. The shell command is not necessarily running in the same chroot, with the same path visibility or permissions, as the Postfix daemon.
localhost selects the Unix-socket path
In a mysql_table map, setting hosts = localhost tells the MySQL client to use its default Unix socket. Prefixing the value with inet: does not reliably turn that name into a TCP connection; the hostname still resolves to the socket behavior used for localhost.
To force a TCP connection to the same machine, use the numeric loopback address:
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hosts = 127.0.0.1This distinction matters even when the database and Postfix are on one server. A socket depends on filesystem visibility and socket permissions; TCP depends on a listening address, a port and a database account permitted to connect from the loopback address.
A wrong database name can look like a connection failure
A reported incident traced the symptom to an incorrect database name in a file named mysql-mailbox-domains.cf. Treat that as a configuration check rather than proof of a universal cause: verify the database name, table and query in each map file against the actual MySQL schema. A typo can produce a lookup failure that appears alongside a generic connection message in the mail log.
Choose the fix that fits your installation
| Approach | What changes | Advantages | Costs and risks | Best fit |
|---|---|---|---|---|
| TCP loopback | Set hosts = 127.0.0.1 in each affected MySQL map; make MySQL listen on loopback and allow the map account from that address. |
No socket has to be copied into the jail; survives MySQL socket recreation and is usually the simplest per-map change. | Requires a TCP listener and matching account permissions; verify that MySQL is not bound only to a different address. | Most installations where local TCP is acceptable. |
| Socket inside the chroot | Use hosts = unix:/path/to/socket and make that socket available beneath the Postfix chroot. |
Keeps Unix-socket transport and avoids opening a TCP listener for this lookup. | Hard links or bind mounts require careful permissions and may need to be recreated when MySQL restarts; path handling is distribution-specific. | Environments that require socket transport and can maintain the mount or link. |
| Disable chroot for the affected service | Change that service’s chroot field in master.cf from y to n. |
Lets the daemon see the normal host filesystem, including the existing MySQL socket; no socket plumbing is needed. | Expands the service’s filesystem view and changes the isolation boundary; scope the change to only the service that needs it. | When operational simplicity is more important than chroot isolation. |
The table describes the choices, not a requirement to apply all three. Pick one transport and make the test in the same environment as the daemon before changing anything else.
Fix 1: force a TCP connection over loopback
1. Edit every relevant map
Open each MySQL map referenced by your Postfix configuration and replace a socket-oriented host value with the loopback address:
hosts = 127.0.0.1Do not change only the map named in one log line if other lookups use separate files. The host line must be present in each map that should use TCP. Leave the database name, username, password and query unchanged until connectivity is confirmed; changing several variables at once makes the failure harder to isolate.
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2. Make MySQL listen on loopback
MySQL must have TCP enabled on 127.0.0.1. Set its bind-address to 127.0.0.1 in the server configuration, then restart MySQL using the service-management method provided by your operating system. If the server is bound only to a Unix socket or only to another address, the Postfix map cannot connect to the loopback endpoint.
3. Check the MySQL account’s host permission
MySQL distinguishes the source host used for authentication. The account used by Postfix must be permitted to connect from 127.0.0.1, not merely from localhost as a socket client. Confirm the grant and password for that account before interpreting an authentication error as a Postfix problem.
4. Query the map again
postmap -q <key> mysql:/etc/postfix/<file>.cf
postmap -v -q <key> mysql:/etc/postfix/<file>.cfA successful query returns the value associated with the key. A failed query with verbose output tells you whether the attempt reaches the connection stage or fails later while selecting the database or running the SQL statement. Repeat for each map file.
5. Reload or restart Postfix through your system’s service manager
Once the direct queries work, apply the Postfix configuration using the reload or restart operation appropriate to your distribution. Then submit a controlled test message and watch the mail log. If the map works from the shell but the daemon still fails, do not undo the TCP setting immediately: check whether the affected service remains chrooted and whether its map file is readable inside that environment.
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Fix 2: expose a Unix socket inside the chroot
The mysql_table syntax supports an explicit Unix-socket host:
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hosts = unix:/path/to/socketFor a chrooted daemon, the path must exist from the daemon’s point of view. If the host’s socket is /var/run/mysqld/mysqld.sock, an equivalent socket location must be present under the Postfix jail, for example beneath /var/spool/postfix/var/run/mysqld/. The exact path and mount layout vary by operating system; verify which directory is the chroot root before editing the map.
Keep the socket available after database restarts
A hard link or bind mount can make the host socket visible inside the jail. Both approaches require the target directory to exist and the Postfix process to have permission to traverse the directories and open the socket. A socket file is recreated when MySQL restarts, so a one-time link may become stale or disappear. A bind mount generally avoids changing the socket inode, but it still has to be established early enough for Postfix to use it and kept across boots.
Because the maintenance procedure is platform-specific, verify the arrangement after every MySQL restart and after a system reboot. If the direct postmap test succeeds outside the jail but fails when run in the daemon’s filesystem view, the in-chroot path or its permissions are still wrong.
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Unix sockets are protected by filesystem ownership and mode bits. The account that runs the Postfix lookup must be able to reach the socket and its parent directories. Avoid making the whole MySQL runtime directory broadly writable merely to satisfy the lookup; correct ownership, group membership or a narrowly scoped mount is safer.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Fix 3: disable chroot for one Postfix service
Postfix records per-service settings in master.cf. The chroot column is normally y or n. For the service that performs the failing MySQL lookup, change only that column from y to n, preserving the service name, command, user and other fields. Do not disable chroot globally when one service is sufficient.
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After saving master.cf, reload or restart Postfix with the service manager for your operating system and repeat the map query and a controlled mail test. This option gives the service access to the host’s normal filesystem, so the existing MySQL socket path can remain unchanged. The trade-off is a wider filesystem view than the chrooted arrangement; evaluate that change against your local isolation requirements.
Diagnose the remaining failures
| Observed result | Most likely explanation | Next check |
|---|---|---|
postmap -q fails immediately with a socket-related message. |
The host value selects a Unix socket that is absent, outside the current root, or inaccessible. | Inspect hosts; choose 127.0.0.1 for TCP or provide the socket inside the chroot. |
| The map works outside the jail but Postfix logs a lookup failure. | The daemon’s chroot cannot see the socket or map file. | Test the path from the daemon’s filesystem view, then use TCP, mount the socket inside the jail, or set the service chroot field to n. |
| TCP is selected but the connection is refused. | MySQL is not listening on 127.0.0.1 and its configured port. |
Check bind-address, restart MySQL, and verify the listener before changing Postfix SQL. |
| TCP reaches MySQL but authentication is denied. | The map account is not granted for source host 127.0.0.1, or its credentials are wrong. |
Correct the account’s host grant and credentials, then rerun postmap -v -q. |
| Connection succeeds but the lookup still reports a table problem. | The database name, table, column names or query in the .cf file is wrong. |
Compare the map’s database name and SQL with the actual schema; a reported case involved a wrong database name. |
| One map works and another fails. | The files do not share identical host or database settings. | Run a separate postmap -q test for every referenced map and correct only the failing file. |
Security and maintenance trade-offs
TCP loopback
Loopback TCP keeps traffic on the local machine and avoids the socket-visibility problem. It does, however, require MySQL to listen on a TCP address and requires an account grant for 127.0.0.1. Keep the bind address limited to loopback when remote database access is not needed.
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An in-chroot socket
A socket avoids a TCP listener for this path and can preserve a strict local-only design. Its weakness is lifecycle maintenance: the socket is recreated by MySQL, while a hard link or mount may not be. Treat the mount or link as part of system startup and database-restart operations, and monitor it like any other dependency.
Removing the chroot
Disabling chroot is often the shortest operational fix because it removes the path-visibility mismatch. It also changes the service’s isolation boundary. Limit the change to the affected service and document why it is required so a later hardening change does not silently re-enable the original failure.
A repeatable recovery sequence
- Identify the exact MySQL map named by the Postfix error and locate its
.cffile. - Run
postmap -qfor a known key, then repeat with-vif it fails. - Inspect the map’s
hostsvalue. Treatlocalhostas a Unix-socket request, not as guaranteed TCP. - Verify the database name and query, especially if the failure is reported as a table lookup problem.
- Choose one remedy: set
hosts = 127.0.0.1, provide an explicit in-chrootunix:socket, or set the affectedmaster.cfservice’s chroot field ton. - For TCP, confirm MySQL’s loopback bind and the account grant for
127.0.0.1. - Rerun the map query for every related
.cffile. - Reload or restart Postfix, send a controlled test message, and check the mail log for a successful lookup.
- After a MySQL restart or system reboot, retest socket-based arrangements; a recreated socket can invalidate a hard-link setup.
The practical diagnosis is straightforward once transport and filesystem scope are separated: a map that works from an unrestricted shell is not proof that a chrooted Postfix daemon can reach the same MySQL endpoint. Make the endpoint visible to the daemon, or deliberately move the map to loopback TCP, and verify the result with postmap before returning mail traffic to normal.
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