flush Command in Linux
The Postfix flush service in Linux manages deferred mail delivery. It attempts to deliver emails that have been deferred. The deferred emails are unsent emails held due to the destination server's reachability issue.
The flush service maintains a record of deferred emails by destination. It is useful to improve the performance of SMTP ETRN requests, and its command-line equivalent sendmail -qR or postqueue -f.
Table of Contents
Here is a comprehensive guide to the options available with the flush command −
- Requests Implemented by flush Service
- Implementing flush Service Requests in Linux
- Understanding flush Service Configuration
Requests Implemented by flush Service
The requests the flush service implements are listed below −
| Requests | Description |
|---|---|
| add sitename queueid | It registers the message with a specific queue ID for a destination |
| send_site sitename | It requests the delivery of all the queued mail for a specific destination |
| send_file queueid | It requests the delivery of a specific deferred message |
| refresh | It refreshes the log files and deletes the empty log files |
| purge | It refreshes all the log files |
Implementing flush Service Requests in Linux
In practice, administrators use higher-level Postfix commands to manage deferred mail and log files. Direct interaction with the flush service via these requests is typically handled internally or through the Postfix command set −
Viewing the Queue
To display the contents of the mail queue, use the postqueue command with the -p option −
postqueue -p
A typical output looks like this −
The output displays the queue ID, message arrival time, and sender/recipient information.
Forcing Mail Delivery
To force Postfix to attempt delivery of all deferred mail, use the postqueue command with the -f option −
postqueue -f
Delivering a Deferred Message with a Specified Queue ID
To deliver a deferred message with a specific queue ID, use the -i option with the postqueue command −
postqueue -i 59H95397HE0Q
The -i option implements the traditional sendmail -qI command, by contacting the flush server.
Delivering a Deferred Message for a Specified Destination
To deliver all the deferred messages to a specific destination, use the -s option −
postqueue -s [192.168.28.190]
Understanding flush Service Configuration
Various configuration parameters can be used to modify the behavior of the flush service. Commonly used configuration parameters are listed below −
| Parameter | Description |
|---|---|
| fast_flush_domains | It is an optional parameter to specify the destination domains for the logfile |
| fast_flush_refresh_time | It is used to specify the time after which the non-empty, unread log files are refreshed |
| fast_flush_purge_time | It is used to specify the time after which the empty log files are deleted |
These parameters are specified in the /etc/postfix/main.cf file.
Conclusion
The Postfix flush service in Linux is used to manage the deferred messages. Its equivalent commands are sendmail -qR or postqueue -f. It is specifically used to implement requests to deliver deferred messages by queue ID or destination. Moreover, it is also used to update the flush daemon log files.
In this tutorial, we explained the flush service, the requests it implements, its usage in Linux, and configuration parameters.