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User scripts
While Asuswrt-Merlin only adds a limited number of new features over the original firmware, a lot of customizations can be achieved through the use of user scripts. These will allow you to set up custom firewall rules, create jobs that can be run at scheduled intervals, or start new services.
Those scripts are stored in the internal non-volatile flash in the JFFS partition. Support for these scripts must be enabled, under Administration -> System on the webui.
These shell scripts will be run when certain events occur. They must be saved in /jffs/scripts/.
These scripts will usually run in parallel to the event itself, except when explicitly documented as being a blocking script. In that case, the script will prevent the execution of the event itself until either the script completes, or it times out (current default timeout value is 120 seconds).
Called after all other system services have been started at boot. This is the best place to stop one of these services and restart it with a different configuration, for example. (But be aware that anytime the service gets manually restarted it will revert back to the original setup.)
Called before all system services are stopped, usually on a reboot.
Called before a service event is called (i.e. restart_httpd, restart_wireless, etc...). First argument is the event (typically stop, start or restart), second argument is the target (wireless, httpd, etc...). This is a blocking script, meaning that it will prevent the execution of the event itself until the script either completes or times out.
Called after the WAN interface came up. Good place to put scripts that depend on the WAN interface (e.g. to update an IPv6 tunnel or a dynamic DNS service). The Internet connection is unlikely to be active when this script is run. Add a sleep line to delay running until the connection is complete, or loop until your command succeeds.
Called after the firewall got started and filtering rules have been applied. This is where you will usually put your own custom rules in the filter table (but not the NAT table). The script receives the WAN interface name (e.g. ppp0) as an argument which can be used in the script using $1.
Called after NAT rules (i.e. port forwards and such) have been applied to the NAT table. This is where you will want to put your own NAT table custom rules (e.g. a port forward that only allows connections coming from a specific IP).
Called right after JFFS got mounted, and before any of the services get started. This is the earliest part of the boot process where you can insert something.
Called just before a partition gets mounted. This is run in a blocking call and will block the mounting of the partition for which it is invoked until its execution is complete or it times out. This is done so that it can be used for things like running e2fsck on the partition before mounting. This script is passed the device path being mounted (e.g. /dev/sda1) as an argument which can be used in the script using $1. Since firmware 384.11 a second argument is passed ($2) that contains the filesystem type (e.g. ext3).
Called just after a partition got mounted. The script is passed the mount point (the filesystem path where the partition was mounted, e.g. /tmp/mnt/OPT) as an argument which can be used in the script using $1.
Called just before unmounting a partition. Like pre-mount, this is a blocking script, so be careful with it. The script is passed the mount point as an argument which can be used in the script using $1.
Called whenever a DHCP event occurs on the WAN interface. The type of event is passed as an argument which can be used in the script using $1; possible event types in the version of udhcpc in ASUSWRT are deconfig (when udhcpc starts and when a lease is lost), bound (when a lease and new IP address are acquired), and renew (when a lease is renewed, but the IP did not change).
Called whenever an OpenVPN server gets started/stopped, or an OpenVPN client connects to a remote server. Uses the same syntax/parameters as the "up" and "down" scripts in OpenVPN.
Called at the end of a DDNS update process. This script is also called when setting the DDNS type to "Custom". The script gets passed the WAN IP as an argument, which can be used in the script using $1. When handling a "Custom" DDNS, this script is also responsible for reporting the success or failure of the update process. See the Custom DDNS section for more information.
Called when the scheduled new firmware version availability check detects there's a new firmware available for download. See update notification example for more info.
Note that in addition to these, you can also use the numerous postconf scripts supported by the firmware, which allow you to execute a script between the moment a service's config file is generated and the service is about to be executed. See the Postconf scripts section for more information.
Don't forget to set any script you create as being executable:
chmod a+rx /jffs/scripts/*
And like any UNIX script, they need to start with a shebang:
#!/bin/sh
Also, you must save files with UNIX line endings. Note that Windows's Notepad cannot save with UNIX line endings; use Notepad++ instead. You can also directly edit them on the router through SSH, by using vi or nano, both included in the firmware; they will create files with the proper line endings.
Try running your script manually at first to make sure there is no syntax error in it. You can also insert some code near the top to be able to easily determine whether or not your script ran. For example:
touch /tmp/000wanstarted
You can then easily tell that the script did run by looking for the presence of 000wanstarted in the /tmp directory.
A useful command for debugging user scripts is logger, which will log messages to the system log, visible in the Web UI.