Creating root-level directories and symbolic links on macOS Catalina

One of the changes which came with macOS Catalina was the introduction of a read-only root volume for the OS. For users or environments which were used to using adding directories to the root level of the boot drive, this change meant they could no longer do that.

To address this need, Apple added a new method for creating directories at the root level which leverages Apple File System’s new firmlink functionality. Firmlinks are new in macOS Catalina and are similar in function to Unix symbolic links, but instead of only allowing travel one way (from source to destination) firmlinks allow bi-directional travel.

The use of firmlinks is exclusively reserved for the OS’s own use, but Apple has also made available what are called synthetic firmlinks. These synthetic firmlinks are how the OS enables folks to create directories and symbolic links on the read-only boot volume. For more details, please see below the jump.

To create a synthetic firmlink, you need to do the following:

1. Create a file in the /etc directory named synthetic.conf.
2. Make sure /etc/synthetic.conf has the following permissions:

• root: read, write
• wheel: read
• everyone: read

3. In /etc/synthetic.conf, define the name(s) of the empty directory or symbolic link you want to have appear at the root level.

4. After all desired entries have been made, save the /etc/synthetic.conf file.

5. Restart the Mac to apply the changes.

For example, /etc/synthetic.conf may look like this:

	# create an empty directory named "foo" at / which may be mounted over
	foo
	# create a symbolic link named "bar" at / which points to
	# "System/Volumes/Data/bar", a writeable location at the root of the data volume
	bar System/Volumes/Data/bar
	# create a symbolic link named "baz" at / which points to "Users/me/baz"
	baz Users/me/baz

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synthetic.conf

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Note: In those cases where you’re creating a symbolic link and are including a path, the start point for the directory path is not /. Instead, it is the next directory level down.

To show how this works, I’ve created a directory containing installer packages located at /Users/Shared/installers.

To create a symbolic link at the root level named installers which points to /Users/Shared/installers, I would do the following:

1. Create the /etc/synthetic.conf file if it didn’t already exist.
2. Add the following entry to the /etc/synthetic.conf file:

installers	Users/Shared/installers

3. Reboot the Mac.

Note: Whomever designed this came down on the “tabs” side of the “tabs vs. spaces” debate. When creating the separation between installers and Users/Shared/installers in the /etc/synthetic.conf file, you need to use tabs. If you use spaces instead, the synthetic firmlink won’t be created.

After the reboot, you should see a symbolic link named installers at the root level of the boot volume. When you navigate to it, you should see the contents of /Users/Shared/installers.

To remove the symbolic link, remove the relevant entry from /etc/synthetic.conf and then restart. After the reboot, the installers symbolic link should be missing from the root level of the boot volume.

For more information, please see the synthetic.conf man page. This is available by entering the following command in Terminal on macOS Catalina:

man synthetic.conf

	SYNTHETIC.CONF(5) BSD File Formats Manual SYNTHETIC.CONF(5)
	NAME
	synthetic.conf
	SYNOPSIS
	synthetic.conf — synthetic symbolic link and directory manifest
	DESCRIPTION
	synthetic.conf describes virtual symbolic links and empty directories to
	be created at the root mount point. Because the root mount point is read-
	only as of macOS 10.15, physical files may not be created at this loca-
	tion. All writeable paths must reside on the data volume, which is
	mounted at /System/Volumes/Data.
	synthetic.conf provides a mechanism for some limited, user-controlled
	file-creation at /. The synthetic entities described in this file are
	synthesized by the kernel during early system boot. They are not physi-
	cally present on the disk, but when the system is booted, they behave as
	if they were within certain parameters.
	synthetic.conf is intended to be used for creating mount points at /
	(e.g. for use as NFS mount points in enterprise deployments) and symbolic
	links (e.g. for creating a package manager root without modifying the
	system volume). synthetic.conf is read by apfs.util(8) during early sys-
	tem boot.
	FORMAT
	synthetic.conf specifies a single synthetic entity per line. Each line
	may have one or two columns, separated by a tab character. If a line has
	a single column, it denotes a virtual empty directory to be created at /.
	If a line has two columns, it denotes a symbolic link at / whose link
	target is given in the second column.
	In either case, the first column denotes the name of the entity to be
	created at /.
	A line beginning with the # character indicates a comment that is not
	parsed.
	SYNTHETIC ENTITIES
	Synthetic entities may not be deleted at runtime. In order to delete a
	synthetic entity, it must be removed from synthetic.conf, and the host
	must be rebooted.
	New files and directories may not be created within a synthetic empty
	directory.
	EXAMPLES
	# create an empty directory named "foo" at / which may be mounted over
	foo
	# create a symbolic link named "bar" at / which points to
	# "System/Volumes/Data/bar", a writeable location at the root of the data volume
	bar System/Volumes/Data/bar
	# create a symbolic link named "baz" at / which points to "Users/me/baz"
	baz Users/me/baz
	FILES
	/etc/synthetic.conf
	SEE ALSO
	apfs.util(8) shutdown(8) reboot(2)
	Darwin/macOS 2 July 2019 Darwin/macOS

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synthetic_conf.txt

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