Mount Up: Regulate Root-Level Automounts With Synthetic.conf
The rhythm is the mount, and the mount is the trouble
macOS Automounts with synthetic.conf
One of the more interesting administration changes for macOS is the /etc/synthetic.conf file — a tool that gives you control over creating “virtual” directory mounts in the root filesystem. Whether you’re mapping an SMB share, adding an NFS mount, or creating a placeholder directory, this utility offers power, flexibility, and some unexpected quirks.
What Exactly is synthetic.conf Used For?
The synthetic.conf file allows macOS users to define non-existent directories or symbolic links in the root filesystem. The system processes this file during boot, making it ideal for creating mounts for network shares like SMB or NFS. It’s a lightweight way to automate mounting without relying on external scripts or tools.
However, like any good story, there’s a twist: changes to synthetic.conf require a system reboot to take effect. Yes, even in 2025, we still reboot computers for things like this. Consider it macOS’s way of saying, “Don’t forget your coffee break.”
What You Need to Know
1.Reboot Required
Unlike /etc/fstab on Linux, changes to synthetic.conf aren’t dynamically loaded. You’ll need to reboot the system for macOS to recognize your modifications. No exceptions. Yes, it’s inconvenient, but it’s also a great excuse grab some coffee.
2.Path Limitations
The directories you define are limited to the root filesystem. If you’re trying to get creative and define paths deep within /Volumes or /Users, this isn’t the tool for you. Instead, define the mounts like you normally would. You can, however, use the automount information on this page.
3.Permissions Confusion
Permissions for your mounted share absolutely depend on how you configured the mount. Test it to make sure your users have the correct access. I suggest using the Finder “Connect to Server” (Command-K) to test connectivity before going too deep.
Example: Automatically Mounting an SMB Share to /external
Here’s a real world example: let’s say you have a Synology NAS at 192.168.4.20 with an SMB share called shared\_folder. You want it to be accessible at /external on your Mac. Here’s how to configure it:
Step 1: Update synthetic.conf
Edit (or create) the file /etc/synthetic.conf with the following content:
external
This tells macOS to create a directory called /external at boot.
Step 2: Create an Automount Configuration
Next, create a /etc/auto\_smb file with the automount configuration:
/external -fstype=smbfs ://username:password@192.168.4.20/shared\_folder
Replace username and password with the credentials to access your SMB share. You might need URL encoding for special characters if necessary.
Step 3: Update /etc/auto_master
Append the following line to /etc/auto\_master:
/- /etc/auto\_smb
This tells macOS to use the custom auto\_smb file for mounting directories.
Step 4: Reload Automount and Reboot
Finally, reload automount to test and reboot the system to apply the changes:
sudo automount -vc
sudo reboot
Once your system is back online, your SMB share should be mounted at /external.
Closing Thoughts
While synthetic.conf and coupling with automount is excellent for network shares, they are different than just setting the fstab and calling it good. Some patience—and maybe some trial and error—you’ll have your macOS system humming along with network shares seamlessly integrated.
And hey, if nothing else, the reboot requirement might remind you that sometimes a fresh start is exactly what you need—both for your system and your day.
