How does automount work linux

If neither the direct or offset mount options are given so the mount is considered to be indirect , then the root directory is always a regular directory, otherwise it is a mount trap when it is empty and a regular directory when not empty. Note that direct and offset are treated identically so a concise summary is that the root directory is a mount trap only if the filesystem is mounted direct and the root is empty. Directories created in the root directory are mount traps only if the filesystem is mounted indirect and they are empty.

Directories further down the tree depend on the maxproto mount option and particularly whether it is less than five or not. When maxproto is five, no directories further down the tree are ever mount traps, they are always regular directories. When the maxproto is four or three , these directories are mount traps precisely when they are empty. So: non-empty i. Empty directories are sometimes mount traps, and sometimes not depending on where in the tree they are root, top level, or lower , the maxproto , and whether the mount was indirect or not.

Any directory provided by a filesystem can be designated as a trap. This involves two separate features that work together to allow autofs to do its job. The task of this method is to find the filesystem that should be mounted on the directory and to return it. The VFS is responsible for actually mounting the root of this filesystem on the directory. This functionality is sufficient for some users of mount traps such as NFS which creates traps so that mountpoints on the server can be reflected on the client.

However it is not sufficient for autofs. For that purpose there is another flag. It may return one of three things:. A return value of zero indicates that there is nothing special about this dentry and normal checks for mounts and automounts should proceed. This avoids races. How it determines this is discussed later.

This allows the automount daemon not to get caught in the mount trap. There is a subtlety here. It is possible that a second autofs filesystem can be mounted below the first and for both of them to be managed by the same daemon. It must only return it when a mount has been requested, but has not yet completed.

An RCU-walk is a fast and lightweight process for walking down a filename path i. Its sole purpose is to determine if it would be safe to follow down into any mounted directory and the only reason that it might not be is if an expiry of the mount is underway.

However, if you need to authenticate before accessing the Samba share, automount will not function. Wildcard characters Let's say you have a directory with a number of subdirectories which you want to have auto-mounted individually.

You can also use variables see autofs 5 man page to substitute users and other parameters to be able to create generic file for multiple users. Mounting Other Types of Files Systems 6. The file-system location specifies the user-space binary used to mount the file system, followed by a hash , followed by the location. The entire location must be preceded by a colon :. You should have already set-up password-less authentication via public key encryption.

Be sure that you understand the security implications of this before proceeding. Specify the user-name used to connect To test mounting your SSHFS file system as root issued the following: sudo sshfs user example. Debugging Auto Mount Problems If you are having trouble automounting your file systems, it may be useful to run automount in the foreground.

In addition, the parameter contains the actual path of the autofs filesystem that you are trying to open as a string.

In practice this ioctl is issued once for each autofs filesystem that the user space daemon is managing. As you can see, the ioctlfd field is -1 on entry to the ioctl and is filled out by the kernel. Furthermore, we can confirm with lsof that the file descriptor is now open for the directory. This ioctl is issued to signal that the user space daemon no longer needs to issue ioctls for a given autofs filesystem.

For this ioctl, there are no additional details passed in a structure. The only piece of information needed is the ioctlfd that the daemon is requesting to be closed. In practice, this ioctl is used to close down the temporary autofs filesystem that is used during early initialization to check the various version information.

Otherwise it only appears to be used when shutting down automount. This ioctl is one of two ioctls issued in response to a request from the kernel. This particular ioctl signals that the request processing was successful and the kernel can continue. This contains the same token that was received from the kernel on the pipe where the request originated. This is how the kernel can match a response to its initial request along with the fact that the ioctlfd means its for a particular autofs filesystem.

In practice, this ioctl is used all the time. Every time the user space daemon successfully handles automounting a filesystem, it communicates it to the kernel via this ioctl. This ioctl is the second of the two ioctls issued in response to a request from the kernel.

This command is similar to the ready command described above, but indicates failure. This status code is what the kernel should return to the process that requested the access. Its worth pointing out that the kernel uses negative numbers to communicate errors and therefore the status returned is -2 or -ENOENT.

This ioctl can be used to set the pipe file descriptor that the kernel will use to send notifications to the user space daemon. The other option would be to pass the file descriptor during the actual call to mount 8. This ioctl only makes sense when you already have a valid ioctlfd to issue it against.

If the automount daemon was restarted and some of its autofs filesystems that its supposed to be managing are shadowed by the real mounts and therefore still mounted , there is no way to establish this pipe file descriptor. This is the purpose of this ioctl. Additional data from autofs4-mount-control. On success the call also sets the process group id used to identify the controlling process eg. This ioctl is issued against a specific autofs filesystem and therefore you must have a valid ioctlfd to use before it can be issued.

Requests are not dispatched to the user space daemon and all processes are allowed to see the raw filesystem not just the process id that was given as the pgrp mount option when the filesystem was mounted. More details from autofs4-mount-control. In order to protect mounts against incorrectly setting the pipe descriptor we also require that the autofs mount be catatonic see next call. Observe after re-mounting option ro changed to rw.

Now, the file system is mounted as read-write and now you can write files in it. Note : It is recommended to fsck file system before re mounting it.

You can check file system by running fsck on its volume. Autofs is a client-side service that automatically mounts the appropriate file system. When a client attempts to access a file system that is not presently mounted, the autofs file system intercepts the request and calls automountd to mount the requested directory. Most people use the synchronous option on the NFS server.

For synchronous writes, the server replies to NFS clients only when the data has been written to stable storage.