Linux Networking and Security

A filesystem is the method an operating system uses to organise, store, and retrieve files on a storage device. Linux uses ext4 and XFS most commonly in production, plus virtual filesystems like /proc and /sys that exist only in memory.

An inode is a data structure in a Linux filesystem that stores metadata about a file — its permissions, owner, size, and data block locations — but not its filename. The filename lives in a directory entry that points to the inode number.

Linux file permissions control which users can read, write, or execute a file using a three-level model: owner, group, and other. Each level has three bits that can be independently set, represented as rwxr-xr-- or in octal as 754.

A symbolic link (symlink) is a special file that contains a path pointing to another file or directory. Accessing the symlink transparently accesses the target. Unlike hard links, symlinks can cross filesystems and point to directories.