COEN 252: Computer Forensics

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Presentation transcript:

COEN 252: Computer Forensics Unix File Systems

Unix File System Increasingly important Linux MacOS X Bewildering variety on a laptop Linux versions Free BSD Open BSD Mac

Unix File Systems Almost everything is a file. File has properties such as File type and access permissions. Link count. Ownership & group membership. Date and time of last modification. File name.

Unix File System Owners can change many of these data Including modification time.

Unix File System Based on Inodes. More flexible than tables.

Inodes i_mode (directory IFDIR, block special file (IFBLK), character special file (IFCHR), or regular file (IFREG) i_nlink i_uid (user id) i_gid (group id) i_size (file size in bytes) i_addr (an array that holds addresses of blocks) i_mtime (modification time & date) i_atime (access time & date)

Inodes

Inodes

Unix File System Classical Unix used a file table to mediate between users and their open files. File table had references to the inodes of open files.

Unix File System On-Disk Layout. Superblock contains data on the file system.

Unix File System

Unix File Systems First versions of Unix had a single file system. Unix System V Release 3.0 introduced File System Switch architecture. No longer a tight coupling between kernel and file system.

Unix File Systems SunOS elaborated on this idea. Clear split between file system-dependent and file system-independent kernel. Intermediary layer is the VFS / VOP / veneer layer. Allows disk file systems such as 4.2 BSD FFS, MS-DOS, NFS, RFS.

Unix File Systems Disk Layout not uniform. Ext2 (Linux) file system layout.

Journaling File Systems File systems use caching in order to speed up operations. An unclean dismount can leave the file system in an unclean state. Journaling file system can keep a log, so that they can simply replay the log in order to bring the file system into a consistent state.

Journaling File Systems Log can contain Only records of changes to metadata. Records of changes to metadata and client data. New values of blocks. Research Effort. Not successfully implemented.

Journaling File Systems ext3 (adds journal to ext2) for Linux JFS ReiserFS XFS …

Journaling File Systems Interesting opportunity for forensic investigation. Unfortunately, log entries get purged if too old.