System Install Date (Linux)

Author Name
Hal Pomeranz

Artifact Name
Linux system install date

Operating System

In general it is rare for any Unix-like operating system to record its
system install date. So you’re left with using other artifacts on the
system as a proxy to deduce the install date.

One of the most popular methods for dating the system install is to
look at the time stamps on the SSH host key files under /etc/ssh.

These files are usually generated via the SSH startup script
(/etc/init.d/sshd or similar) during the first boot of the system,
which typically happens immediately after the system install is

$ ls -l /etc/ssh/ssh_host_*
-rw——- 1 root root 668 Jul 14 2007 /etc/ssh/ssh_host_dsa_key
-rw-r–r– 1 root root 590 Jul 14 2007 /etc/ssh/
-rw——- 1 root root 963 Jul 14 2007 /etc/ssh/ssh_host_key
-rw-r–r– 1 root root 627 Jul 14 2007 /etc/ssh/
-rw——- 1 root root 1675 Jul 14 2007 /etc/ssh/ssh_host_rsa_key
-rw-r–r– 1 root root 382 Jul 14 2007 /etc/ssh/

In the example above, it appears that the system was installed on Jul
14, 2007.

If you’d like to see a finer-grained time stamp, try the “stat”
command on any one of the above files: 

$ stat /etc/ssh/ssh_host_key
File: `/etc/ssh/ssh_host_key’
Size: 963 Blocks: 16 IO Block: 4096 regular file
Device: fd00h/64768d Inode: 1837188 Links: 1
Access: (0600/-rw——-) Uid: ( 0/ root) Gid: ( 0/ root)
Access: 2009-11-29 09:49:28.000000000 -0800
Modify: 2007-07-14 11:56:52.000000000 -0700
Change: 2007-07-14 11:56:52.000000000 -0700

The modify and change times generally reflect the file creation date
(on EXT4 file systems, there will be a file creation time stamp). The
access time is the last time is the last time the file was read.

Private key files such as /etc/ssh/ssh_host_key are generally only
read by the system SSH daemon, and then only when the daemon is
(re)started. Since the SSH daemon is fairly stable and is rarely
restarted, the last access time often correlates with the last time
the system was booted.

All of the usual caveats about file-based time stamps apply here. It’s
possible, though uncommon, that a site might choose to regenerate
their SSH host keys on a regular basis (doing this causes problems for
users, so it’s not normal practice). Time stamps can be easily
manipulated with programs like “touch”. Certain backup programs may
alter access times on files. Also modern Linux systems generally use
the “relatime” option on file systems by default, making last access
time information untrustworthy.

File Locations

Forensic Programs of Use
ls, stat


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