The password hash in /etc/shadow file in Linux

The /etc/shadow file stores user passwords as hashes in a particular
format. If you ever want to verify users passwords against this hash
in a non standard way, like from a web app for example, then you need
to understand how it works.

Each row in /etc/shadow is a string with 9 fields separated by ':'. A
typical line looks like this:

binpipe:$6$vb1tLY1qiY$M.1ZCqKtJdfdfdQQCkaJT6wXqjUkFcA/qNxLygg,Lj/:15405:0:99999:7:::

The hash could be some random string like
43137ce13e6fdedf9a23792ae62a0aeda1e075409427d57332

The nine different fields are:

The local username
The password hash, more on this later
Number of days since the start of unix time (01/01/1970) that the
password was last changed
Minimum number of days before the password can be changed
Maximum number of days before the password must be changed. 99999
means that the user will not be forced to change their password
Number of days before forcing the password change that the user
will be warned.
The number of days after expiration that the account will be disabled
Days since the start of unix time that the account has been disabled
Currently unused but reserved for future use

Most of those fields are generally unused by Linux distros. The
important ones are the username and hash. The hash field itself is
comprised of three different fields. They are separated by '$' and
represent:

Some characters which represents the cryptographic hashing
mechanism used to generate the actual hash
A randomly generated salt to safeguard against rainbow table attacks
The hash which results from joining the users password with the
stored salt and running it through the hashing mechanism specified in
the first field

Make sense?

So that first field before the salt and actual hash can have a finite
set of possible values. The standard methods supported by GNU/Linux
are:

Available algorithms:

$1$
md5
$2a$
Blowfish
$2y$
Blowfish, with correct handling of 8 bit characters
$5$
sha256
$6$
sha512

In practice you shouldn't use anything but sha512. The mkpasswd
command will create the hash string for you and can be used by other
programs to check an existing hash. Given the password hash above, if
you wanted to check if a given password matched it you would run the
following command:

mkpasswd --method=sha512 --salt=vb1tLY1qiY PASSWORD

The equivalent using python looks like this:

import crypt

crypt.crypt('PASSWORD', '$6$vb1tLY1qiY')

Both will return an exact copy of the hash. for the given salt and
password. Notice how using the Python library you actually pass in the
method as part of the salt string. A salt string starting with '$5$'
would use sha256 for example.

You can then compare this hash with the hash in /etc/shadow, or
anywhere else that they might be stored. Like in a central datbase if
you are using NSS.

If you don't provide mkpasswd with a salt it will automatically
generate a random salt. This is usually what you want to do if you are
creating a password for the first time.