What is the default SHA512 salt used in passwd and for the hash stored in the shadow file?

Question

I want to add a user using useradd and specify an encrypted password using the -p flag. I learned that the unix system that I am on uses a SHA512 hash for storing passwords in the /etc/shadow file. When I look in /etc/pam.d/common-password, it says this:

# Explanation of pam_unix options:
#
# The "sha512" option enables salted SHA512 passwords.  Without this option,
# the default is Unix crypt.  Prior releases used the option "md5".
#
# The "obscure" option replaces the old `OBSCURE_CHECKS_ENAB' option in
# login.defs.
#
# See the pam_unix manpage for other options.

# here are the per-package modules (the "Primary" block)
password        [success=1 default=ignore]      pam_unix.so sha512

I just need to know what the salt is, so that I can generate the hash and use it with my

useradd ... ... -p INSERT_HASHED_PASS_HERE

Answer 1

Salt is a value that is [ideally] generated randomly which introduces some variety to passwords in the case that two users happen to have the same password. Using a random salt a user could have the same password on multiple systems but it would not be obvious. What you are trying to do seems to circumvent the purpose of salting a hash.

I presume you are trying to generate a password hash once and use it in multiple places for automation purposes.

There are a couple of utilities that can be used to generate a password hash when passed an arbitrary string as a salt. However, as I said, you really negate the purpose of the salt in the first place.