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See this page as a slide show

CT320 Accounts

Accounts and Namespaces

Thanks to:

• Dr. James Walden, NKU,
• Russ Wakefield, CSU,
• and Dr. Indrajit Ray, CSU,

for the contents of these slides.

Topics

1. UNIX User Accounts
2. Passwords
3. User Management
4. Namespaces

UNIX Accounts

• Account Components
  • Username
  • UID
  • Password
  • Home directory
• Account Files
  • /etc/passwd
  • /etc/shadow
  • /etc/group
• Account Management
  • Adding users
  • Removing and disabling users
  • Account/password policies

/etc/passwd, /etc/shadow

Central file(s) describing UNIX user accounts.

/etc/passwd entries

$ getent passwd root
root:x:0:0:root:/root:/bin/bash

$ getent passwd cs160
cs160:$6$nEUJCmhPFNLGLV9A$aWOD7Ha.d14/9Wj9K0zErIpwcQvg8mnidNTD6bBG/RWZLJ4yWaksyTKXn9cXP5omDmGtssYrvMzxoq4ISR91K.:1057:1549:CS160 Instructor:/s/bach/a/class/cs160:/sbin/nologin

$ getent passwd cs160 | tr : '\n'
cs160
$6$nEUJCmhPFNLGLV9A$aWOD7Ha.d14/9Wj9K0zErIpwcQvg8mnidNTD6bBG/RWZLJ4yWaksyTKXn9cXP5omDmGtssYrvMzxoq4ISR91K.
1057
1549
CS160 Instructor
/s/bach/a/class/cs160
/sbin/nologin

Username

• Syntax
  • Each username must be unique.
  • 8 or fewer chars (32 on some systems)
  • Any character except : or newline, theoretically.
• Issues
  • Naming standards
    • applin, j_applin, or neutron?
  • How to ensure that usernames are unique?
  • System uses UIDs internally

UIDs

• UIDs are 32-bit non-negative integers.
• Standards
  • The super-user, root, is UID 0.
  • System accounts have low UIDs (≤1000)
• Uniqueness
  • Multiple usernames can have same UID!
  • Re-using UIDs may give away files to new user.
    • There are billions of 32-bit integers.
  • Distributed systems may require unique UIDs across organizational boundaries.

Password

• Syntax
  • Length: 8 chars (crypt), unlimited (MD5, SHA)
  • Chars: anything except newline, though certain control chars (e.g., backspace) may be interpreted by the system. Spaces are useful for passphrases.
• Stored in “encrypted” (really, hashed) format
  • Hashed: crypt, MD5, SHA

Password Field

Hashing

Good hash algorithms, such as SHA-512, are considered non-reversible, and return radically different results for small input changes:

Common passwords

According to Wikipedia, the most common passwords of 2016 are:

Rainbow Tables

• A simple attack is to use a list of common passwords and try them for every user. Or, just try every word in the dictionary.
  • Unfortunately, this means that you have to hash every word in the dictionary. Hashing is slow!
  • Speed it up by computing, once, hashes for all dictionary words. We call the precomputed hashes a Rainbow table.

Partial MD5 rainbow table:

    e0ebc3c409070d07f1df0f2f4132509e escape
    bafbb2fabbff5876f8bf7834f802936b escaped
    712f32fc42f27433a6db7cba03a980b3 escapee
    1ba8fbf22b249654d5cde753bae85def escaper
    f801cba4d35a3da1501ab3162cbb4dee escapes

Salt

• Salt is the cure for this.
  • Each line in the password file has some random salt bits.
    • $1$BTZEc3ZC$OgrtMuWtLyZ.9AbsXGTey0
  • 48-bit salt, as used with MD5 & SHA, means 2⁴⁸ (~281 trillion) different hashes for each password.
    • Makes pre-computed hash dictionaries impractical.

mkpasswd

mkpasswd can be handy for generating an initial password:

$ mkpasswd
j0Sb?q6Xu
$ mkpasswd
Hei3(uyL2
$ mkpasswd -l 20
bsAtblikuvb~tOs67znx
$ mkpasswd -c 2 -C 2 -s 2 -d 2
HZ42e&+xc
$ mkpasswd -c 10 -C 10 -s 10 -d 10 -l 60
C*jhSb$Vn1Pz7fih4]zPhZQyrep4fV_bbBoc3l"r8@kmh.Ux,dt2v8_w14h>
$ mkpasswd -c 0 -C 0 -d 0 -s 50 -l 50
{^&[;?>:$($]?<=[]~&=$~:$(-<[}!?%=|+_[*~}@|+*<.>.#/

Or, should you let the user choose one?

Password Aging

/etc/login.defs determines password longevity:

$ grep PASS /etc/login.defs
#	PASS_MAX_DAYS	Maximum number of days a password may be used.
#	PASS_MIN_DAYS	Minimum number of days allowed between password changes.
#	PASS_MIN_LEN	Minimum acceptable password length.
#	PASS_WARN_AGE	Number of days warning given before a password expires.
PASS_MAX_DAYS	99999
PASS_MIN_DAYS	0
PASS_MIN_LEN	5
PASS_WARN_AGE	7

• Is this good, from a security point of view?
• What value is PASS_MIN_DAYS?
• Pre-expire the password, to force the user to change it:
  passwd -e smith

Feedback

• I can’t seem to log in:

    $ ssh xyzzy@denver.cs.colostate.edu
    xyzzy@denver.cs.colostate.edu's password:
    Permission denied, please try again.
    xyzzy@denver.cs.colostate.edu's password:
    Permission denied, please try again.
    xyzzy@denver.cs.colostate.edu's password:
    Permission denied (publickey,gssapi-keyex,gssapi-with-mic,password).

• Which was wrong, the account xyzzy or the password?
• Why didn’t it tell me which one?

GID

• GIDs are 32-bit non-negative integers.
• Each user has a default GID.
  • File group ownership set to default GID
  • Temporarily change default GID: newgrp
• Groups are described in /etc/group
  • Users may belong to multiple groups.
  • Format: group name, pw, GID, user list

    wheel:x:10:root,waldenj,bergs

GECOS

• Original use
  • Data for General Electric Comprehensive Operating System
• Current use
  • User information
  • Full name, location, phone number, e-mail

Home Directory

• Users CWD at login time
• Typically where user stores all files

Login Shell

• Process started when user logs in
• Typically a shell like bash, tcsh, ksh, or zsh
  • System users may be different.
  • Disabled accounts have a special “go-away” shell.

$ ssh acushla perl -E \''say $e[8] while @e=getpwent'\' | sort | uniq -c
Host key verification failed.

Changing your Shell

$ cat /etc/shells
/bin/sh
/bin/bash
/usr/bin/sh
/usr/bin/bash
/usr/bin/tmux
/bin/tmux
/usr/bin/zsh
/bin/zsh
/bin/ksh
/bin/rksh
/usr/bin/ksh
/usr/bin/rksh
/bin/csh
/bin/tcsh
/usr/bin/csh
/usr/bin/tcsh
/bin/false
/bin/true
/sbin/nologin
/usr/local/etc/no_access

• The chsh program change your login shell.
• It may restrict your shell to one from /etc/shells.
  • Why?
• Alas, chsh fails here, so you have to send email to the sys admin people. How quaint!

Adding a User

1. Create account with useradd.
2. Lock account until user arrives.
3. User signs account agreement.
4. Set password with the passwd command.
   • Yes, both /etc/passwd and /bin/passwd exist.

Adding a User

• Edit /etc/{passwd,shadow} with vipw.
• Set password with the passwd command.
• Edit /etc/group to add groups.
• Create user home directory.
  • mkdir /home/studenta
  • chown studenta.student /home/studenta
  • chmod u=rwx,go=rx /home/studenta
• Copy default files from /etc/skel: .bashrc, .Xdefaults, .xsession, etc.
• Set e-mail aliases, disk quotas, etc.
• Verify that the account works.

Disabling an Account

• Edit account configuration:
  • Place * in front of encrypted password.
  • Replace shell with nologin program.
• Kill active logins and processes.

Removing a User

• Disable account.
• Change shared passwords (root, etc.)
  • Why are you using @#!?☠$★ shared passwords‽
• Kill active logins and processes.
• Remove:
  • from local databases/files
  • from e-mail aliases
  • mail spool (backup first)
  • crontabs and pending jobs
  • temporary files
  • home directory (backup first)
  • from passwd, shadow, and group
• Or use userdel.

Namespaces

Systems include many namespaces:

• User account names
• Filesystem pathnames
• Hostnames
• Printer names
• Service names

Types of Namespaces

• Flat
  • No duplicates may exist.
  • Example: usernames in /etc/passwd
• Hierarchical
  • Tree-structured namespace like DNS.
  • Duplicates can exist.
  • Ex: www.cs.colostate.edu and www.cs.uidaho.edu

Namespace Problems

• How to select names?
• How to avoid name collisions?
• How to ensure consistency?
• How to distribute names?

Name Selection

• Functional Names
  • mail hostname, ~ct320, student account
• Formula-based Names
  • cvg0141 hostname, student0148 account
• Themed Names
  • constellations (orion, ursa, etc.)
  • Use something big. Seven dwarves doesn’t allow for much expansion.
• Steal from another standard (e.g., .us, .fr, .de)
• No Standard

Name Lifetime

• When are names removed?
  • Immediately after user leaves organization
  • Some fixed time after resource is no longer in use
• When are names re-used?
  • Immediately: functional names (postmaster, president)
  • Never
  • After a while (dangerous)

Namespace Scope

• Geographical scopes
  • Local machine (e.g., /etc/passwd)
  • Local network
  • Organization
  • Global (e.g., DNS)
• Service scopes
  • Single username for UNIX, NT, e-mail
• Transferring scopes
  • Difficult without advance planning.
  • Some names may have to change.

Key Points

• UNIX accounts use /etc/passwd
• Password security: shadow, hashing, salts
• User management: add, disable, remove
• Namespaces: flat and hierarchical
• Name selection policies
• Names are valid in specific scopes

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