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CIT 470: Advanced Network and
System Administration
Accounts and Namespaces
CIT 470: Advanced Network and System Administration
Slide #1
Topics
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Namespaces
Policies: selection, lifetime, scope, security
User Accounts
Directories
LDAP
CIT 470: Advanced Network and System Administration
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Namespaces
A namespace consists of
– A set of unique keys
– A set of attributes associated with each key
Example
– Key = Username
– Attributes
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GECOS
Homedir
Shell
Password
CIT 470: Advanced Network and System Administration
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Namespaces
Systems include many namespaces
User account names.
E-mail addresses.
Filesystem pathnames.
Hostnames.
IP addresses.
Printer names.
Service names.
CIT 470: Advanced Network and System Administration
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Types of Namespaces
Flat
No duplicates may exist.
Ex: usernames in /etc/passwd.
Hierarchical
Tree-structured namespace like DNS.
Duplicates can exist.
Ex: www.nku.edu and www.google.com
CIT 470: Advanced Network and System Administration
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Namespace Problems
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How to select names?
How to avoid name collisions?
How to ensure consistency?
How to distribute names?
CIT 470: Advanced Network and System Administration
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Name Selection
Functional Names
mail hostname, /cit/470, student account
Descriptive names
geographic, print type, customer type
Formula-based Names
cvg0141 hostname, student0148 account
Themed Names
constellations (orion, ursa, etc.)
No Standard
CIT 470: Advanced Network and System Administration
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Name Lifetime
When are names removed?
Immediately after PC, user leaves org.
Set time after resource is no longer in use.
When are names re-used?
Immediately: functional names.
Never.
After a set time: usernames, email addresses.
CIT 470: Advanced Network and System Administration
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Namespace Scope
Geographical scopes
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Local machine. (e.g., /etc/passwd.)
Local network.
Organization.
Global (e.g., DNS.)
Service scopes
– Single username for UNIX, NT, RADIUS, e-mail, VPN?
Transferring scopes
– Difficult without advance planning.
– Some names may have to change.
CIT 470: Advanced Network and System Administration
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Namespace Security
1. What are you trying to protect names from
and why?
2. Do the names need to be protected or just
the attributes?
3. Who can add, change, or delete records?
4. Can the owner of a record change fields
within the record?
CIT 470: Advanced Network and System Administration
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Example Namespace: Usernames
Selection policies
– Descriptive: waldenj, jwalden
– Decriptive + formulaic: waldenj1, jwalden0002
Scope
– Use for every campus (avoids collisions.)
– Use for every service (avoids collisions.)
Lifetime
– Do not reuse until 1 year has passed since email
addresses derive from usernames.
CIT 470: Advanced Network and System Administration
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One Big Database
Centralize namespace in one big database.
– Use SQL or LDAP to store entire namespace.
Derive other namespaces from database.
– Program to generate UNIX accounts.
– Program to generate NT accounts.
– etc.
Advantages
– Consistency
– Ease of making changes, additions, deletions.
CIT 470: Advanced Network and System Administration
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User Account Types
OS files
– UNIX /etc/{passwd,shadow}
– Windows SAM
Network service
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NIS
LDAP
Kerberos
Active Directory
RADIUS
CIT 470: Advanced Network and System Administration
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UNIX Accounts
• Account Components
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• Account Files
Username
UID
Password
Home directory
– /etc/passwd
– /etc/shadow
– /etc/group
• Account Management
– Adding users
– Removing and disabling users
– Account/password policies
CIT 470: Advanced Network and System Administration
Slide #14
/etc/{passwd,shadow}
Central file(s) describing UNIX user accounts.
/etc/passwd
/etc/shadow
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Username
UID
Default GID
GCOS
Home directory
Login shell
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Username
Encrypted password
Date of last pw change.
Days ‘til change allowed.
Days `til change required.
Expiration warning time.
Expiration date.
student:x:1000:1000:Example User,,555-1212,:/home/student:/bin/bash
student:$1$w/UuKtLF$otSSvXtSN/xJzUOGFElNz0:13226:0:99999:7:::
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Username
Syntax
– Each username must be unique.
– Length limits (8 chars on old systems)
– Any character except : or \n.
• Issues
– Naming standards.
– How to ensure that usernames are unique?
– System uses UIDs internally.
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UIDs
• UIDs are 32-bit non-negative integers.
• Standards
– Root is UID 0.
– System accounts have low UIDs (<= 500)
• Uniqueness
– Multiple usernames can have same UID!
– Re-using UIDs may give away files to new user.
– Distributed systems may require unique UIDs
across organizational boundaries.
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Password
Syntax
– Length: unlimited (MD5,SHA1), 8 chars (crypt)
– Chars: anything except \n, though certain control
chars may be interpreted by system.
Stored in “encrypted” format.
– Hashed: crypt, MD5, SHA1
– Salted: 12-bit salt means 4096 different hashes
for each password
CIT 470: Advanced Network and System Administration
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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
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GECOS
Original use
– Data for General Electric Comprehensive OS
Current use
– User information.
– Full name, location, phone number, e-mail.
CIT 470: Advanced Network and System Administration
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Home Directory
• User’s CWD at login time.
• Typically where user stores all files.
CIT 470: Advanced Network and System Administration
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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 noshell program.
CIT 470: Advanced Network and System Administration
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Adding a User
1. Create account with adduser.
2. Lock account until user arrives.
3. User signs account agreement.
4. Set passwd with passwd.
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Adding a User
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Edit /etc/{passwd,shadow} with vipw.
Set passwd with passwd command.
Edit /etc/group to add groups.
Create user home directory.
1. mkdir /home/studenta
2. chown studenta.student /home/studenta
3. chmod 755 /home/studenta
5. Copy default files from /etc/skel
.bashrc, .Xdefaults, .xsession, etc.
6. Set e-mail aliases, disk quotas, etc.
7. Verify that the account works.
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Disabling an Account
• Edit account configuration:
– Place * in front of encrypted password.
– Replace shell with nologin program.
• Kill active logins and processes.
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Removing a User
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Disable account.
Change shared passwords (root, etc.)
Kill active logins and processes.
Remove from local databases/files.
Remove from e-mail aliases.
Remove mail spool (backup first.)
Remove crontabs and pending jobs.
Remove temporary files.
Remove home directory (backup first.)
Remove from passwd, shadow, and group.
CIT 470: Advanced Network and System Administration
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What is a Directory?
Directory: A collection of information that is
primarily searched and read, rarely modified.
Directory Service: Provides access to directory
information.
Directory Server: Application that provides a
directory service.
CIT 470: Advanced Network and System Administration
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Directories vs. Databases
Directories are optimized for reading.
– Databases balanced for read and write.
Directories are tree-structured.
– Databases typically have relational structure.
Directories are usually replicated.
– Databases can be replicated too.
Both are extensible data storage systems.
Both have advanced search capabilities.
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System Administration Directories
Types of directory data
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Accounts
Mail aliases and lists (address book)
Cryptographic keys
IP addresses
Hostnames
Printers
Common directory services
– DNS, LDAP, NIS
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Advantages of Directories
Make administration easier.
– Change data only once: people, accounts, hosts.
Unify access to network resources.
– Single sign on.
– Single place for users to search (address book)
Improve data management
– Improve consistency (one location vs many)
– Secure data through only one server.
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NIS: Network Information Service
Originally called Sun Yellow Pages
– Clients run ypbind
– Servers run ypserv
– Data stored under /var/yp on server.
Server shares NIS maps with clients
– Each UNIX file may provide multiple maps
– passwd: passwd.byname, passwd.byuid
Slave servers replicate master server content.
Easy to use, but insecure, difficult to extend.
CIT 470: Advanced Network and System Administration
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LDAP
Lightweight Directory Access Protocol
– Lightweight compared to X.500 directories.
– Directory, not a database, service.
– Access Protocol, not a directory itself.
CIT 470: Advanced Network and System Administration
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LDAP Clients and Servers
LDAP Clients
– Standalone directory browsers.
– Embedded clients (mail clients, logins, etc.)
– Cfg /etc/nsswitch.conf on UNIX to use LDAP.
Common LDAP servers
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OpenLDAP
Fedora Directory Server (formerly Sun, Netscape)
Mac Open Directory
Microsoft ActiveDirectory
Novell eDirectory (NDS)
CIT 470: Advanced Network and System Administration
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LDAP Structure
An LDAP directory is made of entries.
– Entries may be employee records, hosts, etc.
Each entries consists of attributes.
– Attributes can be names, phone numbers, etc.
– objectClass attribute identifies entry type.
Each attribute is a type / value pair.
– Type is a label for the information stored (name)
– Value is value for the attribute in this entry.
– Attributes can be multi-valued.
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Tree-structure of LDAP Directories
CIT 470: Advanced Network and System Administration
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LDAP Schemas
Schemas specify allowed objectClasses and attributes.
CIT 470: Advanced Network and System Administration
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LDIF
LDAP Interchange Format.
– Standard text format for storing LDAP
configuration data and directory contents.
LDIF Files
– Collection of entries separated by blank lines.
– Mapping of attribute names to values.
Uses
– Import new data into directory.
– Export directory to LDIF files for backups.
CIT 470: Advanced Network and System Administration
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LDIF Output Example
CIT 470: Advanced Network and System Administration
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Distinguished Names
Distinguished Names (DNs)
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Uniquely identify an LDAP entry.
Provides path from LDAP root to the named entry.
Similar to an absolute pathname.
dn:cn=Jeff Foo,ou=Sales,dc=plainjoe,dc=org
Relative DNs (RDNs)
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Any unique attribute pair in directory’s container.
ex: cn=Jeff Foo OR username=fooj
Similar to a relative pathname.
Except may have multiple components.
cn=Jane Smith+ou=Sales
cn=Jane Smith+ou=Engineering
CIT 470: Advanced Network and System Administration
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LDAP Client/Server Interaction
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Client requests to bind to server.
Server accepts/denies bind request.
Client sends search request.
Server returns zero or more dir entries.
Server sends result code with any errors.
Client sends an unbind request.
Server sends result code and closes socket.
CIT 470: Advanced Network and System Administration
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LDAP Operations
Client Session Operations
– Bind, unbind, and abandon
Query and Retrieval Operations
– Search and compare
Modification Operations
– Add, modify, modifyRDN, and delete
CIT 470: Advanced Network and System Administration
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Authentication
Anonymous Authentication
Binds with empty DN and password.
Simple Authentication
Binds with DN and password. Cleartext.
Simple Authentication over SSL/TLS
Use SSL to encrypt simple authentication.
Simple Authentication and Security Layer
SASL is an extensible security scheme.
SASL mechanisms: Kerberos, GSSAPI, SKEY
CIT 470: Advanced Network and System Administration
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Distributed Directories
• Use multiple LDAP servers.
• Why distribute?
– Throughput
• More servers can reduce load on any single server.
– Latency
• Have local server serve local data to LAN.
• Only use WAN for non-local data on other servers.
– Administrative Boundaries
• Let each side administrate their own directory.
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OpenLDAP
Open source LDAPv3 server.
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LDAP server: slapd
Client commands: ldapadd, ldapsearch
Backend storage: BerkeleyDB
Backend commands: slapadd, slapcat
Schemas: /etc/openldap/schema
Data: /var/lib/ldap
Configuration files
– Client: /etc/openldap/ldap.conf
– Server: /etc/openldap/slapd.conf
CIT 470: Advanced Network and System Administration
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Building an OpenLDAP Server
1. Install OpenLDAP.
2. Configure LDAP for your domain.
Change suffix, rootdn, rootpw options.
vim /etc/openldap/slapd.conf
3. Start server
Immediate: /sbin/service ldap start
Permanent: /sbin/chkconfig --level 35 ldap on
4. Add data with ldapadd
5. Verify functionality with ldapsearch
CIT 470: Advanced Network and System Administration
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LDAP Authentication
1. Configure server with schema + user data.
2. Point clients to hostname and rootDN of svr.
/etc/ldap.conf and
/etc/openldap/ldap.conf
3. Verify server access with ldapsearch
4. Configure clients to use LDAP auth
/etc/nsswitch.conf
passwd: files ldap
shadow: files ldap
group: files ldap
CIT 470: Advanced Network and System Administration
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References
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Brian Arkills, LDAP Directories Explained: An Introduction and
Analysis, Addison-Wesley, 2003.
Gerald Carter, LDAP System Administration, O’Reilly, 2003.
J. Heiss, “Replacing NIS with Kerberos and LDAP,”
http://www.ofb.net/~jheiss/krbldap/, 2004.
LDAP Howtos, Links, and Whitepapers, http://www.bind9.net/ldap/,
2005.
http://www.ldapman.org/, 2005.
Luiz Malere, “Linux LDAP HOWTO,”
http://www.tldp.org/HOWTO/LDAP-HOWTO/, 2004.
OpenLDAP, OpenLDAP Administrator’s Guide,
http://www.openldap.org/devel/admin/, 2005.
RedHat, Red Hat Enterprise Linux 4 Reference Guide, Chapter 13,
http://www.redhat.com/docs/manuals/enterprise/RHEL-4-Manual/refguide/, 2005.
CIT 470: Advanced Network and System Administration
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