Transcript Introduction

CIT 470: Advanced Network and
System Administration
Remote Administration
CIT 470: Advanced Network and System Administration
Slide #1
Topics
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Network Access
SSH
Key-based Authentication
Console Access
X-Windows
VNC and NX
SSH tunneling
CIT 470: Advanced Network and System Administration
Slide #2
Network Access
Most tasks can be done from the shell.
File management.
Disk/volume management.
Troubleshooting and viewing logs.
Installing/removing software.
Start/stop network services.
Reboot/shutdown.
All we need is a way to invoke a shell across
the network.
CIT 470: Advanced Network and System Administration
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telnet
Ubiquitous network terminal protocol
telnet hostname
Similar protocols
rlogin –l user hostname
rsh –l user hostname command
Insecure
Data, including passwords, sent in the clear.
rlogin/rsh use ~/.rhosts for access w/o
passwords.
CIT 470: Advanced Network and System Administration
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ssh
Secure Shell
Replaces
telnet
ftp
rlogin
rsh
rcp
CIT 470: Advanced Network and System Administration
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SSH Security Features
CIT 470: Advanced Network and System Administration
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SSH: Protocols and Products
• SSH v1
– Insecure, obsolete.
– Do not use.
• SSH v2
– Current version.
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OpenSSH
SSH Tectia
F-secure SSH
Putty
WinSCP
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SSH Features
Secure login
ssh –l user host
Secure remote command execution
ssh –l user host command
Secure file transfer
sftp –l user host
scp file user@host:/tmp/myfile
Port forwarding
ssh –L 110:localhost:110 mailhost
CIT 470: Advanced Network and System Administration
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The Problem of Passwords
1. Good passwords are hard to remember.
2. Password transferred to remote system.
3. Automating remote access with passwords
is difficult.
CIT 470: Advanced Network and System Administration
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Public Key Cryptography
Two keys
– Private key known only to owner.
– Public key available to anyone.
Applications
– Confidentiality:
• Sender enciphers using recipient’s public key,
• Receiver deciphers using their private key.
– Integrity/authentication:
• Sender enciphers using own private key,
• Recipient deciphers using sender’s public key.
CIT 470: Advanced Network and System Administration
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Key-based Authentication
SSH uses public-key authentication
Private key stored in your machine.
Public key stored on remote machines.
Public-key login protocol
1. Client sends server a login request.
2. Server issues a challenge.
3. Client responds with computation based on
challenge and private key.
4. Server checks response with public key.
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Using key-based authentication
1. Generate a public/private key pair.
ssh-keygen
Encrypted key files: id_dsa, id_dsa.pub
2. Copy public key to remote host
Copy to ~/.ssh/authorized_keys.
3. Login to remote host
ssh –l user remote
CIT 470: Advanced Network and System Administration
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Keys are more secure than Passwords
1. Need to have two items to login: key file
and passphrase.
2. Neither key nor passphrase is sent to
remote host.
3. Machine-generated cryptographic keys are
infeasible to guess, unlike passwords.
CIT 470: Advanced Network and System Administration
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SSH Agent
Problem: you have to enter passphrase to
decrypt the key each time you use ssh.
Solution: SSH Agent
> ssh-agent $SHELL
> ssh-add Enter passphrase for
/home/jw/.ssh/id_dsa: ********
Identity added:
/home/you/.ssh/id_dsa
(/home/jw/.ssh/id_dsa)
> ssh –l jw host
CIT 470: Advanced Network and System Administration
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SSH Agent Features
Agent support for entire session.
Start ssh-agent on initial shell.
X: ~/.xsession (Often enabled by default.)
Multiple keys
ssh-add keyfile
ssh-add –l
Remove keys
ssh-add –d keyfile
ssh-add –D
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Remote Access when Server is Down
• Problem: No network access to host.
• Solutions:
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Go to computer room and bring host up.
Specialized hardware (network boot / power).
Virtual machines.
Console servers.
CIT 470: Advanced Network and System Administration
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Console Servers
Console
– Main input / output device for computer.
– Historically: serial terminal.
– Typically: keyboard/mouse + monitor.
Server allows access to multiple consoles.
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Console access: BIOS, Bootloader, Kernel
Eliminates need for keyboards, mice, monitors.
Serial line to each machine from server.
One user has R/W, other users have R access.
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Console Hardware
Console servers solution
– Commercial: Cisco, Cyclades,
Xyplex
– Open source: Conserver +
serial expander card
Hardware issues
– Connectors: DB-9, DB-25,
RJ-45
– Encoding: 8N1, 7E1
– Speeds: 9600 – 230k
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X-Windows
• Network-based windowing system.
• Server
– Handles user input and graphical display.
– Runs on the machine with display unit.
• Client
– Graphical applications are clients.
– Can run on a different machine than server.
• Set DISPLAY environment variable.
• Or use –display command line option.
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Window Manager
X client that provides features like:
– Move, resize, iconify, and kill windows.
– Window title bars.
– Popup menus.
Example window managers
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twm: Tab, primitive early window manager
mwm: Motif, found on commercial UNIXes
fvwm: Free, fast, very customizable.
WindowMaker: NeXT-like, see also AfterStep.
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TWM Screenshot
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FVWM Screenshot
CIT 470: Advanced Network and System Administration
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WindowMaker
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Desktops
CDE
Common desktop env for commercial UNIXes.
Gnome
Standard Linux desktop based on GTK+.
KDE
Windows-like free desktop based on QT.
Xfce
Lightweight desktop, also based on GTK+.
CIT 470: Advanced Network and System Administration
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X-Windows Security
Why do we need security?
An evil client can capture/create any X events.
Even if you’re not using any network clients!
Host authentication
Limit who can start clients by IP address.
Set by xhost + or xhost - commands.
Token authentication
Only clients with token can access server.
Set by the xauth command.
CIT 470: Advanced Network and System Administration
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X-Windows Security
Tunneling + host authentication.
All clients appear to be from localhost.
Therefore disable remote clients with xhost –
Use ssh client to tunnel X: ssh –X host
Server must have X11Forwarding set to yes.
Use echo DISPLAY to test if X forwarding is on.
Note that local users can still attack X session.
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VNC: Virtual Network Computing
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Why VNC?
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Remote desktop access.
Helpdesk: control a remote desktop.
Persistent desktop.
Use same desktop from multiple clients.
Need Linux access from Windows.
Need Windows access from Linux.
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What is VNC?
• Open remote desktop protocol.
• Many implementations
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RealVNC: VNC from original researchers.
TightVNC: VNC with high compression.
VNCj: Java VNC, can run within web browser.
PalmVNC: VNC for Palm Pilots.
UltraVNC: enhanced VNC, only for Windows.
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Using VNC
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Start VNC server
UNIX: vncserver
Win: Start menu>Programs>RealVNC>VNCServer
Write down server name and display number.
It will look something like unix3:1
Start VNC client
UNIX: vncviewer
Win: Start menu>Programs>RealVNC>VNCViewer
Enter server and display to connect to (from step 2).
A VNC remote desktop should appear.
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Configuring and Troubleshooting
• On UNIX, VNC stores files under ~/.vnc
• Configuration: xstartup
– Indicates which X clients to start with server.
– Typically includes vncconfig application.
• Configuration: passwd
– Contains VNC server session password.
• Log files: host:display#.log
– Any errors should appear in these logs.
CIT 470: Advanced Network and System Administration
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Securing VNC
VNC does not provide encryption.
Use ssh tunneling to encrypt login + data:
ssh –L 5901:remotehost:5901 remotehost
vncviewer localhost:1
CIT 470: Advanced Network and System Administration
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Tunneling
Tunneling: Encapsulation of one network
protocol in another protocol
– Carrier Protocol: protocol used by network
through which the information is travelling
– Encapsulating Protocol: protocol (GRE, IPsec,
L2TP) that is wrapped around original data
– Passenger Protocol: protocol that carries original
data
CIT 470: Advanced Network and System Administration
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ssh Tunneling
SSH can tunnel TCP connections
– Carrier Protocol: IP
– Encapsulating Protocol: ssh
– Passenger Protocol: TCP on a specific port
POP-3 forwarding
ssh -L 110:pop3host:110 -l user pop3host
– Uses ssh to login to pop3host as user
– Creates tunnel from port 110 (leftmost port #) on
localhost to port 110 (rightmost post #)of pop3host
– User configures mail client to use localhost as POP3
server, then proceeds as normal
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NX
Advantages over VNC:
Speed: fast enough to use over dialup.
Built-in ssh encryption.
Disadvantages
Immature code; hard to install + set up.
GPL client/server for Linux only.
Free Windows client; commercial server.
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References
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Daniel J. Barrett, Robert G. Byrnes, Richard E. Silverman, SSH, The Secure Shell, 2nd edition,
O’Reilly, 2005.
Conserver, http://www.conserver.com/
John Fisher, “Secure X Windows,” CIAC 2316, http://www.ciac.org/ciac/documents/ciac2316.html,
1995.
David K.Z. Harris, “Zonker’s Greater Scroll of Console Knowledge,”
http://www.conserver.com/consoles/, 2005.
Brian Hatch, “SSH Host Key Protection,” http://www.securityfocus.com/infocus/1806, 2004.
No Machine NX, http://www.nomachine.com/
OpenSSH, http://www.openssh.com/
Real VNC, http://www.realvnc.com/
RedHat, Red Hat Enterprise Linux 4 Reference Guide,
http://www.redhat.com/docs/manuals/enterprise/RHEL-4-Manual/ref-guide/, 2005.
Daniel Robbins, “OpenSSH key management,” http://www-128.ibm.com/developerworks/library/lkeyc.html, 2001.
runeb, “Crash Course in X Windows Security,” http://bau2.uibk.ac.at/matic/ccxsec.htm
Carla Schroeder, Linux Cookbook, O’Reilly, 2004.
Carla Schroeder, “FreeNX ups the Remote Linux Desktop Ante,” Enterprise Networking Planet,
http://www.enterprisenetworkingplanet.com/netos/print.php/3508951, 2005.
Glen Turner, “Linux Remote Serial Console HOWTO,” http://www.tldp.org/HOWTO/RemoteSerial-Console-HOWTO/index.html, 2003.
Webmin, http://www.webmin.com/
Window Managers for X, http://xwinman.org/
CIT 470: Advanced Network and System Administration
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