Remote Access
Objectives
• Secure Remote Access
• Harden File Transfer Protocol (FTP)
• Protect Directory Services
cs490ns-cotter
2
Remote Access – into Linux
• vnc – (Virtual Network Connection)
– vnc-server (Fedora / CentOS) – tigervnc-server
•
•
•
•
Establish password for communications
Decide which ports to use (multiple channels available)
Channel 0 = 5900; channel 1 = 5901, etc.
Start service “vncserver :1”
– vnc clients
• Linux: vncviewer
• Windows: tightVNC, ultraVNC, etc.
Remote Access – into Linux
• Windows – tightVNC login
Remote Access – into Linux
TightVNC client
Remote Access – into Linux
• noMachine nx
– nx server available from NoMachine.com
• Uses SSH for data transport
• Runs on port 7100, but all data goes through SSH
(22)
• Uses SSH login (regular user)
– Nx clients
• Linux
• Windows
Remote Access – into Linux
• Windows nx login
Remote Access – into Linux
nx client
Secure Remote Access
• Windows NT includes User Manager to
allow dial-in access, while Windows 2003
uses Computer Management for
Workgroup access and Active Directory for
configuring access to the domain
• Windows XP, 2003 Remote Access
Policies can lock down a remote access
system to ensure that only those intended
to have access are actually granted it
cs490ns-cotter
9
Secure Remote Access
• Linux – Remote access is not part of the
kernel. Available as support programs.
– Unsecured – telnet, ftp
– Secured – SSH, PuTTY, etc.
cs490ns-cotter
10
Tunneling Protocols
• Tunneling: Technique of encapsulating one
packet of data within another type to create a
secure link of transportation.
cs490ns-cotter
11
Secure Transmission Protocols
• PPTP and other similar protocols provide
a secure mechanism for preventing
eavesdroppers from viewing transmissions
• PPTP derived from Point-to-Point Protocol
cs490ns-cotter
12
Point-to-Point Protocol
• Initially designed to support serial connections
(modems) to the Internet
– Standard serial port provides only layer 1 protocol
support (no layer 2 – data link)
– PPP (and the earlier Serial Line Internet Protocol)
were designed as a layer 2 protocol that can carry
IP datagrams through a serial (modem) connection.
• Functions:
–
–
–
–
Encapsulation
Link Control Protocol
Authentication
Network Control Protocols
cs490ns-cotter
13
Point-to-Point Tunneling
Protocol (PPTP)
• Widely deployed tunneling protocol
• Developed by Microsoft
• Based on the Point-to-Point Protocol
(PPP)
• Client connects to a network access
server (NAS) to initiate connection
• Extension to PPTP is Link Control Protocol
(LCP), which establishes, configures, and
tests the connection
cs490ns-cotter
14
PPTP (cont)
client
client
Dial-up
link
PPTP
PPP
PPP
NAS
cs490ns-cotter
Internet
PPTP
Server
15
PPTP (cont)
• Used to encapsulate network protocols
over a TCP/IP network
– Used to carry PPP (which can encapsulate IP,
IPX, Net BEUI, etc.)
– Control messages
• Manages VPN connections
– Supports Authentication, Encryption
– Data packets
cs490ns-cotter
16
Layer 2 Tunneling Protocol
(L2TP)
• Represents a merging of features of PPTP
with Cisco’s Layer 2 Forwarding Protocol
(L2F), which itself was originally designed
to address some of the weaknesses of
PPTP
• Unlike PPTP, which is primarily
implemented as software on a client
computer, L2TP can also be found on
devices such as routers.
cs490ns-cotter
17
L2TP (cont)
cs490ns-cotter
18
L2TP (cont)
1. User initiates a PPP connection to the ISP, using the analog telephone system
or ISDN.
2. The ISP network L2TP Access Concentrator (LAC) accepts the connection at
the POP and the PPP link is established.
3. After the end user and the L2TP Network Server (LNS) negotiate Link protocol,
the LAC partially authenticates the end user with CHAP or PAP. If the user is
not a Virtual Private dialup Network (VPDN) client, authentication continues,
and the client will access the Internet or other contacted service. If the
username is a VPDN client, the mapping will name a specific endpoint (the
LNS).
4. The tunnel end points, the LAC and the LNS, authenticate each other before
any sessions are attempted within a tunnel. Alternatively, the LNS can accept
tunnel creation without any tunnel authentication of the LAC.
5. Once the tunnel exists, an L2TP session is created for the end user.
6. The LAC will propagate the LCP negotiated options and the partially
authenticated CHAP/PAP information to the LNS. The LNS will funnel the
negotiated options and authentication information directly to the virtual access
interface. If the options configured on the virtual template interface does not
match the negotiated options with the LAC, the connection will fail, and a
disconnect is sent to the LAC.
cs490ns-cotter
19
PPPoE: PPP over Ethernet
• Point to Point Protocol over Ethernet
• RFC 2516
• Only Ethernet Framing and MAC address are
relevant (Nothing from CSMA/CD protocol!)
• Ability to connect a collection of hosts over a
simple bridging access device to a remote
access concentrator
• Access control, billing and Type of service: on a
per user basis
• Each PPP session learns Ethernet address of
remote peer with a unique session identifier
cs490ns-cotter
20
PPPoE: cont’d
• Revisit Ethernet Frame
– DestAddr | SourceAddr | Ether_type | Payload |
Checksum
• DestAddr,SourceAddr: 6 byte MAC
address (universally unique)
• Ether_type: 2 bytes
• Payload 46 – 1500 bytes
• Checksum (FCS) 4 bytes
cs490ns-cotter
21
Two stages of PPPoE:
Discovery Stage
– Host (client) may discover all Remote access
concentrators and select one
• A session_id is established
• Ether_type: 0x8863
– Four messages:
•
•
•
•
Initiation (from host to RAC)
Offer from (RAC to host)
Request from (host to RAC)
Confirm (from RAC to host): contains session ID
cs490ns-cotter
22
Session Stage
– Now transmit PPP-LCP message to establish PPP
session
– First generated by host
•
•
•
•
•
•
•
DestAddr: Access Concentrator
SourceAddr: host
Ether_type: 0x8864
SessionID: as obtained from Discovery Stage
Length: <rest of packet>
PPP protocol: 0xc021 (PPP-LCP)
Now PPP payload!
– Continue with other PPP messages (PAP, IP-CP
etc) to get to the IP datagram transmission phase
cs490ns-cotter
23
Authentication Technologies
• Authenticating a transmission to ensure
that it comes from an approved sender
can provide an increased level of security
for remote access users
• Examples:
– Radius
– Tacacs+
cs490ns-cotter
24
(RADIUS)
• Remote Authentication Dial-In User Service
• Originally defined to enable centralized
authentication and access control for PPP sessions
• Requests are forwarded to a single RADIUS server
• Supports authentication, authorization, and auditing
functions
• After connection is made, RADIUS server adds an
accounting record to its log and acknowledges the
request
• Allows company to maintain user profiles in a central
database that all remote servers can share
cs490ns-cotter
25
(TACACS+)
• Terminal Access Control Access Control
System
• Industry standard protocol specification
that forwards username and password
information to a centralized server
• Whereas communication between a NAS
and a TACACS+ server is encrypted,
communication between a client and a
NAS is not
cs490ns-cotter
26
IPSec
• Designed to create and support a
cryptographically protected communication
channel
• This connection is called a Security
Association.
– SA defined by a set of security parameters
referenced in a Security Parameter Index
(SPI) and a destination host address.
IP Security (IPSec)
• Considered to be a transparent security
protocol operating at layer 3
– Transparent to applications, users, and software
• Provides three areas of protection that
correspond to three IPSec protocols:
– Key management (ISAKMP/IKE - Oakley)
• A way to establish an SA
– Authentication (AH)
• A way to ensure the integrity of the communication
– Confidentiality (ESP)
• A way to ensure the privacy of the communication.
cs490ns-cotter
28
IPsec Protocols
• Internet Key Exchange
– Internet Security Association and Key Mgmt protocol
– OAKLEY – Based on Diffie-Hellman
– RFC 2408
• Authentication Header Protocol
– RFC2402
– Message digest based on data + key
• Encapsulating Security Payload Protocol
– RFC 2406
– Uses public key to encrypt, ICV to authenticate
cs490ns-cotter
29
IPSec (cont)
• Once an SA has been established, IPSec
provides a 2 phase security association
• Supports 2 Phase 1 modes:
– Transport mode encrypts or authenticates only the data
portion (payload) of each packet, yet leaves the header
unencrypted.
– Tunnel mode encrypts both the header and data portion
• IPSec accomplishes transport and tunnel modes
by adding new headers to the IP packet
• In tunnel mode the entire original packet is then
treated as the data portion of the new packet
cs490ns-cotter
30
IPsec Transport Mode
Orig. header
cs490ns-cotter
Auth.
header
TCP
Orig. Payload
31
IPsec Tunnel Mode
Original Packet
New header Orig. header
TCP
Orig. Payload
New Payload
New Packet
cs490ns-cotter
32
IPSec (cont)
• Supports 2 Phase 2 Associations
– Authentication
– Encryption
• Both Authentication Header (AH) and
Encapsulating Security Payload (ESP) can be
used with Transport or Tunnel mode, creating
four possible transport mechanisms:
– AH in transport mode / tunnel mode
– ESP in transport mode / tunnel mode
cs490ns-cotter
33
Authentication Header
Next = tcp
AH len
Reserved
SPI (Security Parameters Index)
Sequence Number
Authentication Data (SHA-1, etc.)
- Integrity Check Value -
Hash Message Auth Code
Secret Key Null pad
36363636
X
O
X
O
xor’d key
5c5c5c5c
Msg to Hash (full* IP Packet)
Hash Function
xor’d key
hash
Hash Function
Integrity Check Value
AH in Transport Mode
Orig IP
header
Authentication
header (AH)
TCP
Orig.
Payload
Authenticated packet
cs490ns-cotter
36
AH in Tunnel Mode
Original Packet
New IP
header
Authentication
header (AH)
Orig IP
header
TCP
Orig.
Payload
encrypted
Authenticated
cs490ns-cotter
37
ESP in Transport Mode
encrypted
Orig IP
header
ESP
TCP
header
Orig.
Payload
trailer
ESP
authentication
Authenticated
cs490ns-cotter
38
ESP in Tunnel Mode
encrypted
New IP
header
ESP
header
Orig IP
header
TCP
Orig.
Payload
trailer
ESP
authentication
Authenticated
cs490ns-cotter
39
Internet Key Exchange
• ISAKMP – Internet Security Association and Key
Management Protocol
– A framework to define and exchange the data elements (fiitds)
needed to support secure communications
• IKE – Internet Key Exchange
– Definition of a protocol that workws within the ISAKMP
framework to establish an SA.
• DOI – Domain of Interpretation.
– Provides the specifications for a particular use of ISAKMP.
– Original intent twas that there would be many different DOIs for
many different applications that might use ISAKMP
IKE – 2 Phases
• Phase 1
– Known as the ISAKMP SA
– Mutual authentication and establish session keys.
– Based on names, k]public keys, and/or shared
secrets.
• Phase 2
– Known as ESP or AH SA
– Uses keys established in phase 1 to develop multiple
separate channels
Phase 1 IKE – 2 modes
• Aggressive Mode
– Establishes mutual authentication and
session key establishment in 3 messages
– Simple and quick
• Main Mode
– Uses 6 messages to establish mutual
authentication and session key,
– Allows for selection of cryptographic protocols
IKE – Aggressive Mode
ga mod p, “Alice”, crypto proposal
Bob
Alice
gb mod p, crypto choice, proof for “Bob”
Proof for Alice
IKE – Main Mode
Crypto suites I support
Parameter
Negotiation
Crypto suite I choose
gb mod p
Bob
Alice
ga mod p
Diffie-Hellman
exchange
gab mod p, proof for “Alice”
gab mod p, proof for “Bob”
Send ID’s,
Authenticate,
etc.
IKE Phase 1 Key Types
• How do we authenticate? What info is used?
• Four Authentication Methods
–
–
–
–
Original Public Key Encryption
Revised Public Key Encryption
Public Key Signature
Pre-shared Key
• Two modes for each method
– Aggressive Mode
– Main Mode
Basic Phase 1 Association
• Revised Public key – main mode
Crypto Proposal
Crypto Proposal Accepted
KA = hash (nonceA, cookieA)
{nonceA}Bob, KA{ga mod p}, KA{Alice}, KA{Alice cert}
K = f(gab mod p, nonceA, nonceB, CookieA, CookieB)
K{proof I’m Alice}
K{proof I’m Bob}
Bob
Alice
KB = hash (nonceB, cookieB)
{nonceB}Alice, KB{gb mod p}, KB{Bob}
Phase 2 IKE – IPSec SA
• Quick Mode
Phase 1 SA
X, Y, CP, traffic, SPIA, nonceA, [ga mod p]
X, Y, ack
Bob
Alice
X, Y, CPA, traffic, SPIB, nonceB, [gb mod p]
Virtual Private Networks (VPNs)
• Takes advantage of using the public
Internet as if it were a private network
• Allow the public Internet to be used
privately
• Prior to VPNs, organizations were forced
to lease expensive data connections from
private carriers so employees could
remotely connect to the organization’s
network
cs490ns-cotter
48
VPNs (cont)
• Two common types of VPNs include:
– Remote-access VPN or virtual private dial-up
network (VPDN): user-to-LAN connection
used by remote users
– Site-to-site VPN: multiple sites can connect to
other sites over the Internet
• VPN transmissions achieved through
communicating with endpoints
– An endpoint can be software on a local
computer, a dedicated hardware device such
as a VPN concentrator, or even a firewall
cs490ns-cotter
49
VPNs (cont)
cs490ns-cotter
50
VPN (IPSec) Issues - NAT
• NAT objective is to change (map) private
IP addresses into public IP addresses
• AH includes IP addrs in hash.
• ESP encrypts original header, but not new
header
• Possible with ESP in tunnel mode, but
issues with IKE
• If possible, do IPSec outside of NAT, or in
a shared box.
Hardening File Transfer
Protocol (FTP)
• Three ways to work with FTP:
– Web browser
– FTP client
– Command line
• FTP servers can be configured to allow
unauthenticated users to transfer files
(called anonymous FTP or blind FTP)
cs490ns-cotter
52
FTP (cont)
• Vulnerabilities associated with using FTP
– FTP does not use encryption
– Files being transferred by FTP are vulnerable to
man-in-the-middle attacks
• Use secure FTP to reduce risk of attack
– Secure FTP is a term used by vendors to describe
encrypting FTP transmissions
• Most secure FTP products use Secure Socket
Layers (SSL) to perform the encryption
cs490ns-cotter
53
FTP (cont)
• FTP active mode
– Client connects from any random port >1,024 (PORT
N) to FTP server’s command port, port 21 (Step 1)
– Client starts listening to PORT N+1 and sends the
FTP command PORT N+1 to the FTP server
• FTP passive mode
– Client initiates both connections to server
– When opening an FTP connection, client opens two
local random unprivileged ports >1,024
cs490ns-cotter
54
FTP (cont)
cs490ns-cotter
55
FTP (Implicit / Explicit
Encryption)
• Implicit Encryption
– FTP server / client assume an encryption mechanism
(TLS / sslv1, sslv2, sslv3, etc.)
– No option to use non-encrypted channel provided
– Control port: 990; Data port: 989
– More secure – less flexible
• Explicit Encryption
– FTP server / client negotiates over encryption. May
choose encryption or not
– More flexible – potentially less secure
cs490ns-cotter
56
VSFTPd – Linux FTP Server
• Designed from scratch to be a more
secure FTP server.
– Secure coding techniques enforced
– Many options to control default behavior
– Incorporates encryption.
• Reference documents
– man vsftpd.conf – very helpful in describing
options not shown in default config file
– google …
cs490ns-cotter
57
Implementing VSFTPd ssl
• Verify that installed version of vsftpd supports
ssl (is binary linked to ssl library?)
– ldd /usr/sbin/vsftpd | grep ssl
– should return: libssl.so.6 => /lib/libssl.so.6
(0x0012c000)
• Verify a digital certificate & keys
– cat /etc/pki/tls/certs/vsftpd.pem
– or perhaps cat /etc/ssl/certs/vsftpd.pem
• Secure key / certificate to root only
– chmod 600 /etc/pki/tls/certs/vsftpd.pem
cs490ns-cotter
58
Implementing VSFTPd ssl
• If necessary, make a certificate and key
– cd /etc/pki/tls/certs
– make vsftpd.pem
– or
– cd /etc/ssl/certs
– openssl req –x509 –nodes –days 730 –
newkey \ rsa:1024 –keyout
//etc/ssl/certs/vsftpd.pem \
-out /etc/ssl/certs/vsfptf.pem
cs490ns-cotter
59
Implementing VSFTPd ssl
• Make changes / additions to vsftpd.conf
– ssl_enable=YES
– allow_anon_ssl=NO
– force_local_data_ssl=NO
– force_local_logins_ssl=YES
– ssl_tls=YES
– ssl_sslv2=NO
– ssl_sslv3=NO
– rsa_cert_file=/etc/pki/tls/certs/vsftpd.pem
cs490ns-cotter
60
Get an FTP Client
• Windows
–
–
–
–
coreFTP lite – free
smartFTP - $
turboFTP - $
many others …
• Linux
–
–
–
–
cs490ns-cotter
Kftpgrabber – ssl/tls
Kasablanca – ssl/tls
gftp – ssh
etc.
61
References:
•
•
•
•
•
•
•
PPP – rfc 1661, 1663, (Forouzan, Ch15)
PPTP – rfc 2637, Microsoft
PPPoE – rfc 2516
L2TP – rfc 2661, 3931, Cisco
RADIUS – rfc 2865
IPSec – rfc 2411 (2401 – 2412)
Google…
cs490ns-cotter
62
Summary
• FTP protocol has several security vulnerabilities
- it does not natively use encryption and is
vulnerable to man-in-the-middle attacks
• FTP can be hardened by using secure FTP
(which encrypts using SSL)
• Protecting remote access transmissions is
particularly important in today’s environment as
more users turn to the Internet as the
infrastructure for accessing protected
information
cs490ns-cotter
63
Summary
• Authenticating a transmission to ensure
it came from the sender can provide
increased security for remote access
users
• SSH is a UNIX-based command
interface and protocol for securely
accessing a remote computer
cs490ns-cotter
64