VPN and IPsec Network Administration Using Linux Virtual Private Network and IPSec 04/2009
What is VPN? VPN is an emulation of a private Wide Area Network (WAN) using shared or public IP facilities. A typical VPN consists of two or more private intranets connected by logical tunnels using a public network. These tunnels enables the two ends to exchange data similar to point to point communication.
Site to Site VPN
VPN Components Security Mechanisms: Payload Encryption User Authentication Resource Authorization VPN Tunnelling Protocols: IP Security (IPsec) Point to Point Tunneling Protocol (PPTP) Layer 2 Tunneling Protocol (L2TP)
Encryption The process of converting data into a form that can be read only by the intented receiver. Public key encryption: Sender uses the receiver's public key to encrypt the message Receiver uses its private key to decrypt the the encoded message Pretty Good Privacy (PGP), Data Encryption Standared (DES)
Authentication To verify the identity of the user. Username / password Secret key encryption Public key encryption
Authorization Granting or denying access to resources to a user. The user must be identified and authenticated. Access to resource can be based on source and destination IP addresses, port addresses, and group affiliations time, day, date, application, service, authentication method, etc.
IPSec Internet Protocol Security (IPSec) RFC 2401 Open Internet Standard Transmission security (data encryption) User authentication Operate at the Network Layer
PPTP Point to Point Tunneling Protocol: Microsoft, 3COM and Ascend Communications Proposed as an alternative to IPSec Operate at Layer 2 (Data Link Layer) Used for secure transmission of Windows based traffic RFC 1171
L2TP Layer 2 Tunneling Protocol: Cisco Systems A combination of Layer 2 forwarding and PPTP Offer strong encryption of data
Tunneling Components Target network network that contains resources for remote access Initiator node remote client or server that initiates the VPN session HA (home agent) software at the network access node (router) in the target network. FA (foreign agent) software at the initiator node or at the network access node (router) of the network to which the initiator node belongs
Tunnel Operations The initiator sends a connection request to the FA FA authenticate the user FA forwards request to the HA of the target network HA verify the supply information and sends back information for FA to establish a tunnel The initiator starts forwarding data packets to FA
Tunnel operation 2 FA creates the tunnel header and the routable protocol header for the data packet FA encrypts the data and appends tunnel header and routable protocol header to the data FA forward the resulting packet to HA HA strips off the headers and decrypts the data HA forward the original data packet to the intended destination node.
Tunneled Packet
What is IPSec? IPSec (Internet Protocol Security) refers to a suite of protocols: AH Authentication Header Protocol ESP Encapsulating Security Payload Protocol IKE (or ISAKMP/Oakley) Internet Key Exchange or Internet Security Association and Key Management Protocol
IPSec SA Security Association (SA) is fundamental to IPSec. An SA is a unidirectional (simplex) logical connection between two IPsec systems. An SA consists of: Security parameter index (SPI) IP Destination Address Security Protocol
Security Association Security parameter index (SPI) A 32 bit vlaue used to identify different Sas with the same destination address and security protocol. The SPI is carried in the header of the security protocol (AH or ESP) IP Destination Address This address can be a unicast, broadcast or multicast IP address. Current SA management mechanism is defined only forunicast addresses. Security Protocol This can be either AH or ESP
IPSec SA Databases An IPSec SA uses two databases: The Security Association Database (SAD) maintains the information related to each SA. This information includes the algorithm keys, SA lifespan, and sequence numbers. The Security Policy Database (SPD), maintains the information about security services along with an ordered list of inbound and outbound policy entries.much like firewall rules and packet filters, these entries define what traffic must be processed and what traffic must be ignored per IPSec standards.
Authentication Header AH is used to provide integrity and authentication to IP datagrams Replay protection is alos possible AH is used in two modes: transport mode and tunnel mode AH is identified by protocol number 51
Encapsulating Security Payload ESP is used to provide integrity check, authentication, and encryption to IP datagrams Optional replay protection is also possible ESP is used in two modes: Transport mode and tunnel mode ESP is identified by protocol number 50
AH format
ESP Format
Transport Mode
Transport Mode
Tunnel Mode
Tunnel Mode
Internet Key Exchange Protocol Previousely referred to as ISAKMP/Oakley Supports automated negotiation of Security Associations Supports automated generation and refresh of crypotographic keys Uses parts of ISAKMP and prarts of Oakley and SKEME key exchange protocols to provide management of keys and security associations for the IPSec AH and ESP protocols.
IKE Version 1 RFC 2409 The Internet Key Exchange RFC 2408 Internet Security Association and Key Management Protocol (ISAKMP) RFC 2407 The Internet IP Security Domain of Interpretation for ISAKMP RFC 2412 The OAKLEY Key Determination Protocol RFC 2411 IP Security Document Roadmap
IKE Phases Internet Key Exchange negotiation operates in two separate phases: Phase 1 Authenticating the other IPsec gateway Negotiating an IKE SA with the other gateway Setup a secure two way tunnel for IPSec using ISAKMP to handle phase 2 negotiation (Internet Security Association and Key Management Protocol)
IKE Phase 2 Phase 2 Using the ISAKMP SA, negotiate the set of Security Paramenters for the IPSec (ESP and/or AH) tunnel (ESP/AH keys are unidirectional) Creating the IPSec tunnel Both phases use UDP port 500 for their negotiations ESP and AH protocols do not have ports
Free/Open IPSec Implementation FreeS/WAN http://www.freeswan.org (no longer active) OpenSWAN http://www.openswan.org (based on FreeS/WAN)