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An Introduction to the Tunneling Protocols

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An Introduction to the Tunneling Protocols

Alexandre Dulaunoy and various

ASBL CSRRT-LU (Computer Security Research and Response Team Luxembourg)

http://www.csrrt.org/

10th February 2006

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Definition of Tunneling Protocols

”A tunneling protocol is a network protocol which encapsulates one protocol or session inside another. Protocol A is encapsulated within protocol B, such that A treats B as though it were a data link layer. Tunneling may be used to transport a network protocol through a network which would not otherwise support it.

Tunnelling may also be used to provide various types of VPN functionality such as private addressing.”

e.g. : a transit network unsupporting a specific protocol (DECnet), the need to provide confidentiality and integrity for a specific network stream.

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Types of Tunneling Protocols

Stream oriented (e.g. SSL/TLS)

Encapsulation/Tunneling is done bytes per bytes.

Datagram ”packet” oriented (e.g. IPsec, PPP, PPTP) Encapsulation/Tunneling is done per packet and packets are (often) standalone entities.

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IP headers

proto : 4 (IPinIP), 47 (GRE), 50 (IPsec ESP), 51 (IPsec AH)

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Generic Routing Encapsulation (GRE)

described in RFC2784

A first try to generalize and standardize the approach to encapsulate an arbitrary network layer protocol in another arbitrary network layer protocol.

GRE header includes minimal information like an ether type.

Cryptographic Integrity, Confidentiality is *not* provided by GRE.

Various specific requirements are required when forwarding/decapsulating the packet. (TTL and alike)

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PPTP

described in RFC2637

encapsulate network frame in PPP using the GRE tunneling protocol.

use TCP port 1723 to manage the GRE tunnels. At least two sessions are required to establish one tunnel.

Sessions are authenticated using MSCHAP-v2.

Confidentiality and Integrity can be provided using MPPE (RFC3078) but various security flaws/issue exist.

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IPsec

IPsec a suite of tunneling (and non-tunneling) protocols to secure network connection.

A (very) complex suite of protocols described by different RFCs.

IPsec is a framework to build secure network connection (you can choose between the encryption, the various modes, authentication,...).

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IPsec : the framework

AH or ESP

Authentication mode (AH) or Encapsulation Security Payload (ESP). AH is just doing authentication but ESP is doing authentication and encryption.

Tunnel mode or Transport mode

Transport Mode provides a secure connection between two endpoints (only IP payload) in Tunnel mode is the entire IP packets being encapsulated. (Tunnel or Transport mode ? ip type is always tunnel, the rest is transport...)

Authentication and Encryption mode (AES+SHA1 ? AES+MD5)

Authentication calculates an Integrity Check Value (ICV) over the packet’s contents, with crypto hash MD5 or SHA-1. The shared secret key used, and this allows the recipient to compute the ICV in the same way. by so authenticate both ends.

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IPsec : the framework 2nd part

IKE or manual keys

Various shared secrets (keys) must be shared between the various ends. You can manually set the various required keys by using a “second channel” to distribute the secrets. This can be very complicated with a large meshed vpn networks. IKE (a possible) key exchange is proposed in the IPsec framework in order to exchange info.

Main mode or agressive mode

Main mode is a 6-ways initial exchange to setup the IKE key exchange. The main mode is secure but can generate compatibility issues between implementation. Agressive mode is simpler but somewhat weaker...

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IPsec AH/Transport mode

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IPsec AH/Tunnel mode

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IPsec HMAC (AH auth)

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IPsec ESP/Transport mode

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IPsec ESP/Tunnel mode

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IPsec a final use

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Q and A

Thanks for listening.

http://www.csrrt.org.lu/

adulau@foo.be

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