l2tpd - L2TP for Unix

Land of confusion

Overview of L2TP protocol

Layer 2 Tunnelling Protocol

Product of the PPP Extensions working group of the IETF

Largely builds on the work done with L2F at Cisco

Backwards compatible with L2F

L2TP features

Encapsulates PPP

Utilizes UDP

Reliable Signalling channel ("Control Connection")

Unreliable Data Channel

Data Channel sequencing

Tunnel level authentication

Applications for L2TP

Half of a secure VPN implementation

Overlay network

Avoid University firewalls

Remote static IP address

Global addressing behind NAT

L2TP Header Format

1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|T|L|x|x|S|x|O|P|x|x|x|x|Version|+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Length (opt.) |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Tunnel ID |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Session ID |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Sequence Number (Ns) |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Expected Sequence Number (Nr) |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Offset Size (opt.) |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Offset Pad (opt.) |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

T=Type bit Set for control packetsL=Length bit Set if Length field is presentS=Sequence bit Set if sequence numbers are present, always set for Control messagesO=Offset bit Set if offset field is present Rarely usedP=Priority bit Set if this packet should be given preferential treatment I've never seen this usedVersion Set to 2 currently, 1 indicates L2F as it used substantially the same header format

L2TP Header Format

1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|T|L|x|x|S|x|O|P|x|x|x|x|Version|+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Length (opt.) |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Tunnel ID |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Session ID |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Sequence Number (Ns) |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Expected Sequence Number (Nr) |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Offset Size (opt.) |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Offset Pad (opt.) |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Length Total length of L2TP packetTunnel ID Identifies Tunnel that this packet is associated withSession ID Identifies Session within the tunnel that this packet is associated withSequence Number (Ns) Start with 0, increment for each packet in tunnelExpected Sequence Number (Nr) Sequence number expected in next packet from peerOffset Size Size, in octets of Offset Pad fieldOffset Pad Undefined “filler”

L2TP AVP Format

1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|M|H| rsvd | Length |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Vendor ID |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Attribute Type |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+| Attribute Value +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...[until length is reached... |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

M=Mandatory bit If you don't understand this AVP, shut down the tunnel or sessionH=Hidden bit This attribute is "hidden"Length Overall length of the AVP, including these headers (minimum value of 6)Vendor ID 0 for IETF defined AVP'sAttribute Type What attribute is this?Attribute Value Actual attribute data (if Length field is 6, this field is absent)

L2TP Message Types

1 - SCCRQ - Start Control Connection ReQuest2 - SCCRP - Start Control Connection RePly3 - SCCCN - Start Connection Control CoNnected4 - StopCCN - Stop Control Connection Notification6 - HELLO - HELLO7 - OCRQ - Outgoing Call ReQuest8 - OCRP - Outgoing Call RePly9 - OCCN - Outgoing Call CoNnected10 - ICRQ - Incoming Call ReQuest11 - ICRP - Incoming Call RePly12 - ICCN - Incoming Call CoNnected14 - CDN - Call Disconnect Notification15 - WEN - Wan Error Notify16 - SLI - Set Link Info

The Future of L2TP - L2TPv3

New data channel header format

Clarify ambiguities in current standard

Encapsulates other types of frames than PPP (work with PWE3 - Pseudo Wire Emulation Edge to Edge)

Ethernet

Frame relay

Circuit emulation

Overview of l2tpd software package

History of l2tpd

Originally written by Mark Spencer http://marko.net last version 0.60

Forked in 2000 by Scott Balmos and David Stipp, hosted on Sourceforge http://sf.net/projects/l2tpd

Turned over control in Jan. 2002 to me

Obtained support of Mark Spencer for further development as well

Further detail at http://www.l2tpd.org/history.html

l2tpd features

Tunnel level authentication (currently broken)

Works as both LNS and LAC (only with PPP on same system)

"Autodial"

pre-configured peers

"interactive" control (named pipe)

Known bugs

Challenge Response authentication on tunnels broken

Data channel sequencing support doesn't exist

pty handling is horrible, though works after a fashion

Session and tunnel shutdown works, barely

l2tpd - current work directions

Bugfixing, bugfixing, bugfixing

Easily integrated new features (data sequencing)

Knobs for interoperability

Did I mention bugfixing?

No significant new feature work

l2tpd-devel

What I'm calling the "next-generation" l2tpd

No working code available yet

kernel-module

pppd plugin

l2tpd daemon

"Interactive" use of l2tpd

echo "t 204.255.233.225" > /var/run/l2tp-control

Standard output of l2tpd gives you a tunnel ID:control_finish: Connection established to 204.255.233.225, 1701. Local: 17767, Remote: 86.

echo "c 17767" > /var/run/l2tp-control

/etc/l2tp/l2tpd.conf[global]port = 1701

[lns default]ip range = 192.168.0.1-192.168.0.2lac = 10.0.0.0 - 10.255.255.255hidden bit = norefuse chap = yesrequire authentication = yesppp debug = yes

[lac cindsl01]lns = 204.255.233.225autodial = yes

[lac iostest]lns = 199.170.85.70

l2tpd compared to other tunnelling technologies

Why L2TP vs. IPSec Tunnel Mode

Multi-Protocol

Authentication flexibility

Works via NAT

Multi-Link PPP avoids MTU issues

Encryption

Encryption

Encryption

Protocol overhead

L2TP IPSec Tunnel

Why L2TP vs. PPTP

Open standard

Not Microsoft - 'nuff said

Weak encryption

Microsoft

Non-standard

L2TP PPTP

Why L2TP vs. GRE or IPIP

Scaleability

Control protocol mechanism

Authentication

Multi-protocol

Low protocol overhead

Simple configuration

No dependancy on PPP

L2TP GRE/IPIP

Why L2TP vs. VTun

Interoperability

Open standard

Can carry varied types of traffic

Open

Encryption

Traffic Shaping

L2TP VTun