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NAME | SYNOPSIS | DESCRIPTION | EXAMPLES | INTEROPERABILITY | SEE ALSO | AUTHOR | COLOPHON |
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IP-L2TP(8) Linux IP-L2TP(8)
ip-l2tp - L2TPv3 static unmanaged tunnel configuration
ip [ OPTIONS ] l2tp { COMMAND | help }
ip l2tp add tunnel
remote ADDR local ADDR
tunnel_id ID peer_tunnel_id ID
[ encap { ip | udp } ]
[ udp_sport PORT ] [ udp_dport PORT ]
[ udp_csum { on | off } ]
[ udp6_csum_tx { on | off } ]
[ udp6_csum_rx { on | off } ]
ip l2tp add session [ name NAME ]
tunnel_id ID session_id ID peer_session_id ID
[ cookie HEXSTR ] [ peer_cookie HEXSTR ]
[ l2spec_type { none | default } ]
[ seq { none | send | recv | both } ]
ip l2tp del tunnel tunnel_id ID
ip l2tp del session tunnel_id ID session_id ID
ip l2tp show tunnel [ tunnel_id ID ]
ip l2tp show session [ tunnel_id ID.B ] [ session_id ID ]
NAME := STRING
ADDR := { IP_ADDRESS | any }
PORT := { NUMBER }
ID := { NUMBER }
HEXSTR := { 8 or 16 hex digits (4 / 8 bytes) }
The ip l2tp commands are used to establish static, or so-called
unmanaged L2TPv3 ethernet tunnels. For unmanaged tunnels, there
is no L2TP control protocol so no userspace daemon is required -
tunnels are manually created by issuing commands at a local
system and at a remote peer.
L2TPv3 is suitable for Layer-2 tunneling. Static tunnels are
useful to establish network links across IP networks when the
tunnels are fixed. L2TPv3 tunnels can carry data of more than one
session. Each session is identified by a session_id and its
parent tunnel's tunnel_id. A tunnel must be created before a
session can be created in the tunnel.
When creating an L2TP tunnel, the IP address of the remote peer
is specified, which can be either an IPv4 or IPv6 address. The
local IP address to be used to reach the peer must also be
specified. This is the address on which the local system will
listen for and accept received L2TP data packets from the peer.
L2TPv3 defines two packet encapsulation formats: UDP or IP. UDP
encapsulation is most common. IP encapsulation uses a dedicated
IP protocol value to carry L2TP data without the overhead of UDP.
Use IP encapsulation only when there are no NAT devices or
firewalls in the network path.
When an L2TPv3 ethernet session is created, a virtual network
interface is created for the session, which must then be
configured and brought up, just like any other network interface.
When data is passed through the interface, it is carried over the
L2TP tunnel to the peer. By configuring the system's routing
tables or adding the interface to a bridge, the L2TP interface is
like a virtual wire (pseudowire) connected to the peer.
Establishing an unmanaged L2TPv3 ethernet pseudowire involves
manually creating L2TP contexts on the local system and at the
peer. Parameters used at each site must correspond or no data
will be passed. No consistency checks are possible since there is
no control protocol used to establish unmanaged L2TP tunnels.
Once the virtual network interface of a given L2TP session is
configured and enabled, data can be transmitted, even if the peer
isn't yet configured. If the peer isn't configured, the L2TP data
packets will be discarded by the peer.
To establish an unmanaged L2TP tunnel, use l2tp add tunnel and
l2tp add session commands described in this document. Then
configure and enable the tunnel's virtual network interface, as
required.
Note that unmanaged tunnels carry only ethernet frames. If you
need to carry PPP traffic (L2TPv2) or your peer doesn't support
unmanaged L2TPv3 tunnels, you will need an L2TP server which
implements the L2TP control protocol. The L2TP control protocol
allows dynamic L2TP tunnels and sessions to be established and
provides for detecting and acting upon network failures.
ip l2tp add tunnel - add a new tunnel
tunnel_id ID
set the tunnel id, which is a 32-bit integer value.
Uniquely identifies the tunnel. The value used must match
the peer_tunnel_id value being used at the peer.
peer_tunnel_id ID
set the peer tunnel id, which is a 32-bit integer value
assigned to the tunnel by the peer. The value used must
match the tunnel_id value being used at the peer.
remote ADDR
set the IP address of the remote peer. May be specified as
an IPv4 address or an IPv6 address.
local ADDR
set the IP address of the local interface to be used for
the tunnel. This address must be the address of a local
interface. May be specified as an IPv4 address or an IPv6
address.
encap ENCAP
set the encapsulation type of the tunnel.
Valid values for encapsulation are: udp, ip.
udp_sport PORT
set the UDP source port to be used for the tunnel. Must be
present when udp encapsulation is selected. Ignored when
ip encapsulation is selected.
udp_dport PORT
set the UDP destination port to be used for the tunnel.
Must be present when udp encapsulation is selected.
Ignored when ip encapsulation is selected.
udp_csum STATE
(IPv4 only) control if IPv4 UDP checksums should be
calculated and checked for the encapsulating UDP packets,
when UDP encapsulating is selected. Default is off.
Valid values are: on, off.
udp6_csum_tx STATE
(IPv6 only) control if IPv6 UDP checksums should be
calculated for encapsulating UDP packets, when UDP
encapsulating is selected. Default is on.
Valid values are: on, off.
udp6_csum_rx STATE
(IPv6 only) control if IPv6 UDP checksums should be
checked for the encapsulating UDP packets, when UDP
encapsulating is selected. Default is on.
Valid values are: on, off.
ip l2tp del tunnel - destroy a tunnel
tunnel_id ID
set the tunnel id of the tunnel to be deleted. All
sessions within the tunnel must be deleted first.
ip l2tp show tunnel - show information about tunnels
tunnel_id ID
set the tunnel id of the tunnel to be shown. If not
specified, information about all tunnels is printed.
ip l2tp add session - add a new session to a tunnel
name NAME
sets the session network interface name. Default is
l2tpethN.
tunnel_id ID
set the tunnel id, which is a 32-bit integer value.
Uniquely identifies the tunnel into which the session will
be created. The tunnel must already exist.
session_id ID
set the session id, which is a 32-bit integer value.
Uniquely identifies the session being created. The value
used must match the peer_session_id value being used at
the peer.
peer_session_id ID
set the peer session id, which is a 32-bit integer value
assigned to the session by the peer. The value used must
match the session_id value being used at the peer.
cookie HEXSTR
sets an optional cookie value to be assigned to the
session. This is a 4 or 8 byte value, specified as 8 or 16
hex digits, e.g. 014d3636deadbeef. The value must match
the peer_cookie value set at the peer. The cookie value is
carried in L2TP data packets and is checked for expected
value at the peer. Default is to use no cookie.
peer_cookie HEXSTR
sets an optional peer cookie value to be assigned to the
session. This is a 4 or 8 byte value, specified as 8 or 16
hex digits, e.g. 014d3636deadbeef. The value must match
the cookie value set at the peer. It tells the local
system what cookie value to expect to find in received
L2TP packets. Default is to use no cookie.
l2spec_type L2SPECTYPE
set the layer2specific header type of the session.
Valid values are: none, default.
seq SEQ
controls sequence numbering to prevent or detect out of
order packets. send puts a sequence number in the default
layer2specific header of each outgoing packet. recv
reorder packets if they are received out of order.
Default is none.
Valid values are: none, send, recv, both.
ip l2tp del session - destroy a session
tunnel_id ID
set the tunnel id in which the session to be deleted is
located.
session_id ID
set the session id of the session to be deleted.
ip l2tp show session - show information about sessions
tunnel_id ID
set the tunnel id of the session(s) to be shown. If not
specified, information about sessions in all tunnels is
printed.
session_id ID
set the session id of the session to be shown. If not
specified, information about all sessions is printed.
Setup L2TP tunnels and sessions
site-A:# ip l2tp add tunnel tunnel_id 3000 peer_tunnel_id 4000 \
encap udp local 1.2.3.4 remote 5.6.7.8 \
udp_sport 5000 udp_dport 6000
site-A:# ip l2tp add session tunnel_id 3000 session_id 1000 \
peer_session_id 2000
site-B:# ip l2tp add tunnel tunnel_id 4000 peer_tunnel_id 3000 \
encap udp local 5.6.7.8 remote 1.2.3.4 \
udp_sport 6000 udp_dport 5000
site-B:# ip l2tp add session tunnel_id 4000 session_id 2000 \
peer_session_id 1000
site-A:# ip link set l2tpeth0 up mtu 1488
site-B:# ip link set l2tpeth0 up mtu 1488
Notice that the IP addresses, UDP ports and tunnel / session ids
are matched and reversed at each site.
Configure as IP interfaces
The two interfaces can be configured with IP addresses if only IP
data is to be carried. This is perhaps the simplest
configuration.
site-A:# ip addr add 10.42.1.1 peer 10.42.1.2 dev l2tpeth0
site-B:# ip addr add 10.42.1.2 peer 10.42.1.1 dev l2tpeth0
site-A:# ping 10.42.1.2
Now the link should be usable. Add static routes as needed to
have data sent over the new link.
Configure as bridged interfaces
To carry non-IP data, the L2TP network interface is added to a
bridge instead of being assigned its own IP address, using
standard Linux utilities. Since raw ethernet frames are then
carried inside the tunnel, the MTU of the L2TP interfaces must be
set to allow space for those headers.
site-A:# ip link set l2tpeth0 up mtu 1446
site-A:# ip link add br0 type bridge
site-A:# ip link set l2tpeth0 master br0
site-A:# ip link set eth0 master br0
site-A:# ip link set br0 up
If you are using VLANs, setup a bridge per VLAN and bridge each
VLAN over a separate L2TP session. For example, to bridge VLAN ID
5 on eth1 over an L2TP pseudowire:
site-A:# ip link set l2tpeth0 up mtu 1446
site-A:# ip link add brvlan5 type bridge
site-A:# ip link set l2tpeth0.5 master brvlan5
site-A:# ip link set eth1.5 master brvlan5
site-A:# ip link set brvlan5 up
Adding the L2TP interface to a bridge causes the bridge to
forward traffic over the L2TP pseudowire just like it forwards
over any other interface. The bridge learns MAC addresses of
hosts attached to each interface and intelligently forwards
frames from one bridge port to another. IP addresses are not
assigned to the l2tpethN interfaces. If the bridge is correctly
configured at both sides of the L2TP pseudowire, it should be
possible to reach hosts in the peer's bridged network.
When raw ethernet frames are bridged across an L2TP tunnel, large
frames may be fragmented and forwarded as individual IP fragments
to the recipient, depending on the MTU of the physical interface
used by the tunnel. When the ethernet frames carry protocols
which are reassembled by the recipient, like IP, this isn't a
problem. However, such fragmentation can cause problems for
protocols like PPPoE where the recipient expects to receive
ethernet frames exactly as transmitted. In such cases, it is
important that frames leaving the tunnel are reassembled back
into a single frame before being forwarded on. To do so, enable
netfilter connection tracking (conntrack) or manually load the
Linux netfilter defrag modules at each tunnel endpoint.
site-A:# modprobe nf_defrag_ipv4
site-B:# modprobe nf_defrag_ipv4
If L2TP is being used over IPv6, use the IPv6 defrag module.
Unmanaged (static) L2TPv3 tunnels are supported by some network
equipment vendors such as Cisco.
In Linux, L2TP Hello messages are not supported in unmanaged
tunnels. Hello messages are used by L2TP clients and servers to
detect link failures in order to automate tearing down and
reestablishing dynamic tunnels. If a non-Linux peer supports
Hello messages in unmanaged tunnels, it must be turned off to
interoperate with Linux.
Linux defaults to use the Default Layer2SpecificHeader type as
defined in the L2TPv3 protocol specification, RFC3931. This
setting must be consistent with that configured at the peer. Some
vendor implementations (e.g. Cisco) default to use a
Layer2SpecificHeader type of None.
ip(8)
James Chapman <[email protected]>
This page is part of the iproute2 (utilities for controlling
TCP/IP networking and traffic) project. Information about the
project can be found at
⟨http://www.linuxfoundation.org/collaborate/workgroups/networking/iproute2⟩.
If you have a bug report for this manual page, send it to
[email protected], [email protected]. This page was
obtained from the project's upstream Git repository
⟨https://git.kernel.org/pub/scm/network/iproute2/iproute2.git⟩ on
2024-06-14. (At that time, the date of the most recent commit
that was found in the repository was 2024-06-11.) If you
discover any rendering problems in this HTML version of the page,
or you believe there is a better or more up-to-date source for
the page, or you have corrections or improvements to the
information in this COLOPHON (which is not part of the original
manual page), send a mail to [email protected]
iproute2 19 Apr 2012 IP-L2TP(8)
Pages that refer to this page: ip(8)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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