Tunnel Interfaces
This article touches on ‘classic’ IP tunneling protocols.
GRE is often seen as a one size fits all solution when it comes to classic IP tunneling protocols, and for a good reason. However, there are more specialized options, and many of them are supported by VyOS. There are also rather obscure GRE options that can be useful.
All those protocols are grouped under ‘interfaces tunnel’ in VyOS. Let’s take a closer look at the protocols and options currently supported by VyOS.
IPIP
This is one of the simplest types of tunnels, as defined by RFC2003. It takes an IPv4 packet and sends it as a payload of another IPv4 packet. For this reason, there are no other configuration options for this kind of tunnel.
An example:
set interfaces tunnel tun0 encapsulation ipip
set interfaces tunnel tun0 local-ip 192.0.2.10
set interfaces tunnel tun0 remote-ip 203.0.113.20
set interfaces tunnel tun0 address 192.168.100.200
IP6IP6
This is the IPv6 counterpart of IPIP. I’m not aware of an RFC that defines this encapsulation specifically, but it’s a natural specific case of IPv6 encapsulation mechanisms described in RFC2473.
It’s not likely that anyone will need it any time soon, but it does exist.
An example:
set interfaces tunnel tun0 encapsulation ipip
set interfaces tunnel tun0 local-ip 2001:db8:aa::1/64
set interfaces tunnel tun0 remote-ip 2001:db8:aa::2/64
set interfaces tunnel tun0 address 2001:db8:bb::1/64
IPIP6
In the future this is expected to be a very useful protocol (though there are other proposals).
As the name implies, it’s IPv4 encapsulated in IPv6, as simple as that.
An example:
set interfaces tunnel tun0 encapsulation ipip6
set interfaces tunnel tun0 local-ip 2001:db8:aa::1/64
set interfaces tunnel tun0 remote-ip 2001:db8:aa::2/64
set interfaces tunnel tun0 address 192.168.70.80
6in4 (SIT)
6in4 uses tunneling to encapsulate IPv6 traffic over IPv4 links as defined in RFC4213. The 6in4 traffic is sent over IPv4 inside IPv4 packets whose IP headers have the IP protocol number set to 41. This protocol number is specifically designated for IPv6 encapsulation, the IPv4 packet header is immediately followed by the IPv6 packet being carried. The encapsulation overhead is the size of the IPv4 header of 20 bytes, therefore with an MTU of 1500 bytes, IPv6 packets of 1480 bytes can be sent without fragmentation. This tunneling technique is frequently used by IPv6 tunnel brokers like Hurricane Electric.
An example:
set interfaces tunnel tun0 encapsulation sit
set interfaces tunnel tun0 local-ip 192.0.2.10
set interfaces tunnel tun0 remote-ip 192.0.2.20
set interfaces tunnel tun0 address 2001:db8:bb::1/64
A full example of a Tunnelbroker.net config can be found at here.
Generic Routing Encapsulation (GRE)
A GRE tunnel operates at layer 3 of the OSI model and is repsented by IP protocol 47. The main benefit of a GRE tunnel is that you are able to route traffic across disparate networks. GRE also supports multicast traffic and supports routing protocols that leverage multicast to form neighbor adjacencies.
Configuration
A basic configuration requires a tunnel source (local-ip), a tunnel destination (remote-ip), an encapsulation type (gre), and an address (ipv4/ipv6). Below is a configuration example taken from a VyOS router and a Cisco IOS router. The main difference between these two configurations is that VyOS requires you explicitly configure the encapsulation type. The Cisco router defaults to ‘gre ip’ otherwise it would have to be configured as well.
VyOS Router:
set interfaces tunnel tun100 address '10.0.0.1/30'
set interfaces tunnel tun100 encapsulation 'gre'
set interfaces tunnel tun100 local-ip '198.51.100.2'
set interfaces tunnel tun100 remote-ip '203.0.113.10'
Cisco IOS Router:
interface Tunnel100
ip address 10.0.0.2 255.255.255.252
tunnel source 203.0.113.10
tunnel destination 198.51.100.2
Tunnel keys
GRE is also the only classic protocol that allows creating multiple tunnels with the same source and destination due to its support for tunnel keys. Despite its name, this feature has nothing to do with security: it’s simply an identifier that allows routers to tell one tunnel from another.
An example:
set interfaces tunnel tun0 local-ip 192.0.2.10
set interfaces tunnel tun0 remote-ip 192.0.2.20
set interfaces tunnel tun0 address 10.40.50.60/24
set interfaces tunnel tun0 parameters ip key 10
set interfaces tunnel tun0 local-ip 192.0.2.10
set interfaces tunnel tun0 remote-ip 192.0.2.20
set interfaces tunnel tun0 address 172.16.17.18/24
set interfaces tunnel tun0 parameters ip key 20
GRE-Bridge
While normal GRE is for layer 3, GRE-Bridge is for layer 2. GRE-Bridge can encapsulate Ethernet frames, thus it can be bridged with other interfaces to create datalink layer segments that span multiple remote sites.
Layer 2 GRE example:
set interfaces bridge br0
set interfaces tunnel tun0 encapsulation gre-bridge
set interfaces tunnel tun0 local-ip 192.0.2.10
set interfaces tunnel tun0 remote-ip 192.0.2.20
set interfaces tunnel tun0 parameters ip bridge-group bridge br0
set interfaces ethernet eth1 bridge-group br0
As you can see, the bridge-group option for tunnels is in a rather unusual place, different from all other interfaces.
Troubleshooting
GRE is a well defined standard that is common in most networks. While not inherently difficult to configure there are a couple of things to keep in mind to make sure the configuration performs as expected. A common cause for GRE tunnels to fail to come up correctly include ACL or Firewall configurations that are discarding IP protocol 47 or blocking your source/desintation traffic.
1. Confirm IP connectivity between tunnel local-ip and remote-ip:
vyos@vyos:~$ ping 203.0.113.10 interface 198.51.100.2 count 4
PING 203.0.113.10 (203.0.113.10) from 198.51.100.2 : 56(84) bytes of data.
64 bytes from 203.0.113.10: icmp_seq=1 ttl=254 time=0.807 ms
64 bytes from 203.0.113.10: icmp_seq=2 ttl=254 time=1.50 ms
64 bytes from 203.0.113.10: icmp_seq=3 ttl=254 time=0.624 ms
64 bytes from 203.0.113.10: icmp_seq=4 ttl=254 time=1.41 ms
--- 203.0.113.10 ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 3007ms
rtt min/avg/max/mdev = 0.624/1.087/1.509/0.381 ms
2. Confirm the link type has been set to GRE:
vyos@vyos:~$ show interfaces tunnel tun100
tun100@NONE: <POINTOPOINT,NOARP,UP,LOWER_UP> mtu 1476 qdisc noqueue state UNKNOWN group default qlen 1000
link/gre 198.51.100.2 peer 203.0.113.10
inet 10.0.0.1/30 brd 10.0.0.3 scope global tun100
valid_lft forever preferred_lft forever
inet6 fe80::5efe:c612:2/64 scope link
valid_lft forever preferred_lft forever
RX: bytes packets errors dropped overrun mcast
2183 27 0 0 0 0
TX: bytes packets errors dropped carrier collisions
836 9 0 0 0 0
3. Confirm IP connectivity across the tunnel:
vyos@vyos:~$ ping 10.0.0.2 interface 10.0.0.1 count 4
PING 10.0.0.2 (10.0.0.2) from 10.0.0.1 : 56(84) bytes of data.
64 bytes from 10.0.0.2: icmp_seq=1 ttl=255 time=1.05 ms
64 bytes from 10.0.0.2: icmp_seq=2 ttl=255 time=1.88 ms
64 bytes from 10.0.0.2: icmp_seq=3 ttl=255 time=1.98 ms
64 bytes from 10.0.0.2: icmp_seq=4 ttl=255 time=1.98 ms
--- 10.0.0.2 ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 3008ms
rtt min/avg/max/mdev = 1.055/1.729/1.989/0.395 ms
Virtual Tunnel Interface (VTI)
Set Virtual Tunnel Interface
set interfaces vti vti0 address 192.168.2.249/30
set interfaces vti vti0 address 2001:db8:2::249/64
Results in:
vyos@vyos# show interfaces vti
vti vti0 {
address 192.168.2.249/30
address 2001:db8:2::249/64
description "Description"
}
Warning
When using site-to-site IPsec with VTI interfaces, be sure to disable route autoinstall
set vpn ipsec options disable-route-autoinstall
More details about the IPsec and VTI issue and option disable-route-autoinstall https://blog.vyos.io/vyos-1-dot-2-0-development-news-in-july
The root cause of the problem is that for VTI tunnels to work, their traffic selectors have to be set to 0.0.0.0/0 for traffic to match the tunnel, even though actual routing decision is made according to netfilter marks. Unless route insertion is disabled entirely, StrongSWAN thus mistakenly inserts a default route through the VTI peer address, which makes all traffic routed to nowhere.