8.13: IPv6 Examples Without a Router

In this section we present a few IPv6 experiments that can be done without an IPv6 connection and without even an IPv6 router. Without a router, we cannot use SLAAC or DHCPv6. We will instead use link-local addresses, which require the specification of the interface along with the address, and manually configured unique-local (8.3 Network Prefixes) addresses. One practical problem with link-local addresses is that application documentation describing how to include a specification of the interface is sometimes sparse.

8.12.1 ping6

The IPv6 analogue of the familiar ping command, used to send ICMPv6 Echo Requests, is ping6 on Linux and Mac systems and ping -6 on Windows. The ping6 command supports an option to specify the interface; eg -I eth0; as noted above, this is mandatory when sending to link-local addresses. Here are a few ping6 examples:

ping6 ::1: This pings the host’s loopback address; it should always work.

ping6 -I eth0 ff02::1: This pings the all-nodes multicast group on interface eth0. Here are two of the answers received:

• 64 bytes from fe80::3e97:eff:fe2c:2beb (this is the host I am pinging from)
• 64 bytes from fe80::2a0:ccff:fe24:b0e4 (a second Linux host)

Answers were also received from a Windows machine and an Android phone. A VoIP phone – on the same subnet but supporting IPv4 only – remained mute, despite VoIP’s difficulties with IPv4 NAT that would be avoided with IPv6. In lieu of the interface option -I eth0, the “zone-identifier” syntax ping6 ff02::1%eth0 also usually works; see the following section.

ping6 -I eth0 fe80::2a0:ccff:fe24:b0e4: This pings the link-local address of the second Linux host answering the previous query; again, the %eth0 syntax should also work. The destination interface identifier here uses the now-deprecated EUI-64 format; note the “ff:fe” in the middle. Also note the flipped seventh bit of the two bytes 02a0; the destination has Ethernet address 00:a0:cc:24:b0:e4.

8.12.2 TCP connections using link-local addresses

The next experiment is to create a TCP connection. Some commands, like ping6 above, may provide for a way of specifying the interface as a command-line option. Failing that, RFC 4007 [https://tools.ietf.org/html/rfc4007.html] defines the concept of a zone identifier that is appended to the IPv6 address, separated from it by a “%” character, to specify the link involved. On Linux systems the zone identifier is most often the interface name, eg eth0 or ppp1. Numeric zone identifiers are also used, in which case it represents the number of the particular interface in some designated list and can be called the zone index. On Windows systems the zone index for an interface can often be inferred from the output of the ipconfig command, which should include it with each link-local address. The use of zone identifiers is often restricted to literal (numeric) IPv6 addresses, perhaps because there is little demand for symbolic link-local addresses.

The following link-local address with zone identifier creates an ssh connection to the second Linux host in the example of the preceding section:

ssh fe80::2a0:ccff:fe24:b0e4%eth0

That the ssh service is listening for IPv6 connections can be verified on that host by netstat -a | grep -i tcp6. That the ssh connection actually used IPv6 can be verified by, say, use of a network sniffer like WireShark (for which the filter expression ipv6 or ip.version == 6 is useful). If the connection fails, but ssh works for IPv4 connections and shows as listening in the tcp6 list from the netstat command, a firewall-blocked port is a likely suspect.

8.12.3 Manual address configuration

The use of manually configured addresses is also possible, for either global or unique-local (ie not connected to the Internet) addresses. However, without a router there can be no Prefix Discovery, 8.6.2 Prefix Discovery, and this may create subtle differences.

The first step is to pick a suitable prefix; in the example below we use the unique-local prefix fd37:beef:cafe::/64 (though this particular prefix does not meet the randomness rules for unique-local prefixes). We could also use a globally routable prefix, but here we do not want to mislead any hosts about reachability.

Without a router as a source of Router Advertisements, we need some way to specify both the prefix and the prefix length; the latter can be thought of as corresponding to the IPv4 subnet mask. One might be forgiven for imagining that the default prefix length would be /64, given that this is the only prefix length generally allowed (8.3 Network Prefixes), but this is often not the case. In the commands below, the prefix length is included at the end as the /64. This usage is just slightly peculiar, in that in the IPv4 world the slash notation is most often used only with true prefixes, with all bits zero beyond the slash length. (The Linux ip command also uses the slash notation in the sense here, to specify an IPv4 subnet mask, eg 10.2.5.37/24. The ifconfig and Windows netsh commands specify the IPv4 subnet mask the traditional way, eg 255.255.255.0.)

Hosts will usually assume that a prefix configured this way with a length represents an on-link prefix, meaning that neighbors sharing the prefix are reachable directly via the LAN.

We can now assign the low-order 64 bits manually. On Linux this is done with:

• host1: ip -6 address add fd37:beef:cafe::1/64 dev eth0
• host2: ip -6 address add fd37:beef:cafe::2/64 dev eth0

Macintosh systems can be configured similarly except the name of the interface is probably en0 rather than eth0. On Windows systems, a typical IPv6-address-configuration command is

netsh interface ipv6 add address "Local Area Connection" fd37:beef:cafe::1/64

Now on host1 the command

ssh fd37:beef:cafe::2

should create an ssh connection to host2, again assuming ssh on host2 is listening for IPv6 connections. Because the addresses here are not link-local, /etc/host entries may be created for them to simplify entry.

Assigning IPv6 addresses manually like this is not recommended, except for experiments.

On a LAN not connected to the Internet and therefore with no actual routing, it is nonetheless possible to start up a Router Advertisement agent (8.6.1 Router Discovery), such as radvd, with a manually configured /64 prefix. The RA agent will include this prefix in its advertisements, and reasonably modern hosts will then construct full addresses for themselves from this prefix using SLAAC. IPv6 can then be used within the LAN. If this is done, the RA agent should also be configured to announce only a meaningless route, such as ::/128, or else nodes may falsely believe the RA agent is providing full Internet connectivity.