Debian 7 how are IPv6 link local addresses set

dhclientipv6linux-networking

It seems like when dhclient runs on eth0 I get an IPv4 address from the DHCP server and a Scope:Link IPv6 address attached to eth0:

inet6 addr: fe80::a00:27ff:fed0:4d41/64 Scope:Link

But I can't see from dhclient-script how that address is being added. On another interface with a static IP address, I'd like to add a link local IPv6 address, and I was wondering if there was a generic command to do that without knowing the mac.

Edit:
It looks like the kernel assigns the link local address when you do "ip link set dev ethX up" or "ifconfig ethX up". However, in my case I had a cable plugged in to the interface that was DHCP'ing and no cable plugged into the interface I was setting up statically. Can't verify until Monday but I'm guessing the kernel does not assign link local addresses to the interface if there's no link.

Best Answer

Link local addresses are derived from the MAC address of the device. They are auto-generated as a part of bringing the interface up. Auto-configuration includes a discovery process to ensure that the address is unique on the network.

A similar process is used to auto-configure routable addresses when a router advertisement is available. These addresses may be regenerated periodically to provide privacy.

RFC 4862 specifies the processes to be followed.

Related Solutions

Debian – Disable IPv6 on Debian VPS (Virtuozzo!)

I've only tested this on Ubuntu, but you could try the following:

echo 1 > /proc/sys/net/ipv6/conf/all/disable_ipv6

And if this appears to work, you can make it permanent by adding the following to /etc/sysctl.conf:

net.ipv6.conf.all.disable_ipv6=1

Ubuntu – Configure static IPv6 on Ubuntu

Default Gateways work a bit differently on IPv6.

What I suggest you do is fire up tcpdump and watch it for Router Advertisements (you might want to analyse the capture in WireShark - much easier). This is how your IPv6 host knows how to get out to the internet. Also watch for Neighbor Solicitation as this is the replacement for ARP.

If you see these requests coming in, but not going back out, then your internal firewall is blocking IPv6 traffic. If you see them going out (requests), but not back in, then you need to make sure that your prefix length is correct (that's the /64 part. Although I've seen a lot of /64s being passed out that are actually a /48).

Big Important Bold Bit

ICMP is absolutally critical for IPv6 functionality. In IPv4 it can be blocked without huge impact, but in IPv6, the whole network relies on ICMP for all its functionality. Don't block it.

In reference to

My provider gave me a "/64" (whatever that is - I'm still confused) of IPv6 addresses

That's your Prefix Length, previously known as your Subnet Mask that's used for CIDR. The lower the number, the more IP addresses you have. For IPv6, /64 and /48 are very common and should give you more IP addresses than you will ever need in your entire life.

Best Answer

Related Solutions

Debian – Disable IPv6 on Debian VPS (Virtuozzo!)

Ubuntu – Configure static IPv6 on Ubuntu

Big Important Bold Bit

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