[GLLUG] basic IPv6 questions

Carles Pina i Estany carles at pina.cat
Sun Oct 3 19:12:56 UTC 2021

Hi Andy,

On Oct/03/2021, Andy Smith via GLLUG wrote:

> On Sun, Oct 03, 2021 at 02:57:51PM +0200, Carles Pina i Estany via GLLUG wrote:
> > I have a Raspberry pi connected to a BT router that recently has
> > "switched" to ipv6 only (yay?!). This is helping me to test a Gandi
> > server ipv6 configuration.
> > 
> > Raspberry pi: /sbin/ifconfig eth0:
> First of all you should get into the habit of using "ip address" and
> "ip route" commands instead of "ifconfig" and "route".


> So, I understand that there may be a muscle memory issue with

I have solved some muscle memory with "alias" :-)

Not an excuse, you are 100% right, just for a Sunday laugh:

I have human parsing issues! I read the output of "ifconfig" and "route"
without any effort. I need some effort for the output of "ip a", "ip r"
and I miss some parameters sometimes.

It's about 20 years that I moved from "pine" to "mutt" and I still use
"p" to start "mutt":
carles at pinux:~$ alias p
alias p='LANG=en_GB.UTF8 mutt'
carles at pinux:~$

(my system's locale is in Catalan but I prefer mutt in English)

I've followed with interest your email (THANKS EVER SO MUCH!). I'll add
some comments in some places with some small question or for me to
confirm that I understood some parts correctly...

> immediately reaching for "ifconfig" etc — I experienced the same
> myself and still do with some of the "netstat" replacements — but
> there's nothing to be done about that except try to learn the new
> tools!
> > eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
> >         inet6 fe80::56d8:5a6c:fc11:16f1  prefixlen 64  scopeid 0x20<link>
> >         inet6 2a00:23c6:2c01:b801:2817:ffe3:d3aa:5d8c  prefixlen 64  scopeid 0x0<global>
> >         ether b8:27:eb:b0:9d:76  txqueuelen 1000  (Ethernet)
> > Gandi server: /sbin/ifconfig eth0:
> > eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
> >         inet  netmask  broadcast
> >         inet6 2001:4b98:dc2:53:216:3eff:fe82:b1fb  prefixlen 64  scopeid 0x0<global>
> >         inet6 fe80::216:3eff:fe82:b1fb  prefixlen 64  scopeid 0x20<link>
> >         ether 00:16:3e:82:b1:fb  txqueuelen 1000  (Ethernet)
> > -inet6 2a00 and 2001 ipv6 addresses: they are global ipv6 routeable from
> >  the internet (google.com is 20aa:..., dns.google is 2001:...).
> Yes.

As said below by you and thanks to "sipcalc 2000::/3":
Anything from 2000:0000:0000:0000:0000:0000:0000:0000 to
3fff:ffff:ffff:ffff:ffff:ffff:ffff:ffff belongs to the global unicast

> >  by the DHCP server of each network (or static configuration).
> Yes, although dynamic IPv6 configuration is more often done by
> StateLess Address Autoconfiguration (SLAAC) rather than DHCP. SLAAC

I suspect that 20 years ago I studied this at uni. Sadly I've forgotten
many things and I think that we never did a hands on setting up of an
ipv6 network :-(

> works through announcements by the network segment's router(s)
> telling devices on that network segment which addresses they can
> choose and what their default gateway should be.
>     https://en.wikipedia.org/wiki/IPv6_address#Stateless_address_autoconfiguration

I've read also the linked (and specially the first two paragraphs):

If the Raspberry pi + BT router used SLAAC is this more or less what
-Raspberry pi sends a broadcast using NDP probably type "Router
Solicitation (Type 133)"
-Router probably answers with a "Router Advertisement (Type 134)"

The Router Advertisement includes the IP of the router (in the "Source

> DHCPv6 does exist though so it is possible that this is used in
> addition or instead of SLAAC.

good to know!

When I had to setup a Linux box in a LAN I sometimes use
isc-dhcp-server. If I wanted to setup ipv6 devices with SLAAC: what
would be the way to go?

For example, the last time that I had to do this I used isc-dhcp-server
for very basic things like:
-Setup the DNS of the clients
-For some of the clients a static assigned IP (e.g. host with MAC
address X is always the IP Y)
-Setup the gateway of the clients (some clients didn't have a gateway,
some had a gateway)

Is this something that could be done using SLAAC? Or should be done with
a DHCPv6 server?

> > -Any difference between 2a00 and 2001? Any other addresses like this?
> They are conceptually as similar as and in
> IPv4m, for example. Just different globally routable address blocks.
> Out of the entire 128-bit space of IPv6 addresses, 2000::/3 has been
> allocated so far for global unicast addresses, so every public
> address you see should be in the range:
> 	$ sipcalc 2000::/3

thanks for this!

> > -From the server I would be able to ping
> >  2a00:23c6:2c01:b801:2817:ffe3:d3aa:5d8c if BT wanted (can I ask them?
> >  is it in the router configuration? Doing a traceroute I don't quite get
> >  into the local home router IP, I don't think so)
> In theory you or anyone should be able to ping
> 2a00:23c6:2c01:b801:2817:ffe3:d3aa:5d8c but there might be a


> > -fe80:: are local in the network IPs. For Gandi: this gets generated out
> >  of the MAC address. And not for the Pi, why not? (how to set this up?)
> Your pi:
> >         inet6 fe80::56d8:5a6c:fc11:16f1  prefixlen 64  scopeid 0x20<link>
> >         ether b8:27:eb:b0:9d:76  txqueuelen 1000  (Ethernet)
> Your server:
> >         inet6 fe80::216:3eff:fe82:b1fb  prefixlen 64  scopeid 0x20<link>
> >         ether 00:16:3e:82:b1:fb  txqueuelen 1000  (Ethernet)
> I think that your raspberry pi might be using stable privacy
> addresses (RFC7217) to choose a random interface ID (IID) per link
> not based on MAC address, then using that for link-local and for any
> autoconfigured addresses, so that your MAC address is not
> transmitted outside your network.
>     https://newbedev.com/how-to-avoid-exposing-my-mac-address-when-using-ipv6
> Normally the link local address is generated as a "Modified EUI-64

To confirm:

Link local address: a private address like (starting with
fe80:: in ipv6). Valid only in the network segment, not routeable

Global address: network 2000::/3 public routeable

> identifier"; it will have ff:fe in the middle and would have the 7th
> bit flipped, so we'd expect the EUI-64 for b8:27:eb:b0:9d:76 to be:
> Insert ff:fe	→ b827:ebff:feb0:9d76
> 'b8' as bits	→ 10111000
> flip 7th		→ 10111010
> back to hex		→ ba27:ebff:feb0:9d76
> Your IID on your pi clearly isn't that, but it also doesn't have the
> ff:fe or the flipped 7th bit, so it's not EUI-64. Looking at "ip -6
> a" (short hand for "ip address show") might tell you some more info;
> it will at least say whether the address is temporary or not.

2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether b8:27:eb:b0:9d:76 brd ff:ff:ff:ff:ff:ff
    inet6 2a00:23c6:2c01:b801:2817:ffe3:d3aa:5d8c/64 scope global dynamic mngtmpaddr noprefixroute 
       valid_lft 287sec preferred_lft 107sec
    inet6 fe80::56d8:5a6c:fc11:16f1/64 scope link 
       valid_lft forever preferred_lft forever

does "dynamic" imply "temporary"?

> Sometimes only the global scope addresses use the private
> identifiers but some implementations derive the link scope address
> this way too, as it is possible for your link address to be
> published outside, e.g. in email headers. I think this is what is
> happening here but perhaps you can confirm by inspecting your local
> configuration.

Does the "ip a" that I showed answer the question? (else I'll dig more)

> > pi at raspberrypi:~ $ traceroute -n -6 google.com
> > traceroute to google.com (2a00:1450:4009:817::200e), 30 hops max, 80 byte packets
> >  1  2a00:23c6:2c01:b801:ee6c:9aff:fea3:a231  5.056 ms  4.732 ms  4.549 ms
> > 
> > I'm not quite matching the first hop with the routing table. Why not?
> This is down to how traceroute works. Traceroute relies on receiving
> an ICMP packet from each hop. It only knows the name of the hop based
> on the source address of the ICMP packet. Even though you talk to
> fe80::ee6c:9aff:fea3:a231 as your default gateway, it generates an
> ICMP packet with its global address in it
> (2a00:23c6:2c01:b801:ee6c:9aff:fea3:a231) because the packet that
> caused this ICMP to be generated had source address
> 2a00:23c6:2c01:b801:2817:ffe3:d3aa:5d8c.

So, if I had a different ipv6 device in this network (when I'm there in
10 days I will test it): the traceroute first hop will be different
depending on the device that I initiate the ipv6?

> > ipv4 I would be able to see the gateway there (unless the gateway is not
> > answering ICMP, but if it's fe80::ee6c:9aff:fea3:a231 it does answer
> > ICMP).
> In IPv4 the device generally wouldn't have multiple valid source
> addresses to choose from. In an IPv4 network without NAT, if you are
> and your router is, then when you send a packet to
> google at it has source address This
> generates ICMP at your router that wants to go back to you at
> so it will choose source address for that ICMP.
> Even if your host had an extra private address and
> router had a private address, router is not going to
> spontaneously decide to source an ICMP packet from
> when it sends it to even if you did set your default gateway
> to be So in fact IPv4 is working exactly the same as
> IPv6 in this regard it's just that you don't tend to use IPv4 that
> way.

I think that I understood it!

BTW, in the Raspberry pi:
pi at raspberrypi:~ $ ip a
inet6 2a00:23c6:2c01:b801:2817:ffe3:d3aa:5d8c/64 scope global dynamic mngtmpaddr noprefixroute

Do I have all this IPs:
$ sipcalc 2a00:23c6:2c01:b801:2817:ffe3:d3aa:5d8c/64

Network range		- 2a00:23c6:2c01:b801:0000:0000:0000:0000 -

So I can just assign more IPs if I want to the eth0 and ping to other
IPs from other devices in the network? (and outside the network if the
firewall allowed it?)

> https://bitfolk.com/ -- No-nonsense VPS hosting

This email is a fantastic reason to consider bitfolk (and the same with
other emails in the past years from companies that I might be

Carles Pina i Estany

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