Building Custom Raspberry Pi SD Card Images


Pi4 Gadget



  • Find somewhere to host a 500mb file
  • Keep it up to date when new versions are released




Virtual Raspberry Pi in a terminal window




#!/usr/bin/expect -f
set timeout -1
set imageName [lindex $argv 0]
  puts "No Image file provided"
set cwd [file normalize [file dirname $argv0]]
set imagePath [file join $cwd $imageName]
expect "login: "
send "pi\n"
expect "Password: "
send "raspberry\n"



proc slurp {file} {
    set fh [open $file r]
    set ret [read $fh]
    close $fh
    return $ret
expect "# "
send "$file\n"
send "EOF\n"


There is a version of the output from here.


A Personal Offline Certificate Authority

I had a slight scare in the run up to Christmas. I went to use the VPN on my phone to connect into my home network and discovering that the certificate that identifies both my phone and the one for the server had expired the day before.

This wouldn’t have been a problem except I couldn’t find where I’d stashed the files that represent the CA I had used to create the certificates. There was a short panic until I got home that evening and found them on a old decommissioned server that luckily I hadn’t got round to scrapping properly yet.

This led me to think of a better place to store these files. I wanted to have a (relatively) secure offline place to store them, but also somewhere that could handle the actual signing of certificates and the rest of the admin (I normally end up Googling the instructions for openssl each time I need to do this).

A simple approach would be to just store the files on a encrypted USB Mass Storage device, but I wanted something a little bit more automated.


Recycling my ever useful Raspberry Pi Zero as a USB Ethernet Gadget instructions again with a Raspberry Pi Zero (note not a Zero W) gets me a device that has no direct internet connection, but that can be plugged into nearly any machine and accessible via a local network connection.

RTC attached to a Raspberry Pi Zero in a Pimoroni case

One little niggle is that working with certificates requires an accurate clock on the device. Raspbian by defaults sets it’s clock via NTP over the network since there is no persistent clock on the pi. The fix for this is a i2c battery backed up hardware clock. You can pick these up from a number of places, but I grabbed one of these from amazon.

To enable the RTC you need to add the following to /boot/config.txt


And comment out the first if block in /lib/udev/hwclock-set

#    exit 0

Now we have a reliable system clock we can go about setting up the CA.


The first version just requires me to ssh in to the pi and use openssl on the command line. This was enough to get me started again, but I was looking for something a bit more user friendly.

I had a look round for a web interface to openssl and found a few different options

But they all requires a whole bunch of other things like OpenLDAP, MySQL and Apache which is all a bit too heavy weight for a Pi Zero.

A web form collecting data for a certificate

So I decided to write my own, a bit of poking around and I found the node-openssl-cert module on npm which looked like it should be able to handle everything I need.

Combined with express and I now have a form I can fill in with the subject details and hit submit.

The page then downloads a PKCS12 format bundle which contains the CA cert, Client cert and Client key all protected by a supplied password. I can then use openssl to break out the parts I need or just import the whole thing into Android.

At the moment I’ve just copied the existing CA key & cert out of the existing CA directory structure to get this to work. I intend to update the code to make use of the serial number and index tracking so if needed I can generate a Certificate Revocation List if needed, also potentially allow the downloading of previously generated certs.

You can find the project on github here and I hope to find some time to write up some end to end instructions for setting it all up.

The interesting bit was how to download a file from a XMLHttpRequest, you can see that trick here.


I originally titled this as “A Secure Offline Certificate Authority”. I changed it because this isn’t really any more secure than a USB key you just keep the CA key & cert on, and probably less secure than if you encrypted that drive. It is true that the CA key cert are not accessible from the host machine without SSHing to the device, but the CA key & cert are still just stored on the Pi’s SDCard so if anybody has physical access to it then it’s game over.

I could look at i2c or SPI secure elements that could be used to store the private key but the real solution to this is an ASCI or FPGA combined with a secure element, but that is all overkill for what I needed here.

Problems with a IPv6 only network

In my last post I talked about running a pure IPv6 network, as part of my ISP building project, but still allowing access to resources on the internet currently only available via IPv4.

This works well assuming all the clients on the local network are IPv6 capable, unfortunately this is not always the case. There are legacy devices that do not understand IPv6.

This is a real problem with IoT devices that are either no longer being maintained or just that have hardware that is incapable of using anything other than IPv4. There is also a small problem that a IP cam with a IPv6 address is probably available to the world with out some firewall rules or a ACL limiting access to the local /64, but those are problems for another day…

Another issue is hard coded IPv4 addresses in legacy applications, this is a problem even if the OS/device supports both IPv4 & IPv6 but is only connected via IPv6.

There is are a few of solution to both these problems.

Dual Stack networks

The simplest is to just run a dual stack network supplying both IPv4 & IPv6 all the way from the end device to the edge of the ISPs network. While this works it either means using lots of IPv4 addresses in the ISPs internal network and probably 2 layers of NAT (one at the customers router and then CGNAT at the edge of the ISPs network) assuming the ISP is not handing out publicly routed IPv4 addresses directly to customers.


464XLAT -> 4 to 6 to 4 X transLATion and the X can be either a Client or Provider.

464CLAT comes in 2 main types

464CLAT – on host

464CLAT works by doing the conversion from IPv4 to IPv6 on the client. This works for sites that are only available via IPv4 and for hard coded IPv4 addresses. It all happens in the TCP/IP stack on the host OS. For this to work the OS needs a way to work out what the DNS64 prefix is.

This paper discusses methods for a client to determine the NAT64 prefix. It suggests that the simple method of looking up the IPv6 address of a hostname known to only have a IPv4 entry and using returned address (This behaviour is described in rfc7050).

There is an important step to validate that the address is returned is valid, because DNS64 breaks DNSSEC it takes a few extra steps.

  • Make a IPv6 (AAAA) look up for ipv4only.arp with DNSSEC checking disabled
  • Make a reverse (PTR) lookup of the IPv6 address 64:ff9b::c000:ab
  • This should return a pointer to a fully qualified domain name for a host in the ISPs domain (
  • Finally make a IPv6 (AAAA) lookup for this hostname, which should return the same IPv6 address and a valid DNSSEC signature (RRSIG AAAA) record signed by the ISPs domain keys.

All of the required parts for this are running on my test network, I just need to attach some client devices to see if it works.

464CLAT – on router (Stateless translation)

This is a form of dual stack network, but it limits the dual stack to just the customer side of their router.

In this case the client network runs both native IPv6 and a RFC1918 range IPv4 and the clients router does the conversion by adding the IPv6 prefix. As well as running DHCPv6/SLAAC for IPv6 addressing it also runs DHCPv4 and hands out IPv4 addresses.

This works for devices that can’t support IPv6 (e.g. cheap IoT devices) and you still get to run a IPv6 only network in the ISP network.

The router generates it’s own prefix from the publicly routed IPv6 range it’s been allocated for it’s client side network. It then uses this prefix to generate 1 to 1 IPv6 addresses for each IPv4 address it hands out.

Jool can also be used to handle this in SIIT mode, you have to tell it what the input IPv4 range is and the IPv6 prefix it can work with.

modprobe jool_siit
jool_siit instance add "example" --iptables
jool_siit -i "example" eamt add fd12:3456:789a:ff46:2::/96
sysctl -w net.ipv4.ip_forward=1
sysctl -w net.ipv6.conf.ppp0.accept_ra=2
sysctl -w net.ipv6.conf.all.forwarding=1
ip6tables -t mangle -A PREROUTING -j JOOL_SIIT --instance "example"
iptables  -t mangle -A PREROUTING -j JOOL_SIIT --instance "example"

Here I’ve hardcoded the IPv6 prefix, but for a real deployment we would need to use a delegated IPv6 prefix that is routed to the gateway. This should be run using the script option for dhcp6c when the prefixes are delegated.

interface ppp0 {
	send ia-na 0;
	send ia-pd 0;
	script "/etc/dhcp6c/";
id-assoc na {
id-assoc pd 0 {
  prefix-interface eth1 {
    sla-len 0;
    sla-id 1;

This config takes the /64 prefix and assigns it to eth1 for clients to use as part of SLAAC. I’m still working out how to extract the second /96 prefix and pass it to the script.

We can then update the script that starts jool as follows and point to it in the dhcp6c.conf file so it gets run once the network is up.

#IPv6=`ip -o -6 a show dev eth1 | awk '/global/ { print $4 }'`
#PREFIX=`subnetcalc $IPv6 -n | awk '/Network/ { print $3}'`
IPv4=`ip -4 -o a show dev eth1 | awk '/global/ {print $4}'`
SUBNET=`subnetcalc $IPv4 -n | awk '/Network/ {print $3}'`
#need to make sure this only runs once
modprobe jool_siit
jool_siit instance add "example" --iptables
jool_siit -i "example" eamt add fd12:3456:789a:ff46:2::/96 $SUBNET/24
sysctl -w net.ipv4.conf.all.forwarding=1
sysctl -w net.ipv6.conf.all.forwarding=1


This is usually just the NAT64 we talked about in the last post running in the ISPs network.

IPv6 only network with IPv4 access (DNS64/NAT64)

Continuing the theme of building my own ISP I started looking at running a IPv6 only network.

As IPv4 addresses become increasingly scarce it won’t be possible to hand out publicly routeable addresses to every user. The alternatives are things like CGNAT but that has a bunch of problems.

On the other hand the default suggested IPv6 allocation per user is a /48 subnet (which is 65,5536 /64 subnets each containing 18,446,744,073,709,551,616 addresses) which should be more than enough for anybody. More and more services slowly are making them selves available via IPv6.

So rather than run a dual stack IPv4/IPv6 network with a double NAT’d (at the home router and again at the ISP’s CGNAT link to the rest of the internet ) IPv4 address, we can run a pure IPv6 ISP and offer access to IPv4 via a NAT64 gateways to allow access to those services that are still IPv4 only.


This is the part that converts IPv4 addresses to IPv6 addresses.

The local device looking to connect makes a DNS request for the hostname of the remote device. If there is no native AAAA (IPv6 address) entry for the hostname, the DNS64 server will generate one based on converting the IPv4 address to hex and prepending a IPv6 prefix. The prefix can be anything with at least /96 (allowing enough room for all IPv4 addresses) but there is a pre-defined address range of 64:ff9b::/96.

So if looking up the remote hostname returns then the mapped IPv6 address would be 64:ff9b::c0a8:0105 ( -> c0.a8.01.05 in hex)

As of version 9.8 of bind support for DN64 is built in and configured by adding the following:

dns64 64:ff9b::/96 {
  clients { any; };
  mapped { !10/8; any;};
  break-dnssec yes;

Running your own means you can control who can access the server (using the clients directive and control which IP address ranges are mapped or excluded).

All this IP address mapping does break DNSSEC but since most clients rely on their recursive DNS servers to validate DNSSEC record rather than doing it directly this is less of a problem.

There are also a number of public DNS servers that support DNS64 including one run by Google.

To test you can use dig and point to the DNS64 server. e.g. to get a IPv6 mapped address for ( &

$ dig @2001:4860:4860::6464 -t AAAA

; <<>> DiG 9.11.5-P4-5.1-Raspbian <<>> @2001:4860:4860::6464 -t AAAA
; (1 server found)
;; global options: +cmd
;; Got answer:


;; ANSWER SECTION:		235	IN	CNAME		285	IN	AAAA	64:ff9b::d43a:edfe		285	IN	AAAA	64:ff9b::d43a:e9fe

;; Query time: 133 msec
;; SERVER: 2001:4860:4860::6464#53(2001:4860:4860::6464)
;; WHEN: Mon Feb 03 21:40:50 GMT 2020
;; MSG SIZE  rcvd: 124

We can see that is d43ae9fe in hex and has been added to the 64:ff9b::/96 prefix to make 64:ff9b::d43a:e9fe


This is the part that actually does the mapping between the IPv6 address of the initiating device and the IPv4 address of the target device.

There are a few different implementations of NAT64 for Linux

I decided to give Jool a go first based on a presentation I found.

I had to build Jool from source, but this wasn’t particularly tricky and once installed I followed the Stateful NAT64 instructions. There were 2 bits missing from this that caused a few problems.

The first was because my host machine has multiple IPv4 addresses I needed to add the right address to the `pool4`. When adding the the address you also need to specify a range of ports to use and these need to be excluded from ephemeral local port range.

The ephemeral range can be between 1024 and 65535 and you can check what the current range is set to with sysctrl

$ sysctl net.ipv4.ip_local_port_range
net.ipv4.ip_local_port_range = 32768	60999

For a proper deployment this range needs reducing so that you can commit enough ports for all the NAT64 connections that will pass through the gateway. You can also add multiple IPv4 addresses.

Quick aside: The whole point of this exercise is to reduce the number of publicly routable IPv4 addresses that we need. To make this work we are always going to need some, but we will share a small number at one point at the edge of the network to be used as the NAT64 egress point, but as more and more services move over to supporting IPv6 this number will decrease.

Because I’m only playing at the moment, I’m just going to use the 61000-65535 range and leave the ephemeral ports alone. I will still be able to host 4500 connections.

To make starting it all easier I wrote a short script that

  • Loads the module
  • Enables IPv4 and IPv6 routing
  • Sets up jool with the default IPv6 prefix
  • Adds the iptables entries to intercept the packets
  • Adds the IPv4 output address to the jool config with the port range for TCP, UDP and ICMP
modprobe jool

sysctl -w net.ipv4.conf.all.forwarding=1
sysctl -w net.ipv6.conf.all.forwarding=1

jool instance add "example" --iptables  --pool6 64:ff9b::/96

ip6tables -t mangle -A PREROUTING -j JOOL --instance "example"
iptables -t mangle -A PREROUTING -j JOOL --instance "example"

jool -i "example" pool4 add -i 61000-65535
jool -i "example" pool4 add -t 61000-65535
jool -i "example" pool4 add -u 61000-65535

Putting it together

To make it all work I need to set the DNS server handed out to my ISP customers point to the DNS64 instance and to make sure that 64:ff9b::/96 gets routed via the gateway machine.

To test I pinged

$ ping6 -c 2
PING (64:ff9b::d43a:e9fe) 56 data bytes
64 bytes from 64:ff9b::d43a:e9fe: icmp_seq=1 ttl=54 time=18.1 ms
64 bytes from 64:ff9b::d43a:e9fe: icmp_seq=2 ttl=54 time=23.0 ms

--- 64:ff9b::d43a:e9fe ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 3ms
rtt min/avg/max/mdev = 18.059/20.539/23.019/2.480 ms
Network diagram
Network layout

Getting out past the firewall

Ahhh, the joys of a IT departments that think everybody just uses Word/Excel/Outlook and just browses to Facebook at lunchtime.

Networks that transparently proxy HTTP/HTTPS (probably with man in the middle TLS CA certs deployed to all the machines, but that is an entirely different problem) and block everything else really do not work in the modern world where access to places like GitHub via SSH or devices connecting out via MQTT are needed.

One possible solution to the SSH problem is a bastion host. This is a machine that can be reached from the internal network but is also allowed to connect to the outside world. This allows you to use this machine as a jumping off point to reach services blocked by the firewall.

The simple way is to log into the bastion, and then from the shell connect on to your intended external host, this works for targets you want a shell on but not for things like cloning/updating git repositories. We also want to automate as much of this as possible.

The first step is to set up public/private key login for the bastion machine. To do this we first generate a key pair with the ssh-keygen command.

$ ssh-keygen -f ~/.ssh/bastion -t ecdsa -b 521
Generating public/private ecdsa key pair.
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in ~/.ssh/bastion.
Your public key has been saved in ~/.ssh/
The key fingerprint is:
SHA256:3Cfr60QRNbkRHDt6LEUUcemhKFmonqDlgEETgZl+H8A hardillb@tiefighter
The key's randomart image is:
+---[ECDSA 521]---+
|oB+      ..+X*.. |
|= .E    . .o++o  |
|.o  .  . o..+= . |
|....o...o..=o..  |
|  .=.o..S.* +    |
|  . ..o  . *     |
|          o      |
|         o       |
|         .+.     |

In this case we want to leave the passphrase blank because we want to use this key as part of automation of other steps, normally you should use a passphrase to protect access should the keys be compromised.

Once generated you can copy it to the `~/.ssh/authorized_keys` file on the bastion machine using the ssh-copy-id command

$ ssh-copy-id -i ~/.ssh/bastion user@bastion

Once that is in place we should be able to use the key to log straight into the bastion machine. We can now use the `-J` option to specify the bastion as a jump point to reach a remote machine.

$ ssh -J user@bastion user@remote.machine

We can also add this as an entry in the `.ssh/config` file which is more useful for things like git where it’s harder to get at the actual ssh command line.

Host bastion
 HostName bastion
 User user
 IdentityFile ~/.ssh/bastion

Host github
 User git
 IdentityFile ~/.ssh/github
 ProxyCommand ssh -W %h:%p bastion

This config will proxy all git commands working with remote repositories on via the bastion machine, using the bastion key to authenticate with the bastion machine and the github key to authenticate with github. This is all totally transparent to git.

ssh also supports a bunch of other useful ticks, such as port forwarding from either end of the connection.

It also can proxy other protocols using the Socks tunnelling protocol which means it can be used as poor man’s VPN in some situations. To enable Socks proxying you can use -D option to give a local port number or the DynamixProxy directive in the ~/.ssh/config file. This option is really useful with web browser that supports Socks proxies as it means you can point the browser at a local port and have it surf the web as if it was the remote machine.

All of this still works if you are using a bastion machine.

Custom hardware

Combining all this really useful SSH capability with a the Raspberry Pi gadgets makes it possible to carry a bastion host with you. Using a Raspberry Pi Zero W or even a full sized Pi 4 that can be configured to join a more open WiFi network (e.g. a visitor or testing network) you can have a device you just plug into a spare USB port that will give you a jumping off point to the outside world while still being connected to the more restricted internal network with all that provides. Just don’t tell IT security ;-).

This works well because even the most locked down machine normally still allows USB network adapters to be used.