Proxy DHCP

Somebody should have thought of this, and it turns out that somebody did.

A couple of years ago, I wrote about setting up a network boot server on my home LAN. I said this meant “taking the job of doing DHCP away from the router and doing it myself on a Linux box”.

Reader, I was wrong.

Setting up my new connection from BT today, I really didn’t want to keep running DHCP separately on a Raspberry Pi. What I wanted was for the Pi to just talk to clients which wanted to do network boot, and leave the BT router to do DHCP for ordinary devices.

This exists, and it’s called proxy DHCP. dnsmasq on the Pi can handle it; this tutorial is the one I got working. Sadly proxy DHCP doesn’t seem to honour options, so I had to take the path prefix (option 210) and manually prepend it to all the paths in my boot config.

This is all back up and working now, and hopefully should prove useful and less disruptive for a few years yet to come.

Now that i have native IPv6 and DHCP is back on the router, the Pi ceases to be a single point of failure, though it’s been rock solid for nearly three years now.

VirtualBox does network boot, of course

VirtualBox does network boot, of course

IPv6 at home from BT

It works – 10/10 on; can’t really say fairer than that. I did however enjoy the comedy reverse DNS:

$ curl   (05/04 19:19)

Oddly enough, doesn’t resolve forwards to every IPv6 address on their network, so why bother claiming the reverse?

IPv6 only IMAP server

Because why not.

I have an IPv6 only virtual machine hosted at Mythic Beasts. Sadly, I don’t have much time to play with it these days, so I’m retiring it next month. Meanwhile, I’ve got something working which I thought ought to be possible but failed at the last time I tried…

I participate in the minority sport of hosting my own e-mail. That means I run an IMAP server which mail clients can view and search messages using. For myself, I have native IPv6 on my home internet connection, but when I’m out and about, my mobile phone is still IPv4 only, and most of my relatives who I host mail for are v4 only too. Mythic and others have blogged at length on hosting websites behind their IPv4-to-IPv6 reverse proxy, but can it handle IMAP?

$ nc -vv 993                                 (01/04 17:06)
DNS fwd/rev mismatch: !=
DNS fwd/rev mismatch: != [] 993 (imaps) open

The fact that they’ve got it listening on the standard port for IMAP over SSL is encouraging. Let’s give it a go…


In addition to your IPv6 only VM, you’ll need a domain and you’ll need to point some DNS. The examples below assume you’ve CNAMEd to

Part 1: Install and configure Dovecot

Couldn’t be easier if we’re using Debian:

# apt-get install dovecot-imapd

Configuring Dovecot to properly store mail is beyond the scope of this article, so we’ll just do some minimal setup.

Part 2: SSL

It’s 2017, and SSL certificates are free. So let’s get some.

# apt-get install dehydrated apache2 dehydrated-apache2

The Apache server is only necessary because I’m taking the path of least resistance to getting SSL certificates from letsencrypt, which involves them verifying control over the domains using HTTP based challenges. You could set up Mythic’s DNS API to eliminate the need for this.

Once you’ve done this, edit /etc/dehydrated/domains.txt to include your domain (e.g. and add /etc/dehydrated/conf.d/ containing CONTACT_EMAIL=””.

Having done this, you can run “dehydrated -c”. I’d recommend not running it as root, which means fixing the directory permissions when it falls over the first time. All that sorted, we should now have a directory /var/lib/dehydrated/certs/ That means we can go ahead and edit /etc/dovecot/conf.d/11-ssl.conf to read:

ssl = required
ssl_cert = </var/lib/dehydrated/certs/
ssl_key = </var/lib/dehydrated/certs/

While you’re there, you should set port=0 on the non-SSL IMAP listener in 10-master.conf, to stop it listening on non-SSL ports.

All that sorted, we can restart Dovecot:

# service dovecot restart

And now we can jump over to a client machine and try it out:

$ ncat --ssl -v 993
Ncat: Version 6.47 ( )
Ncat: SSL connection to 2a00:1098:0:82:1000:3b:1:1:993.
Ncat: SHA-1 fingerprint: 2FB3 9166 A7B7 6552 6215 963C 43D0 824E A02F 9FB3

Assuming all is well, you’ll see the OK response from Dovecot, and when you hit Ctrl-C, you’ll get a line logged in /var/log/syslog on the server complaining about how your client hung up.

At this point, you should be able to try it with a real mail client, e.g. Thunderbird. And you’ll notice that everything in the garden is lovely, except that the logs show all connections coming from Mythic’s proxy, rather than their true source IPs.

Part 3: Proper IP addresses in the logs

Mythic’s proxy supports the PROXY protocol to enable this, but does Dovecot? As it turns out, they added support in 2.2.19, but Debian stable has 2.2.13. Backports to the rescue:

# apt-get install -t jessie-backports dovecot-imapd

Now we need to turn on PROXY support for using the Mythic control panel. Having done that, I was very impressed with Dovecot – the logging in /var/log/syslog walked me through fixing each mistake I made, starting with needing to configure Dovecot for PROXY:

Edit /etc/dovecot/conf.d/10-master.conf and add haproxy = yes to the imaps listener. The defaults are sensibly secure: to avoid clients spoofing IP addresses, you must provide a whitelist of clients allowed to speak to listeners with haproxy support turned on. That means setting haproxy_trusted_networks in the same file. You can find the necessary IPv6 addresses to space-separate on this page.

Having done that and restarted Dovecot, I moved my laptop over to my guest wifi network (where there is no IPv6), and restarted Thunderbird…

$ grep dovecot /var/log/syslog
Apr  1 17:17:55 test-box dovecot: imap-login: Login: user=<david>, method=PLAIN, rip=, lip=, mpid=1234, TLS, session=<...>

Result! The IP quoted as “rip” (remote IP) is the IPv4 one I’m running Thunderbird on. Interestingly, Dovecot has chosen to log the IPv4 of Mythic’s proxy as the local IP (lip).

Blast from the past

I was surprised to find my less-than-a-year-old HP Microserver Gen8 taking over half an hour to come back from each reboot. Closer inspection showed the system clock jumping back to 2016 every time it restarted, which forces an fsck when Linux realises its disks have been modified “in the future”. Checking >1TB of disk takes a long time.

It’s been a while since I had to replace a CMOS battery, but it fixed the problem:


The Stuff Should Just Work Epiphany

It doesn’t apply to everyone who works in the IT industry, but as most of us get older, we lose our tolerance for things which don’t Just Work, at least in our home lives. When you’ve spent all day working with new and sharp-edged technology, you really want to come home and have your appliances act like appliances, not like computers which misbehave and need debugging. Having said that, a related phenomenon I’ve experienced over the past year is that when you are promoted far enough to stop doing coding for a living, you may have a resurgence of interest in hacking around with stuff in your spare time…

Installing a new Raspberry Pi

This has got very slick. These days, if you buy a new Pi with one of their SD cards, it boots to a nice menu asking which operating system you’d like to install – including Microsoft’s IoT offering, which looks interesting. I haven’t tried it yet, though, as my first Pi 3 is destined for media centre duties, making OSMC the OS of choice.


GWR, you’re fired

Blissful, unimaginable spaaace!

Blissful, unimaginable spaaace!

So I’m off to spend the weekend with friends in London, as you do.

There are a few things I really want to get done on the train, involving my laptop. Odds of getting a seat with a plug socket and enough space to work, on a Friday night peak time GWR train from Oxford to London Paddington? Approaching zero unless I pay double for first class.

But wait, capitalism to the rescue! Almost uniquely in the country, Oxford station now has multiple competing routes into London, courtesy of Chiltern going via Oxford Parkway and into London Marylebone. And unlike GWR, it’s on time, and I have not just a table, socket and reasonably roomy seat, but three other unoccupied seats around me.

GWR, I’m never taking your route to London again.

(The WiFi is crocked, though … couldn’t all be perfect, now could it.)

Cheaper isn’t always better

Getting data abroad on EE pay as you go has always been a miserable experience, assuming you’re too disorganised to sort it out in advance. You land somewhere foreign, you just need a few MB to get Google Maps up and find your hotel, and if their captive portal even manages to load, it’s sorely lacking in a button which says “shut up and take my money”. They also apply rather mean time limits – yes, you can have 75MB of data, but it only lasts 24 hours. WTF? I wanted that much for the whole trip.

For longer trips abroad, none of this matters as you can simply buy a local SIM at the airport. This is especially easy for me with my OnePlus Two, as it has twin SIM slots so you don’t miss out on the odd text or urgent call from home.

However, for just four full days on the ground in Seville – much of it spent in an underground hotel conference centre with excellent WiFi – what to do?

Google Maps is really good these days at downloading the surrounding area while you’re on WiFi, which almost solves the problem, but not quite.

However, I remembered that I have an AAISP SIM in my second slot. In addition to its clever SIP pass through, it can also do data roaming. The prices are high – 10p/MB – but it’s pay as you go, billed in arrears by direct debit, and you can set hard limits at their end as well as on the phone itself.

This worked out really nicely, and I used 28.98MB over the four days. Which means I saved 11p vs paying EE their £3, and probably a lot more because I would have needed data over more than a 24 hour period. Well done AAISP.

Memo to self: must ditch EE for someone less annoying.

Abroad with Monzo

I’m in Spain this week for ApacheCon, and Monzo is definitely delivering on the promise – no foreign usage fees, just the MasterCard exchange rate. I got some cash out when I landed, and the rate was €1.16 for a pound. The Post Office seem to require a minimum spend of £400 to give a worse rate (€1.1266 for a pound)  – and who has time to get their holiday money in advance in this day and age?

Getting cash out at the airport

Getting cash out at the airport

As ever with these things, avoid ATM and chip and PIN machines offering to charge you in GBP – they’re highly unlikely to give a better rate.