Category Archives: SysAdmin

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:


HPE Microserver Gen8

It was about time I pensioned off the tired old Core2 Duo desktop running as my home fileserver. It sucked up sufficient electricity that it was worth having a Raspberry Pi sat on top of it, to issue a wake-on-LAN before running various tasks (e.g. backups) and turn it off again afterwards. It was also starting to develop reliability issues – who knew buying used-up hardware for a nominal £1 would barely give three years’ service…

HP’s Microservers have a good reputation as a basic home NAS box, and the £60 cashback offer running in November certainly helps: I got the Gen8 with a dual core 2.3Ghz Celeron and 4GB of RAM for £120 after cashback. Here it is:

HPE Gen8 Microserver

HPE Gen8 Microserver

It looks quite swish and is very quiet, especially if you select the power-saving options in the BIOS – it also puts out reassuringly little heat. The BIOS is quite nicely laid out and easy to follow, though it does seem to lack the classic “discard changes and exit” option.

HPE Microserver Gen8 BIOS

HPE Microserver Gen8 BIOS


It has four SATA bays which are inside the front door and have trays to slide the disks in and out with. They’re not hot-swap apparently they are as long as you don’t use the inbuilt RAID controller!, but at least physically moving disks in and out isn’t a problem: they even supply a little tool to handle the screws with. I put the boot disk from my old server in the leftmost slot, and the two 1.5TB halves of my main RAID array in slots 2 and 3. It booted from the first disk and Just Worked, though the BIOS takes a while to wade through all the checks.

I believe it has some sort of built in hardware RAID controller, but I prefer to stick to good ol’ fashioned Linux software RAID (newer, cooler solutions are also available): you can set up an array on any old hardware, plug the disks into something new and different, and it all comes back to life trivially. Linux is really good at moving to new hardware, and after an fsck* (even that could have been avoided if I’d set the system clock right) it was all ready to go. Try doing that with hardware RAID, where moving disks to a new controller is often impossible without wiping them and starting over.


It has no less than three network ports on the back – two are ordinary dual NICs and the third is for HP’s “iLO” remote management stuff. Since I’ve run out of ports on my router I haven’t had a chance to try that out. Linux recognised the NICs as eth2 and eth3, but that might just be a hangover from the installation starting life on other hardware.

Other thoughts

I’ve only had it a few days, but it’s been solid and reliable so far … here’s hoping it outlasts its Core2 predecessor by a good few years.

*File System ChecK – it’s a Linux command. Obviously.

Sip2Sim and the OnePlus Two

Andrews and Arnold do an interesting service where they supply a SIM card which connects to VOIP at their end. Annoyingly, they don’t have a sensibly usable set of 07… UK mobile numbers they can route onto VOIP to go with the service, but since my OnePlus Two has two SIM slots, that seemed like a way to give it a punt…

Double SIM carding it ... like a pro

Double SIM carding it … like a pro (this is the drawer from the OnePlus Two)

The particular variant of SIM I ordered (O2/EU Voice) doesn’t push out to Nano SIM, instead requiring a pair of scissors and a steady hand (or a proper tool, but who has one of those?) As you can see from the picture, I got away with the scissors and it even worked afterwards:

Custom network name and two signal strength indicators

Custom network name and two signal strength indicators

Android has some pretty impressive native support for more than one SIM, and shows two signal strength selectors as you’d expect. As you can see at the top left, the SIM networks (operator names) are shown with a pipe separating them. For some reason you can’t fiddle with this on the control pages, but you can set it when ordering and contact support to alter it.

I ordered a London number on AAISP’s VOIP service to go with the SIM, and that all works as expected. Texts are a bit clunky (presenting as from “SIP2SIM”), but it looks like that may be configurable/changable.

Mobile data appears to go via NAT and emerge from an IP address registered to Manx Telecom.

The two things I really wanted to play with are setting up my own Asterisk again, and using the roaming to get decent data on the train up north. I’ll report back when I’ve had a play.

Bytemark charging for IPv4 addresses

We‘ve had a server at Bytemark for many years (since the summer of 2009). For the most part they’ve been good, Towards the end of last year, they started charging for extra IPv4 addresses. This came as no great surprise, given the near-exhaustion of this finite resource, but our modest /27 suddenly represented a 1/3 increase in the cost of our server.

Fortunately, Bytemark agreed to waive the increase until our next annual renewal, after I got a bit shirty about mid-term price increases. Which left us with the joy of consolidating our usage. They’re quite right to point out that the advent of SNI support in all modern browsers, and things like sslh mean we don’t need so many addresses any more, but having parcelled them out to my six users in blocks of 4 (making firewalling easier as everyone had a /30), I had a long and tedious consolidation exercise to carry out.

Happily, many reboots and much faffing later, we’re nearly there and should be able to hand back 16 addresses of our 32 next month, thus cutting our cost by £192+VAT/year.

This has prompted me to take a closer look at going all-IPv6. I’ve ordered a small IPv6 only VM from Mythic Beasts to play with. Teething troubles aside, it works quite well, with inbound proxying for the websites and NAT64 for outbound access to IPv4 services. Running just the one IP stack feels much cleaner and easier to administer, and opens up the possibility of using an IP address per website/service with no danger of running out.

Edit: Bytemark’s original announcement failed to mention that it’s £1 plus VAT for each IPv4 address. Sigh.

Raspberry Pi print & scan server

I was pleasantly surprised at how easy it was (with a bit of Googling) to set up the Raspberry Pi 2 I’d had gathering dust on my desk as a print server. My increasingly venerable USB-only Canon MP480 works just fine under Debian, but having to boot my desktop to use it was becoming tedious – much of what I get done these days is laptop-based or even done on my phone.

Having set it all up, I can scan and print from any Linux box on the network (theoretically from Windows too, only I have none of those left), and, with the addition of the CUPS Print app, I can print PDFs and the like directly from my Android phone.

Update – putting some IPv6-only DNS on the Pi and pointing a Windows 10 VM at it via IPP, printing Just Works. Even more impressively, Windows drivers for your printer do not need to exist, as long as they exist for CUPS on the Pi. I just chose the generic HP PostScript printer suggested in the article and it works perfectly, which is handy as Canon have no intention of providing Win10 drivers.

E-mail for the last three days – my part in its downfall

You know Tuesday afternoon is going well when your mother calls and asks why she hasn’t received any e-mail for the last three days. Come to think of it, I hadn’t had much e-mail lately either.

Five minutes later on the mailserver (IP addresses and domain names changed to protect the guilty)…

$ exim4 -bhc

>>> check hosts = ${sg {${lookup sqlite{ /path/to/my/sqlite.database SELECT host FROM blacklist WHERE domain = '$domain'; }}}{\n}{: }}
>>> no IP address found for host (during SMTP connection from (somewhere) [])
>>> in dns_again_means_nonexist? no (option unset)
>>> host in ""? list match deferred for
>>> deny: condition test deferred in ACL "acl_check_rcpt"
451 Temporary local problem - please try later
LOG: H=(somewhere) [] F=<> temporarily rejected RCPT <>

$ host
Host not found: 2(SERVFAIL)

Yes,  if you have a host-based blacklist which contains names as well as IP addresses, Exim will defer messages if it gets a “temporary” DNS failure when looking up names on the list. So not only did the owner of screw me over by sending me spam, but their broken DNS deferred all of my incoming mail. Excellent. I’ll let you know if the internet comes up with a solution to this other than the per-domain one.

Yubico Yubikey 4: PGP, U2F and other things

I recently bought myself a Yubikey 4, as previously mentioned.

Here’s what I’ve managed to do with it so far …

General observations

Since this is very new hardware, it comes as no surprise that the versions of the Yubikey tools in Debian stable aren’t new enough to talk to it. I used the Windows versions off the Yubico website to have a poke around the settings, but if the device is already in OTP+U2F+CCID mode, then that should cover all bases.

The device arrived just over 1 business day after I’d ordered, sent from a UK distributor – having had to order and pay in dollars via PayPal, it wasn’t clear that this would be so snappy. Well done Yubico.

Physically, it fits nicely onto my keyring and is barely any more noticeable than the two small USB drives and other bits and pieces hanging on there. Though its arrival did inspire me to purge my keyring of quite a lot of keys I’d been carrying around for years and almost never using. Weight is a concern when you’re dangling your keys on the end of a USB-connected device (mostly in case they pull it out of the port rather than the risk of physical damage in this case), so digging out an extension cable to let the keys rest on my desk was necessary to connect to my desktop. Laptops are naturally more convenient here.

U2F (Universal Second Factor)

I didn’t buy it for this particularly – until there’s Firefox support, U2F is a bit too edge-case for me. However, with the addition of the relevant udev rules (easily Google-able), it does work with Google Chrome on Debian and is a more convenient alternative to the Google-Authenticator based two-factor auth I already had enabled on my Github and Google accounts. The nice thing is that both services will fall back to accepting codes from the app on your phone if a U2F token isn’t present, so it’s low-risk to turn this on. The Yubikey is more convenient than having to dig out my phone and transcribe a code, though (not least because a security-conscious nerd like me now has over a screenful of entries in the app) – and is also impervious to the phone running out of battery or otherwise failing.


Other than confirming it works on Yubico’s demo site, I’ve not found a use for this just yet. It’s worth noting that, if I understand correctly, it relies on Yubico having the corresponding public key to the private key embedded in the device, and thus uses require authenticating against Yubico’s servers. It is possible to insert your own key onto the device and run your own auth service, but that seemed a bit much effort for one person (and hard to convince myself it’d be more secure in the long run). Meanwhile, I see I could use the Yubikey as a second factor for key-based SSH authentication, or even a second factor for sudo. But in both cases it would require Yubico’s servers being up to authenticate the second factor, and I’m not sure what my backup plan for that would be. At the very least, I’d want to own a second Yubikey as fallback before locking down any service to the point where I couldn’t get in without one.


This was the main use-case I bought the Yubikey for. Previously I’d had a single RSA PGP key which I used for both encrypting and signing. The difficulty with this set-up is that you end up wanting the private key key on half a dozen machines which you regularly use for e-mail, but the more places you have it, the more likely it is to end up on a laptop that gets stolen or similar.

For my new key, I’ve switched to a more paranoid setup which involves keeping the master key completely offline, and using sub-keys on the Yubikey to sign/encrypt/authenticate – these operations are carried out by a dedicated chip on the Yubikey and the keys can’t be retrieved from it. You’re still required to enter a PIN to do these operations, in addition to having physical possession of the Yubikey and plugging it in. However, the nice trick about the sub-keys is that in the event of the Yubikey being lost/stolen, you can simply revoke them and replace them with new ones – without affecting your master key’s standing in the web of trust.

The Yubikey 4 supports 4096 bit RSA PGP keys – unlike its predecessors which were capped to 2048 bits. To make all this work, you need to use the 2.x series of GnuPG – 1.x has a 3072 bit limit on card-based keys and even that turned out to be more theoretical than achievable. I couldn’t initially persuade GnuPG 2.x (invoked as gpg2 on Debian/Ubuntu) to recognise the Yubikey, but a combination of installing the right packages (gnupg-agent, libpth20, pinentry-curses, libccid, pcscd, scdaemon, libksba8) and backporting the libccid configuration from a newer version finally did the trick, with gpg2 –card-status displaying the right thing. Note that if running Gnome, some extra steps may be required to stop it interfering with gnupg-agent (but you should get a clear warning/error from gpg2 –card-status if that’s the case).

I generated my 4096 bit subkeys on an offline machine (a laptop booted from a Debian live CD) and backed them up before using “keytocard” in gpg to write them to the Yubikey.

I got into a bit of a tangle by specifying a 5 digit PIN – gpg (1.x) let me set it but then refused to accept it, saying it was too short! Fortunately I’d set a longer admin PIN and the admin PIN can be used to reset the main PIN. I’m not sure if that was just a gpg 1.x bug, but I’ve gone for longer PINs to be safe.

Having waded through all that, the pay-off is quite impressive: Enigmail, my preferred solution for PGP and e-mail, works flawlessly with this set-up. It correctly picks and uses sub-keys marked as for encryption purposes, and it prompts for the Yubikey PIN exactly as you’d expect when signing, encrypting and decrypting. It’s worth having a read of the documentation, and giving some consideration to the “touch to sign” facility. This goes some way towards making up for the Yubikey’s weakness compared to some more traditional PGP smart cards: it doesn’t have a trusted keypad for you to enter your PIN, so there’s always the risk of it being intercepted by a keylogger. But touch to sign means malware can’t ask the key to sign/encrypt/decrypt without you touching the button on the key for each operation. In any case, this set-up is a quantum leap over my old one and means I can make more use of PGP on more machines, with less risk of my key being compromised.

It’s worth noting that the Yubikey is fully supported on Windows and is recognised by GnuPG there too. This set-up might finally persuade me that a Windows machine can be trusted with doing encrypted e-mail.

Making the Yubikey 4 talk to GPG on Debian Jessie

I’ve just bought myself a Yubikey 4 to experiment with.

The U2F and OTP features are of some interest, but the main thing I bought it for was PGP via GnuPG. I was disappointed to discover that it works (at least as far as gpg –card-status showing the device) on current Ubuntu (15.10) and even on Windows (!), but not Debian stable (Jessie). Still, this is quite a new device…

In every case on Debian/Ubuntu, you need to apt-get install pcscd

For the non-internet-connected machine on which I generate my master PGP key and do key signing, I can just use the Ubuntu live CD, but since my day-to-day laptop and desktop are Debian, this does need to work on Debian stable for it to be of much use to me. A bit of digging revealed this handy matrix of devices, and the knowledge that support for the Yubikey 4 was added to libccid in 1.20. Meanwhile, jessie contains 1.4.18-1. Happily, a bit more digging revealed that retrieving and using the testing package’s version of /etc/libccid_Info.plist was enough to make it all start working:

[david@jade:~]$ gpg --card-status                            (28/11 20:20)
gpg: detected reader `Yubico Yubikey 4 OTP+U2F+CCID 00 00'
Application ID ...: XXX
Version ..........: 2.1
Manufacturer .....: unknown
Serial number ....: XXX
Name of cardholder: David North
Language prefs ...: en
Sex ..............: male
URL of public key : [not set]
Login data .......: david
Private DO 1 .....: [not set]
Private DO 2 .....: [not set]
Signature PIN ....: not forced
Key attributes ...: 2048R 2048R 2048R
Max. PIN lengths .: 127 127 127
PIN retry counter : 3 0 3
Signature counter : 0
Signature key ....: [none]
Encryption key....: [none]
Authentication key: [none]
General key info..: [none]

I’ve raised a bug asking if the new config can be backported wholesale to stable, but meanwhile, you can copy the file around to make it work.

For U2F to work in Google Chrome, I needed to fiddle with udev as per the suggestions you can find if you Google about a bit. The OTP support works straight away, of course, as it’s done by making the device appear as a USB keyboard.

When my beeper goes off

As regular readers will know, I run a co-located server with a few friends. I’m also responsible for things like the WiFi network at church. Inevitably, these things run just fine for weeks, months or even years at a time – and then fail. The first I hear of it is usually from a would-be user, sometimes days later.

The world of open-source has a solution to this, and it’s called Nagios (actually, I think the cool kids have jumped ship to the more recent Icinga, but I haven’t played with that). It’s a monitoring system – the idea is that you install it somewhere, tell it what computers / devices / websites to monitor, then it can e-mail you or do something else to alert you when things fail or go offline.

The really nice thing about it is that it’s all pluggable – you can write your own checks as scripts, and also provide your own notification commands, for example to send you a text.

The only problem is, where to put it? Hosting it on the co-lo box I’m trying to monitor is a bit self-defeating. Fortunately this is a rather good use for that Raspberry Pi I had lying around – I found a convenient internet link to plug it into, removed all the GUI packages, and it’s running Nagios just fine. So far, without really even trying, I’ve got it monitoring 76 services across 29 hosts. Some of the checks are a bit random – e.g. a once-per-day check on whether any of my domain names have expired – but now it’s possible to buy ten-year renewals it’s nice to have an eye on this sort of thing, as who knows if one’s registrar will still have the right contact details to send reminders in a decade?*

So far it’s working a treat, texting me about problems so I can fix them before users notice, although if I add many more checks I suspect I’ll have found an excuse to invest in a Raspberry Pi 2 to keep up with the load.

If you’re doing it right, you’ll configure Nagios to know which routers and computers stand between it and the rest of the internet (and hence what it’s trying to monitor). This means it won’t generate a slew of alerts if its internet connection dies, just one. It also means you get a useful map of what you’re monitoring:

* I’m pretty sure Mythic will do, because they’re great, but it never hurts to have a backup in place.