systemd, bridges, containers, and IPv6

I apologize for the quickie here; it’s late, I’m tired, and I just cobbled together a solution to a problem that’s been bothering me since this weekend. First, some backstory since I know you guys appreciate it when I go off on tangents. (And if you don’t, you can just scroll passed this nonsense.)

I’ve been migrating most of my services on my home file server into their own containers. I already did this on my VPS, and I plan on doing it to others eventually, because the service isolation is somewhat helpful and prevents stupid mistakes from doing equally stupid things to the host. Containers aren’t a panacea, of course, and the man pages specifically warn against using them as a security apparatus. It’s not effective, so they say, but I’m also a believer in defense-in-depth.

Anyway, the intent has been mostly a matter of isolation. I have a Minecraft server running, among other things, and I’ve been wanting to isolate it from the rest of the system. Not only does this make configuration somewhat easier since I can have service-specific user accounts without polluting the host passwd entries, but it means I can provide some package isolation as well (no Java on the host–yay!). This isn’t without its shortcomings, mind you, and it’s been one Hell of an interesting battle, particularly when IPv6 is thrown into the mix. But I digress, and the server-specific configuration is a welcome post for another day.

In the coming weeks, I’ll see about making a post on using systemd-nspawn for container isolation, why I chose it over Docker and LXC, how to get it working with user namespaces, and how to configure it with IPv6 addresses routed to each container (rather than through auto-config). Hint: you need routable prefixes like a /48 or /56–trying to (ab)use the neighbor discovery proxy and the respective sysctl settings to segment parts of your /64 non-routable prefix won’t work. Don’t even bother, because it’s not worth the misery.

Meanwhile, back to business: Since I do quite a bit of development of various sorts on my desktop, I also wanted to muck about with containers (which I’ve done for isolating builds), but I wanted to bridge the container with my network-facing interface in a manner that it would happily pull down IPv4 and IPv6 addresses from DHCP. My network, on both IP protocols, is serviced by DHCPv4 and DHCPv6 with more-or-less static assignments tossed out to the appropriate systems with a few exceptions. Specifically, connected devices either get their automatically configured IPv6 address, a static address plus an automatically configured address, or some mix (or absence) thereof. Ideally, containers should receive these addresses as well and behave just as an ordinary OS instance or virtual machine would.

Unfortunately, if you follow the systemd-networkd guides for Arch, you’ll quickly wind up in a circumstance where your network is only going to be half-configured. You might be able to get the IPv4 addresses you expect, if you configure the bridge for DHCP, but your IPv6 range isn’t necessarily going to work even if it does accumulate a set of addresses. Bummer.

Oh, and pay attention to casing: If you enter any of the values entirely in lowercase even by accident, nothing will work. I made this mistake, and my eyes never saw anything wrong because I was too focused on reading “bridge” to notice that it should have been entered as “Bridge.” The obsession, in this case, with the word “bridge” apparently served as an override for sensible parsing that included letter case, but this is a fairly common problem when you’ve been steeped in a similar class of issues for days on multiple systems; it’s a similar phenomenon to semantic satiation wherein words begin to look “strange” if you stare at them too long.

For the purposes of this discussion, we’ll assume that we’ve created files in a similar spirit to the Arch guide for systemd-networkd bridges: vbr0.netdev,, and whatever the name is of your hardware interface (mine is enp6s0, so I named its configuration rather creatively:

As with the guide, our files are as follows:

# vbr0.netdev

(Notice DHCP=yes above. If you need static assignments, change this accordingly.)

And finally:


Oh! What’s this IPv6AcceptRouterAdvertisements=no? More importantly why do we need it? We want to accept IPv6 RAs on our network, don’t we?

The answer might surprise you: Yes and no. Yes, we need router advertisements, but no, we don’t need them on the bridge’s slave device. (Bonus: You won’t find this in the current documentation.) If you fail to add this option, your physical ethernet device will collect an autoconfiguration address, probably accumulate the appropriate route information, configure itself for IPv6, all while your bridge device does the same thing. You’ll be left with an interface that works for IPv4, is configured for both IPv4 and IPv6, but refuses to do anything with its IPv6 assignments. Obviously, if your network isn’t running radvd or similar, you won’t need this, and you certainly won’t need this if you’re not using IPv6. However, if you’re not running IPv6, you probably aren’t reading this article, are you?

If you’ve made it this far, I assume you’re interested in the final moment of truth and the culmination of our journey together. I therefore present to you a systemd-nspawn container with DHCP assignments for IPv4 and IPv6 auto configuration (addresses redacted for privacy):

[root@buildbot network]# ip addr
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever
2: host0@if5: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
    link/ether 1e:21:2b:fc:51:b9 brd ff:ff:ff:ff:ff:ff link-netnsid 0
    inet brd scope global dynamic host0
       valid_lft 938sec preferred_lft 938sec
    inet6 [prefix redacted]:664e:9f3d/128 scope global noprefixroute 
       valid_lft forever preferred_lft forever
    inet6 [prefix redacted]:fefc:51b9/64 scope global mngtmpaddr noprefixroute 
       valid_lft forever preferred_lft forever
    inet6 fe80::1c21:2bff:fefc:51b9/64 scope link 
       valid_lft forever preferred_lft forever

If you haven’t bothered planning your IPv6 migration, you really ought to. The inevitable is coming sooner or later. You do want to be prepared, don’t you?

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Updates Suck (Part 2): KDE Plasma 5.5 breaks task manager minimized application display

If you’ve recently updated to KDE Plasma 5.5, you may have noticed that minimizing your applications no longer produces a noticeable effect in the task manager. (In actuality, the status effect of minimized versus non-minimized applications is limited to a thin, gray outline around the application title–hardly something that conveys application state well enough to process visually at first glance.)

The fault of this change lies with this bug report posted almost 3 years ago. After much discussion, it was decided that 1) the “disabled” state of minimized applications was a mistake, 2) the minimized state makes it too difficult for some people to locate the gray scale application icon, and 3) no one wanted to add a configuration option to enable or disable this feature, so the next best thing was to rip it out. Now, I’ll be honest: Maybe I just happen to have better-than-average pattern matching capabilities in my visual cortex (doubtful), but the gray scale icons for minimized applications really don’t bother me. What does bother me is this change: It takes me twice as long to find minimized applications on the task manager. To say nothing of the fact that two or three people complaining incessantly for 3 years can effect such a substantial change on the rest of us who actually liked the feature…

Humorously, I didn’t notice this change during the initial update. Since I spread my work out across multiple desktops, I don’t often minimize my applications. This explains, at least in part, why it took three days for me to discover this problem and make a post about it (and why I blamed it on plasma-shell’s propensity to crash all the time, although that seemed like a reasonable explanation in lieu of anything more substantive).

The good news is that I traced it down to this commit which turned off the minimized gray scale display. As expected, removing this patch returns the task manager’s display of minimized applications to its previous state. The bad news is that, yes, you do need to recompile plasma-desktop from source. If you happen to use Arch Linux, I’ve put together a PKGBUILD containing the fix based off the PKGBUILD in ABS (hence why I haven’t yet changed any of the credits–I should note that this is not an official package). I’ll keep this updated as best as I can until such time that upstream finally settles on a better idea than a 1 pixel gray outline for non-minimized applications.

The rest of you will have to apply the patch and build the sources yourself. Sorry about that.

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Updates Suck (Part 1): Firefox 43, HTML5 breakage, and more

Sometimes, updates are awful. They break things, they undo configuration options you were sure you had set at some point in the past (either resetting them to defaults or removing them entirely).

I updated my Arch install for the second time in two days in the hopes that some minor bugs would be resolved (more on that in another post), but I hadn’t noticed that Firefox was included in the update list (version 42 to 43). Sadly, not only did this break Firefox’s appearance using oxygen-gtk, but it also broke HTML5 video. Every video on Youtube refused to load following the updates unless Flash was enabled (I have it disabled), and I couldn’t figure out why. A new profile proved somewhat useful: Youtube would work–unless I copied my old profile over in bits and pieces.

After about a half hour of exploring, I found that the culprit was actually in the profile’s prefs.js. In particular, because I had found some instructions on enabling HTML5 under Linux, apparently there were some configuration options that are no longer used and conflict with Firefox 43, which enables many of these by default.

If you encounter a similar problem, the fix should be relatively easy. Simply remove:

user_pref("", true);
user_pref("media.fragmented-mp4.gmp.enabled", true);

From your prefs.js, restart Firefox, and verify that it still works. If not, try removing all of the media.* preferences. If that still doesn’t work and you’re a user of FlashBlock, try uninstalling it and removing any residual preferences (also from prefs.js).

I used to recommend using FlashBlock in combination with NoScript to reduce the browser’s attack surface, but as fewer and fewer sites I encounter actively use Flash, I find that disabling the plugin (settings, addons, plugins) works best for me. Unfortunately, the author included a new feature to block HTML5 video by default, and I’ve found that it tends to cause breakage with some sites, most notably Youtube. Since then, I’ve had the plugin disabled, but as I no longer have a need for it, why keep it around?

Hopefully, if you’ve encountered a similar problem with HTML5 video, the suggested fixes here will work for you. If not, feel free to leave a comment or pester me on Twitter (@zancarius).

(I still haven’t gotten Firefox 43 to play nicely with oxygen-gtk3, so I may eventually consider abandoning the theme–though I find breeze-gtk still contains a variety of display bugs that make it unpleasant to use.)

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