Wandering Thoughts archives


Systemd's fate will be decided by whether or not it works

I have recently been hearing a bunch of renewed grumbling about systemd, probably provoked by the release of RHEL 7 (with a contributing assist from the Debian decision for it and Ubuntu's decision to go along with Debian). There are calls for a boycott or moving away from systemd-using Linuxes, perhaps to FreeBSD, for example. My personal view is that such things misread the factors that will drive both sides of the decision about systemd, that will sway many people either passively for or actively against it.

What it all comes down to is that operating systems are commodities and this commodification extends to the init system. For most people, the purpose of an OS, a Linux distribution, a method of configuring the network, and an init system is to run your applications and keep your system going without causing you heartburn (ideally all of them will actually help you). For (some) management and organizations, an additional purpose is making things not their fault. Technical specifics are almost always weak influences at best.

(It is worth saying explicitly that this is as it should be. The job of computer systems is to serve the needs of the organization; they can and must be judged on how well they do that. Other factors are secondary. Note that this doesn't mean that other factors are irrelevant; in a commodity market, there may be many solutions that meet the needs and so you can choose among them based on secondary factors.)

This cuts both ways. On the one hand, it means that generally no one is really going to care if you run FreeBSD instead of Linux (or Linux X instead of Linux Y) because you want to, provided that everything keeps working or at most that things are only slightly worse from their perspective. On the other hand, it also means that most sysadmins don't care deeply about the technical specifics of what they're running provided that it works.

You can probably see where this is going. If (and only if) systemd works, most people won't care about it. Most sysadmins are not going to chuck away perfectly good RHEL 7, Debian, or Ubuntu systems on the principle that systemd is icky, especially if this requires them to step down to systems that are less attractive apart from not having systemd. In fact most sysadmins are probably only vaguely aware of systemd, especially if things just work on their systems.

On the other hand, if systemd turns out to make RHEL 7, Debian, or Ubuntu machines unstable we will see a massive pushback and revolt. No amount of blandishment from any of the powers that be can make sysadmins swallow things that give them heartburn; a glowing example of this is SELinux, which Red Hat has been trying to push on people for ages with notable lack of success. If Red Hat et al cannot make systemd work reliably and will not replace it posthaste, people will abandon them for other options that work (be those other Linuxes or FreeBSD). And if systemd works well only in some environments, only people in those environments will have the luxury of ignoring it.

That is why I say that systemd's fate will be decided by whether or not it works. If it does work, inertia means that most sysadmins will accept it because it is part of a commodity that they've already picked for other reasons and they likely don't care about the exact details of said commodity. If it doesn't work, that's just it; people will move to systems that do work in one way or another, because that's the organizational imperative (systems that don't work are expensive space heaters or paperweights).

Sidebar: The devil's advocate position

What I've written is only true in the moderate term. In the long term, systemd's fate is in the hands of both Linux distribution developers in general and the people who can write better versions of what it does. If those people are and remain dissatisfied with systemd, it's very likely to eventually get supplanted and replaced. Call this the oyster pearl story of Linux evolution, where people not so much scratch an itch (in the sense of a need) as scratch an irritation.

linux/SystemdFateDecisionSource written at 23:17:36; Add Comment

The kernel should not generate messages at the behest of the Internet

Here is a kernel message that one of my machines logged recently:

sit: Src spoofed ->

Did I say 'a message'? Actually, no, I meant 493 messages in a few days (and it would be more if I had not used iptables to block these packets). Perhaps you begin to see the problem here. This message shows two failures. The first is that it's not usefully ratelimited. This exact message was repeated periodically, often in relatively close succession and with no intervening messages, yet it was not effectively ratelimited and suppressed.

(The kernel code uses net_warn_ratelimited() but this is clearly not ratelimited enough.)

The second and more severe failure is the kernel should not default to logging messages at the behest of the Internet. If you have a sit tunnel up for 6to4, anyone on the Internet can flood your kernel logs with their own version of this message; all they have to do is hand-craft a 6to4 packet with the wrong IPv6 address. As we've seen here, such packets can probably even be generated by accident or misconfiguration or perhaps funny routing. Allow me to be blunt: the kernel should not be handing this power to people on the Internet. Doing so is a terrible idea for all of the usual reasons that giving Internet strangers any power over your machine is a bad idea.

These messages should not be generated by default (at any logging level, because there is no logging level that means 'only log messages that are terrible ideas'). If the kernel wants to generate them, it can and should be controlled by a sysctl or a sysfs option or the like that defaults to off. People who really really want to know can then turn it on; the rest of us will leave it off in our usual great indifference to yet another form of Internet badness.

(Since I haven't been this quite this harsh on kernel messages earlier, I'll admit it: my attitude on kernel messages has probably steadily gotten stricter and more irritated over time. I should probably write down my thoughts on good kernel messages sometime.)

Sidebar: what this message means

A 6to4 encapsulated packet has two addresses; the outside IPv4 address and the inner IPv6 address. The kernel insists that the inner IPv6 address is the IPv4 address's 6to4 address. Here the outside source is but the inner 6to4 address in 2002::/16 is for the IPv4 address You can get a similar message if the destination address has a mismatch between the IPv4 address and the 6to4 IPv6 address.

(To decode the 6to4 IPv6 address, take off the leading 2002: bit and then convert the next four hex octets to decimal bytes; each byte is one digit in the address. So the source is claimed to be 4d.4d.4d.07 aka We can follow the same procedure for the destination address, getting (hex) aka decimal, which matches the outer IPv4 address.)

linux/OutsideKernelMessagesNo written at 00:44:19; Add Comment

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