Where is 'localhost'? (Docker networking)

Building up to the devproxy2 release, I set things up to test in Docker, using a known version of Go.  Nothing worked for quite some time.

Core problem: localhost inside the container is not the host network.  I had to configure it to listen on all addresses (by asking Go to connect to ":8080" without a host address), per this other Stack Overflow answer. If I had thought to check the exact error message client side (curl: empty reply from server), I could have solved this one first, not last.

Once I fixed the core problem, everything started working, and I didn’t go back and test the other things I had tried.  So everything else is only a potential problem.

Potential problem: localhost inside the container is not the host network.  I configured it to connect to host.docker.internal instead, per this Stack Overflow answer.  This is the same as the core problem, but in the other direction.

Potential problem: according to my notes, the default network bridge in Docker doesn’t have Internet access. Creating a user-defined bridge solves this.  Furthermore, using Docker Compose automatically creates a user-defined bridge for the stack.  By the time I solved the core problem, I was already using Docker Compose for the networking feature.

Obvious (2009)

Editor's Note: this is a post from my old Serendipity weblog, "Pawprints of the Mind," which was originally posted almost 12 years ago, on 2009-04-28. The text below is reproduced verbatim from the original post.

When things like Amazon's 1-click patent come to light, suddenly there's a mob of Slashdotters decrying the idea as 'obvious'. My thoughts on the rsync protocol were the same: knowing that there was a program to transfer changed parts of files over the network, I turned my mind to figuring out how the changed parts could be detected. Later I came across a description of the actual rsync protocol, and it was indeed fairly close to my idea for it. Therefore rsync is obvious.

Or is it? The solution may be relatively obvious, but the problem itself was not something that ever crossed my mind before running into the program that solves it. The invisibility of the problem ends up hiding the solution as well.

Apple seems to be actively searching for non-obvious problems: the ones people don't even think of as problems until the iWhatever comes out and redefines the entire market for whatevers. The iPod's clickwheel seems innovative until you realize it's basically a mousewheel. An analog interface to digital systems. Apple only put that kind of interface on because they happened to see that purely binary controls on existing MP3 players were fairly primitive. Once the iPod was released, nobody wanted to admit to being blind to the problems Apple tackled, so their chosen solution (undoubtedly one of many obvious possibilities) was hailed as pure genius.

It seems that the magic is not in who you are, it's in what you choose to think about. If you've never asked, "How quick can we make it for customers to order?" then you'll never end up with 1-click shopping.

Using zerofree to reduce Ubuntu/Debian OVA file size

Editor's note [2020-11-21]: this is an update to a 2014 post on the same topic.  Since then, the mount command is also required.

When exporting an OVA file from VirtualBox's "Export Appliance" feature, any disk block that isn't filled with 0 has to be included in the resulting file.  Normally, an OS doesn't create those blocks as it works; when a file is deleted, the space is marked as free, but retains its original contents until it is re-allocated for another file.  If there are a lot of files deleted, a significant amount of non-empty "empty" space can accumulate.

Running the zerofree program writes data of value 0 over all space marked as free, allowing that free space to be excluded from the OVA file again.

The following applies to Ubuntu 18.04 (bionic) and 20.04 (focal) at least; I forget exactly how far back it goes, but the way to get at a command line changed at some point, and there was a period where poweroff didn't actually work in the pre-boot environment.

Regardless, here's how to run zerofree on Debian, Ubuntu, or derivatives.

State of the Linux Desktop (2009)

Editor's Note: this is a post from my old Serendipity weblog, "Pawprints of the Mind," which was originally posted nearly 12 years ago, on 2009-05-18.  History since then has unfolded differently in some details, but the broad strokes seem to be accurate.  The text below is reproduced verbatim from the original post.

The Linux desktop is a curious thing. Years ago, the annual prediction was that that year was finally going to be the Year of the Linux Desktop, and Windows would be kicked out of market dominance for good. World domination by open source, and all that.

Linux has come a long way since then. Monitors can be largely auto-configured, fonts can be anti-aliased, CDs can be burned without fiddling with ide-scsi, USB works without having to care about your kernel version, and even media keyboards and mice with tilt-wheels seem to work these days.

But the "desktop" has come a long way since then, too. Various OS X versions introduced technologies like Display PDF, Quartz Extreme, Exposé, and other things that Windows Vista copied with WPF and Aero. Linux eventually duplicated them in Compiz, Beryl, Compiz-Fusion, or something. Somewhere in there. It kinda-sorta works now, if you don't want thumbnails of minimized windows.

Now that Linux has joined the current decade, Microsoft is readying Windows 7, with features like thumbnails of windows on the taskbar (with Aero), the 'recent documents' of an application available from the taskbar button, and so forth. Apple certainly has their own plans. And both of them have decent sound and MIDI support, where starting up one audio application is actually not likely to silence another's sound.

So, the "Linux Desktop" is still behind, and always has been. One problem here seems to be that it's not about innovation. It's about copying everyone else's modern technology with ancient components. It's about solving problems again that already have better solutions in other systems, just to see how closely Linux can approximate them with its decades-old core design.

And then someone, or maybe a few people, whine about the solution and come up with an alternative. The alternatives vie for dominance, and the rest of the desktop world moves on while the Linux landscape settles into one predominant choice, if they're lucky. If they're unlucky, you get things like the KDE/Gnome situation.

Due to Linux's philosophy about neither inventing wheels nor including batteries, there's a large number of components with their own maintenance and release schedules, and most of them target "current" versions of other components. For any given complex program, there's a relatively small window of time between the components it uses being released, and becoming obsolete with future releases. Every time components break compatibility, a new version of all programs dependent on them have to be released to deal with the changes. The more changes dealt with, the more complex the program becomes, so compatibility with older versions also gets cut out after a time.

And sometimes, a sweeping change comes through, and no compatibility is maintained between the two versions: App 2.0 will run on Foo 3.0 and newer, and App 1.4 will only run on Foo 2.5 and below.

From a software developer's point of view, then, supporting Linux is a risky proposition. You have to either watch the work in progress on the entire stack, and guess what will be completed and be important to your application, or you end up being taken by surprise when one of your components updates and your application doesn't run on an ever-growing range of Linux systems until you have time to update it.

Apple and Microsoft may have their own problems, but they have the clout and the guts to say, "We need to migrate to doing things this way, and it'll be good for at least 5 years." Look at Windows 95–Me, or NT4–XP. Whereas the rate of change in Linux effectively limits compatibility to about a year at the most. So most serious software gets developed for other platforms, and most serious users are on those platforms as well. Then familiarity begets desire for those platforms over Linux.

In short, Linux is sort of the Perl of operating systems: messy and aging. People working on Linux desktop technologies seem to be doing it to prove that they can. And secretly, they hope to get horses fast enough to put that Ford guy out of business.

Defense in Depth: Read-Only Document Root

One security rule I live by as a sysadmin, which admittedly causes me a lot of trouble with PHP apps, is that the running web server user is never able to write to files within the document root.

Any user-file upload/download mechanism goes somewhere else, so that read access can do a permission check, and be delivered through readfile() or a redirect to a CDN, not passed through a complex mechanism that may decide to execute the file's contents.

Configuration files are similarly locked down.  A project that configures itself with PHP code is a project that offers arbitrary code execution to anyone who can change that configuration file.

I don't let the web user install new code to /bin; we shouldn't let the web user install new code to /srv/www. It's the server, not a system administrator.

Not coincidentally, our document root is never the root of a git repository.  It's typically placed in a public folder, allowing us to have a special writable folder like cache both within the project/deployment, and outside of the document root.

MySQL on MacOS: Discoveries

For a long time, I’ve installed MySQL within my development virtual machine (VM), loaded it with a minimal subset of production data, and pointed the database hostname to 127.0.0.1 through the VM’s /etc/hosts file.

However, working from home, I realized that the my slower DSL connection did not affect the amount of time it took to pull a fresh subset of production data into the VM.

I finally went to the trouble of installing MySQL on the host side, and configuring everything to work with that, so that filling it does not require the data to cross any hypervisor boundaries at all.  And I learned some things!

1. The installer sets it up to launch as _mysql user
2. Using the wrong CA certificate causes padding errors
3. MySQL 8.0.14+ allows a separate "administrative management" port
4. MySQL’s AES_ENCRYPT() function makes the worst possible choice by default
5. The speedup is dominated by MySQL's disk access

(Not) Learning Rust

I want Rust to succeed.  It seems like the best gambit for fixing C, where the slightest mistake is likely to be a security hole… or to be turned into one by a newer compiler version.

On the other hand, actually learning it has proven extremely difficult.  Very little of Rust is explained in terms of something else; and all of the basic mechanics of the language feel weird.

After the Rust Book and the Cookbook and maybe some other stuff from Awesome Rust, I went through the tutorial about writing very bad linked lists; there were places that were like, “Can we do X? [code that does X].”  I would stop there, run that code, and try to solve the error myself.  I never could. I would read on, and it would be, “The error tells us we need to call F” so I’d try to figure out how to call F.  I never could figure it out, even with that hint. And I still didn’t have the capability to go from error message to even discovering F, let alone understanding that F would apply to the error.

I threw myself at the wall several times, and failed to advance past the part where one writes a number-guessing game for the CLI.

Moreover, I don’t have a motivating project to “learn Rust” with.  It’s mostly an exercise in following other people’s code, breaking it, or trying to improve it, and ending up with incomprehensible and unactionable errors.  The compiler errors try to be helpful, but if the code starts out far enough off the rails, it’s just flailing, same as me.

When a survey comes in that says “Opinions of Rust are very favorable, but people aren’t using it,” I understand.  I understand completely.  We all want something to fix entire bug classes, but keep the bare-metal performance, and Rust is basically without competition there.  But we can’t all devote enough time and energy to learning Rust from scratch.