#robustness principle

[sonofafinch]

Be conservative in what you do, be liberal in what you accept from others.

-Jon Postel

[rubypostit]

Be conservative in what you do, be liberal in what you accept from others (often reworded as "Be conservative in what you send, be liberal in what you accept").

[appliedcoffeetechnology]

Peeve: Servers who Lie About File Types and the Clients who Believe Them.

I just needed to print some past statements from the web site of a major business bank. Their web site makes these available in plain, ordinary PDF documents. Click on the document and the web browser complains that it doesn't know what to do with a .cgi file. Yes, that's right, there's a Perl script or something serving these files (nothing wrong with that) and the web server is not setting the headers appropriately. No big deal. Save the file, rename so that it ends in .pdf (there's another WTF, PDF readers that check for a file extension instead of noticing that the first four bytes are an ASCII encoding of the characters "%PDF" and realizing that there's a pretty good chance that this really is a PDF) and there's no problem.

Same problem with the postal service, though with them this is not a recent problem. They've had their server configured incorrectly for years to the point that I ended up automating a fix on my end so that I could print shipping labels.

What we really need is some application of the robustness principle (Robert Braden, RFC 1122 section 1.2.2, "be liberal in what you accept, and conservative in what you send"). Server people, stop lying about what kind of data you're sending. There's no advantage to doing so and it breaks things. If you're sending a PDF, say you're sending a PDF. Client people, you probably shouldn't even be looking at the file name extension (especially on platforms where file name extensions have never been required) when all of the file formats you support have magic numbers at the start that tell you what you're dealing with.