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htaccess/no-406-fallback ​

Severity: warning · Status: enforced · Applies to: ids/**/.htaccess

What ​

Prefer ending a content negotiation block with an unconditional redirect to your HTML documentation, rather than a rule that returns 406 Not Acceptable.

Why ​

Answering 406 is a defensible choice. The identifier genuinely cannot serve what was asked for, and saying so is honest — some authors mean exactly that, which is why this is a warning and not an error.

One shape is worse than the rest, and it is the one that gets copied from ontology templates:

apache
RewriteCond %{HTTP_ACCEPT} .+
RewriteRule ^$ - [R=406,L]

The condition .+ matches any non-empty Accept header. Every HTTP client sends one — browsers, curl, triple stores, crawlers. So this rule matches essentially every request that reaches it, and returns an error instead of a redirect.

It appears to work while you are testing, because your earlier rules catch the formats you thought to check. It fails for everything you did not: a browser sending an Accept header your text/html condition does not quite match, a client sending a wildcard media range, a tool sending application/*. Those users get a 406 and have no idea why.

This has returned 406 for real requests here, in an identifier that repeated the fallback several times over. Hundreds of identifiers carry the catch-all shape.

And a 406 is usually just a worse answer. If somebody asks for a format you do not publish, giving them your human-readable documentation tells them what does exist. An error tells them nothing.

Wrong ​

apache
RewriteEngine on

RewriteCond %{HTTP_ACCEPT} text/turtle
RewriteRule ^$ https://example.org/vocab.ttl [R=303,L]

RewriteCond %{HTTP_ACCEPT} text/html
RewriteRule ^$ https://example.org/index.html [R=303,L]

# Everything else gets an error
RewriteCond %{HTTP_ACCEPT} .+
RewriteRule ^$ - [R=406,L]
apache
RewriteEngine on

RewriteCond %{HTTP_ACCEPT} text/turtle
RewriteRule ^$ https://example.org/vocab.ttl [R=303,L]

RewriteCond %{HTTP_ACCEPT} application/ld\+json
RewriteRule ^$ https://example.org/vocab.jsonld [R=303,L]

# Default: anything else, including browsers, gets the documentation.
# No condition, so it always matches.
RewriteRule ^$ https://example.org/index.html [R=303,L]

How to fix ​

Delete the 406 rule and make sure the last rule in the block is unconditional and points at something useful.

Then check the cases the 406 was hiding:

sh
curl -sI -H 'Accept: */*' http://localhost:8080/my-vocab/
curl -sI                  http://localhost:8080/my-vocab/

Neither should return 406.

How to check ​

Run w3id-check with --rule htaccess/no-406-fallback to check this rule on its own; without it the tool runs every rule, as the pull request checks do.

sh
node tools/check/bin/w3id-check.js --rule htaccess/no-406-fallback ids/my-project

The check distinguishes the two cases. A 406 behind a condition that matches any Accept header gets the stronger message; a 406 behind a specific condition is reported as the narrower, considered choice it probably is. Neither is an error.

Testing your changes covers installing the tool, and what else is worth checking by hand.

See also ​