Chasing SSL certificate chains to build a chain file
Supposes that you have some shiny new SSL certificates for some reason. These new certificates need a chain of intermediate certificates in order to work with everything, but for some reason you don't have the right set. In ideal circumstances you'll be able to easily find the right intermediate certificates on your SSL CA's website and won't need the rest of this entry.
Okay, let's assume that your SSL CA's website is an unhelpful swamp pit. Fortunately all is not lost, because these days at least some SSL certificates come with the information needed to find the intermediate certificates. First we need to dump out our certificate, following my OpenSSL basics:
openssl x509 -text -noout -in WHAT.crt
This will print out a bunch of information. If you're in luck (or possibly always), down at the bottom there will be a 'Authority Information Access' section with a 'CA Issuers - URI' bit. That is the URL of the next certificate up the chain, so we fetch it:
(In case it's not obvious: for this purpose you don't have to worry if this URL is being fetched over HTTP instead of HTTPS. Either your certificate is signed by this public key or it isn't.)
Generally or perhaps always this will not be a plain text file like your certificate is, but instead a binary blob. The plain text format is called PEM; your fetched binary blob of a certificate is probably in the binary DER encoding. To convert from DER to PEM we do:
openssl x509 -inform DER -in <WGOT-FILE>.crt -outform PEM -out intermediate-01.crt
Now you can inspect intermediate-01.crt in the same to see if it needs a further intermediate certificate; if it does, iterate this process. When you have a suitable collection of PEM format intermediate certificates, simply concatenate them together in order (from the first you fetched to the last, per here) to create your chain file.
PS: The Qualys SSL Server Test is a good way to see how correct your certificate chain is. If it reports that it had to download any certificates, your chain of intermediate certificates is not complete. Similarly it may report that some entries in your chain are not necessary, although in practice this rarely hurts.
Sidebar: Browsers and certificate chains
As you might guess, some but not all browsers appear to use this embedded intermediate certificate URL to automatically fetch any necessary intermediate certificates during certificate validation (as mentioned eg here). Relatedly, browsers will probably not tell you about unnecessary intermediate certificates they received from your website. The upshot of this can be a HTTPS website that works in some browsers but fails in others, and in the failing browser it may appear that you sent no additional certificates as part of a certificate chain. Always test with a tool that will tell you the low-level details.
(Doing otherwise can cause a great deal of head scratching and frustration. Don't ask how I came to know this.)
My reactions to Python's warnings module
A commentator on my entry on the warnings problem pointed out the existence of the warnings module as a possible solution to my issue. I've now played around with it and I don't think it fits my needs here, for two somewhat related reasons.
The first reason is that it simply makes me nervous to use or even take over the same infrastructure that Python itself uses for things like deprecation warnings. Warnings produced about Python code and warnings that my code produces are completely separate things and I don't like mingling them together, partly because they have significantly different needs.
The second reason is that the default formatting that the warnings module uses is completely wrong for the 'warnings produced from my program' case. I want my program warnings to produce standard Unix format (warning) messages and to, for example, not include the Python code snippet that generated them. Based on playing around with the warnings module briefly it's fairly clear that I would have to significantly reformat standard warnings to do what I want. At that point I'm not getting much out of the warnings module itself.
All of this is a sign of a fundamental decision in the warnings module: the warnings module is only designed to produce warnings about Python code. This core design purpose is reflected in many ways throughout the module, such as in the various sorts of filtering it offers and how you can't actually change the output format as far as I can see. I think that this makes it a bad fit for anything except that core purpose.
In short, if I want to log warnings I'm better off using general logging and general log filtering to control what warnings get printed. What features I want there are another entry.