ssh-keygen ignoring -t parameter

Question

I am trying to generate a new keypair using the following command

ssh-keygen -t rsa -f id_rsa

According to the manpages, the -t param should indicate the format of the key:

ssh-keygen [-q] [-b bits] [-t dsa | ecdsa | ed25519 | rsa] [-N new_passphrase] [-C comment] [-f output_keyfile]
[...]
The type of key to be generated is specified with the -t option.  If invoked without any arguments, ssh-keygen will generate
     an RSA key.
[...]
-t dsa | ecdsa | ed25519 | rsa
     Specifies the type of key to create.  The possible values are “dsa”, “ecdsa”, “ed25519”, or “rsa”

However, the command still outputs a key with the newer ed25519 format:

$ head id_rsa
-----BEGIN OPENSSH PRIVATE KEY-----
[...]

How do I generate a proper RSA type key?

Answer 1

Meta: this isn't a programming question or problem and probably belongs on a different Stack, but I'm not sure which so I'll leave it for anyone who wants to suggest a move.

-t selects the type (aka algorithm) of the key, not necessarily the format of the file it is stored in (except for the long-obsolete and now-deleted type rsa1 which was tied to SSH1 format). You probably have 7.8, which uses 'new' file format by default for all types whereas before it was the default only for ed25519. The new file format does (and already did) support all types, but in older versions was used for other types only if you specified -o. See the release notes which describe how to go back to 'legacy' format, or the ssh-keygen man page under -m.

Note OpenSSH new format uses a much better PBE (password-based encryption) scheme than OpenSSL's legacy PEM format(s), the previous default. OpenSSH is also capable of reading (though not creating) PKCS8-encrypted PEM format which you can create or convert with OpenSSL (except ed25519) and is better but still not as good as OpenSSH new format. (Warning: ssh-keygen -m PKCS8 is a misnomer and actually uses X.509/PKIX SPKI format not PKCS8, which can be confusing.) If you're interested in the security of these key files, I'll dig up some of the existing Qs on that topic; I'm sure I've seen at least a dozen.