Yes, ftp.ruby-lang.org is down.
You can find more information at http://www.ruby-lang.org/en/news/2013/08/06/status-issue/ and ruby/www.ruby-lang.org#259
In the meantime, you can find a mirror at http://mirrorservice.org/sites/ftp.ruby-lang.org/pub/ruby/ , but do note the instructions on the link above about making sure you have something safe:
"If you decide to download Ruby from one of various mirrors, please be vigilant and verify the MD5 checksums of any files you download match MD5 checksum in the release notes for that version. Obviously, you should also verify that the release notes are coming from a trusted source. We have no reason to believe that www.ruby-lang.org was compromised (it runs on different hardware than ftp.ruby-lang.org) but it’s worth double-checking the release notes against the oldest version you can find on archive.org, just to be safe."
How to make rvm work under these conditions,; easy but takes a while because the github download is huge:
Pick a git tag from https://github.com/ruby/ruby/releases
Assuming you pick version tag v1_9_3_0 to install.
export ruby_release_tag=v1_9_3_0
Install it like so:
rvm install ruby-1.9.3-t${ruby_release_tag} --verify-downloads 1
If you want to install a point version, you must have at least one other point version already installed to use this method. Otherwise, use method 2.
Download the .tar.bz2 from a mirror such as http://mirrorservice.org/sites/ftp.ruby-lang.org/pub/ruby/
This should also work with a tar.gz download from github, but you need to make it a
tar.bzfile:gunzip < source.tar.gz | bzip2 > source.tar.bz2. If you download a zip from github, unzip it, and then tar.bz the result.
Put it in $rvm_path/archives/
Install it like so:
rvm install ruby-1.9.999-whatever --disable-binary