Can't open FTP URL when building vignette

I have an R package with a vignette that demonstrates a function that downloads cDNA fasta files from Ensembl’s FTP site. Everywhere else the URL used by the function works, both on my lab’s Linux server and my own computer, yet on Travis CI, that same URL can’t be opened. Here’s the log:

Why does this happen? Is it because of poor network in Travis instances and it takes too long to download the file (up to 80MB)?

Your program doesn’t give any details for the error so I cannot say anything for sure. Make it provide details, and/or, if you control the server, look at its logs. Ideally, you want to see a full transcript of the FTP session.

A common problem with using FTP from Travis is a server not allowing control and data connections from different IPs.

I don’t control the server; the package simply puts together that FTP URL with Ensembl version number and species and downloads a file using that URL, though the multiple IPs problem sounds like a plausible explanation.