"Curl : (33) HTTP server doesn't seem to support byte ranges. Cannot resume."

Question

Given an online file which I can download via my web browser.

I run a curl on it, with

mkdir -p ./data
curl -L -C - 'http://www.ngdc.noaa.gov/mgg/global/relief/ETOPO1/data/ice_surface/grid_registe‌​red/netcdf/readme_etopo1_netcdf.txt' -o ./data/countries.zip 

I optain the following error message:

curl: (33) HTTP server doesn't seem to support byte ranges. Cannot resume.

How to fix that ? Other downloading tools welcome.

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Note:

• -L: follows redirects
• -C - : continues previously unfinished download

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Edit: this error message appears when the file to download already exist AND is already complete. It also stop the ongoing script. My requirement are :

• if the file doesn't exist, then download.
• if the file does exist but incomplete, continue the download where it stopped.
• if the file does exist and is complete, pass silently to next command. (no fail)

How could I do so ?

Answer 1

I tried running this command twice:

curl -L -C - 'http://www.naturalearthdata.com/http//www.naturalearthdata.com/download/10m/cultural/ne_10m_admin_0_sovereignty.zip' -o countries.zip

and got the following output:

$ curl -L -C - 'http://www.naturalearthdata.com/http//www.naturalearthdata.com/download/10m/cultural/ne_10m_admin_0_sovereignty.zip' -o countries.zip
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
100 5225k  100 5225k    0     0   720k      0  0:00:07  0:00:07 --:--:--  836k
$ curl -L -C - 'http://www.naturalearthdata.com/http//www.naturalearthdata.com/download/10m/cultural/ne_10m_admin_0_sovereignty.zip' -o countries.zip
** Resuming transfer from byte position 5351381
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
$ echo $?
0

So, it looks like the "resume" is working fine. Since you posted the question back in May, it is entirely possible that cURL has fixed their bug or that the webserver in question has updated their support for HTTP range requests.

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As you noted in the comments, the bug still occurs with the ngdc.noaa.gov website. I checked my curl and it is doing the same thing. Therefore the bug is still in curl.

With Wireshark I checked what is going on in the HTTP protocol. Basically, when curl makes the request to resume the completed file, the server sends back an HTTP 416 error ("Requested Range Not Satisfiable"). In the case of naturalearthdata.com, the CDN they use adds a Content-Range header specifying the exact length of the file. ngdc.noaa.gov does not add this header. Note that the addition of Content-Range in HTTP 416 responses is optional per RFC 2616.

curl uses Content-Range to determine if the download is complete. If the header is missing, curl assumes that the server doesn't support range downloads and spits out that error message.

I've reported this as a bug to the libcurl mailing list. We'll see what they say. In the meantime, here are two possible workarounds:

• Use a different downloader. I use aria2c often, which is a very nice command-line download utility with support for multiple connections and resumed downloads. It may make your downloads faster by utilizing more of your connection (assuming the server supports it), and I've checked that aria2c doesn't suffer from the same bug as curl.
• Use curl -I <URL> | grep Content-Length | cut -d' ' -f 2 to obtain the length of the file, and check that against your downloaded file size, before running curl.