nbdkit-curl-plugin - nbdkit curl plugin (HTTP, FTP and other protocols)
nbdkit -r curl [url=]http://example.com/disk.imgnbdkit-curl-plugin is a plugin for nbdkit(1) which turns content served over HTTP, FTP, and more, into a Network Block Device. It uses a library called libcurl (also known as cURL) to read data from URLs. The exact list of protocols that libcurl can handle depends on how it was compiled, but most versions will handle HTTP, HTTPS, FTP, FTPS and more (see: curl -V).
Note: This plugin supports writes. However for HTTP, you may not want nbdkit to issue PUT requests to the remote server (which probably doesn't understand them). To force nbdkit to use a readonly connection, pass the -r flag. Using the -r flag also enables NBD multi-conn, which usually performs much better (if the client supports it).
Although this plugin can access SFTP (ie. SSH) servers, it is much better to use nbdkit-ssh-plugin(1). This plugin can be used to access file:/// URLs, but you should use nbdkit-file-plugin(1) instead.
nbdkit -r curl http://example.com/disk.imgserves the remote disk image as NBD on TCP port 10809 (to control ports and protocols used to serve NBD see nbdkit(1)).
(nbdkit ≥ 1.18)
Configure CA bundle for libcurl. See CURLOPT_CAINFO(3) for details.
Pass empty string in order to not use the default certificate store that libcurl is compiled with.
(nbdkit ≥ 1.18)
Set CA certificates directory location for libcurl. See CURLOPT_CAPATH(3) for more information.
(nbdkit ≥ 1.34)
Open up to N connections to the web server. The default is 16. Connections are shared between all NBD clients, so you may wish to increase this if you expect many simultaneous NBD clients (or a single client using many multi-conn connections).
See "NBD CONNECTIONS AND CURL HANDLES" below.
(nbdkit ≥ 1.12)
Set a cookie in the request header when connecting to the remote server.
A typical example is:
cookie='vmware_soap_session="52a01262-bf93-ccce-d379-8dabb3e55560"'This option can be used at most once. It only works for HTTP and HTTPS transports. To set multiple cookies you must concatenate them yourself, eg:
cookie='name1=content1; name2=content2'See CURLOPT_COOKIE(3) for more information about this. The format is quite strict and must consist of key=value, each cookie separated by exactly "; " (semicolon and space).
If the cookie is used for authentication then passing it on the command line is not secure on shared machines. Use the alternate +FILENAME syntax to pass it in a file, - to read the cookie interactively, or -FD to read it from a file descriptor.
(nbdkit ≥ 1.26)
Enable cookie processing (eg. when following redirects), starting with an empty list of cookies. This is equivalent to calling CURLOPT_COOKIEFILE(3) with an empty string.
(nbdkit ≥ 1.26)
Enable cookie processing (eg. when following redirects), prepopulating cookies from the given file. The file can contain cookies in any format supported by curl, see CURLOPT_COOKIEFILE(3). Cookies sent by the server are not saved when nbdkit exits.
(nbdkit ≥ 1.26)
Enable cookie processing (eg. when following redirects), prepopulating cookies from the given file, and writing server cookies back to the file when the NBD handle is closed. The file can contain cookies in any format supported by curl, see CURLOPT_COOKIEJAR(3).
There is some advice on the internet telling you to set this to /dev/null, but you should not do this because it can corrupt /dev/null. If you don't want a cookiejar, omit this option. If you want to enable cookie processing without updating a permanent cookiejar, use the cookiefile= option instead.
(nbdkit ≥ 1.22, not Windows)
Run SCRIPT (a command or shell script fragment) to generate the HTTP/HTTPS cookies. cookie-script cannot be used with cookie. See "HEADER AND COOKIE SCRIPTS" below.
(nbdkit ≥ 1.26)
Do not follow redirects from the server. The default is true (follow redirects).
You can follow redirects but avoid redirecting to a less secure protocol (eg. HTTPS redirecting to FTP) by using the protocols parameter instead.
(nbdkit ≥ 1.22)
For HTTP/HTTPS, send a custom header, or remove a header that curl has added. To add a custom header:
header='X-My-Name: John Doe'To remove a header that curl has added, add the header followed by a colon and no value:
header='User-Agent:'To add a custom header that has no value, you have to use a semicolon instead of colon. This adds an X-Empty: header with no value:
header='X-Empty;'See CURLOPT_HTTPHEADER(3). You can use this option multiple times in order to add several headers. Note this sends the header in all requests, even when following a redirect, which can cause headers (eg. containing sensitive authorization information) to be sent to hosts other than the one originally requested.
(nbdkit ≥ 1.22, not Windows)
Run SCRIPT (a command or shell script fragment) to generate the HTTP/HTTPS headers. header-script cannot be used with header. See "HEADER AND COOKIE SCRIPTS" below.
(nbdkit ≥ 1.34)
Force curl to use a particular HTTP protocol. The default is none meaning curl will negotiate the best protocol with the server. The other settings are mainly for testing. See CURLOPT_HTTP_VERSION(3) for details.
(nbdkit ≥ 1.36)
Force curl to use only IPv4 (ipresolve=v4), only IPv6 (ipresolve=v6) or any IP version supported by your system (ipresolve=any). The default is any. See CURLOPT_IPRESOLVE(3).
Set the password to use when connecting to the remote server.
Note that passing this on the command line is not secure on shared machines.
Ask for the password (interactively) when nbdkit starts up.
Read the password from the named file. This is a secure method to supply a password, as long as you set the permissions on the file appropriately.
Read the password from file descriptor number FD, inherited from the parent process when nbdkit starts up. This is also a secure method to supply a password.
(nbdkit ≥ 1.12)
Limit the protocols that are allowed in the URL. Use this option for extra security if the URL comes from an untrusted source and you want to avoid security isues in the more obscure protocols that curl supports. (See qemu CVE-2013-0249 for an example of a security bug introduced by allowing unrestricted protocols).
For example if you only intend HTTP and HTTPS URLs to be used, then add this parameter: protocols=http,https
This restriction also applies if the plugin follows a redirect to another protocol (eg. you start with an https:// URL which the server redirects to ftp://). To prevent redirects altogether see the followlocation parameter.
The value of this parameter is a comma-separated list of protocols, for example protocols=https, or protocols=http,https, or protocols=file,ftp,gopher. For a list of known protocols, see the libcurl manual (CURLOPT_PROTOCOLS_STR(3)), and in nbdkit ≥ 1.40 the output of:
nbdkit curl --dump-pluginThe default is to allow any protocol.
(nbdkit ≥ 1.20)
Set the proxy. See CURLOPT_PROXY(3).
(nbdkit ≥ 1.12)
Set the proxy username and password.
Provide custom host name to IP address resolution. You can supply this option as many times as needed. See CURLOPT_RESOLVE(3) for the full details of this option.
Don't verify the SSL certificate of the remote host.
Set the SSL ciphers and TLS version. For further information see CURLOPT_SSL_CIPHER_LIST(3) and CURLOPT_SSLVERSION(3).
(nbdkit ≥ 1.20)
Enable TCP keepalives.
(nbdkit ≥ 1.20)
Enable Nagle’s algorithm. Small writes on the network socket will not be sent immediately but will be held in a local buffer and coalesced if possible. This is more efficient for the network but can cause increased latency.
The default (in libcurl ≥ 7.50.2) is that Nagle’s algorithm is disabled for better latency at the expense of network efficiency.
Set the timeout for requests.
Use the default libcurl timeout for requests.
Select TLSv1.3 ciphers available. See CURLOPT_TLS13_CIPHERS(3) and https://curl.se/docs/ssl-ciphers.html
(nbdkit ≥ 1.10)
Instead of using a TCP connection, connect to the server over the named Unix domain socket. See CURLOPT_UNIX_SOCKET_PATH(3).
The URL of the remote disk image. This is passed to libcurl directly via CURLOPT_URL(3).
This parameter is required.
url= is a magic config key and may be omitted in most cases. See "Magic parameters" in nbdkit(1).
Set the username to use when connecting to the remote server. This may also be set in the URL (eg. http://foo@example.com/disk.img)
(nbdkit ≥ 1.22)
Send user-agent header when using HTTP or HTTPS. The default is no user-agent header.
nbdkit ≤ 1.32 used a simple model where a new NBD connection would create a new libcurl handle. Since a libcurl handle maintains a small cache of connections, this meant that the number of HTTP connections would be a small multiple of the number of incoming NBD connections and the total would not be limited (assuming http: or https: URL).
nbdkit 1.34 changed to using a fixed pool of libcurl handles shared across all NBD connections. You can control the maximum number of curl handles in the pool with the connections parameter (default 4). Since each curl handle maintains a small cache of connections, this meant that the number of HTTP connections would be a small multiple of the connections parameter. If there are more than 4 incoming NBD connections, they will contend for the libcurl handles, unless you adjust connections.
nbdkit ≥ 1.36 changed again to use a curl multi handle (libcurl-multi(3)). Now the connections parameter controls the maximum number of HTTP connections made to the remote server (CURLMOPT_MAX_TOTAL_CONNECTIONS(3)). This is more efficient especially with HTTP/2 and HTTP/3, where each HTTP connection can contain a very large number of streams (typically up to 100) multiplexed over one connection. The default for connections was raised to 16.
While the header and cookie parameters can be used to specify static headers and cookies which are used in every HTTP/HTTPS request, the alternate header-script and cookie-script parameters can be used to run an external script or program to generate headers and/or cookies. This is particularly useful to access services which require an authorization token. In addition the header-script-renew and cookie-script-renew parameters allow you to renew the authorization token by rerunning the script periodically.
header-script is incompatible with header, and cookie-script is incompatible with cookie.
The header script should print zero or more HTTP headers, each line of output in the same format as the header parameter. The headers printed by the script are passed to CURLOPT_HTTPHEADER(3).
In the following example, an imaginary web service requires authentication using a token fetched from a separate login server. The token expires after 60 seconds, so we also tell the plugin that it must renew the token (by re-running the script) if more than 50 seconds have elapsed since the last request:
nbdkit curl https://service.example.com/disk.img \
header-script='
printf "Authorization: Bearer "
curl -s -X POST https://auth.example.com/login |
jq -r .token
' \
header-script-renew=50The cookie script should print a single line in the same format as the cookie parameter. This is passed to CURLOPT_COOKIE(3).
Within the header-script and cookie-script the following shell variables are available:
$iterationThe number of times that the script has been called. The first time the script is called this contains 0.
$urlThe URL as passed to the plugin.
VMware ESXi’s web server can expose both VMDK and raw format disk images, but requires you to log in using HTTP Basic Authentication. While you can use the user and password parameters to send HTTP Basic Authentication headers in every request, tests have shown that it is faster to accept the cookie which the server returns and send that instead. (It is not clear why it is faster, but one theory is that VMware has to do a more expensive username and password check each time.)
The web server can be accessed as below. Since the cookie expires after a certain period of time, we use cookie-script-renew, and because the server uses a self-signed certificate we must use --insecure and sslverify=false.
SERVER=esx.example.com
DCPATH=data
DS=datastore1
GUEST=guest-name
URL="https://$SERVER/folder/$GUEST/$GUEST-flat.vmdk?dcPath=$DCPATH&dsName=$DS"
nbdkit curl "$URL" \
cookie-script='
curl --head -s --insecure -u root:password "$url" |
sed -ne "{ s/^Set-Cookie: \([^;]*\);.*/\1/ip }"
' \
cookie-script-renew=500 \
sslverify=falseAccessing objects like container layers from Docker Hub requires that you first fetch an authorization token, even for anonymous access. These tokens expire after about 5 minutes (300 seconds) so must be periodically renewed.
You will need this authorization script (/tmp/auth.sh):
#!/bin/sh -
IMAGE=library/fedora
curl -s "https://auth.docker.io/token?service=registry.docker.io&scope=repository:$IMAGE:pull" |
jq -r .tokenYou will also need this script to get the blobSum of the layer (/tmp/blobsum.sh):
#!/bin/sh -
TOKEN=`/tmp/auth.sh`
IMAGE=library/fedora
curl -s -X GET -H "Authorization: Bearer $TOKEN" \
"https://registry-1.docker.io/v2/$IMAGE/manifests/latest" |
jq -r '.fsLayers[0].blobSum'Both scripts must be executable, and both can be run on their own to check they are working. To run nbdkit:
IMAGE=library/fedora
BLOBSUM=`/tmp/blobsum.sh`
URL="https://registry-1.docker.io/v2/$IMAGE/blobs/$BLOBSUM"
nbdkit curl "$URL" \
header-script=' printf "Authorization: Bearer "; /tmp/auth.sh ' \
header-script-renew=200 \
--filter=gzipNote that this exposes a tar file over NBD. See also nbdkit-tar-filter(1).
This prints out the headers and cookies generated by the header-script and cookie-script options, which can be useful when debugging these scripts.
This prints out additional information about the total time taken to do name resolution, connect to the remote server, etc. The information is printed in the debug output before nbdkit exits. The output will look like:
nbdkit: debug: times (-D curl.times=1):
nbdkit: debug: name resolution : 0.128442 s
nbdkit: debug: connection : 4.945213 s
nbdkit: debug: SSL negotiation : 4.291362 s
nbdkit: debug: pretransfer : 0.104137 s
nbdkit: debug: first byte received : 56.115269 s
nbdkit: debug: data transfer : 222.633831 s
nbdkit: debug: redirection time : 0.000000 sThe cumulative time taken to perform each step is shown (summed across all HTTP connections). The redirection time is the total time taken doing HTTP redirections. For further information see "TIMES" in curl_easy_getinfo(3).
This enables very verbose curl debugging. See CURLOPT_VERBOSE(3). This is mainly useful if you suspect there is a bug inside libcurl itself.
This enhances -D curl.verbose=1 by printing connection and transfer IDs next to each debug message. As this has some overhead it is not enabled by default.
The plugin.
Use nbdkit --dump-config to find the location of $plugindir.
nbdkit-curl-plugin first appeared in nbdkit 1.2.
curl(1), libcurl(3), CURLOPT_CAINFO(3), CURLOPT_CAPATH(3), CURLOPT_COOKIE(3), CURLOPT_COOKIEFILE(3), CURLOPT_COOKIEJAR(3), CURLOPT_FOLLOWLOCATION(3), CURLOPT_HTTPHEADER(3), CURLOPT_IPRESOLVE(3), CURLOPT_PROXY(3), CURLOPT_RESOLVE(3), CURLOPT_SSL_CIPHER_LIST(3), CURLOPT_SSLVERSION(3), CURLOPT_TCP_KEEPALIVE(3), CURLOPT_TCP_NODELAY(3), CURLOPT_TLS13_CIPHERS(3), CURLOPT_URL(3), CURLOPT_UNIX_SOCKET_PATH(3), CURLOPT_USERAGENT(3), CURLOPT_VERBOSE(3), nbdkit(1), nbdkit-extentlist-filter(1), nbdkit-file-plugin(1), nbdkit-retry-filter(1), nbdkit-retry-request-filter(1), nbdkit-ssh-plugin(1), nbdkit-torrent-plugin(1), nbdkit-plugin(3), http://curl.haxx.se, https://curl.se/docs/ssl-ciphers.html
Richard W.M. Jones
Parts derived from Alexander Graf's "QEMU Block driver for CURL images".
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