Provided by: nbdkit_1.24.1-2ubuntu4_amd64 
NAME
nbdkit-curl-plugin - nbdkit curl plugin (HTTP, FTP and other protocols)
SYNOPSIS
nbdkit -r curl [url=]http://example.com/disk.img
DESCRIPTION
"nbdkit-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.
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.
EXAMPLE
nbdkit -r curl http://example.com/disk.img
serves the remote disk image as NBD on TCP port 10809 (to control ports and protocols used to serve NBD
see nbdkit(1)).
PARAMETERS
cainfo=FILENAME
(nbdkit ≥ 1.18)
Configure CA bundle for libcurl. See CURLOPT_CAINFO(3) for details.
capath=PATH
(nbdkit ≥ 1.18)
Set CA certificates directory location for libcurl. See CURLOPT_CAPATH(3) for more information.
cookie=COOKIE
cookie=+FILENAME
cookie=-
cookie=-FD
(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.
cookie-script=SCRIPT
cookie-script-renew=SECS
(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.
header=HEADER
(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.
header-script=SCRIPT
header-script-renew=SECS
(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.
password=PASSWORD
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.
password=-
Ask for the password (interactively) when nbdkit starts up.
password=+FILENAME
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.
password=-FD
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.
protocols=PROTO,PROTO,...
(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"
The value of this parameter is a comma-separated list of protocols. The following protocols are
known: dict, file, ftp, ftps, gopher, http, https, imap, imaps, ldap, ldaps, mqtt, pop3, pop3s, rtmp,
rtmpe, rtmps, rtmpt, rtmpte, rtmpts, rtsp, scp, sftp, smb, smbs, smtp, smtps, telnet, tftp.
The default is to allow any protocol.
proxy=PROXY
(nbdkit ≥ 1.20)
Set the proxy. See CURLOPT_PROXY(3).
proxy-password=PASSWORD
proxy-password=-
proxy-password=+FILENAME
proxy-password=-FD
proxy-user=USERNAME
(nbdkit ≥ 1.12)
Set the proxy username and password.
sslverify=false
Don't verify the SSL certificate of the remote host.
tcp-keepalive=true
(nbdkit ≥ 1.20)
Enable TCP keepalives.
tcp-nodelay=false
(nbdkit ≥ 1.20)
Disable Nagle’s algorithm.
timeout=SECS
Set the timeout for requests.
timeout=0
Use the default libcurl timeout for requests.
unix-socket-path=PATH
(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).
[url=]URL
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).
user=USERNAME
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")
user-agent=USER-AGENT
(nbdkit ≥ 1.22)
Send user-agent header when using HTTP or HTTPS. The default is no user-agent header.
HEADER AND COOKIE SCRIPTS
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".
Header script
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=50
Cookie script
The cookie script should print a single line in the same format as the "cookie" parameter. This is
passed to CURLOPT_COOKIE(3).
Header and cookie script shell variables
Within the "header-script" and "cookie-script" the following shell variables are available:
$iteration
The number of times that the script has been called. The first time the script is called this
contains 0.
$url
The URL as passed to the plugin.
Example: VMware ESXi cookies
VMware ESXi’s web server can expose both VMDK and raw format disk images, but requires you to log in
using HTTP Basic Authentication. While you can use the "user" and "password" parameters to send HTTP
Basic Authentication headers in every request, tests have shown that it is faster to accept the cookie
which the server returns and send that instead. (It is not clear why it is faster, but one theory is
that VMware has to do a more expensive username and password check each time.)
The web server can be accessed as below. Since the cookie expires after a certain period of time, we use
"cookie-script-renew", and because the server uses a self-signed certificate we must use --insecure and
"sslverify=false".
SERVER=esx.example.com
DCPATH=data
DS=datastore1
GUEST=guest-name
URL="https://$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=false
Example: Docker Hub authorization tokens
Accessing 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 .token
You 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=gzip
Note that this exposes a tar file over NBD. See also nbdkit-tar-filter(1).
DEBUG FLAGS
-D curl.scripts=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.
-D curl.verbose=1
This enables very verbose curl debugging. See CURLOPT_VERBOSE(3). This is mainly useful if you
suspect there is a bug inside libcurl itself.
FILES
$plugindir/nbdkit-curl-plugin.so
The plugin.
Use "nbdkit --dump-config" to find the location of $plugindir.
VERSION
"nbdkit-curl-plugin" first appeared in nbdkit 1.2.
SEE ALSO
curl(1), libcurl(3), CURLOPT_CAINFO(3), CURLOPT_CAPATH(3), CURLOPT_COOKIE(3), CURLOPT_HTTPHEADER(3),
CURLOPT_PROXY(3), CURLOPT_TCP_KEEPALIVE(3), CURLOPT_TCP_NODELAY(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-readahead-filter(1), nbdkit-retry-filter(1),
nbdkit-ssh-plugin(1), nbdkit-torrent-plugin(1), nbdkit-plugin(3), http://curl.haxx.se.
AUTHORS
Richard W.M. Jones
Parts derived from Alexander Graf's "QEMU Block driver for CURL images".
COPYRIGHT
Copyright (C) 2014-2020 Red Hat Inc.
LICENSE
Redistribution and use in source and binary forms, with or without modification, are permitted provided
that the following conditions are met:
• Redistributions of source code must retain the above copyright notice, this list of conditions and
the following disclaimer.
• Redistributions in binary form must reproduce the above copyright notice, this list of conditions and
the following disclaimer in the documentation and/or other materials provided with the distribution.
• Neither the name of Red Hat nor the names of its contributors may be used to endorse or promote
products derived from this software without specific prior written permission.
THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES,
INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
DAMAGE.
nbdkit-1.24.1 2021-01-20 nbdkit-curl-plugin(1)