Difference between revisions of "Curl/manual"

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Latest revision as of 01:53, 26 April 2007

The text that follows is a near-replica of work obtained from a different source. Please see the "Source" section found at the bottom of this article for the appropriate reference.

If you need more information about the source of this information or wish to voice a concern, please feel free to contact me. Thank you!

The cURL Manual by the Haxx Team.

USING PASSWORDS

FTP

  • To ftp files using name+passwd, include them in the URL like:
curl ftp://name:passwd@machine.domain:port/full/path/to/file
  • or specify them with the -u flag like
curl -u name:passwd ftp://machine.domain:port/full/path/to/file

FTPS

It is just like for FTP, but you may also want to specify and use SSL-specific options for certificates etc.

HTTP

The HTTP URL doesn't support user and password in the URL string. Curl does support that anyway to provide a ftp-style interface and thus you can pick a file like: 

curl http://name:passwd@machine.domain/full/path/to/file

or specify user and password separately like in 

curl -u name:passwd http://machine.domain/full/path/to/file

HTTP offers many different methods of authentication and curl supports several: Basic, Digest, NTLM and Negotiate. Without telling which method to use, curl defaults to Basic. You can also ask curl to pick the most secure ones out of the ones that the server accepts for the given URL, by using --anyauth.

NOTE! Since HTTP URLs don't support user and password, you can't use that style when using Curl via a proxy. You _must_ use the -u style fetch during such circumstances.

HTTPS

Probably most commonly used with private certificates, as explained below.

GOPHER

Curl features no password support for gopher.

PROXY

  • Get an ftp file using a proxy named my-proxy that uses port 888:
curl -x my-proxy:888 ftp://ftp.leachsite.com/README
  • Get a file from a HTTP server that requires user and password, using the same proxy as above:
curl -u user:passwd -x my-proxy:888 http://www.get.this/
  • Some proxies require special authentication. Specify by using -U as above:
curl -U user:passwd -x my-proxy:888 http://www.get.this/

See also the environment variables Curl support that offer further proxy control.

RANGES

With HTTP 1.1 byte-ranges were introduced. Using this, a client can request to get only one or more subparts of a specified document. Curl supports this with the -r flag.

  • Get the first 100 bytes of a document:
curl -r 0-99 http://www.get.this/
  • Get the last 500 bytes of a document:
curl -r -500 http://www.get.this/

Curl also supports simple ranges for FTP files as well. Then you can only specify start and stop position.

  • Get the first 100 bytes of a document using FTP:
curl -r 0-99 ftp://www.get.this/README

UPLOADING

FTP

  • Upload all data on stdin to a specified ftp site:
curl -T - ftp://ftp.upload.com/myfile
  • Upload data from a specified file, login with user and password:
curl -T uploadfile -u user:passwd ftp://ftp.upload.com/myfile
  • Upload a local file to the remote site, and use the local file name remote too:
curl -T uploadfile -u user:passwd ftp://ftp.upload.com/
  • Upload a local file to get appended to the remote file using ftp:
curl -T localfile -a ftp://ftp.upload.com/remotefile
  • Curl also supports ftp upload through a proxy, but only if the proxy is configured to allow that kind of tunneling. If it does, you can run curl in a fashion similar to:
curl --proxytunnel -x proxy:port -T localfile ftp.upload.com

HTTP

  • Upload all data on stdin to a specified http site:
curl -T - http://www.upload.com/myfile

Note that the http server must have been configured to accept PUT before this can be done successfully.

For other ways to do http data upload, see the POST section below.

VERBOSE / DEBUG

If curl fails where it isn't supposed to, if the servers don't let you in, if you can't understand the responses: use the -v flag to get verbose fetching. Curl will output lots of info and what it sends and receives in order to let the user see all client-server interaction (but it won't show you the actual data). 

curl -v ftp://ftp.upload.com/

To get even more details and information on what curl does, try using the --trace or --trace-ascii options with a given file name to log to, like this: 

curl --trace trace.txt www.haxx.se

DETAILED INFORMATION

Different protocols provide different ways of getting detailed information about specific files/documents. To get curl to show detailed information about a single file, you should use -I/--head option. It displays all available info on a single file for HTTP and FTP. The HTTP information is a lot more extensive.

For HTTP, you can get the header information (the same as -I would show) shown before the data by using -i/--include. Curl understands the -D/--dump-header option when getting files from both FTP and HTTP, and it will then store the headers in the specified file.

  • Store the HTTP headers in a separate file (headers.txt in the example):
curl --dump-header headers.txt curl.haxx.se

Note that headers stored in a separate file can be very useful at a later time if you want curl to use cookies sent by the server. More about that in the cookies section.

POST (HTTP)

It's easy to post data using curl. This is done using the -d option. The post data must be urlencoded.

  • Post a simple "name" and "phone" guestbook.
curl -d "name=Rafael%20Sagula&phone=3320780" \
              http://www.where.com/guest.cgi

How to post a form with curl, lesson #1:

Dig out all the <input> tags in the form that you want to fill in. (There's a perl program called formfind.pl on the curl site that helps with this).

If there's a "normal" post, you use -d to post. -d takes a full "post string", which is in the format 

<variable1>=<data1>&<variable2>=<data2>&...

The 'variable' names are the names set with "name=" in the <input> tags, and the data is the contents you want to fill in for the inputs. The data must be properly URL encoded. That means you replace space with + and that you write weird letters with %XX where XX is the hexadecimal representation of the letter's ASCII code.

  • Example:
(page located at http://www.formpost.com/getthis/

<form action="post.cgi" method="post">
<input name=user size=10>
<input name=pass type=password size=10>
<input name=id type=hidden value="blablabla">
<input name=ding value="submit">
</form>

We want to enter user 'foobar' with password '12345'.

To post to this, you enter a curl command line like: 

curl -d "user=foobar&pass=12345&id=blablabla&ding=submit" http://www.formpost.com/getthis/post.cgi

While -d uses the application/x-www-form-urlencoded mime-type, generally understood by CGI's and similar, curl also supports the more capable multipart/form-data type. This latter type supports things like file upload.

-F accepts parameters like -F "name=contents". If you want the contents to be read from a file, use <@filename> as contents. When specifying a file, you can also specify the file content type by appending ';type=<mime type>' to the file name. You can also post the contents of several files in one field. For example, the field name 'coolfiles' is used to send three files, with different content types using the following syntax: 

curl -F "coolfiles=@fil1.gif;type=image/gif,fil2.txt,fil3.html" \
              http://www.post.com/postit.cgi

If the content-type is not specified, curl will try to guess from the file extension (it only knows a few), or use the previously specified type (from an earlier file if several files are specified in a list) or else it will using the default type 'text/plain'.

Emulate a fill-in form with -F. Let's say you fill in three fields in a form. One field is a file name which to post, one field is your name and one field is a file description. We want to post the file we have written named "cooltext.txt". To let curl do the posting of this data instead of your favourite browser, you have to read the HTML source of the form page and find the names of the input fields. In our example, the input field names are 'file', 'yourname' and 'filedescription'. 

curl -F "file=@cooltext.txt" -F "yourname=Daniel" \
          -F "filedescription=Cool text file with cool text inside" \
              http://www.post.com/postit.cgi

To send two files in one post you can do it in two ways:

1. Send multiple files in a single "field" with a single field name: 

curl -F "pictures=@dog.gif,cat.gif"

2. Send two fields with two field names: 

curl -F "docpicture=@dog.gif" -F "catpicture=@cat.gif"

To send a field value literally without interpreting a leading '@' or '<', or an embedded ';type=', use --form-string instead of -F. This is recommended when the value is obtained from a user or some other unpredictable source. Under these circumstances, using -F instead of --form-string would allow a user to trick curl into uploading a file.

REFERRER

 A HTTP request has the option to include information about which address
 that referred to actual page.  Curl allows you to specify the
 referrer to be used on the command line. It is especially useful to
 fool or trick stupid servers or CGI scripts that rely on that information
 being available or contain certain data.

       curl -e www.coolsite.com http://www.showme.com/

 NOTE: The referer field is defined in the HTTP spec to be a full URL.

USER AGENT

 A HTTP request has the option to include information about the browser
 that generated the request. Curl allows it to be specified on the command
 line. It is especially useful to fool or trick stupid servers or CGI
 scripts that only accept certain browsers.

 Example:

 curl -A 'Mozilla/3.0 (Win95; I)' http://www.nationsbank.com/

 Other common strings:
   'Mozilla/3.0 (Win95; I)'     Netscape Version 3 for Windows 95
   'Mozilla/3.04 (Win95; U)'    Netscape Version 3 for Windows 95
   'Mozilla/2.02 (OS/2; U)'     Netscape Version 2 for OS/2
   'Mozilla/4.04 [en] (X11; U; AIX 4.2; Nav)'           NS for AIX
   'Mozilla/4.05 [en] (X11; U; Linux 2.0.32 i586)'      NS for Linux

 Note that Internet Explorer tries hard to be compatible in every way:
   'Mozilla/4.0 (compatible; MSIE 4.01; Windows 95)'    MSIE for W95

 Mozilla is not the only possible User-Agent name:
   'Konqueror/1.0'             KDE File Manager desktop client
   'Lynx/2.7.1 libwww-FM/2.14' Lynx command line browser

COOKIES

 Cookies are generally used by web servers to keep state information at the
 client's side. The server sets cookies by sending a response line in the
 headers that looks like 'Set-Cookie: <data>' where the data part then
 typically contains a set of NAME=VALUE pairs (separated by semicolons ';'
 like "NAME1=VALUE1; NAME2=VALUE2;"). The server can also specify for what
 path the "cookie" should be used for (by specifying "path=value"), when the
 cookie should expire ("expire=DATE"), for what domain to use it
 ("domain=NAME") and if it should be used on secure connections only
 ("secure").

 If you've received a page from a server that contains a header like:
       Set-Cookie: sessionid=boo123; path="/foo";

 it means the server wants that first pair passed on when we get anything in
 a path beginning with "/foo".

 Example, get a page that wants my name passed in a cookie:

       curl -b "name=Daniel" www.sillypage.com

 Curl also has the ability to use previously received cookies in following
 sessions. If you get cookies from a server and store them in a file in a
 manner similar to:

       curl --dump-header headers www.example.com

 ... you can then in a second connect to that (or another) site, use the
 cookies from the 'headers' file like:

       curl -b headers www.example.com

 While saving headers to a file is a working way to store cookies, it is
 however error-prone and not the preferred way to do this. Instead, make curl
 save the incoming cookies using the well-known netscape cookie format like
 this:

       curl -c cookies.txt www.example.com

 Note that by specifying -b you enable the "cookie awareness" and with -L
 you can make curl follow a location: (which often is used in combination
 with cookies). So that if a site sends cookies and a location, you can
 use a non-existing file to trigger the cookie awareness like:

       curl -L -b empty.txt www.example.com

 The file to read cookies from must be formatted using plain HTTP headers OR
 as netscape's cookie file. Curl will determine what kind it is based on the
 file contents.  In the above command, curl will parse the header and store
 the cookies received from www.example.com.  curl will send to the server the
 stored cookies which match the request as it follows the location.  The
 file "empty.txt" may be a nonexistent file.

 Alas, to both read and write cookies from a netscape cookie file, you can
 set both -b and -c to use the same file:

       curl -b cookies.txt -c cookies.txt www.example.com

PROGRESS METER

 The progress meter exists to show a user that something actually is
 happening. The different fields in the output have the following meaning:

 % Total    % Received % Xferd  Average Speed          Time             Curr.
                                Dload  Upload Total    Current  Left    Speed
 0  151M    0 38608    0     0   9406      0  4:41:43  0:00:04  4:41:39  9287

 From left-to-right:
  %             - percentage completed of the whole transfer
  Total         - total size of the whole expected transfer
  %             - percentage completed of the download
  Received      - currently downloaded amount of bytes
  %             - percentage completed of the upload
  Xferd         - currently uploaded amount of bytes
  Average Speed
  Dload         - the average transfer speed of the download
  Average Speed
  Upload        - the average transfer speed of the upload
  Time Total    - expected time to complete the operation
  Time Current  - time passed since the invoke
  Time Left     - expected time left to completion
  Curr.Speed    - the average transfer speed the last 5 seconds (the first
                  5 seconds of a transfer is based on less time of course.)

 The -# option will display a totally different progress bar that doesn't
 need much explanation!

SPEED LIMIT

 Curl allows the user to set the transfer speed conditions that must be met
 to let the transfer keep going. By using the switch -y and -Y you
 can make curl abort transfers if the transfer speed is below the specified
 lowest limit for a specified time.

 To have curl abort the download if the speed is slower than 3000 bytes per
 second for 1 minute, run:

       curl -Y 3000 -y 60 www.far-away-site.com

 This can very well be used in combination with the overall time limit, so
 that the above operation must be completed in whole within 30 minutes:

       curl -m 1800 -Y 3000 -y 60 www.far-away-site.com

 Forcing curl not to transfer data faster than a given rate is also possible,
 which might be useful if you're using a limited bandwidth connection and you
 don't want your transfer to use all of it (sometimes referred to as
 "bandwidth throttle").

 Make curl transfer data no faster than 10 kilobytes per second:

       curl --limit-rate 10K www.far-away-site.com

   or

       curl --limit-rate 10240 www.far-away-site.com

 Or prevent curl from uploading data faster than 1 megabyte per second:

       curl -T upload --limit-rate 1M ftp://uploadshereplease.com

 When using the --limit-rate option, the transfer rate is regulated on a
 per-second basis, which will cause the total transfer speed to become lower
 than the given number. Sometimes of course substantially lower, if your
 transfer stalls during periods.

CONFIG FILE

 Curl automatically tries to read the .curlrc file (or _curlrc file on win32
 systems) from the user's home dir on startup.

 The config file could be made up with normal command line switches, but you
 can also specify the long options without the dashes to make it more
 readable. You can separate the options and the parameter with spaces, or
 with = or :. Comments can be used within the file. If the first letter on a
 line is a '#'-letter the rest of the line is treated as a comment.

 If you want the parameter to contain spaces, you must inclose the entire
 parameter within double quotes ("). Within those quotes, you specify a
 quote as \".

 NOTE: You must specify options and their arguments on the same line.

 Example, set default time out and proxy in a config file:

       # We want a 30 minute timeout:
       -m 1800
       # ... and we use a proxy for all accesses:
       proxy = proxy.our.domain.com:8080

 White spaces ARE significant at the end of lines, but all white spaces
 leading up to the first characters of each line are ignored.

 Prevent curl from reading the default file by using -q as the first command
 line parameter, like:

       curl -q www.thatsite.com

 Force curl to get and display a local help page in case it is invoked
 without URL by making a config file similar to:

       # default url to get
       url = "http://help.with.curl.com/curlhelp.html"

 You can specify another config file to be read by using the -K/--config
 flag. If you set config file name to "-" it'll read the config from stdin,
 which can be handy if you want to hide options from being visible in process
 tables etc:

       echo "user = user:passwd" | curl -K - http://that.secret.site.com

EXTRA HEADERS

 When using curl in your own very special programs, you may end up needing
 to pass on your own custom headers when getting a web page. You can do
 this by using the -H flag.

 Example, send the header "X-you-and-me: yes" to the server when getting a
 page:

       curl -H "X-you-and-me: yes" www.love.com

 This can also be useful in case you want curl to send a different text in a
 header than it normally does. The -H header you specify then replaces the
 header curl would normally send. If you replace an internal header with an
 empty one, you prevent that header from being sent. To prevent the Host:
 header from being used:

       curl -H "Host:" www.server.com

FTP and PATH NAMES

 Do note that when getting files with the ftp:// URL, the given path is
 relative the directory you enter. To get the file 'README' from your home
 directory at your ftp site, do:

       curl ftp://user:passwd@my.site.com/README

 But if you want the README file from the root directory of that very same
 site, you need to specify the absolute file name:

       curl ftp://user:passwd@my.site.com//README

 (I.e with an extra slash in front of the file name.)

FTP and firewalls

The FTP protocol requires one of the involved parties to open a second connction as soon as data is about to get transfered. There are two ways to do this.

The default way for curl is to issue the PASV command which causes the server to open another port and await another connection performed by the client. This is good if the client is behind a firewall that don't allow incoming connections. 

curl ftp.download.com

If the server for example, is behind a firewall that don't allow connections on other ports than 21 (or if it just doesn't support the PASV command), the other way to do it is to use the PORT command and instruct the server to connect to the client on the given (as parameters to the PORT command) IP number and port.

The -P flag to curl supports a few different options. Your machine may have several IP-addresses and/or network interfaces and curl allows you to select which of them to use. Default address can also be used: 

curl -P - ftp.download.com

Download with PORT but use the IP address of our 'le0' interface (this does not work on windows): 

curl -P le0 ftp.download.com

Download with PORT but use 192.168.0.10 as our IP address to use: 

curl -P 192.168.0.10 ftp.download.com

NETWORK INTERFACE

Get a web page from a server using a specified port for the interface: 

curl --interface eth0:1 http://www.netscape.com/

or 

curl --interface 192.168.1.10 http://www.netscape.com/

HTTPS

Secure HTTP requires SSL libraries to be installed and used when curl is built. If that is done, curl is capable of retrieving and posting documents using the HTTPS protocol.

Example: 

curl https://www.secure-site.com

Curl is also capable of using your personal certificates to get/post files from sites that require valid certificates. The only drawback is that the certificate needs to be in PEM-format. PEM is a standard and open format to store certificates with, but it is not used by the most commonly used browsers (Netscape and MSIE both use the so called PKCS#12 format). If you want curl to use the certificates you use with your (favourite) browser, you may need to download/compile a converter that can convert your browser's formatted certificates to PEM formatted ones. This kind of converter is included in recent versions of OpenSSL, and for older versions Dr Stephen N. Henson has written a patch for SSLeay that adds this functionality. You can get his patch (that requires an SSLeay installation) from his site at: http://www.drh-consultancy.demon.co.uk/

Example on how to automatically retrieve a document using a certificate with a personal password: 

curl -E /path/to/cert.pem:password https://secure.site.com/

If you neglect to specify the password on the command line, you will be prompted for the correct password before any data can be received.

Many older SSL-servers have problems with SSLv3 or TLS, that newer versions of OpenSSL etc is using, therefore it is sometimes useful to specify what SSL-version curl should use. Use -3, -2 or -1 to specify that exact SSL version to use (for SSLv3, SSLv2 or TLSv1 respectively): 

curl -2 https://secure.site.com/

Otherwise, curl will first attempt to use v3 and then v2.

To use OpenSSL to convert your favourite browser's certificate into a PEM formatted one that curl can use, do something like this (assuming netscape, but IE is likely to work similarly):

  1. You start with hitting the 'security' menu button in netscape.
  2. Select 'certificates->yours' and then pick a certificate in the list
  3. Press the 'export' button
  4. enter your PIN code for the certs
  5. select a proper place to save it

Run the 'openssl' application to convert the certificate. If you cd to the openssl installation, you can do it like: 

% ./apps/openssl pkcs12 -in [file you saved] -clcerts -out [PEMfile]

RESUMING FILE TRANSFERS

To continue a file transfer where it was previously aborted, curl supports resume on http(s) downloads as well as ftp uploads and downloads.

  • Continue downloading a document:
       curl -C - -o file ftp://ftp.server.com/path/file
  • Continue uploading a document(*1):
       curl -C - -T file ftp://ftp.server.com/path/file
  • Continue downloading a document from a web server(*2):
       curl -C - -o file http://www.server.com/

(*1) = This requires that the ftp server supports the non-standard command
       SIZE. If it doesn't, curl will say so.

(*2) = This requires that the web server supports at least HTTP/1.1. If it
       doesn't, curl will say so.

TIME CONDITIONS

HTTP allows a client to specify a time condition for the document it requests. It is If-Modified-Since or If-Unmodified-Since. Curl allow you to specify them with the -z/--time-cond flag.

For example, you can easily make a download that only gets performed if the remote file is newer than a local copy. It would be made like: 

curl -z local.html http://remote.server.com/remote.html

Or you can download a file only if the local file is newer than the remote one. Do this by prepending the date string with a '-', as in: 

curl -z -local.html http://remote.server.com/remote.html

You can specify a "free text" date as condition. Tell curl to only download the file if it was updated since yesterday: 

curl -z yesterday http://remote.server.com/remote.html

Curl will then accept a wide range of date formats. You always make the date check the other way around by prepending it with a dash '-'.

DICT

For fun try 

curl dict://dict.org/m:curl
curl dict://dict.org/d:heisenbug:jargon
curl dict://dict.org/d:daniel:web1913

Aliases for 'm' are 'match' and 'find', and aliases for 'd' are 'define' and 'lookup'. For example, 

curl dict://dict.org/find:curl

Commands that break the URL description of the RFC (but not the DICT protocol) are 

curl dict://dict.org/show:db
curl dict://dict.org/show:strat

Authentication is still missing (but this is not required by the RFC)

LDAP

If you have installed the OpenLDAP library, curl can take advantage of it and offer ldap:// support.

LDAP is a complex thing and writing an LDAP query is not an easy task. I do advice you to dig up the syntax description for that elsewhere. Two places that might suit you are:

  1. Netscape's "Netscape Directory SDK 3.0 for C Programmer's Guide Chapter 10: Working with LDAP URLs"
  2. RFC 2255, "The LDAP URL Format"

To show you an example, this is now I can get all people from my local LDAP server that has a certain sub-domain in their email address: 

curl -B "ldap://ldap.frontec.se/o=frontec??sub?mail=*sth.frontec.se"

If I want the same info in HTML format, I can get it by not using the -B (enforce ASCII) flag.

ENVIRONMENT VARIABLES

Curl reads and understands the following environment variables: 

http_proxy, HTTPS_PROXY, FTP_PROXY, GOPHER_PROXY

They should be set for protocol-specific proxies. General proxy should be set with 

ALL_PROXY

A comma-separated list of host names that shouldn't go through any proxy is set in (only an asterisk, '*' matches all hosts) 

NO_PROXY

If a tail substring of the domain-path for a host matches one of these strings, transactions with that node will not be proxied.

The usage of the -x/--proxy flag overrides the environment variables.

NETRC

Unix introduced the .netrc concept a long time ago. It is a way for a user to specify name and password for commonly visited ftp sites in a file so that you don't have to type them in each time you visit those sites. You realize this is a big security risk if someone else gets hold of your passwords, so therefore most unix programs won't read this file unless it is only readable by yourself (curl doesn't care though).

Curl supports .netrc files if told so (using the -n/--netrc and --netrc-optional options). This is not restricted to only ftp, but curl can use it for all protocols where authentication is used.

A very simple .netrc file could look something like: 

machine curl.haxx.se login iamdaniel password mysecret

CUSTOM OUTPUT

To better allow script programmers to get to know about the progress of curl, the -w/--write-out option was introduced. Using this, you can specify what information from the previous transfer you want to extract.

To display the amount of bytes downloaded together with some text and an ending newline: 

curl -w 'We downloaded %{size_download} bytes\n' www.download.com

KERBEROS4 FTP TRANSFER

Curl supports kerberos4 for FTP transfers. You need the kerberos package installed and used at curl build time for it to be used.

First, get the krb-ticket the normal way, like with the kauth tool. Then use curl in way similar to: 

curl --krb4 private ftp://krb4site.com -u username:fakepwd

There's no use for a password on the -u switch, but a blank one will make curl ask for one and you already entered the real password to kauth.

TELNET

The curl telnet support is basic and very easy to use. Curl passes all data passed to it on stdin to the remote server. Connect to a remote telnet server using a command line similar to: 

curl telnet://remote.server.com

And enter the data to pass to the server on stdin. The result will be sent to stdout or to the file you specify with -o.

You might want the -N/--no-buffer option to switch off the buffered output for slow connections or similar.

Pass options to the telnet protocol negotiation, by using the -t option. To tell the server we use a vt100 terminal, try something like: 

curl -tTTYPE=vt100 telnet://remote.server.com

Other interesting options for it -t include: 

- XDISPLOC=<X display> Sets the X display location.

- NEW_ENV=<var,val> Sets an environment variable.

NOTE: the telnet protocol does not specify any way to login with a specified user and password so curl can't do that automatically. To do that, you need to track when the login prompt is received and send the username and password accordingly.

Persistent connections

Specifying multiple files on a single command line will make curl transfer all of them, one after the other in the specified order.

libcurl will attempt to use persistent connections for the transfers so that the second transfer to the same host can use the same connection that was already initiated and was left open in the previous transfer. This greatly decreases connection time for all but the first transfer and it makes a far better use of the network.

Note that curl cannot use persistent connections for transfers that are used in subsequence curl invokes. Try to stuff as many URLs as possible on the same command line if they are using the same host, as that'll make the transfers faster. If you use a http proxy for file transfers, practically all transfers will be persistent.

Source

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