Download [UPD] A File Using Curl

Curl is a command line tool for doing all sorts of URL manipulations and transfers, but this particular document will focus on how to use it when doing HTTP requests for fun and profit. This documents assumes that you know how to invoke curl --help or curl --manual to get basic information about it.

Curl is not written to do everything for you. It makes the requests, it gets the data, it sends data and it retrieves the information. You probably need to glue everything together using some kind of script language or repeated manual invokes.

Download A File Using Curl

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HTTP is the protocol used to fetch data from web servers. It is a simple protocol that is built upon TCP/IP. The protocol also allows information to get sent to the server from the client using a few different methods, as will be shown here.

The client, curl, sends an HTTP request. The request contains a method (like GET, POST, HEAD etc), a number of request headers and sometimes a request body. The HTTP server responds with a status line (indicating if things went well), response headers and most often also a response body. The "body" part is the plain data you requested, like the actual HTML or the image etc.

The host name is usually resolved using DNS or your /etc/hosts file to an IP address and that is what curl will communicate with. Alternatively you specify the IP address directly in the URL instead of a name.

Each protocol curl supports operates on a default port number, be it over TCP or in some cases UDP. Normally you do not have to take that into consideration, but at times you run test servers on other ports or similar. Then you can specify the port number in the URL with a colon and a number immediately following the host name. Like when doing HTTP to port 1234:

The port number you specify in the URL is the number that the server uses to offer its services. Sometimes you may use a proxy, and then you may need to specify that proxy's port number separately from what curl needs to connect to the server. Like when using an HTTP proxy on port 4321:

The simplest and most common request/operation made using HTTP is to GET a URL. The URL could itself refer to a web page, an image or a file. The client issues a GET request to the server and receives the document it asked for. If you issue the command line

You can ask the remote server for ONLY the headers by using the --head (-I) option which will make curl issue a HEAD request. In some special cases servers deny the HEAD method while others still work, which is a particular kind of annoyance.

A single curl command line may involve one or many URLs. The most common case is probably to just use one, but you can specify any amount of URLs. Yes any. No limits. You will then get requests repeated over and over for all the given URLs.

Forms are the general way a website can present an HTML page with fields for the user to enter data in, and then press some kind of 'OK' or 'Submit' button to get that data sent to the server. The server then typically uses the posted data to decide how to act. Like using the entered words to search in a database, or to add the info in a bug tracking system, display the entered address on a map or using the info as a login-prompt verifying that the user is allowed to see what it is about to see.

The data you send to the server MUST already be properly encoded, curl will not do that for you. For example, if you want the data to contain a space, you need to replace that space with %20, etc. Failing to comply with this will most likely cause your data to be received wrongly and messed up.

HTTP Authentication is the ability to tell the server your username and password so that it can verify that you are allowed to do the request you are doing. The Basic authentication used in HTTP (which is the type curl uses by default) is plain text based, which means it sends username and password only slightly obfuscated, but still fully readable by anyone that sniffs on the network between you and the remote server.

Sometimes your HTTP access is only available through the use of an HTTP proxy. This seems to be especially common at various companies. An HTTP proxy may require its own user and password to allow the client to get through to the Internet. To specify those with curl, run something like:

An HTTP request may include a 'referer' field (yes it is misspelled), which can be used to tell from which URL the client got to this particular resource. Some programs/scripts check the referer field of requests to verify that this was not arriving from an external site or an unknown page. While this is a stupid way to check something so easily forged, many scripts still do it. Using curl, you can put anything you want in the referer-field and thus more easily be able to fool the server into serving your request.

At times, you will see that getting a page with curl will not return the same page that you see when getting the page with your browser. Then you know it is time to set the User Agent field to fool the server into thinking you are one of those browsers.

If you use curl to POST to a site that immediately redirects you to another page, you can safely use --location (-L) and --data/--form together. Curl will only use POST in the first request, and then revert to GET in the following operations.

Browsers typically support at least two other ways of redirects that curl does not: first the html may contain a meta refresh tag that asks the browser to load a specific URL after a set number of seconds, or it may use JavaScript to do it.

The way the web browsers do "client side state control" is by using cookies. Cookies are just names with associated contents. The cookies are sent to the client by the server. The server tells the client for what path and host name it wants the cookie sent back, and it also sends an expiration date and a few more properties.

Many applications and servers use this method to connect a series of requests into a single logical session. To be able to use curl in such occasions, we must be able to record and send back cookies the way the web application expects them. The same way browsers deal with them.

Curl has a full blown cookie parsing engine built-in that comes in use if you want to reconnect to a server and use cookies that were stored from a previous connection (or hand-crafted manually to fool the server into believing you had a previous connection). To use previously stored cookies, you run curl like:

Curl's "cookie engine" gets enabled when you use the --cookie option. If you only want curl to understand received cookies, use --cookie with a file that does not exist. Example, if you want to let curl understand cookies from a page and follow a location (and thus possibly send back cookies it received), you can invoke it like:

Curl has the ability to read and write cookie files that use the same file format that Netscape and Mozilla once used. It is a convenient way to share cookies between scripts or invokes. The --cookie (-b) switch automatically detects if a given file is such a cookie file and parses it, and by using the --cookie-jar (-c) option you will make curl write a new cookie file at the end of an operation:

Curl supports encrypted fetches when built to use a TLS library and it can be built to use one out of a fairly large set of libraries - curl -V will show which one your curl was built to use (if any!). To get a page from an HTTPS server, simply run curl like:

In the HTTPS world, you use certificates to validate that you are the one you claim to be, as an addition to normal passwords. Curl supports client- side certificates. All certificates are locked with a pass phrase, which you need to enter before the certificate can be used by curl. The pass phrase can be specified on the command line or if not, entered interactively when curl queries for it. Use a certificate with curl on an HTTPS server like:

curl also tries to verify that the server is who it claims to be, by verifying the server's certificate against a locally stored CA cert bundle. Failing the verification will cause curl to deny the connection. You must then use --insecure (-k) in case you want to tell curl to ignore that the server cannot be verified.

It should be noted that curl selects which methods to use on its own depending on what action to ask for. -d will do POST, -I will do HEAD and so on. If you use the --request / -X option you can change the method keyword curl selects, but you will not modify curl's behavior. This means that if you for example use -d "data" to do a POST, you can modify the method to a PROPFIND with -X and curl will still think it sends a POST . You can change the normal GET to a POST method by simply adding -X POST in a command line like:

This is similar to FTP, but you can use the --key option to specify a private key to use instead of a password. Note that the private key may itself be protected by a password that is unrelated to the login password of the remote system; this password is specified using the --pass option. Typically, curl will automatically extract the public key from the private key file, but in cases where curl does not have the proper library support, a matching public key file must be specified using the --pubkey option.

HTTP offers many different methods of authentication and curl supports several: Basic, Digest, NTLM and Negotiate (SPNEGO). 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! According to the URL specification, HTTP URLs can not contain a user and password, so that style will not work when using curl via a proxy, even though curl allows it at other times. When using a proxy, you must use the -u style for user and password.

curl supports both HTTP and SOCKS proxy servers, with optional authentication. It does not have special support for FTP proxy servers since there are no standards for those, but it can still be made to work with many of them. You can also use both HTTP and SOCKS proxies to transfer files to and from FTP servers. e24fc04721