cURL is a command line tool to get or Submit data using URL syntax.

If you work as a developer or in support, you need to be aware of Using the cURL command to troubleshoot web applications. cURL is a cross-platform utility that you can use on Windows, MAC and UNIX.

The following are some of the most commonly used syntaxes. An example should help you.

Verify if you can connect to the URL

If you are on a UNIX system and are trying to connect the external URL, you must first verify that you can access the URL through curl.

curl yoururl.com

No output is thrown. However, if the server fails to connect, you will see errors that could not resolve the host.

Save URL/URI output to file

If you need to save the URL or URI content to a specific file, you can use the following syntax

curl https://yoururl.com > yoururl.html

ex:

[[email protected] tmp]# curl https://gf.dev > /tmp/gfhtml
  % Total % Received % Xferd Average Speed ​​Time Time Time Current
                                 Download Upload Total Spent Left Speed
100 18557 0 18557 0 0 72565 0 --:--:-- --:--:-- --:--:-- 72772
[[email protected] tmp]#

The example above saves the entire contents of gf.dev to /tmp/gf.html

Show request and response header

If you’re having trouble and want to validate, you’ll get the expected request and response header.

curl -v yoururl.com

Ex:

[[email protected] tmp]# curl -v https://geekflare.com
* About to connect() to geekflare.com port 443 (#0)
* Trying 104.25.134.107...
* Connected to geekflare.com (104.25.134.107) port 443 (#0)
* Initializing NSS with certpath: sql:/etc/pki/nssdb
* CAfile: /etc/pki/tls/certs/ca-bundle.crt
  CApath: none
* SSL connection using TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256
* Server certificate:
* subject: CN=ssl371609.cloudflaressl.com,OU=PositiveSSL Multi-Domain,OU=Domain Control Validated
* start date: Nov 07 00:00:00 2019 GMT
* expire date: May 15 23:59:59 2020 GMT
* common name: ssl371609.cloudflaressl.com
* issuer: CN=COMODO ECC Domain Validation Secure Server CA 2,O=COMODO CA Limited,L=Salford,ST=Greater Manchester,C=GB
> GET/HTTP/1.1
> User agent: curl/7.29.0
> Host: geekflare.com
> */*
>
< HTTP/1.1 200 OK
< Date: Sat, 09 Nov 2019 19:41:37 GMT
< Content type: text/html; charset=UTF-8
< Transfer encoding: chunked
< Connection: keep alive
< Set cookie: __cfduid=d2ce6cd359ebc0b6eb5ff3a454ed042021573328497; expires=Sun, 08-Nov-20 19:41:37 GMT; path=/; domain=.geekflare.com; HttpOnly; Safe
< Vary: Accept Encoding
< Link: <https://geekflare.com/wp-json/>; rel="https://api.w.org/"
< Link: <https://geekflare.com/>; rel=shortlink
< X-SRCache Fetch Status: HIT
< X-SRCache Store Status: BYPASS
< X-Frame Options: SAMEORIGIN
< X-Powered-By: EasyEngine v4.0.12
< Via: 1.1 google
< CF Cache Status: DYNAMIC
< Strict Transport Security: max-age=15552000; preload
< X-Content-Type-Options: nosniff
< Alt-Svc: h3-23=":443"; ma=86400
< Expect CT: max-age=604800, report-uri="https://report-uri.cloudflare.com/cdn-cgi/beacon/expect-ct"
< Server: cloudflare
< CF RAY: 533243e4bcd4bbf4-LHR
<

Download at a limit rate

If you are working on optimization and want to see how much time it takes to download at a certain speed, you can:

curl –-limit-rate 2000B

Ex:

curl --limit-rate 2000B https://gf.dev

Using a proxy to connect

Very handy when you work on the DMZ server where you need to connect to the outside world through a proxy.

curl --proxy yourproxy:port https://yoururl.com

Test URL with injecting header

You can use curl by including a header with your data to test or troubleshoot the issue at hand. Let's see the following example to request it with Content-Type.

curl --header 'Content-Type: application/json' http://yoururl.com

If you do the above, ask curl to pass Content-Type as application / json in the request header.

Display only response header

If you are troubleshooting and want to quickly check the response header, you can use the following syntax.

curl --head http://yoururl.com

Ex:

[[email protected] tmp]# curl --head https://chandan. ok
HTTP/1.1 200 OK
Date: Sat, 09 Nov 2019 19:51:23 GMT
Content type: text/html
Connection: keep alive
Set cookie: __cfduid=d3cb2c7b8e566ad99c870b0af12b0f1eb1573329083; expires=Sun, 08-Nov-20 19:51:23 GMT; path=/; domain=.chandan.io; HttpOnly
X-GUploader-UploadID: AEnB2Uo96JhvJmR2zYUL-Ndh2ta3UD_ykQAB5C7O8cjZQhCf-GxHQ0MsodSzRnl3guSN3ywAYNjtWcPXfwDXjLg3bQ-P5vQMOA
Expires: Sat, 09 Nov 2019 20:51:23 GMT
Cache control: public, max-age=3600
Last-Modified: Mon, 06 Aug 2018 10:45:47 GMT
x-goog-generation: 1533552347482034
x-goog-metageneration: 1
x-goog-stored-content-encoding: identity
x-goog-stored-content-length: 24620
x-goog-hash: crc32c=DpDPAQ==
x-goog-hash: md5=cIP/3rusdUx12Zla1kf1yA==
x-goog-storage-class: MULTI_REGIONAL
Accept Ranges: bytes
CF Cache Status: DYNAMIC
Expect CT: max-age=604800, report-uri="https://report-uri.cloudflare.com/cdn-cgi/beacon/expect-ct"
Server: cloudflare
CF RAY: 53325234dc2fbb9a-LHR

[[email protected] tmp]#

Connect HTTPS/SSL URL and ignore any SSL certificate error

If you try to access a URL secured with SSL/TLS certificates and it matches the wrong certificate or CN, you will get the following error.

curl: (51) Unable to communicate securely with peer: requested domain name does not match the server's certificate.

Good news, you can tell cURL to ignore the certification error with --insecure flag.

curl --insecure https://yoururl.com

Connect using a specific protocol (SSL/TLS)

Very handy for testing if a specific URL can pass a specific SSL/TLS protocol.

How to connect with SSL v3

curl --sslv3 https://yoururl.com

and for different TLS versions

curl --tlsv1 https://example.com
curl --tlsv1.0 https://example.com
curl --tlsv1.1 https://example.com
curl --tlsv1.2 https://example.com
curl --tlsv1.3 https://example.com

Download file from FTP Server

You can also use curl to download the file by providing the username and password.

curl -u user:password -O ftp://ftpurl/style.css

You can always use “-v” with any syntax to print in verbose mode.

Using Host Header

The Host header is useful for testing the destination URL over IP when the requested content is only available if the Host header matches. Or if you want to test the application with Load Balancer IP/URL.

curl --header 'Host: targetapplication.com' https://192.0.0.1:8080/

How about cURL online?

Yes, this is possible with the following tools. You can run cURL remotely.

Online CURL - a lightweight tool for getting the URL online and the ability to add the following options.

--connect-timeout
--cookies
--data
--headers
--head
--location
--max-time
--proxy
--request
--user
--url
--user-agent

cURL command line builder - This one is different. It helps you create the curl command, which allows you to input information into a nice UI. Below is the cURL.

command

cURL is a useful utility to troubleshoot a real-time connectivity issue and I hope the above will help you. If you want to learn more, I would recommend Linux Command Line Basics online course.