About This Article
This article was created using an automated generation workflow powered by generative AI. It is structured based on the official curl manual and HTTP specifications, focusing on observing the entry point of redirects via headers to facilitate initial triage for SSO, short URLs, and link failures.Verification Status: 📘 Confirmed with official curl and HTTP specifications; external communication not verified.
When a URL redirects to another URL, instead of using-Lto follow everything right away, checking the status andLocationfirst helps you understand what is actually happening. Particularly with SSO, shortened URLs, HTTP-to-HTTPS upgrades, CDN changes, and public URL updates, simply visualizing "where you are redirected" makes troubleshooting much smoother.
Success Criteria for This Guide
You will have succeeded if you can confirm the3xxandLocation:included in the initial response, and observe how curl follows the destination when you add-L.
Try This First
curl -sS -D - -o /dev/null https://httpbin.org/redirect/1
Check Here
3xxIf a status andLocation:appear, that response itself is instructing you where to go next.
There are three key points to inspect:
What is the status code?
Location:Is the Location a relative or absolute URL?Has the hostname changed?
This allows you to verify the phenomenon of "opening a link opens a different screen" not just by how it looks in the browser, but as an HTTP response.
Change One Thing
curl -sS -L -D - -o /dev/null https://httpbin.org/redirect/1
-LAdding tells curl to follow the Location header. If multiple sets of headers are displayed, observe the intermediate statuses as well.
Next, change the redirect limit.
curl -sS -L -D - -o /dev/null https://httpbin.org/redirect/3
By confirming how the header blocks increase when comparing 1 hop to 3 hops, you will understand the difference between "looking only at the final URL" and "inspecting the intermediate path."
Why Does This Happen?
HTTP redirection is a mechanism where the server returns a 3xx status and aLocationheader, allowing the client to determine the next request.
What is important here is thatredirects are not mere URL substitutions. With 301, 302, 303, 307, and 308, how the client handles the method in the subsequent request is not always the same. While this difference can be easy to miss if you only look at GET requests, it matters significantly for form submissions and API POST requests.
Furthermore, when the hostname changes at the redirect destination, other conditions such as authentication credentials, cookies, proxies, certificates, and network controls may come into play.
graph LR A[元URL] -->|3xx + Location| B[次のURL] B -->|3xx + Location| C[さらに次のURL] C -->|200など| D[最終応答]
Check Failures and Boundary Conditions Too
It is also important not to investigate under the assumption that a redirect always exists.
curl -sS -D - -o /dev/null https://example.com/
Location:If there is no, that fact itself is the result. Moreover, issues like DNS failures, TLS errors, or proxy rejections stop before a 3xx status occurs, so suspecting only redirects will not solve them.
-LAdding from the beginning tends to focus your attention solely on the final outcome, making it easy to overlook abnormal intermediate jump destinations or unnecessary redirects.
Where Can This Be Applied in Administrative and Office Work?
1. Verifying Internal Portal Links
When clicking an old URL redirects to a new site, you can check where the original URL is forwarding. When organizing legacy links left in departmental procedural manuals or Excel spreadsheets, this serves as material to identify the official new URL rather than just checking whether the link opens.
2. Initial Triage of SSO Inquiries
For inquiries such as "getting stuck in a login screen loop" or "not returning to the original screen after authentication," this serves as a starting point to see if the redirect chain has become too long or if it is routing to an unexpected hostname.
However, do not casually share actual requests containing authentication cookies or tokens. Always verify using public URLs or pre-authentication entry points first.
3. Checking Shortened URLs in Emails and Documents
Before opening a shortened URL directly in a browser, checking just theLocationheader lets you grasp the initial forwarding destination. While this does not guarantee safety, it serves as an initial check to see "where it is trying to lead you."
4. Material for Inquiries to Web Administrators
Instead of simply saying "it won't open," organizing details such as:
The initial URL
The initial status
The Location header
The number of redirects
The final status
makes it much easier to communicate specifics to web administrators or vendors.
Summary for Professional Work
This technique is effective for investigating SSO flows, shortened URLs, HTTP-to-HTTPS upgrades, CDN switches, and legacy internal links. To avoid blindly sending requests with authentication headers or cookies to unknown redirect destinations, always verify the target destination first.
As an operational procedure, the safest sequence is:Inspect the entry point without-Lfirst -> Follow the chain with-Lif necessary -> Identify the point where the host changes.
curl manual: https://curl.se/docs/manpage.html
RFC 9110: https://www.rfc-editor.org/rfc/rfc9110
