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OpenSSL is a powerful tool for working with SSL/TLS, but it can also be used to send custom HTTP requests, which is very useful for debugging and learning how HTTP(S) works at a low level.

 

1. How It Works

Normally, tools like curl or your browser handle everything behind the scenes: they perform the TLS handshake and build the HTTP request. With OpenSSL, you manually create the HTTP request, and OpenSSL only handles the encrypted connection.

• TLS Handshake: OpenSSL’s s_client command establishes a secure (TLS) connection with the server.


• Send Request: You pipe (or redirect) a raw HTTP request into s_client. The server processes it and sends a response over the secure channel.

 

2. Sending a GET Request

Here’s how you send a simple GET request to www.bing.com:

(
printf “GET / HTTP/1.1rn”
printf “Host: www.bing.comrn”
printf “Connection: closern”
printf “rn”
) | openssl s_client -connect www.bing.com:443 -servername www.bing.com -quiet

 

Key points:

• Each line ends with rn.


• Host header is required for HTTP/1.1.


• The empty line at the end separates headers from the body (no body in GET).


• -servername ensures the correct certificate (SNI).


• -quiet hides handshake details and shows just the HTTP response. It adds -ign_eof automatically


• -ign_eof wait until get response from server

 

3. Sending a POST Request

POST requests have a body. For example, sending JSON to httpbin.org:

echo -n ‘{“name”: “Kevin”}’ > body.json
BODYLEN=$(wc -c < body.json)
(
printf "POST /post HTTP/1.1rn"
printf "Host: httpbin.orgrn"
printf "Content-Type: application/jsonrn"
printf "Content-Length: %drn" "$BODYLEN"
printf "Connection: closern"
printf "rn"
cat body.json
) | openssl s_client -connect httpbin.org:443 -servername httpbin.org -quiet

Key points:

• Set Content-Type and Content-Length correctly.


• Use cat to add the POST body.

Note: it may throw error “unexpected eof while reading:../ssl/record/rec_layer_s3.c:317:”, it appears due to server closed the connection immediately, normally it can be ignored.

4. Uploading a File (PUT Example)

You can upload files (e.g., to Azure Blob Storage) with PUT. Suppose you have a file /tmp/test.txt and an Azure SAS URL:

FILE=”test.txt”
ACCOUNT=”youraccount”
CONTAINER=”yourcontainer”
BLOB=”test.txt”
SAS=”your-sas-token”
LENGTH=$(wc -c < "$FILE")
(
printf "PUT /%s/%s?%s HTTP/1.1rn" "$CONTAINER" "$BLOB" "$SAS"
printf "Host: %s.blob.core.windows.netrn" "$ACCOUNT"
printf "x-ms-blob-type: BlockBlobrn"
printf "Content-Length: %drn" "$LENGTH"
printf "Connection: closern"
printf "rn"
cat "$FILE"
) | openssl s_client -connect "$ACCOUNT.blob.core.windows.net:443" -servername "$ACCOUNT.blob.core.windows.net" -quiet

5. Common Pitfalls & Tips

• Always end headers with rn, and include an empty line before the body.


• Content-Length must exactly match the body size (in bytes).


• Use the correct Host and -servername for TLS/SNI.


• If you get 400 Bad Request, check header formatting and line endings.


• Use -quiet or -ign_eof to see the HTTP response clearly.

 

Summary:

OpenSSL lets you see and control every detail of your HTTPS requests. It’s great for learning, debugging, and troubleshooting complex HTTP or SSL/TLS issues. Start simple—try GET and POST—then experiment with more advanced requests!