How to make an HTTP request with telnet

One of the most frequent interview question for tech professionals, especially system administrators and developers is – “tell us what happens when you type a URL in a browser?”. Skipping the DNS resolution part, we can understand the client to server HTTP communication with telnet. The simplest case is a GET request to a path with a HOST header.As an example, let us make an http request to an AWS service which responds back with our public IP address –

$ telnet 80
GET / HTTP/1.1

Here is the full transaction –

daniel@hidmo:~$ telnet 80
Connected to
Escape character is '^]'.
GET / HTTP/1.1

HTTP/1.1 200 OK
Date: Sat, 14 Sep 2019 12:51:55 GMT
Server: lighttpd/1.4.41
Content-Length: 14
Connection: keep-alive
Connection closed by foreign host.

Notice how the server closes the connection after waiting for a few seconds, that is because the keep-alive is enabled on the server side as shown from the server response – “Connection: keep-alive“. With keep-alive we can make additional http calls with out going through the whole 3-way TCP handshake.

Disable keep-alive on client side

If for some reason, we want to close the connection on the client side immediately we can pass “Connection: Close” as part of the http header in the request.

References –

Telnet manpage

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