Error “curl: (1) Received HTTP/0.9 when not allowed” (SOLVED)

cURL is a popular command line utility that supports many network protocols.

curl supports HTTP with numerous options and variations. It can speak HTTP version 0.9, 1.0, 1.1, 2 and 3 depending on build options and the correct command line options.

But when the remote host does respond with HTTP/0.9, curl fails with an error that HTTP/0.9 is not allowed, e.g .:

curl 109.169.23.79:8083
curl: (1) Received HTTP/0.9 when not allowed

The thing is that since curl 7.66.0, HTTP/0.9 is disabled by default. But this is not a problem, as this protocol can be re-enabled with an option.

The --http0.9 option tells curl to handle HTTP version 0.9 response normally.

HTTP/0.9 is a completely headerless response and therefore you can also connect with this to non-HTTP servers and still get a response since curl will simply transparently downgrade - if allowed.

An example of a command with this option:

curl --http0.9 109.169.23.79:8083

What versions of HTTP does cURL support? How to select the HTTP version in cURL

As mentioned, cURL supports the following versions of HTTP: 0.9, 1.0, 1.1, 2, and 3.

The following options are used to select them:

--http0.9

Considered just above

-0, --http1.0

Tells curl to use HTTP version 1.0 instead of using its internally preferred HTTP version.

This option overrides --http1.1 and --http2.

--http1.1

Tells curl to use HTTP version 1.1.

This option overrides -0, --http1.0 and --http2. Added in 7.33.0.

--http2-prior-knowledge

Tells curl to issue its non-TLS HTTP requests using HTTP/2 without HTTP/1.1 Upgrade. It requires prior knowledge that the server supports HTTP/2 straight away. HTTPS requests will still do HTTP/2 the standard way with negotiated protocol version in the TLS handshake.

--http2-prior-knowledge requires that the underlying libcurl was built to support HTTP/2. This option overrides --http1.1 and -0, --http1.0 and --http2. Added in 7.49.0.

--http2

Tells curl to use HTTP version 2.

See also --http1.1 and --http3. --http2 requires that the underlying libcurl was built to support HTTP/2. This option overrides --http1.1 and -0, --http1.0 and --http2-prior-knowledge. Added in 7.33.0.

--http3

WARNING: this option is experimental. Do not use in production.

Tells curl to use HTTP version 3 directly to the host and port number used in the URL. A normal HTTP/3 transaction will be done to a host and then get redirected via Alt-SVc, but this option allows a user to circumvent that when you know that the target speaks HTTP/3 on the given host and port.

This option will make curl fail if a QUIC connection cannot be established, it cannot fall back to a lower HTTP version on its own.

See also --http1.1 and --http2. --http3 requires that the underlying libcurl was built to support HTTP/3. This option overrides --http1.1 and -0, --http1.0 and --http2 and --http2-prior-knowledge. Added in 7.66.0.