aio-libs/aiohttp · error · BadHttpMessage

Missing 'Host' header in request.

Error message

Missing 'Host' header in request.

What it means

Raised by HttpRequestParser.parse_message (aiohttp/http_parser.py:717) when an HTTP/1.1 request has no Host header. RFC 9110 section 7.2 makes Host mandatory for HTTP/1.1. HTTP/1.0 requests are not required to send Host (though encouraged), so this only fires for version 1.1.

Source

Thrown at aiohttp/http_parser.py:717

            if not url.absolute:
                # authority-form is only allowed with CONNECT
                # https://www.rfc-editor.org/info/rfc9112/#section-3.2.3-1
                raise InvalidURLError(
                    path.encode(errors="surrogateescape").decode("latin1")
                )

        # read headers
        (
            headers,
            raw_headers,
            close,
            compression,
            upgrade,
            chunked,
        ) = self.parse_headers(lines[1:])

        if version_o == HttpVersion11 and hdrs.HOST not in headers:
            raise BadHttpMessage("Missing 'Host' header in request.")

        if close is None:  # then the headers weren't set in the request
            if version_o <= HttpVersion10:  # HTTP 1.0 must asks to not close
                close = True
            else:  # HTTP 1.1 must ask to close.
                close = False

        return RawRequestMessage(
            method,
            path,
            version_o,
            headers,
            raw_headers,
            close,
            compression,
            upgrade,
            chunked,
            url,

View on GitHub (pinned to c0ef574e29)

Solutions

1. Include a 'Host' header in every HTTP/1.1 request.
2. If you intend HTTP/1.0 semantics, send 'HTTP/1.0' in the request line (Host not required).
3. Use a real HTTP client (aiohttp.ClientSession) which sets Host automatically from the URL.

Example fix

# before - HTTP/1.1 with no Host
sock.send(b'GET / HTTP/1.1\r\n\r\n')
# after - include Host
sock.send(b'GET / HTTP/1.1\r\nHost: example.com\r\n\r\n')
# or downgrade to HTTP/1.0 if Host is unavailable
sock.send(b'GET / HTTP/1.0\r\n\r\n')

def host_ok(version: str, headers) -> bool:
    if version == 'HTTP/1.1':
        return any(k.lower() == 'host' for k in headers)
    return True  # HTTP/1.0 does not require Host
assert host_ok(version_token, outgoing_headers)

When it happens

Trigger: An HTTP/1.1 request line ('... HTTP/1.1') with no 'Host' header in the header block. Common with raw socket/netcat clients, telnet, or HTTP/1.0 clients that were bumped to 1.1 without adding Host.

Common situations: netcat/telnet health checks, embedded clients that build requests by hand, load-balancer probes that strip Host, or proxy misconfiguration that drops the header.

Related errors

• Duplicate '{name}' header found.
• Invalid Content-Length header: {content_length_hdr!r}
• Got more than {limit} bytes when reading: {line!r}.
• Bad line ending, expected CRLF
• Transfer-Encoding can't be present with Content-Length

AI-assisted analysis of aio-libs/aiohttp@c0ef574e29 (2026-08-04). Data as JSON: /data/errors/b8e6e4c1d436f40f.json. Report an issue: GitHub.