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https://issues.apache.org/jira/browse/HTTPCLIENT-2401?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18029993#comment-18029993
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Oleg Kalnichevski edited comment on HTTPCLIENT-2401 at 10/15/25 8:10 AM:
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This is what RFC 9110 states.
```
7.8. Upgrade
The "Upgrade" header field is intended to provide a simple mechanism for
transitioning from HTTP/1.1 to some other protocol on the same connection.
A client MAY send a list of protocol names in the Upgrade header field of a
request to invite the server to switch to one or more of the named protocols,
in order of descending preference, before sending the final response. A server
MAY ignore a received Upgrade header field if it wishes to continue using the
current protocol on that connection. Upgrade cannot be used to insist on a
protocol change.
```
Upgrade header is an _invitation_. This is perfectly appropriate for a client
to invite the opposite endpoint to upgrade and it should be perfectly fine for
the opposite endpoint to _ignore_ the invitation.
[~Laxika] Please report this defect to the developers of the proxy.
was (Author: olegk):
This is what RFC 9110 states.
{noformat}
7.8. Upgrade
The "Upgrade" header field is intended to provide a simple mechanism for
transitioning from HTTP/1.1 to some other protocol on the same connection.
A client MAY send a list of protocol names in the Upgrade header field of a
request to invite the server to switch to one or more of the named protocols,
in order of descending preference, before sending the final response. A server
MAY ignore a received Upgrade header field if it wishes to continue using the
current protocol on that connection. Upgrade cannot be used to insist on a
protocol change.
{noformat}
Upgrade header is an _invitation_. This is perfectly appropriate for a client
to invite the opposite endpoint to upgrade and it should be perfectly fine for
the opposite endpoint to _ignore_ the invitation.
[~Laxika] Please report this defect to the developers of the proxy.
> RequestUpgrade interceptor tries to upgrade the connection when using proxies
> -----------------------------------------------------------------------------
>
> Key: HTTPCLIENT-2401
> URL: https://issues.apache.org/jira/browse/HTTPCLIENT-2401
> Project: HttpComponents HttpClient
> Issue Type: Bug
> Components: HttpClient (classic)
> Affects Versions: 5.5.1
> Reporter: Lakatos Gyula
> Priority: Minor
> Time Spent: 10m
> Remaining Estimate: 0h
>
> The {{RequestUpgrade}} interceptor tries to upgrade the connection to TLS 1.2
> when proxies are being used. Most proxies don't support HTTPS connections,
> only HTTP tunneling (via {{{}CONNECT{}}}).
> Here is a trivial example that reproduces the error:
> {code:java}
> package com.example.proxy.apachehttpclient5;
> import org.apache.hc.client5.http.classic.methods.HttpGet;
> import org.apache.hc.client5.http.impl.classic.CloseableHttpClient;
> import org.apache.hc.client5.http.impl.classic.HttpClients;
> import org.apache.hc.core5.http.HttpHost;
> import org.apache.hc.core5.http.io.entity.EntityUtils;
> import java.io.IOException;
> public class Main {
> public static void main(String[] args) throws IOException {
> try (final CloseableHttpClient httpclient = HttpClients.custom()
> .setProxy(new HttpHost("proxy.scrape.do", 8080))
> .build()) {
> final HttpGet request = new HttpGet("http://crawler-test.com/";);
> request.setHeader("Proxy-Authorization", "Basic xyz");
> String responseContent = httpclient.execute(request, response ->
> EntityUtils.toString(response.getEntity()));
> System.out.println(responseContent);
> }
> } {code}
> The proxy service returns with a 400 Bad Request.
> As a workaround, the logic in the {{RequestUpgrade}} interceptor can be
> disabled with a {{RequestConfig}} that sets {{protocolUpgradeEnabled}} to
> false. However, I think the default behavior is still incorrect in this case
> and is highly unexpected. It took me a significant amount of digging around
> to figure out what's going on in the background.
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