[jira] [Updated] (FLINK-39761) Missing 'Connection: close' header on '304 Not Modified' responses causes proxy connection pool poisoning

["ASF GitHub Bot (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/FLINK-39761?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

ASF GitHub Bot updated FLINK-39761:
-----------------------------------
    Labels: pull-request-available  (was: )

> Missing 'Connection: close' header on '304 Not Modified' responses causes proxy connection pool poisoning
> ---------------------------------------------------------------------------------------------------------
>
>                 Key: FLINK-39761
>                 URL: https://issues.apache.org/jira/browse/FLINK-39761
>             Project: Flink
>          Issue Type: Bug
>          Components: Runtime / REST, Runtime / Web Frontend
>    Affects Versions: 1.20.4, 2.2.1
>            Reporter: Nagy Attila Bálint
>            Assignee: Nagy Attila Bálint
>            Priority: Major
>              Labels: pull-request-available
>
> *Overview:*
> When the Flink Web UI / History Server serves static files (e.g., .css, .js) and receives an If-Modified-Since request matching the file's modification time, it correctly generates a {{304 Not Modified}} response.
> However, the server immediately drops the TCP connection without including a {{Connection: close}} HTTP header in the response.
> This violates HTTP/1.1 keep-alive [expectations|https://www.rfc-editor.org/info/rfc2068/#section-19.7.1] and causes *connection pool poisoning* in downstream reverse proxies (such as Apache Knox or Nginx).
> *Impact:*
> Because HTTP/1.1 assumes persistent connections by default, reverse proxies receive the 304 response and place the connection back into their reusable connection pool.
> When the proxy attempts to reuse this connection for the very next request, it hits an unexpected end of stream because Flink has already severed the TCP connection.
> In Apache Knox, this manifests as a NoHttpResponseException and results in intermittent HTTP 500 Server Errors being served to the end user.
> *Root Cause:*
> The issue originates in {{org.apache.flink.runtime.rest.handler.legacy.files.StaticFileServerHandler}}.
> current master branch (2.2.1+) [link|https://github.com/apache/flink/blob/45295cf62608ca172b83ac42d9128d027a91d06a/flink-runtime/src/main/java/org/apache/flink/runtime/rest/handler/legacy/files/StaticFileServerHandler.java#L312]
> 1.20.4 branch [link|https://github.com/apache/flink/blob/release-1.20.4/flink-runtime/src/main/java/org/apache/flink/runtime/rest/handler/legacy/files/StaticFileServerHandler.java#L312]
> In the {{sendNotModified}} method, the code creates a {{DefaultFullHttpResponse}} and immediately attaches a {{ChannelFutureListener.CLOSE}} listener to the write operation.
> However, it fails to set the {{Connection: close}} header on the response object before flushing it to the client.
> {code:java}
> public static void sendNotModified(ChannelHandlerContext ctx) {
>     FullHttpResponse response = new DefaultFullHttpResponse(HTTP_1_1, NOT_MODIFIED);
>     setDateHeader(response);
>     // BUG: Missing explicit Connection: close header here before closing the channel.
>     // Proxies assume the connection is kept alive.
>     // close the connection as soon as the error message is sent.
>     ctx.writeAndFlush(response).addListener(ChannelFutureListener.CLOSE);
> }
> {code}
> *Proposed Solution:*
> To comply with HTTP/1.1 specifications and prevent proxy connection pool poisoning, the Flink server must explicitly communicate that the connection is being closed.
> The fix is simply to add the Connection: close header before flushing the response:
> {code:java}
> public static void sendNotModified(ChannelHandlerContext ctx) {
>     FullHttpResponse response = new DefaultFullHttpResponse(HTTP_1_1, NOT_MODIFIED);
>     setDateHeader(response);
>     
>     // Explicitly notify the client that the connection will be dropped
>     response.headers().set(HttpHeaderNames.CONNECTION, HttpHeaderValues.CLOSE);
>     // close the connection as soon as the error message is sent.
>     ctx.writeAndFlush(response).addListener(ChannelFutureListener.CLOSE);
> }
> {code}



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