Unavailable downstream service results in HTTP 500

When a downstream service is unavailable for whatever reason, HTTP 500 is emitted rather than 503, and an exception is thrown

io.netty.channel.AbstractChannel$AnnotatedConnectException: finishConnect(..) failed with error(-111): Connection refused: /127.0.0.1:8556
Caused by: java.net.ConnectException: finishConnect(..) failed with error(-111): Connection refused
        at io.netty.channel.unix.Errors.newConnectException0(Errors.java:166) ~[netty-transport-native-unix-common-4.1.132.Final.jar!/:4.1.132.Final]
        at io.netty.channel.unix.Errors.handleConnectErrno(Errors.java:131) ~[netty-transport-native-unix-common-4.1.132.Final.jar!/:4.1.132.Final]
        at io.netty.channel.unix.Socket.finishConnect(Socket.java:359) ~[netty-transport-native-unix-common-4.1.132.Final.jar!/:4.1.132.Final]
        at io.netty.channel.epoll.AbstractEpollChannel$AbstractEpollUnsafe.doFinishConnect(AbstractEpollChannel.java:715) ~[netty-transport-classes-epoll-4.1.132.Final.jar!/:4.1.132.Final]
        at io.netty.channel.epoll.AbstractEpollChannel$AbstractEpollUnsafe.finishConnect(AbstractEpollChannel.java:692) ~[netty-transport-classes-epoll-4.1.132.Final.jar!/:4.1.132.Final]
        at io.netty.channel.epoll.AbstractEpollChannel$AbstractEpollUnsafe.epollOutReady(AbstractEpollChannel.java:567) ~[netty-transport-classes-epoll-4.1.132.Final.jar!/:4.1.132.Final]
        at io.netty.channel.epoll.EpollEventLoop.processReady(EpollEventLoop.java:491) ~[netty-transport-classes-epoll-4.1.132.Final.jar!/:4.1.132.Final]
        at io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:399) ~[netty-transport-classes-epoll-4.1.132.Final.jar!/:4.1.132.Final]
        at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:998) ~[netty-common-4.1.132.Final.jar!/:4.1.132.Final]
        at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74) ~[netty-common-4.1.132.Final.jar!/:4.1.132.Final]
        at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30) ~[netty-common-4.1.132.Final.jar!/:4.1.132.Final]
        at java.base/java.lang.Thread.run(Thread.java:1583) ~[na:na]

SCG WebFlux 4.3.x

Similar issue observed when DNS resolution fails in case of DNS based routes in Kubernetes (service not published)

Start by tracing Spring Cloud Gateway's WebFlux handling for downstream connection and DNS-resolution failures in the 4.3.x path. Reproduce a route with an unavailable downstream service, then verify that the response is HTTP 503 and no exception escapes as HTTP 500.