MetaMask Support / HTTP OPTIONS Support
It would appear that MetaMask is not compatible with dshackle either and I believe the root cause for that incompatibility is that dshackle does not support HTTP OPTIONS requests.
Request from the MetaMask Extension:

You'll notice it's /eth/ in the screenshot, but /eth doesn't work either (I was just testing)
The reply is a 404
I think the only way around this right now is to place dshackle behind a proxy like haproxy or nginx and force those to handle HTTP OPTIONS requests.
Ok so, I went ahead of put an NGINX server in front of dshackle and I handled the HTTP options request. The next thing that MetaMask does is ask for the net_version...
Their request seems perfectly legit:
{"id":1057264140543346,"jsonrpc":"2.0","method":"net_version","params":[]}
However, it throws an error about it not being a string or an Integer....
2020-19-08 20:58:17.265 | ERROR | HttpServerOperations | [id: 0x55e8f2b9, L:/172.17.0.2:8545 - R:/172.17.0.1:57982] Error finishing response. Closing connection
io.infinitape.etherjar.rpc.RpcException: RPC Error -32700: ID must be String or Integer
at io.emeraldpay.dshackle.proxy.ReadRpcJson.apply(ReadRpcJson.kt:144) ~[classes/:?]
at io.emeraldpay.dshackle.proxy.ReadRpcJson.apply(ReadRpcJson.kt:39) ~[classes/:?]
at reactor.core.publisher.FluxMap$MapSubscriber.onNext(FluxMap.java:100) [reactor-core-3.3.5.RELEASE.jar:3.3.5.RELEASE]
at reactor.core.publisher.FluxHandle$HandleSubscriber.onNext(FluxHandle.java:112) [reactor-core-3.3.5.RELEASE.jar:3.3.5.RELEASE]
at reactor.core.publisher.FluxMap$MapConditionalSubscriber.onNext(FluxMap.java:213) [reactor-core-3.3.5.RELEASE.jar:3.3.5.RELEASE]
at reactor.core.publisher.FluxDoFinally$DoFinallySubscriber.onNext(FluxDoFinally.java:123) [reactor-core-3.3.5.RELEASE.jar:3.3.5.RELEASE]
at reactor.core.publisher.FluxHandleFuseable$HandleFuseableSubscriber.onNext(FluxHandleFuseable.java:178) [reactor-core-3.3.5.RELEASE.jar:3.3.5.RELEASE]
at reactor.core.publisher.FluxContextStart$ContextStartSubscriber.onNext(FluxContextStart.java:96) [reactor-core-3.3.5.RELEASE.jar:3.3.5.RELEASE]
at reactor.core.publisher.Operators$MonoSubscriber.complete(Operators.java:1755) [reactor-core-3.3.5.RELEASE.jar:3.3.5.RELEASE]
at reactor.core.publisher.MonoCollectList$MonoCollectListSubscriber.onComplete(MonoCollectList.java:121) [reactor-core-3.3.5.RELEASE.jar:3.3.5.RELEASE]
at reactor.core.publisher.FluxPeek$PeekSubscriber.onComplete(FluxPeek.java:252) [reactor-core-3.3.5.RELEASE.jar:3.3.5.RELEASE]
at reactor.core.publisher.FluxMap$MapSubscriber.onComplete(FluxMap.java:136) [reactor-core-3.3.5.RELEASE.jar:3.3.5.RELEASE]
at reactor.netty.channel.FluxReceive.onInboundComplete(FluxReceive.java:366) [reactor-netty-0.9.7.RELEASE.jar:0.9.7.RELEASE]
at reactor.netty.channel.ChannelOperations.onInboundComplete(ChannelOperations.java:367) [reactor-netty-0.9.7.RELEASE.jar:0.9.7.RELEASE]
at reactor.netty.http.server.HttpServerOperations.onInboundNext(HttpServerOperations.java:489) [reactor-netty-0.9.7.RELEASE.jar:0.9.7.RELEASE]
at reactor.netty.channel.ChannelOperationsHandler.channelRead(ChannelOperationsHandler.java:96) [reactor-netty-0.9.7.RELEASE.jar:0.9.7.RELEASE]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379) [netty-transport-4.1.49.Final.jar:4.1.49.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365) [netty-transport-4.1.49.Final.jar:4.1.49.Final]
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357) [netty-transport-4.1.49.Final.jar:4.1.49.Final]
at reactor.netty.http.server.HttpTrafficHandler.channelRead(HttpTrafficHandler.java:214) [reactor-netty-0.9.7.RELEASE.jar:0.9.7.RELEASE]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379) [netty-transport-4.1.49.Final.jar:4.1.49.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365) [netty-transport-4.1.49.Final.jar:4.1.49.Final]
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357) [netty-transport-4.1.49.Final.jar:4.1.49.Final]
at io.netty.channel.CombinedChannelDuplexHandler$DelegatingChannelHandlerContext.fireChannelRead(CombinedChannelDuplexHandler.java:436) [netty-transport-4.1.49.Final.jar:4.1.49.Final]
at io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:323) [netty-codec-4.1.34.Final.jar:4.1.34.Final]
at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:297) [netty-codec-4.1.34.Final.jar:4.1.34.Final]
at io.netty.channel.CombinedChannelDuplexHandler.channelRead(CombinedChannelDuplexHandler.java:251) [netty-transport-4.1.49.Final.jar:4.1.49.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379) [netty-transport-4.1.49.Final.jar:4.1.49.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365) [netty-transport-4.1.49.Final.jar:4.1.49.Final]
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357) [netty-transport-4.1.49.Final.jar:4.1.49.Final]
at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1410) [netty-transport-4.1.49.Final.jar:4.1.49.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379) [netty-transport-4.1.49.Final.jar:4.1.49.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365) [netty-transport-4.1.49.Final.jar:4.1.49.Final]
at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:919) [netty-transport-4.1.49.Final.jar:4.1.49.Final]
at io.netty.channel.epoll.AbstractEpollStreamChannel$EpollStreamUnsafe.epollInReady(AbstractEpollStreamChannel.java:796) [netty-transport-native-epoll-4.1.34.Final-linux-x86_64.jar:4.1.34.Final]
at io.netty.channel.epoll.EpollEventLoop.processReady(EpollEventLoop.java:427) [netty-transport-native-epoll-4.1.34.Final-linux-x86_64.jar:4.1.34.Final]
at io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:328) [netty-transport-native-epoll-4.1.34.Final-linux-x86_64.jar:4.1.34.Final]
at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:989) [netty-common-4.1.49.Final.jar:4.1.49.Final]
at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74) [netty-common-4.1.49.Final.jar:4.1.49.Final]
at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30) [netty-common-4.1.49.Final.jar:4.1.49.Final]
at java.lang.Thread.run(Thread.java:834) [?:?]
Given that the max size of a standard integer is 2147483647 and they're using numbers as large as 1057264140543346, that would suggest that we need to step it up to Int64 or long?
Yeah, it was designed for microservices and HTTP access was added as an option, so no HTTP OPTIONS. It can be added, though it will also need config options, but it low priority because usually you have an ingress dispatched in front of it, like Nginx and others.
Adding it as a nice-to-have feature.
Alright, so NGINX aside...should I open a new issue for support for long ids?
added issue, you can add to it