**TL;DR**: When upgrading from `Bitcoin Core 0.20.1` to `Bitcoin Core 0.21.0`, a downstream application using the HTTP JSON-RPC API started getting HTTP 403 "Forbidden".

Turns out it is because I have `rpcallowip=::/0` in `bitcoin.conf` which used to work but does not anymore.
As a workaround, `rpcallowip=0.0.0.0/0` works.

-----

I run bitcoind in docker container1, and some application using the HTTP JSON-RPC API of bitcoind is in docker container2. They are on the same "docker network".

In `bitcoin.conf` I have:
```
rpcauth=<...>
rpcbind=0.0.0.0:8332
rpcallowip=::/0
```
(bitcoind RPC is not actually exposed to the public internet; it is up to the docker layer and above to ensure this.)

The application in container2 is connecting through HTTP accessing an URL such as `http://user:pass@172.19.1.10:8332/`.

This setup used to work when running `Bitcoin Core 0.20.1` in container1.

It no longer works with `Bitcoin Core 0.21.0`.

-----

If I change my `bitcoin.conf` as below, it starts working again using 0.21:
```
rpcauth=<...>
rpcbind=0.0.0.0:8332
rpcallowip=0.0.0.0/0
```

So `rpcallowip=::/0` used to work but does not anymore; `rpcallowip=0.0.0.0/0` works.

-----

Easy way to test:
`curl --data-binary '{"jsonrpc":"1.0","id":"curltext","method":"getblockchaininfo","params":[]}' -H 'content-type:text/plain;' http://user:pass@172.19.1.10:8332/`
for (const std::string& strFile : args.GetArgs("-loadblock")) {
        vImportFiles.push_back(fs::PathFromString(strFile));
    }
    chainman.m_thread_load = std::thread(&util::TraceThread, "initload", [=, &chainman, &args, &node] {
         Impor
        ImportBlocks(chainman, vImportFiles);
        if (args.GetBoolArg("-stopafterblockimport", DEFAULT_STOPAFTERBLOCKIMPORT)) {
            LogPrintf("Stopping after block import\n");
            StartShutdown();
            return;
        }
         Start 
        if (!StartIndexBackgroundSync(node)) {
            bilingual_str err_str = _("Failed to start indexes, shutting down..");
            chainman.GetNotifications().fatalError(err_str.original, err_str);
            return;
        }
         Load 
        if (auto* pool{chainman.ActiveChainstate().GetMempool()}) {
            LoadMempool(*pool, ShouldPersistMempool(args) ? MempoolPath(args) : fs::path{}, chainman.ActiveChainstate(), {});
            pool->SetLoadTried(!chainman.m_interrupt);
        }
    });
     Wait for genesis block to be processed
    {
        WAIT_LOCK(g_genesis_wait_mutex, lock);
         We pr
         Impor
         check 
        while (!fHaveGenesis && !ShutdownRequested()) {