**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/`
if (args.IsArgSet("-externalip")) {
        
        if (args.SoftSetBoolArg("-discover", false))
            LogPrintf("%s: parameter interaction: -externalip set -> setting -discover=0\n", __func__);
    }
    if (args.GetBoolArg("-blocksonly", DEFAULT_BLOCKSONLY)) {
        
        if (args.SoftSetBoolArg("-whitelistrelay", false))
            LogPrintf("%s: parameter interaction: -blocksonly=1 -> setting -whitelistrelay=0\n", __func__);
        
        if (args.SoftSetArg("-maxmempool", ToString(DEFAULT_BLOCKSONLY_MAX_MEMPOOL_SIZE_MB)))
            LogPrintf("%s: parameter interaction: -blocksonly=1 -> setting -maxmempool=%d\n", __func__, DEFAULT_BLOCKSONLY_MAX_MEMPOOL_SIZE_MB);
    }
    
    if (args.GetBoolArg("-whitelistforcerelay", DEFAULT_WHITELISTFORCERELAY)) {
        if (args.SoftSetBoolArg("-whitelistrelay", true))
            LogPrintf("%s: parameter interaction: -whitelistforcerelay=1 -> setting -whitelistrelay=1\n", __func__);
    }
    if (args.IsArgSet("-onlynet")) {
        const auto onlynets = args.GetArgs("-onlynet");
        bool clearnet_reachable = std::any_of(onlynets.begin(), onlynets.end(), [](const auto& net) {
            const auto n = ParseNetwork(net);
            return n == NET_IPV4 || n == NET_IPV6;
        });
        if (!clearnet_reachable && args.SoftSetBoolArg("-dnsseed", false)) {
            LogPrintf("%s: parameter interaction: -onlynet excludes IPv4 and IPv6 -> setting -dnsseed=0\n", __func__);
        }
    }
}
**