**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/`
#else
    SetConsoleCtrlHandler(consoleCtrlHandler, true);
#endif
    std::set_new_handler(new_handler_terminate);
    return true;
}
bool AppInitParameterInteraction(const ArgsManager& args)
{
    const CChainParams& chainparams = Params();
    
    
    
    
    
    ChainType chain = args.GetChainType();
    if (chain == ChainType::SIGNET) {
        LogPrintf("Signet derived magic (message start): %s\n", HexStr(chainparams.MessageStart()));
    }
    bilingual_str errors;
    for (const auto& arg : args.GetUnsuitableSectionOnlyArgs()) {
        errors += strprintf(_("Config setting for %s only applied on %s network when in [%s] section.") + Untranslated("\n"), arg, ChainTypeToString(chain), ChainTypeToString(chain));
    }
    if (!errors.empty()) {
        return InitError(errors);
    }
    
    bilingual_str warnings;
    for (const auto& section : args.GetUnrecognizedSections()) {
        warnings += strprintf(Untranslated("%s:%i ") + _("Section [%s] is not recognized.") + Untranslated("\n"), section.m_file, section.m_line, section.m_name);
    }