Why does /etc/hosts not work anymore for some websites on Big Sur? [closed]

For years I’ve used /etc/hosts to adjust IP address. Sometimes it’s for prototyping, or adding an address for development purposes that isn’t ready to go into public DNS. And sometimes I block certain domains that I don’t want to visit by redirecting them to incorrect IP addresses. Some are invasive tracking websites, e.g., connect.facebook.net. Others I keep blocked as an anti-procrastination measures, for example news sites such as politico.com which I might otherwise spend all day reading when I meant to be working.

But now on macOS 11 Big Sur, I notice that some /etc/hosts entries are ignored by Safari. DNS lookups with gethostbyname() correctly show the block/override address, but Safari displays the public site anyway.

What on earth is going on? What’s changed with /etc/hosts and can I keep using it to override DNS entries when doing development work?

macOS 11 added support for what is officially called “Service binding and parameter specification via the DNS (DNS SVCB and HTTPSSV)”.

Now, when you visit a website, it’s not just the typical DNS A host-to-ip-address record that’s consulted, but a brand-new HTTPS DNS record is checked too. It’s not just a name entry; it’s a brand-new record type (#65), to go along with the more familiar A and CNAME and MX.

These new HTTPS DNS records can indicate that the site supports HTTPS, including protocol versions and IP addresses. That way, typing in a bare domain name gives the https:// version of the site right away, maybe even on HTTP/2 or HTTP/3, skipping the old-fashioned HTTP redirect. There’s even a draft option for domain operators to tell your computer to bypass any local DNS settings and use a specific server for all future DNS queries involving their domain.

There are many pro-performance intentions here, and some pro-privacy ones too.

But there is a fatal privacy and security flaw in both the specification and implementation: it removes the ability for users to override domain name lookups in /etc/hosts, even when faced with actively malicious domain name operators.

To see how this is working in action:

The version of dig that comes with macOS doesn’t directly support these new records, but you can see whether they exist with

$ dig -t type65 www.politico.com
…
;; QUESTION SECTION:
;www.politico.com.      IN  TYPE65

;; ANSWER SECTION:
www.politico.com.   53806   IN  CNAME   www.politico.com.cdn.cloudflare.net.
www.politico.com.cdn.cloudflare.net. 300 IN TYPE65 \# 58 0001000001000302683200040008681210CA681211CA000600202606 47000000000000000000681210CA2606470000000000000000006812 11CA
…

I don’t know how to parse that, but wireshark does if I packet-capture it

Domain Name System (response)
    Queries
        www.politico.com.cdn.cloudflare.net: type HTTPS, class IN
    Answers
        www.politico.com.cdn.cloudflare.net: type HTTPS, class IN
            Name: www.politico.com.cdn.cloudflare.net
            Type: HTTPS (HTTPS Specific Service Endpoints) (65)
            Class: IN (0x0001)
            Time to live: 300 (5 minutes)
            Data length: 58
            SvcPriority: 1
            TargetName: <Root>
            SvcParams
                SvcParam: ALPN
                    SvcParamKey: ALPN (1)
                    SvcParamValue length: 3
                    ALPN length: 2
                    ALPN: h2
                SvcParam: IPv4 Hint
                    SvcParamKey: IPv4 Hint (4)
                    SvcParamValue length: 8
                    IP: 104.18.16.202
                    IP: 104.18.17.202
                SvcParam: IPv6 Hint
                    SvcParamKey: IPv6 Hint (6)
                    SvcParamValue length: 32
                    IP: 2606:4700::6812:10ca
                    IP: 2606:4700::6812:11ca

So that’s what’s happening:

Safari on Big Sur can load some websites you’ve blocked in /etc/hosts, because it gets their IP addresses from these new HTTPS records
It can only do that for some sites, because most domain name operators haven’t set this up yet. It looks like Cloudflare has done this for everyone on their platform; fortunately most domain name operators, including the advertising/tracking/malware giants, haven’t caught on to this yet.

For now, you can keep using /etc/hosts for domain names that you fully control.

In the meantime, for other domains, you have some options:

you could run a local DNS server or firewall on your home network that blocks these requests
you could configure a local DNS resolver daemon on your mac, and use it to block these requests
you could switch to a Linux distribution where a configurable local resolver daemon is the default
you could stop using Safari, although other apps using the default macOS networking stack may continue silently bypassing /etc/hosts

Chrome has run some trials for this but does not appear to have implemented it yet. Firefox has started implementing it but doesn’t seem to have gotten too far.

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