Some websites bypass hosts file in Safari

An answer on Stack Overflow Why does /etc/hosts not work anymore for some websites on Big Sur? (unfortunately closed) gives as answer a new DNS feature added in order to speed up access to sites that support HTTPS, but which unfortunately may workaround the hosts file.

I quote the answer here:

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:

1. 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
2. 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.