Revert "net/dns: set IPv4 auto mode in NM, so it lets us set DNS."

This reverts commit 7d16c8228b.

I have no idea how I ended up here. The bug I was fixing with this change
fails to reproduce on Ubuntu 18.04 now, and this change definitely does
break 20.04, 20.10, and Debian Buster. So, until we can reliably reproduce
the problem this was meant to fix, reverting.

Part of #1875

Signed-off-by: David Anderson <dave@natulte.net>

net/dns/nm.go

@@ -183,25 +183,6 @@ func (m *nmManager) trySet(ctx context.Context, config OSConfig) error {
 	// settings here, but that led to #1870.
 
 	ipv4Map := settings["ipv4"]
-	// In some versions of NM, you can only provide DNS configuration
-	// if the interface is in "auto" or "manual" mode (defaults to
-	// "disabled").
-	//
-	// In "manual" mode we'd have to also give NM all our IP and
-	// routing configuration, which we don't want to do. So instead,
-	// we set the ipv4 mode to "auto", and then tell it to ignore any
-	// information it might get from DHCP (which is none, since we
-	// don't do DHCP on tailscale0). This is the documented way to
-	// provide only DNS configuration to NetworkManager.
-	//
-	// Some versions of NM don't need this, and will accept DNS
-	// configuration even for disabled interfaces, but there's no harm
-	// in telling those versions to do DHCP and ignore all results,
-	// hence why we have a single codepath here.
-	ipv4Map["method"] = dbus.MakeVariant("auto")
-	ipv4Map["ignore-auto-routes"] = dbus.MakeVariant(true)
-	ipv4Map["ignore-auto-dns"] = dbus.MakeVariant(true)
-	ipv4Map["never-default"] = dbus.MakeVariant(true)
 	ipv4Map["dns"] = dbus.MakeVariant(dnsv4)
 	ipv4Map["dns-search"] = dbus.MakeVariant(search)
 	// We should only request priority if we have nameservers to set.
@@ -221,9 +202,18 @@ func (m *nmManager) trySet(ctx context.Context, config OSConfig) error {
 	}
 
 	ipv6Map := settings["ipv6"]
-	// Same logic as above for IPv4 - request autoconfiguration, tell
-	// NM to ignore all autoconfiguration results, and provide manual
-	// DNS settings.
+	// In IPv6 settings, you're only allowed to provide additional
+	// static DNS settings in "auto" (SLAAC) or "manual" mode. In
+	// "manual" mode you also have to specify IP addresses, so we use
+	// "auto".
+	//
+	// NM actually documents that to set just DNS servers, you should
+	// use "auto" mode and then set ignore auto routes and DNS, which
+	// basically means "autoconfigure but ignore any autoconfiguration
+	// results you might get". As a safety, we also say that
+	// NetworkManager should never try to make us the default route
+	// (none of its business anyway, we handle our own default
+	// routing).
 	ipv6Map["method"] = dbus.MakeVariant("auto")
 	if len(addrs6) > 0 {
 		ipv6Map["address-data"] = dbus.MakeVariant(addrs6)