Ability to select the dial address for resolvers

[alpominth]

Many DoH, DoT and some DoQ resolvers have more than one IP for connecting:

$ host -t A dns.nextdns.io
dns.nextdns.io is an alias for steering.nextdns.io.
steering.nextdns.io has address 5.161.43.197
steering.nextdns.io has address 170.39.224.134

$ host -t A dns10.quad9.net
dns10.quad9.net has address 149.112.112.10
dns10.quad9.net has address 9.9.9.10

$ host -t A dns.cloudflare.com
dns.cloudflare.com has address 104.16.132.229
dns.cloudflare.com has address 104.16.133.229

$ host -t A dns-unfiltered.adguard.com
dns-unfiltered.adguard.com has address 94.140.14.140
dns-unfiltered.adguard.com has address 94.140.14.141

An option to specify which IP address will be used would be a good idea.

And maybe, RouteDNS could randomize the IP address selection to avoid overcharging the first IP address of a DoH|DoT|DoQ resolver. Maybe an option for doing that.

[cbuijs]

See also #56

[folbricht]

If no bootstrap-address is used, the selection should already be somewhat random. But as mentioned in #56, it'll not balance over requests, but over connections. So if you have a constant stream of queries going there it's likely going to keep using one connection. Once a timeout occurs, it'll re-dial which could end up on a different (or the same) address.

[cbuijs]

Maybe have the ability to use a resolver group for bootstrap as well, potentially create loops, but at least you can create any chain/resolution you want (with boot-strap-addresses).

How I do it now, I create a TCP listener on 127.0.0.1:53053 and point the bootstrap-resolver to it. A snippet of the config:

[bootstrap-resolver]
protocol = "tcp"
address = "127.0.0.1:53053"
query-timeout = 5

[listeners.bootstrap-do53-tcp]
address = "127.0.0.1:53053"
protocol = "tcp"
resolver = "bootstrap-dedup"

[groups.bootstrap-dedup]
type = "request-dedup"
resolvers = ["bootstrap-router"]

[routers.bootstrap-router]
routes = [
        {name = '(^|\.)dns\.server\.com\.$', type = "A", resolver = "bootstrap-a"},
        {name = '(^|\.)dns\.server\.com\.$', type = "AAAA", resolver = "bootstrap-aaaa"},

        {resolver = "nxdomain"},
]

[groups.bootstrap-a]
type = "static-responder"
rcode = 0
answer = [
        ". 3600 IN A 1.2.3.4",
        ". 3600 IN A 4.3.2.1",
        ". 3600 IN A 11.22.33.44",
]

[groups.bootstrap-aaaa]
type = "static-responder"
rcode = 0
answer = [
        ". 3600 IN AAAA 2001:db8::1:2:3:4",
        ". 3600 IN AAAA 2001:db8::4:3:2:1",
        ". 3600 IN AAAA 2001:db8::11:22:33:44",
]

[groups.nxdomain]
type = "static-responder"
rcode = 3

#### REST OF ROUTEDNS CONFIG BELOW ####

This as a construct example, you can build whatever you want with this. Be aware of loops though ;-).

-Chris.