Support more than 4 IP forwarding

It would be nice if we could set more than 4 IP within forward policy (http://knot-resolver.readthedocs.io/en/stable/modules.html#query-policies) Because forward policy is set as default in omnia, we have to cut these additional IP in omnia init script and few users were complaining about it.

And users expect us to always choose a good one, based on how fast they answered in past? (and whether they answered reasonably, etc.)

That never happens with stub, so I would say that in such case, the expectations are wrong if that is the case.

Yes, reputation tracking currently doesn't work with stub nor with forwarding https://gitlab.labs.nic.cz/knot/resolver/issues/125 but I thought it might be desirable in some use cases. Otherwise I can't see why to have so many IPs in the list...

@vcunat Yes, I believe that was what they expected.

added feature label

mentioned in issue #208 (closed)

There's a very fast (250ms by default, 100ms in Omnia) re-transmit that iterates through the list very quickly. We might improve that logic a bit (see #208 (closed)), but generally with caching it should be already good enough. Having more that four (2 IPv4 + 2 IPv6) addresses on that list doesn't make any sense for static network device like router.

If you want to do the selection of fastest FORWARD target, you can do something like this in the init script:

#!/bin/sh                                                                                                                                                                                                                                                                                 

for NS in "$@"; do
    RTT=$(dig +noall +stats +time=1 +retry=0 . IN DNSKEY @$NS | sed -ne 's/;; Query time: \(.*\) msec/\1/p');
    if [ -z "$RTT" ]; then
        RTT=1000 # anything bigger than timeout equals timeout                                                                                                                                                                                                                            
    elif [ "$1" != "${1#*:[0-9a-fA-F]}" ]; then
        RTT=$(($RTT - 20)) # give IPv6 a 20 ms headstart                                                                                                                                                                                                                                  
    fi
    printf "%s;%s\n" "$RTT" "$NS";
done | sort -n | cut -f 2- -d \; | head -4

added wontfix and removed feature labels

@jpavlinec If there's a real use-case, please report it here.

I suppose a significant part of the point is that the init/config scripts on Omnia have to decide somehow which four NSs to keep if it gets more of them.

Well, I think it wouldn't take too much work to extend the NS list, perhaps exclusively for FORWARD+STUB modes and not recursive one, but let's close until we plan to actually implement it.

Is it really common on Omnia to get more than four addresses?

closed

Personally I don't think it's too common, but I can imagine that this can happen in case that someone is using 3G or some other type of fallback connection.

@jpavlinec: that would put all forwarders in there even if e.g. the 3G ones won't be reachable normally?

BTW, a related change: https://gitlab.labs.nic.cz/knot/resolver/blob/ac55abb436da/NEWS#L7