appc,feature/conn25,net: Add DNS response interception for conn25

The new version of app connector (conn25) needs to read DNS responses
for domains it is interested in and store and swap out IP addresses.

Add a hook to dns manager to enable this.
Give the conn25 updated netmaps so that it knows when to assign
connecting addresses and from what pool.
Assign an address when we see a DNS response for a domain we are
interested in, but don't do anything with the address yet.

Updates tailscale/corp#34252

Signed-off-by: Fran Bull <fran@tailscale.com>

feature/conn25/ippool.go

@@ -0,0 +1,61 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+package conn25
+
+import (
+	"errors"
+	"net/netip"
+
+	"go4.org/netipx"
+)
+
+// errPoolExhausted is returned when there are no more addresses to iterate over.
+var errPoolExhausted = errors.New("ip pool exhausted")
+
+// ippool allows for iteration over all the addresses within a netipx.IPSet.
+// netipx.IPSet has a Ranges call that returns the "minimum and sorted set of IP ranges that covers [the set]".
+// netipx.IPRange is "an inclusive range of IP addresses from the same address family.". So we can iterate over
+// all the addresses in the set by keeping a track of the last address we returned, calling Next on the last address
+// to get the new one, and if we run off the edge of the current range, starting on the next one.
+type ippool struct {
+	// ranges defines the addresses in the pool
+	ranges []netipx.IPRange
+	// last is internal tracking of which the last address provided was.
+	last netip.Addr
+	// rangeIdx is internal tracking of which netipx.IPRange from the IPSet we are currently on.
+	rangeIdx int
+}
+
+func newIPPool(ipset *netipx.IPSet) *ippool {
+	if ipset == nil {
+		return &ippool{}
+	}
+	return &ippool{ranges: ipset.Ranges()}
+}
+
+// next returns the next address from the set, or errPoolExhausted if we have
+// iterated over the whole set.
+func (ipp *ippool) next() (netip.Addr, error) {
+	if ipp.rangeIdx >= len(ipp.ranges) {
+		// ipset is empty or we have iterated off the end
+		return netip.Addr{}, errPoolExhausted
+	}
+	if !ipp.last.IsValid() {
+		// not initialized yet
+		ipp.last = ipp.ranges[0].From()
+		return ipp.last, nil
+	}
+	currRange := ipp.ranges[ipp.rangeIdx]
+	if ipp.last == currRange.To() {
+		// then we need to move to the next range
+		ipp.rangeIdx++
+		if ipp.rangeIdx >= len(ipp.ranges) {
+			return netip.Addr{}, errPoolExhausted
+		}
+		ipp.last = ipp.ranges[ipp.rangeIdx].From()
+		return ipp.last, nil
+	}
+	ipp.last = ipp.last.Next()
+	return ipp.last, nil
+}