Initial implementation of ForwardCRD plugin (#4512)

* Add forwardcrd plugin README.md

Co-authored-by: Aidan Obley <aobley@vmware.com>

Signed-off-by: Christian Ang <angc@vmware.com>

* Create forwardcrd plugin

- Place forwardcrd before forward plugin in plugin list. This will avoid
forward from preventing the forwardcrd plugin from handling any queries
in the case of having a default upstream forwarder in a server block (as
is the case in the default kubernetes Corefile).

Co-authored-by: Aidan Obley <aobley@vmware.com>

Signed-off-by: Christian Ang <angc@vmware.com>

* Add Forward CRD

Signed-off-by: Christian Ang <angc@vmware.com>

* Add NewWithConfig to forward plugin

- allows external packages to instanciate forward plugins

Co-authored-by: Aidan Obley <aobley@vmware.com>

Signed-off-by: Christian Ang <angc@vmware.com>

* ForwardCRD plugin handles requests for Forward CRs

- add a Kubernetes controller that can read Forward CRs
- instances of the forward plugin are created based on Forward CRs from
the Kubernetes controller
- DNS requests are handled by calling matching Forward plugin instances
based on zone name
- Defaults to the kube-system namespace to align with Corefile RBAC

Signed-off-by: Christian Ang <angc@vmware.com>

Use klog v2 in forwardcrd plugin

* Refactor forward setup to use NewWithConfig

Co-authored-by: Christian Ang <angc@vmware.com>

Signed-off-by: Edwin Xie <exie@vmware.com>

* Use ParseInt instead of Atoi

- to ensure that the bitsize is 32 for later casting to uint32

Signed-off-by: Christian Ang <angc@vmware.com>

* Add @christianang to CODEOWNERS for forwardcrd

Signed-off-by: Christian Ang <angc@vmware.com>

Co-authored-by: Edwin Xie <exie@vmware.com>

plugin/forward/forward.go

@@ -8,6 +8,7 @@ import (
 	"context"
 	"crypto/tls"
 	"errors"
+	"fmt"
 	"sync/atomic"
 	"time"
 
@@ -16,6 +17,8 @@ import (
 	"github.com/coredns/coredns/plugin/dnstap"
 	"github.com/coredns/coredns/plugin/metadata"
 	clog "github.com/coredns/coredns/plugin/pkg/log"
+	"github.com/coredns/coredns/plugin/pkg/parse"
+	"github.com/coredns/coredns/plugin/pkg/transport"
 	"github.com/coredns/coredns/request"
 
 	"github.com/miekg/dns"
@@ -54,12 +57,127 @@ type Forward struct {
 	Next plugin.Handler
 }
 
+// ForwardConfig represents the configuration of the Forward Plugin. This can
+// be used with NewWithConfig to create a new configured instance of the
+// Forward Plugin.
+type ForwardConfig struct {
+	From             string
+	To               []string
+	Except           []string
+	MaxFails         *uint32
+	HealthCheck      *time.Duration
+	HealthCheckNoRec bool
+	ForceTCP         bool
+	PreferUDP        bool
+	TLSConfig        *tls.Config
+	TLSServerName    string
+	Expire           *time.Duration
+	MaxConcurrent    *int64
+	Policy           string
+	TapPlugin        *dnstap.Dnstap
+}
+
 // New returns a new Forward.
 func New() *Forward {
 	f := &Forward{maxfails: 2, tlsConfig: new(tls.Config), expire: defaultExpire, p: new(random), from: ".", hcInterval: hcInterval, opts: options{forceTCP: false, preferUDP: false, hcRecursionDesired: true}}
 	return f
 }
 
+// NewWithConfig returns a new Forward configured by the provided
+// ForwardConfig.
+func NewWithConfig(config ForwardConfig) (*Forward, error) {
+	f := New()
+	if config.From != "" {
+		zones := plugin.Host(config.From).NormalizeExact()
+		f.from = zones[0] // there can only be one here, won't work with non-octet reverse
+
+		if len(zones) > 1 {
+			log.Warningf("Unsupported CIDR notation: '%s' expands to multiple zones. Using only '%s'.", config.From, f.from)
+		}
+	}
+	for i := 0; i < len(config.Except); i++ {
+		f.ignored = append(f.ignored, plugin.Host(config.Except[i]).NormalizeExact()...)
+	}
+	if config.MaxFails != nil {
+		f.maxfails = *config.MaxFails
+	}
+	if config.HealthCheck != nil {
+		if *config.HealthCheck < 0 {
+			return nil, fmt.Errorf("health_check can't be negative: %s", *config.HealthCheck)
+		}
+		f.hcInterval = *config.HealthCheck
+	}
+	f.opts.hcRecursionDesired = !config.HealthCheckNoRec
+	f.opts.forceTCP = config.ForceTCP
+	f.opts.preferUDP = config.PreferUDP
+	if config.TLSConfig != nil {
+		f.tlsConfig = config.TLSConfig
+	}
+	f.tlsServerName = config.TLSServerName
+	if f.tlsServerName != "" {
+		f.tlsConfig.ServerName = f.tlsServerName
+	}
+	if config.Expire != nil {
+		f.expire = *config.Expire
+		if *config.Expire < 0 {
+			return nil, fmt.Errorf("expire can't be negative: %s", *config.Expire)
+		}
+	}
+	if config.MaxConcurrent != nil {
+		if *config.MaxConcurrent < 0 {
+			return f, fmt.Errorf("max_concurrent can't be negative: %d", *config.MaxConcurrent)
+		}
+		f.ErrLimitExceeded = fmt.Errorf("concurrent queries exceeded maximum %d", *config.MaxConcurrent)
+		f.maxConcurrent = *config.MaxConcurrent
+	}
+	if config.Policy != "" {
+		switch config.Policy {
+		case "random":
+			f.p = &random{}
+		case "round_robin":
+			f.p = &roundRobin{}
+		case "sequential":
+			f.p = &sequential{}
+		default:
+			return f, fmt.Errorf("unknown policy '%s'", config.Policy)
+		}
+	}
+	f.tapPlugin = config.TapPlugin
+
+	toHosts, err := parse.HostPortOrFile(config.To...)
+	if err != nil {
+		return f, err
+	}
+
+	transports := make([]string, len(toHosts))
+	allowedTrans := map[string]bool{"dns": true, "tls": true}
+	for i, host := range toHosts {
+		trans, h := parse.Transport(host)
+
+		if !allowedTrans[trans] {
+			return f, fmt.Errorf("'%s' is not supported as a destination protocol in forward: %s", trans, host)
+		}
+		p := NewProxy(h, trans)
+		f.proxies = append(f.proxies, p)
+		transports[i] = trans
+	}
+
+	// Initialize ClientSessionCache in tls.Config. This may speed up a TLS handshake
+	// in upcoming connections to the same TLS server.
+	f.tlsConfig.ClientSessionCache = tls.NewLRUClientSessionCache(len(f.proxies))
+
+	for i := range f.proxies {
+		// Only set this for proxies that need it.
+		if transports[i] == transport.TLS {
+			f.proxies[i].SetTLSConfig(f.tlsConfig)
+		}
+		f.proxies[i].SetExpire(f.expire)
+		f.proxies[i].health.SetRecursionDesired(f.opts.hcRecursionDesired)
+
+	}
+	return f, nil
+}
+
 // SetProxy appends p to the proxy list and starts healthchecking.
 func (f *Forward) SetProxy(p *Proxy) {
 	f.proxies = append(f.proxies, p)