/*
Copyright 2017 The Kubernetes Authors.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package dns

import (
	"fmt"
	"io"
	"io/ioutil"
	"net"
	"os"
	"path/filepath"
	"strings"

	"k8s.io/api/core/v1"
	utilfeature "k8s.io/apiserver/pkg/util/feature"
	"k8s.io/client-go/tools/record"
	"k8s.io/kubernetes/pkg/apis/core/validation"
	"k8s.io/kubernetes/pkg/features"
	runtimeapi "k8s.io/kubernetes/pkg/kubelet/apis/cri/runtime/v1alpha2"
	kubecontainer "k8s.io/kubernetes/pkg/kubelet/container"
	"k8s.io/kubernetes/pkg/kubelet/util/format"

	"github.com/golang/glog"
)

var (
	// The default dns opt strings.
	defaultDNSOptions = []string{"ndots:5"}
)

type podDNSType int

const (
	podDNSCluster podDNSType = iota
	podDNSHost
	podDNSNone
)

// Configurer is used for setting up DNS resolver configuration when launching pods.
type Configurer struct {
	recorder record.EventRecorder
	nodeRef  *v1.ObjectReference
	nodeIP   net.IP

	// If non-nil, use this for container DNS server.
	clusterDNS []net.IP
	// If non-empty, use this for container DNS search.
	ClusterDomain string
	// The path to the DNS resolver configuration file used as the base to generate
	// the container's DNS resolver configuration file. This can be used in
	// conjunction with clusterDomain and clusterDNS.
	ResolverConfig string
}

// NewConfigurer returns a DNS configurer for launching pods.
func NewConfigurer(recorder record.EventRecorder, nodeRef *v1.ObjectReference, nodeIP net.IP, clusterDNS []net.IP, clusterDomain, resolverConfig string) *Configurer {
	return &Configurer{
		recorder:       recorder,
		nodeRef:        nodeRef,
		nodeIP:         nodeIP,
		clusterDNS:     clusterDNS,
		ClusterDomain:  clusterDomain,
		ResolverConfig: resolverConfig,
	}
}

func omitDuplicates(strs []string) []string {
	uniqueStrs := make(map[string]bool)

	var ret []string
	for _, str := range strs {
		if !uniqueStrs[str] {
			ret = append(ret, str)
			uniqueStrs[str] = true
		}
	}
	return ret
}

func (c *Configurer) formDNSSearchFitsLimits(composedSearch []string, pod *v1.Pod) []string {
	limitsExceeded := false

	if len(composedSearch) > validation.MaxDNSSearchPaths {
		composedSearch = composedSearch[:validation.MaxDNSSearchPaths]
		limitsExceeded = true
	}

	if resolvSearchLineStrLen := len(strings.Join(composedSearch, " ")); resolvSearchLineStrLen > validation.MaxDNSSearchListChars {
		cutDomainsNum := 0
		cutDomainsLen := 0
		for i := len(composedSearch) - 1; i >= 0; i-- {
			cutDomainsLen += len(composedSearch[i]) + 1
			cutDomainsNum++

			if (resolvSearchLineStrLen - cutDomainsLen) <= validation.MaxDNSSearchListChars {
				break
			}
		}

		composedSearch = composedSearch[:(len(composedSearch) - cutDomainsNum)]
		limitsExceeded = true
	}

	if limitsExceeded {
		log := fmt.Sprintf("Search Line limits were exceeded, some search paths have been omitted, the applied search line is: %s", strings.Join(composedSearch, " "))
		c.recorder.Event(pod, v1.EventTypeWarning, "DNSConfigForming", log)
		glog.Error(log)
	}
	return composedSearch
}

func (c *Configurer) formDNSNameserversFitsLimits(nameservers []string, pod *v1.Pod) []string {
	if len(nameservers) > validation.MaxDNSNameservers {
		nameservers = nameservers[0:validation.MaxDNSNameservers]
		log := fmt.Sprintf("Nameserver limits were exceeded, some nameservers have been omitted, the applied nameserver line is: %s", strings.Join(nameservers, " "))
		c.recorder.Event(pod, v1.EventTypeWarning, "DNSConfigForming", log)
		glog.Error(log)
	}
	return nameservers
}

func (c *Configurer) formDNSConfigFitsLimits(dnsConfig *runtimeapi.DNSConfig, pod *v1.Pod) *runtimeapi.DNSConfig {
	dnsConfig.Servers = c.formDNSNameserversFitsLimits(dnsConfig.Servers, pod)
	dnsConfig.Searches = c.formDNSSearchFitsLimits(dnsConfig.Searches, pod)
	return dnsConfig
}

func (c *Configurer) generateSearchesForDNSClusterFirst(hostSearch []string, pod *v1.Pod) []string {
	if c.ClusterDomain == "" {
		return hostSearch
	}

	nsSvcDomain := fmt.Sprintf("%s.svc.%s", pod.Namespace, c.ClusterDomain)
	svcDomain := fmt.Sprintf("svc.%s", c.ClusterDomain)
	clusterSearch := []string{nsSvcDomain, svcDomain, c.ClusterDomain}

	return omitDuplicates(append(clusterSearch, hostSearch...))
}

// CheckLimitsForResolvConf checks limits in resolv.conf.
func (c *Configurer) CheckLimitsForResolvConf() {
	f, err := os.Open(c.ResolverConfig)
	if err != nil {
		c.recorder.Event(c.nodeRef, v1.EventTypeWarning, "CheckLimitsForResolvConf", err.Error())
		glog.Error("CheckLimitsForResolvConf: " + err.Error())
		return
	}
	defer f.Close()

	_, hostSearch, _, err := parseResolvConf(f)
	if err != nil {
		c.recorder.Event(c.nodeRef, v1.EventTypeWarning, "CheckLimitsForResolvConf", err.Error())
		glog.Error("CheckLimitsForResolvConf: " + err.Error())
		return
	}

	domainCountLimit := validation.MaxDNSSearchPaths

	if c.ClusterDomain != "" {
		domainCountLimit -= 3
	}

	if len(hostSearch) > domainCountLimit {
		log := fmt.Sprintf("Resolv.conf file '%s' contains search line consisting of more than %d domains!", c.ResolverConfig, domainCountLimit)
		c.recorder.Event(c.nodeRef, v1.EventTypeWarning, "CheckLimitsForResolvConf", log)
		glog.Error("CheckLimitsForResolvConf: " + log)
		return
	}

	if len(strings.Join(hostSearch, " ")) > validation.MaxDNSSearchListChars {
		log := fmt.Sprintf("Resolv.conf file '%s' contains search line which length is more than allowed %d chars!", c.ResolverConfig, validation.MaxDNSSearchListChars)
		c.recorder.Event(c.nodeRef, v1.EventTypeWarning, "CheckLimitsForResolvConf", log)
		glog.Error("CheckLimitsForResolvConf: " + log)
		return
	}

	return
}

// parseResolveConf reads a resolv.conf file from the given reader, and parses
// it into nameservers, searches and options, possibly returning an error.
func parseResolvConf(reader io.Reader) (nameservers []string, searches []string, options []string, err error) {
	file, err := ioutil.ReadAll(reader)
	if err != nil {
		return nil, nil, nil, err
	}

	// Lines of the form "nameserver 1.2.3.4" accumulate.
	nameservers = []string{}

	// Lines of the form "search example.com" overrule - last one wins.
	searches = []string{}

	// Lines of the form "option ndots:5 attempts:2" overrule - last one wins.
	// Each option is recorded as an element in the array.
	options = []string{}

	lines := strings.Split(string(file), "\n")
	for l := range lines {
		trimmed := strings.TrimSpace(lines[l])
		if strings.HasPrefix(trimmed, "#") {
			continue
		}
		fields := strings.Fields(trimmed)
		if len(fields) == 0 {
			continue
		}
		if fields[0] == "nameserver" && len(fields) >= 2 {
			nameservers = append(nameservers, fields[1])
		}
		if fields[0] == "search" {
			searches = fields[1:]
		}
		if fields[0] == "options" {
			options = fields[1:]
		}
	}

	return nameservers, searches, options, nil
}

func (c *Configurer) getHostDNSConfig(pod *v1.Pod) (*runtimeapi.DNSConfig, error) {
	var hostDNS, hostSearch, hostOptions []string
	// Get host DNS settings
	if c.ResolverConfig != "" {
		f, err := os.Open(c.ResolverConfig)
		if err != nil {
			return nil, err
		}
		defer f.Close()

		hostDNS, hostSearch, hostOptions, err = parseResolvConf(f)
		if err != nil {
			return nil, err
		}
	}
	return &runtimeapi.DNSConfig{
		Servers:  hostDNS,
		Searches: hostSearch,
		Options:  hostOptions,
	}, nil
}

func getPodDNSType(pod *v1.Pod) (podDNSType, error) {
	dnsPolicy := pod.Spec.DNSPolicy
	switch dnsPolicy {
	case v1.DNSNone:
		if utilfeature.DefaultFeatureGate.Enabled(features.CustomPodDNS) {
			return podDNSNone, nil
		}
		// This should not happen as kube-apiserver should have rejected
		// setting dnsPolicy to DNSNone when feature gate is disabled.
		return podDNSCluster, fmt.Errorf(fmt.Sprintf("invalid DNSPolicy=%v: custom pod DNS is disabled", dnsPolicy))
	case v1.DNSClusterFirstWithHostNet:
		return podDNSCluster, nil
	case v1.DNSClusterFirst:
		if !kubecontainer.IsHostNetworkPod(pod) {
			return podDNSCluster, nil
		}
		// Fallback to DNSDefault for pod on hostnetowrk.
		fallthrough
	case v1.DNSDefault:
		return podDNSHost, nil
	}
	// This should not happen as kube-apiserver should have rejected
	// invalid dnsPolicy.
	return podDNSCluster, fmt.Errorf(fmt.Sprintf("invalid DNSPolicy=%v", dnsPolicy))
}

// Merge DNS options. If duplicated, entries given by PodDNSConfigOption will
// overwrite the existing ones.
func mergeDNSOptions(existingDNSConfigOptions []string, dnsConfigOptions []v1.PodDNSConfigOption) []string {
	optionsMap := make(map[string]string)
	for _, op := range existingDNSConfigOptions {
		if index := strings.Index(op, ":"); index != -1 {
			optionsMap[op[:index]] = op[index+1:]
		} else {
			optionsMap[op] = ""
		}
	}
	for _, op := range dnsConfigOptions {
		if op.Value != nil {
			optionsMap[op.Name] = *op.Value
		} else {
			optionsMap[op.Name] = ""
		}
	}
	// Reconvert DNS options into a string array.
	options := []string{}
	for opName, opValue := range optionsMap {
		op := opName
		if opValue != "" {
			op = op + ":" + opValue
		}
		options = append(options, op)
	}
	return options
}

// appendDNSConfig appends DNS servers, search paths and options given by
// PodDNSConfig to the existing DNS config. Duplicated entries will be merged.
// This assumes existingDNSConfig and dnsConfig are not nil.
func appendDNSConfig(existingDNSConfig *runtimeapi.DNSConfig, dnsConfig *v1.PodDNSConfig) *runtimeapi.DNSConfig {
	existingDNSConfig.Servers = omitDuplicates(append(existingDNSConfig.Servers, dnsConfig.Nameservers...))
	existingDNSConfig.Searches = omitDuplicates(append(existingDNSConfig.Searches, dnsConfig.Searches...))
	existingDNSConfig.Options = mergeDNSOptions(existingDNSConfig.Options, dnsConfig.Options)
	return existingDNSConfig
}

// GetPodDNS returns DNS settings for the pod.
func (c *Configurer) GetPodDNS(pod *v1.Pod) (*runtimeapi.DNSConfig, error) {
	dnsConfig, err := c.getHostDNSConfig(pod)
	if err != nil {
		return nil, err
	}

	dnsType, err := getPodDNSType(pod)
	if err != nil {
		glog.Errorf("Failed to get DNS type for pod %q: %v. Falling back to DNSClusterFirst policy.", format.Pod(pod), err)
		dnsType = podDNSCluster
	}
	switch dnsType {
	case podDNSNone:
		// DNSNone should use empty DNS settings as the base.
		dnsConfig = &runtimeapi.DNSConfig{}
	case podDNSCluster:
		if len(c.clusterDNS) != 0 {
			// For a pod with DNSClusterFirst policy, the cluster DNS server is
			// the only nameserver configured for the pod. The cluster DNS server
			// itself will forward queries to other nameservers that is configured
			// to use, in case the cluster DNS server cannot resolve the DNS query
			// itself.
			dnsConfig.Servers = []string{}
			for _, ip := range c.clusterDNS {
				dnsConfig.Servers = append(dnsConfig.Servers, ip.String())
			}
			dnsConfig.Searches = c.generateSearchesForDNSClusterFirst(dnsConfig.Searches, pod)
			dnsConfig.Options = defaultDNSOptions
			break
		}
		// clusterDNS is not known. Pod with ClusterDNSFirst Policy cannot be created.
		nodeErrorMsg := fmt.Sprintf("kubelet does not have ClusterDNS IP configured and cannot create Pod using %q policy. Falling back to %q policy.", v1.DNSClusterFirst, v1.DNSDefault)
		c.recorder.Eventf(c.nodeRef, v1.EventTypeWarning, "MissingClusterDNS", nodeErrorMsg)
		c.recorder.Eventf(pod, v1.EventTypeWarning, "MissingClusterDNS", "pod: %q. %s", format.Pod(pod), nodeErrorMsg)
		// Fallback to DNSDefault.
		fallthrough
	case podDNSHost:
		// When the kubelet --resolv-conf flag is set to the empty string, use
		// DNS settings that override the docker default (which is to use
		// /etc/resolv.conf) and effectively disable DNS lookups. According to
		// the bind documentation, the behavior of the DNS client library when
		// "nameservers" are not specified is to "use the nameserver on the
		// local machine". A nameserver setting of localhost is equivalent to
		// this documented behavior.
		if c.ResolverConfig == "" {
			switch {
			case c.nodeIP == nil || c.nodeIP.To4() != nil:
				dnsConfig.Servers = []string{"127.0.0.1"}
			case c.nodeIP.To16() != nil:
				dnsConfig.Servers = []string{"::1"}
			}
			dnsConfig.Searches = []string{"."}
		}
	}

	if utilfeature.DefaultFeatureGate.Enabled(features.CustomPodDNS) && pod.Spec.DNSConfig != nil {
		dnsConfig = appendDNSConfig(dnsConfig, pod.Spec.DNSConfig)
	}
	return c.formDNSConfigFitsLimits(dnsConfig, pod), nil
}

// SetupDNSinContainerizedMounter replaces the nameserver in containerized-mounter's rootfs/etc/resolve.conf with kubelet.ClusterDNS
func (c *Configurer) SetupDNSinContainerizedMounter(mounterPath string) {
	resolvePath := filepath.Join(strings.TrimSuffix(mounterPath, "/mounter"), "rootfs", "etc", "resolv.conf")
	dnsString := ""
	for _, dns := range c.clusterDNS {
		dnsString = dnsString + fmt.Sprintf("nameserver %s\n", dns)
	}
	if c.ResolverConfig != "" {
		f, err := os.Open(c.ResolverConfig)
		defer f.Close()
		if err != nil {
			glog.Error("Could not open resolverConf file")
		} else {
			_, hostSearch, _, err := parseResolvConf(f)
			if err != nil {
				glog.Errorf("Error for parsing the reslov.conf file: %v", err)
			} else {
				dnsString = dnsString + "search"
				for _, search := range hostSearch {
					dnsString = dnsString + fmt.Sprintf(" %s", search)
				}
				dnsString = dnsString + "\n"
			}
		}
	}
	if err := ioutil.WriteFile(resolvePath, []byte(dnsString), 0600); err != nil {
		glog.Errorf("Could not write dns nameserver in file %s, with error %v", resolvePath, err)
	}
}