all: add local caching

This commit adds config params to enable local DNS response caching and
control its behavior, allow tweaking the cache size, ttl override and
serving stale response.

cmd/ctrld/dns_proxy.go

@@ -13,8 +13,11 @@ import (
 	"github.com/miekg/dns"
 
 	"github.com/Control-D-Inc/ctrld"
+	"github.com/Control-D-Inc/ctrld/internal/dnscache"
 )
 
+const staleTTL = 60 * time.Second
+
 func (p *prog) serveUDP(listenerNum string) error {
 	listenerConfig := p.cfg.Listener[listenerNum]
 	// make sure ip is allocated
@@ -123,6 +126,22 @@ func (p *prog) upstreamFor(ctx context.Context, defaultUpstreamNum string, lc *c
 }
 
 func (p *prog) proxy(ctx context.Context, upstreams []string, failoverRcodes []int, msg *dns.Msg) *dns.Msg {
+	var staleAnswer *dns.Msg
+	serveStaleCache := p.cache != nil && p.cfg.Service.CacheServeStale
+	// Inverse query should not be cached: https://www.rfc-editor.org/rfc/rfc1035#section-7.4
+	if p.cache != nil && msg.Question[0].Qtype != dns.TypePTR {
+		if cachedValue := p.cache.Get(dnscache.NewKey(msg)); cachedValue != nil {
+			answer := cachedValue.Msg.Copy()
+			answer.SetReply(msg)
+			now := time.Now()
+			if cachedValue.Expire.After(now) {
+				ctrld.Log(ctx, proxyLog.Debug(), "hit cached response")
+				setCachedAnswerTTL(answer, now, cachedValue.Expire)
+				return answer
+			}
+			staleAnswer = answer
+		}
+	}
 	upstreamConfigs := p.upstreamConfigsFromUpstreamNumbers(upstreams)
 	resolve := func(n int, upstreamConfig *ctrld.UpstreamConfig, msg *dns.Msg) *dns.Msg {
 		ctrld.Log(ctx, proxyLog.Debug(), "sending query to %s: %s", upstreams[n], upstreamConfig.Name)
@@ -148,12 +167,29 @@ func (p *prog) proxy(ctx context.Context, upstreams []string, failoverRcodes []i
 	for n, upstreamConfig := range upstreamConfigs {
 		answer := resolve(n, upstreamConfig, msg)
 		if answer == nil {
+			if serveStaleCache && staleAnswer != nil {
+				ctrld.Log(ctx, proxyLog.Debug(), "serving stale cached response")
+				now := time.Now()
+				setCachedAnswerTTL(staleAnswer, now, now.Add(staleTTL))
+				return staleAnswer
+			}
 			continue
 		}
 		if answer.Rcode != dns.RcodeSuccess && len(upstreamConfigs) > 1 && containRcode(failoverRcodes, answer.Rcode) {
 			ctrld.Log(ctx, proxyLog.Debug(), "failover rcode matched, process to next upstream")
 			continue
 		}
+		if p.cache != nil {
+			ttl := ttlFromMsg(answer)
+			now := time.Now()
+			expired := now.Add(time.Duration(ttl) * time.Second)
+			if cachedTTL := p.cfg.Service.CacheTTLOverride; cachedTTL > 0 {
+				expired = now.Add(time.Duration(cachedTTL) * time.Second)
+			}
+			setCachedAnswerTTL(answer, now, expired)
+			p.cache.Add(dnscache.NewKey(msg), dnscache.NewValue(answer, expired))
+			ctrld.Log(ctx, proxyLog.Debug(), "add cached response")
+		}
 		return answer
 	}
 	ctrld.Log(ctx, proxyLog.Error(), "all upstreams failed")
@@ -229,6 +265,35 @@ func containRcode(rcodes []int, rcode int) bool {
 	return false
 }
 
+func setCachedAnswerTTL(answer *dns.Msg, now, expiredTime time.Time) {
+	ttl := uint32(expiredTime.Sub(now).Seconds())
+	if ttl < 0 {
+		return
+	}
+
+	for _, rr := range answer.Answer {
+		rr.Header().Ttl = ttl
+	}
+	for _, rr := range answer.Ns {
+		rr.Header().Ttl = ttl
+	}
+	for _, rr := range answer.Extra {
+		if rr.Header().Rrtype != dns.TypeOPT {
+			rr.Header().Ttl = ttl
+		}
+	}
+}
+
+func ttlFromMsg(msg *dns.Msg) uint32 {
+	for _, rr := range msg.Answer {
+		return rr.Header().Ttl
+	}
+	for _, rr := range msg.Ns {
+		return rr.Header().Ttl
+	}
+	return 0
+}
+
 var osUpstreamConfig = &ctrld.UpstreamConfig{
 	Name: "OS resolver",
 	Type: "os",

cmd/ctrld/prog.go

@@ -12,6 +12,7 @@ import (
 	"github.com/miekg/dns"
 
 	"github.com/Control-D-Inc/ctrld"
+	"github.com/Control-D-Inc/ctrld/internal/dnscache"
 )
 
 var errWindowsAddrInUse = syscall.Errno(0x2740)
@@ -22,7 +23,8 @@ var svcConfig = &service.Config{
 }
 
 type prog struct {
-	cfg *ctrld.Config
+	cfg   *ctrld.Config
+	cache dnscache.Cacher
 }
 
 func (p *prog) Start(s service.Service) error {
@@ -32,6 +34,14 @@ func (p *prog) Start(s service.Service) error {
 }
 
 func (p *prog) run() {
+	if p.cfg.Service.CacheEnable {
+		cacher, err := dnscache.NewLRUCache(p.cfg.Service.CacheSize)
+		if err != nil {
+			mainLog.Error().Err(err).Msg("failed to create cacher, caching is disabled")
+		} else {
+			p.cache = cacher
+		}
+	}
 	var wg sync.WaitGroup
 	wg.Add(len(p.cfg.Listener))
 

config.go

@@ -64,10 +64,14 @@ type Config struct {
 
 // ServiceConfig specifies the general ctrld config.
 type ServiceConfig struct {
-	LogLevel   string `mapstructure:"log_level" toml:"log_level"`
-	LogPath    string `mapstructure:"log_path" toml:"log_path"`
-	Daemon     bool   `mapstructure:"-" toml:"-"`
-	AllocateIP bool   `mapstructure:"-" toml:"-"`
+	LogLevel         string `mapstructure:"log_level" toml:"log_level"`
+	LogPath          string `mapstructure:"log_path" toml:"log_path"`
+	CacheEnable      bool   `mapstructure:"cache_enable" toml:"cache_enable"`
+	CacheSize        int    `mapstructure:"cache_size" toml:"cache_size"`
+	CacheTTLOverride int    `mapstructure:"cache_ttl_override" toml:"cache_ttl_override"`
+	CacheServeStale  bool   `mapstructure:"cache_serve_stale" toml:"cache_serve_stale"`
+	Daemon           bool   `mapstructure:"-" toml:"-"`
+	AllocateIP       bool   `mapstructure:"-" toml:"-"`
 }
 
 // NetworkConfig specifies configuration for networks where ctrld will handle requests.
@@ -179,11 +183,8 @@ func (uc *UpstreamConfig) setupDOH3Transport() {
 		if err != nil {
 			return nil, err
 		}
-		localAddr := &net.UDPAddr{IP: net.IPv4zero, Port: 0}
-		if strings.Index(uc.BootstrapIP, ":") > -1 {
-			localAddr = &net.UDPAddr{IP: net.IPv6zero, Port: 0}
-		}
-		udpConn, err := net.ListenUDP("udp", localAddr)
+
+		udpConn, err := net.ListenUDP("udp", nil)
 		if err != nil {
 			return nil, err
 		}

docs/config.md

@@ -33,6 +33,8 @@ If no configuration files found, a default `config.toml` file will be created in
 [service]
     log_level = "info"
     log_path = ""
+    cache_enable = true
+    cache_size = 4096
 
 [network.0]
     cidrs = ["0.0.0.0/0"]
@@ -109,6 +111,31 @@ Relative or absolute path of the log file.
 - Type: string
 - Required: no
 
+### cache_enable
+When `cache_enable = true`, all resolved DNS query responses will be cached for duration of the upstream record TTLs.
+
+- Type: boolean
+- Required: no
+
+### cache_size
+The number of cached records, must be a positive integer. Tweaking this value with care depends on your available RAM. 
+A minimum value `4096` should be enough for most use cases.
+
+An invalid `cache_size` value will disable the cache, regardless of `cache_enable` value.
+
+- Type: int
+- Required: no
+
+### cache_ttl_override
+When `cache_ttl_override` is set to a positive value (in seconds), TTLs are overridden to this value and cached for this long.
+
+- Type: int
+- Required: no
+
+### cache_serve_stale
+When `cache_serve_stale = true`, in cases of upstream failures (upstreams not reachable), `ctrld` will keep serving
+stale cached records (regardless of their TTLs) until upstream comes online.
+
 The above config will look like this at query time.
 
 ```

go.mod

@@ -4,6 +4,7 @@ go 1.19
 
 require (
 	github.com/go-playground/validator/v10 v10.11.1
+	github.com/hashicorp/golang-lru/v2 v2.0.1
 	github.com/kardianos/service v1.2.1
 	github.com/lucas-clemente/quic-go v0.29.1
 	github.com/miekg/dns v1.1.50

go.sum

@@ -112,6 +112,8 @@ github.com/hashicorp/go-uuid v1.0.1/go.mod h1:6SBZvOh/SIDV7/2o3Jml5SYk/TvGqwFJ/b
 github.com/hashicorp/go.net v0.0.1/go.mod h1:hjKkEWcCURg++eb33jQU7oqQcI9XDCnUzHA0oac0k90=
 github.com/hashicorp/golang-lru v0.5.0/go.mod h1:/m3WP610KZHVQ1SGc6re/UDhFvYD7pJ4Ao+sR/qLZy8=
 github.com/hashicorp/golang-lru v0.5.1/go.mod h1:/m3WP610KZHVQ1SGc6re/UDhFvYD7pJ4Ao+sR/qLZy8=
+github.com/hashicorp/golang-lru/v2 v2.0.1 h1:5pv5N1lT1fjLg2VQ5KWc7kmucp2x/kvFOnxuVTqZ6x4=
+github.com/hashicorp/golang-lru/v2 v2.0.1/go.mod h1:QeFd9opnmA6QUJc5vARoKUSoFhyfM2/ZepoAG6RGpeM=
 github.com/hashicorp/hcl v1.0.0 h1:0Anlzjpi4vEasTeNFn2mLJgTSwt0+6sfsiTG8qcWGx4=
 github.com/hashicorp/hcl v1.0.0/go.mod h1:E5yfLk+7swimpb2L/Alb/PJmXilQ/rhwaUYs4T20WEQ=
 github.com/hashicorp/logutils v1.0.0/go.mod h1:QIAnNjmIWmVIIkWDTG1z5v++HQmx9WQRO+LraFDTW64=

internal/dnscache/cache.go

@@ -0,0 +1,76 @@
+package dnscache
+
+import (
+	"strings"
+	"time"
+
+	lru "github.com/hashicorp/golang-lru/v2"
+	"github.com/miekg/dns"
+)
+
+// Cacher is the interface for caching DNS response.
+type Cacher interface {
+	Get(Key) *Value
+	Add(Key, *Value)
+}
+
+// Key is the caching key for DNS message.
+type Key struct {
+	Qtype  uint16
+	Qclass uint16
+	Name   string
+}
+
+type Value struct {
+	Expire time.Time
+	Msg    *dns.Msg
+}
+
+var _ Cacher = (*LRUCache)(nil)
+
+// LRUCache implements Cacher interface.
+type LRUCache struct {
+	cacher *lru.ARCCache[Key, *Value]
+}
+
+func (l *LRUCache) Get(key Key) *Value {
+	v, _ := l.cacher.Get(key)
+	return v
+}
+
+func (l *LRUCache) Add(key Key, value *Value) {
+	l.cacher.Add(key, value)
+}
+
+// NewLRUCache creates a new LRUCache instance with given size.
+func NewLRUCache(size int) (*LRUCache, error) {
+	cacher, err := lru.NewARC[Key, *Value](size)
+	return &LRUCache{cacher: cacher}, err
+}
+
+// NewKey creates a new cache key for given DNS message.
+func NewKey(msg *dns.Msg) Key {
+	q := msg.Question[0]
+	return Key{Qtype: q.Qtype, Qclass: q.Qclass, Name: normalizeQname(q.Name)}
+}
+
+// NewValue creates a new cache value for given DNS message.
+func NewValue(msg *dns.Msg, expire time.Time) *Value {
+	return &Value{
+		Expire: expire,
+		Msg:    msg,
+	}
+}
+
+func normalizeQname(name string) string {
+	var b strings.Builder
+	b.Grow(len(name))
+	for i := 0; i < len(name); i++ {
+		c := name[i]
+		if 'A' <= c && c <= 'Z' {
+			c += 'a' - 'A'
+		}
+		b.WriteByte(c)
+	}
+	return b.String()
+}