diff --git a/config/glider.conf.example b/config/glider.conf.example
index a260e7f..2db7b63 100644
--- a/config/glider.conf.example
+++ b/config/glider.conf.example
@@ -233,6 +233,9 @@ dnsminttl=0
 # size of CACHE
 dnscachesize=4096
 
+# show query log of dns cache
+dnscachelog=True
+
 # custom records
 dnsrecord=www.example.com/1.2.3.4
 dnsrecord=www.example.com/2606:2800:220:1:248:1893:25c8:1946
diff --git a/dns/client.go b/dns/client.go
index 2347546..d43b3a0 100644
--- a/dns/client.go
+++ b/dns/client.go
@@ -109,13 +109,19 @@ func (c *Client) handleAnswer(respBytes []byte, clientAddr, dnsServer, network,
 	}
 
 	ips, ttl := c.extractAnswer(resp)
-	if ttl == 0 { // we got a null result
-		ttl = 600
+	if ttl > c.config.MaxTTL {
+		ttl = c.config.MaxTTL
+	} else if ttl < c.config.MinTTL {
+		ttl = c.config.MinTTL
+	}
+
+	if ttl <= 0 { // we got a null result
+		ttl = 1800
 	}
 
 	c.cache.Set(qKey(resp.Question), valCopy(respBytes), ttl)
-	log.F("[dns] %s <-> %s(%s) via %s, type: %d, %s: %s",
-		clientAddr, dnsServer, network, dialerAddr, resp.Question.QTYPE, resp.Question.QNAME, strings.Join(ips, ","))
+	log.F("[dns] %s <-> %s(%s) via %s, %s/%d: %d %s",
+		clientAddr, dnsServer, network, dialerAddr, resp.Question.QNAME, resp.Question.QTYPE, ttl, strings.Join(ips, ","))
 
 	return nil
 }
@@ -137,12 +143,6 @@ func (c *Client) extractAnswer(resp *Message) ([]string, int) {
 		}
 	}
 
-	if ttl > c.config.MaxTTL {
-		ttl = c.config.MaxTTL
-	} else if ttl < c.config.MinTTL {
-		ttl = c.config.MinTTL
-	}
-
 	return ips, ttl
 }
 
@@ -277,7 +277,7 @@ func (c *Client) AddHandler(h AnswerHandler) {
 func (c *Client) AddRecord(record string) error {
 	r := strings.Split(record, "/")
 	domain, ip := r[0], r[1]
-	m, err := c.MakeResponse(domain, ip)
+	m, err := c.MakeResponse(domain, ip, uint32(c.config.MaxTTL))
 	if err != nil {
 		return err
 	}
@@ -296,10 +296,11 @@ func (c *Client) AddRecord(record string) error {
 }
 
 // MakeResponse makes a dns response message for the given domain and ip address.
-func (c *Client) MakeResponse(domain string, ip string) (*Message, error) {
+// Note: you should make sure ttl > 0.
+func (c *Client) MakeResponse(domain, ip string, ttl uint32) (*Message, error) {
 	ipb := net.ParseIP(ip)
 	if ipb == nil {
-		return nil, errors.New("GenResponse: invalid ip format")
+		return nil, errors.New("MakeResponse: invalid ip format")
 	}
 
 	var rdata []byte
@@ -316,7 +317,7 @@ func (c *Client) MakeResponse(domain string, ip string) (*Message, error) {
 	m := NewMessage(0, Response)
 	m.SetQuestion(NewQuestion(qtype, domain))
 	rr := &RR{NAME: domain, TYPE: qtype, CLASS: ClassINET,
-		TTL: uint32(c.config.MinTTL), RDLENGTH: rdlen, RDATA: rdata}
+		TTL: ttl, RDLENGTH: rdlen, RDATA: rdata}
 	m.AddAnswer(rr)
 
 	return m, nil
diff --git a/go.mod b/go.mod
index d030756..9fb5b0b 100644
--- a/go.mod
+++ b/go.mod
@@ -19,6 +19,7 @@ require (
 	github.com/xtaci/kcp-go/v5 v5.6.1
 	golang.org/x/crypto v0.0.0-20210513164829-c07d793c2f9a
 	golang.org/x/net v0.0.0-20210525063256-abc453219eb5 // indirect
+	golang.org/x/sys v0.0.0-20210603125802-9665404d3644 // indirect
 )
 
 // Replace dependency modules with local developing copy
diff --git a/go.sum b/go.sum
index df9b7ba..b90c019 100644
--- a/go.sum
+++ b/go.sum
@@ -154,8 +154,11 @@ golang.org/x/sys v0.0.0-20201009025420-dfb3f7c4e634/go.mod h1:h1NjWce9XRLGQEsW7w
 golang.org/x/sys v0.0.0-20201101102859-da207088b7d1/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20201119102817-f84b799fce68/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20210423082822-04245dca01da/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
-golang.org/x/sys v0.0.0-20210525143221-35b2ab0089ea h1:+WiDlPBBaO+h9vPNZi8uJ3k4BkKQB7Iow3aqwHVA5hI=
 golang.org/x/sys v0.0.0-20210525143221-35b2ab0089ea/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.0.0-20210601080250-7ecdf8ef093b h1:qh4f65QIVFjq9eBURLEYWqaEXmOyqdUyiBSgaXWccWk=
+golang.org/x/sys v0.0.0-20210601080250-7ecdf8ef093b/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
+golang.org/x/sys v0.0.0-20210603125802-9665404d3644 h1:CA1DEQ4NdKphKeL70tvsWNdT5oFh1lOjihRcEDROi0I=
+golang.org/x/sys v0.0.0-20210603125802-9665404d3644/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/term v0.0.0-20201126162022-7de9c90e9dd1 h1:v+OssWQX+hTHEmOBgwxdZxK4zHq3yOs8F9J7mk0PY8E=
 golang.org/x/term v0.0.0-20201126162022-7de9c90e9dd1/go.mod h1:bj7SfCRtBDWHUb9snDiAeCFNEtKQo2Wmx5Cou7ajbmo=
 golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
diff --git a/service/dhcpd/dhcpd.go b/service/dhcpd/dhcpd.go
index e583a1c..e889c11 100644
--- a/service/dhcpd/dhcpd.go
+++ b/service/dhcpd/dhcpd.go
@@ -67,14 +67,22 @@ func handleDHCP(serverIP net.IP, mask net.IPMask, pool *Pool) server4.Handler {
 		switch mt := m.MessageType(); mt {
 		case dhcpv4.MessageTypeDiscover:
 			replyType = dhcpv4.MessageTypeOffer
-		case dhcpv4.MessageTypeRequest:
+		case dhcpv4.MessageTypeRequest, dhcpv4.MessageTypeInform:
 			replyType = dhcpv4.MessageTypeAck
+		case dhcpv4.MessageTypeRelease:
+			pool.ReleaseIP(m.ClientHWAddr)
+			log.F("[dpcpd] %v released ip %v", m.ClientHWAddr, m.ClientIPAddr)
+			return
+		case dhcpv4.MessageTypeDecline:
+			pool.ReleaseIP(m.ClientHWAddr)
+			log.F("[dpcpd] received decline message from %v", m.ClientHWAddr)
+			return
 		default:
 			log.F("[dpcpd] can't handle type %v", mt)
 			return
 		}
 
-		replyIp, err := pool.AssignIP(m.ClientHWAddr)
+		replyIp, err := pool.LeaseIP(m.ClientHWAddr)
 		if err != nil {
 			log.F("[dpcpd] can not assign IP, error %s", err)
 			return
diff --git a/service/dhcpd/pool.go b/service/dhcpd/pool.go
index c06313a..6b483d7 100644
--- a/service/dhcpd/pool.go
+++ b/service/dhcpd/pool.go
@@ -5,12 +5,21 @@ import (
 	"errors"
 	"math/rand"
 	"net"
+	"sync"
 	"time"
 )
 
 // Pool is a dhcp pool.
 type Pool struct {
 	items []*item
+	mutex sync.RWMutex
+	lease time.Duration
+}
+
+type item struct {
+	ip     net.IP
+	mac    net.HardwareAddr
+	expire time.Time
 }
 
 // NewPool returns a new dhcp ip pool.
@@ -26,15 +35,32 @@ func NewPool(lease time.Duration, start, end net.IP) (*Pool, error) {
 
 	items := make([]*item, 0, e-s+1)
 	for n := s; n <= e; n++ {
-		items = append(items, &item{lease: lease, ip: num2ip(n)})
+		items = append(items, &item{ip: num2ip(n)})
 	}
 	rand.Seed(time.Now().Unix())
-	return &Pool{items: items}, nil
+
+	p := &Pool{items: items, lease: lease}
+	go func() {
+		for now := range time.Tick(time.Second) {
+			p.mutex.Lock()
+			for i := 0; i < len(items); i++ {
+				if !items[i].expire.IsZero() && now.After(items[i].expire) {
+					items[i].mac = nil
+					items[i].expire = time.Time{}
+				}
+			}
+			p.mutex.Unlock()
+		}
+	}()
+
+	return p, nil
 }
 
-// AssignIP assigns an ip to mac from dhco pool.
-func (p *Pool) AssignIP(mac net.HardwareAddr) (net.IP, error) {
-	var ip net.IP
+// LeaseIP leases an ip to mac from dhcp pool.
+func (p *Pool) LeaseIP(mac net.HardwareAddr) (net.IP, error) {
+	p.mutex.Lock()
+	defer p.mutex.Unlock()
+
 	for _, item := range p.items {
 		if bytes.Equal(mac, item.mac) {
 			return item.ip, nil
@@ -43,39 +69,35 @@ func (p *Pool) AssignIP(mac net.HardwareAddr) (net.IP, error) {
 
 	idx := rand.Intn(len(p.items))
 	for _, item := range p.items[idx:] {
-		if ip = item.take(mac); ip != nil {
-			return ip, nil
+		if item.mac == nil {
+			item.mac = mac
+			item.expire = time.Now().Add(p.lease)
+			return item.ip, nil
 		}
 	}
 
 	for _, item := range p.items {
-		if ip = item.take(mac); ip != nil {
-			return ip, nil
+		if item.mac == nil {
+			item.mac = mac
+			item.expire = time.Now().Add(p.lease)
+			return item.ip, nil
 		}
 	}
-	return nil, errors.New("no more ip can be assigned")
+
+	return nil, errors.New("no more ip can be leased")
 }
 
-type item struct {
-	taken bool
-	ip    net.IP
-	lease time.Duration
-	mac   net.HardwareAddr
-}
+// ReleaseIP releases ip from pool according to the given mac.
+func (p *Pool) ReleaseIP(mac net.HardwareAddr) {
+	p.mutex.Lock()
+	defer p.mutex.Unlock()
 
-func (i *item) take(addr net.HardwareAddr) net.IP {
-	if !i.taken {
-		i.taken = true
-		go func() {
-			timer := time.NewTimer(i.lease)
-			<-timer.C
-			i.mac = nil
-			i.taken = false
-		}()
-		i.mac = addr
-		return i.ip
+	for _, item := range p.items {
+		if bytes.Equal(mac, item.mac) {
+			item.mac = nil
+			item.expire = time.Time{}
+		}
 	}
-	return nil
 }
 
 func ip2num(ip net.IP) uint32 {