server exit with nil pointer

[runzexia]

Describe the bug
A clear and concise description of what the bug is.

Log
please add -d to debug log
server:

Format domain b._dns-sd._udp.6.1.255.10.in-addr.arpa. to b._dns-sd._udp.6.1.255.10.in-addr.arpa.
Received DNS query for b._dns-sd._udp.6.1.255.10.in-addr.arpa.:
Exchange message for domain b._dns-sd._udp.6.1.255.10.in-addr.arpa. to dns server 10.233.0.3:53
Format domain db._dns-sd._udp.6.1.255.10.in-addr.arpa. to db._dns-sd._udp.6.1.255.10.in-addr.arpa.
Received DNS query for db._dns-sd._udp.6.1.255.10.in-addr.arpa.:
Exchange message for domain db._dns-sd._udp.6.1.255.10.in-addr.arpa. to dns server 10.233.0.3:53
Format domain lb._dns-sd._udp.6.1.255.10.in-addr.arpa. to lb._dns-sd._udp.6.1.255.10.in-addr.arpa.
Received DNS query for lb._dns-sd._udp.6.1.255.10.in-addr.arpa.:
Exchange message for domain lb._dns-sd._udp.6.1.255.10.in-addr.arpa. to dns server 10.233.0.3:53
 *** invalid answer name b._dns-sd._udp.0.144.31.172.in-addr.arpa. after 12 query for b._dns-sd._udp.0.144.31.172.in-addr.arpa.
 *** invalid answer name db._dns-sd._udp.0.144.31.172.in-addr.arpa. after 12 query for db._dns-sd._udp.0.144.31.172.in-addr.arpa.
 *** invalid answer name b._dns-sd._udp.6.1.255.10.in-addr.arpa. after 12 query for b._dns-sd._udp.6.1.255.10.in-addr.arpa.
 *** invalid answer name lb._dns-sd._udp.6.1.255.10.in-addr.arpa. after 12 query for lb._dns-sd._udp.6.1.255.10.in-addr.arpa.
 *** invalid answer name db._dns-sd._udp.6.1.255.10.in-addr.arpa. after 12 query for db._dns-sd._udp.6.1.255.10.in-addr.arpa.
 *** invalid answer name lb._dns-sd._udp.0.144.31.172.in-addr.arpa. after 12 query for lb._dns-sd._udp.0.144.31.172.in-addr.arpa.
 *** invalid answer name db._dns-sd._udp.54.196.168.192.in-addr.arpa. after 12 query for db._dns-sd._udp.54.196.168.192.in-addr.arpa.
*** error: read udp 10.233.102.169:36633->10.233.0.3:53: i/o timeout
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x603096]

goroutine 51 [running]:
gitlab.alibaba-inc.com/rdc-incubator/kt/pkg/proxy/daemon.(*handler).ServeDNS(0x7ed488, 0x6ab120, 0xc420191800, 0xc420177830)
	/Users/yunlong/go/src/gitlab.alibaba-inc.com/rdc-incubator/kt/pkg/proxy/daemon/dnsserver.go:66 +0x416
gitlab.alibaba-inc.com/rdc-incubator/kt/vendor/github.com/miekg/dns.(*Server).serveDNS(0xc4200f4000, 0xc420191800)
	/Users/yunlong/go/src/gitlab.alibaba-inc.com/rdc-incubator/kt/vendor/github.com/miekg/dns/server.go:687 +0x2cb
gitlab.alibaba-inc.com/rdc-incubator/kt/vendor/github.com/miekg/dns.(*Server).serve(0xc4200f4000, 0xc420191800)
	/Users/yunlong/go/src/gitlab.alibaba-inc.com/rdc-incubator/kt/vendor/github.com/miekg/dns/server.go:572 +0x2a7
gitlab.alibaba-inc.com/rdc-incubator/kt/vendor/github.com/miekg/dns.(*Server).worker(0xc4200f4000, 0xc420191800)
	/Users/yunlong/go/src/gitlab.alibaba-inc.com/rdc-incubator/kt/vendor/github.com/miekg/dns/server.go:244 +0x4d
created by gitlab.alibaba-inc.com/rdc-incubator/kt/vendor/github.com/miekg/dns.(*Server).spawnWorker
	/Users/yunlong/go/src/gitlab.alibaba-inc.com/rdc-incubator/kt/vendor/github.com/miekg/dns/server.go:284 +0x85

client:

2019/07/29 14:27:06 Client address 172.31.144.16
2019/07/29 14:27:06 Deploying proxy deployment kt-connect-daemon-csnsv in namespace default
2019/07/29 14:27:06 Pod status is Pending
2019/07/29 14:27:08 Pod status is Pending
2019/07/29 14:27:10 Pod status is Running
2019/07/29 14:27:10 Success deploy proxy deployment kt-connect-daemon-csnsv in namespace default
2019/07/29 14:27:10 Child, os.Args = [ktctl -d connect]
2019/07/29 14:27:10 Child, cmd.Args = [kubectl --kubeconfig=/Users/runzexia/.kube/config -n default port-forward deployments/kt-connect-daemon-csnsv 2222:22]
Forwarding from 127.0.0.1:2222 -> 22
Forwarding from [::1]:2222 -> 22
2019/07/29 14:27:12 port-forward start at pid: 96311
2019/07/29 14:27:17 Child, os.Args = [ktctl -d connect]
2019/07/29 14:27:17 Child, cmd.Args = [sshuttle --dns --to-ns 10.233.102.169 -e ssh -oStrictHostKeyChecking=no -oUserKnownHostsFile=/dev/null -i /tmp/kt_id_rsa -r root@127.0.0.1:2222 -x 127.0.0.1 10.233.64.0/24 10.233.65.0/24 10.233.0.0/16]
Handling connection for 2222
Warning: Permanently added '[127.0.0.1]:2222' (ECDSA) to the list of known hosts.
server: WARNING: Neither ip nor netstat were found on the server.
client: Connected.
2019/07/29 14:27:19 vpn(sshuttle) start at pid: 96312
2019/07/29 14:27:19 KT proxy start successful
Daemon Start At 96309Connection to 127.0.0.1 closed by remote host.
client: fatal: server died with error code 255
2019/07/29 14:32:17 vpn(sshuttle) exited
E0729 14:33:17.503274   96311 portforward.go:224] lost connection to pod
2019/07/29 14:33:17 port-forward exited