DNS 其实就是一个分布式的树状命名系统，它就像一个去中心化的分布式数据库，存储着从域名到 IP 地址的映射。k8s中利用CoreDNS进行域名解析。
在进行CoreDNS抓包之前先来了解几个概念
完全限定名称
完全限定域名(FQDN)就是互联网上计算机或者主机的完整域名。由主机名、域名、顶级域组成。FQDN= HostName + DomainName
如：域名 www.ayunw.cn ，实际上它应该是 www.ayunw.cn. ，而通常最后的点可以不写。最后的点被称为根域 www就是主机名，ayunw.cn就是域名，而.cn又被称为顶级域(一级域名)，ayunw被称为二级域名，最后的点被称为 根域。
如：www.allen.ayunw.cn. ，其中最后的点被称为 根域(TLD)，cn被称为顶级域(一级域名)，ayunw被称为二级域名，allen被称为三级域名，www被称为主机名。
k8s中，非完全限定名称比如：demo-hello.paas.svc.cluster.local
无类域间路由(CIDR)
如：192.168.1.0/24。想要更好的对CIDR了解的可以自行谷歌查询详情，这里不展开说。
这里，我自己有一个域名叫 www.ayunw.cn ，然后这里我尝试用一个 paas 名称空间下的一个pod对 www.ayunw.cn 做 nslookup 域名解析。并且对某一个coredns的pod进行抓包分析。
为了测试，我这里用一个已经发布好测试的容器。进入容器，查看 /etc/resolv.conf 文件内容
root@demo-hello-perf-dev-v0-5-0-f9f9cd5c9-r27cw:/# cat /etc/resolv.conf nameserver 10.10.0.2 search paas.svc.cluster.local svc.cluster.local cluster.local options ndots:5
在该容器中安装 nslookup 工具，然后对 www.ayunw.cn 域名进行解析
[root@kube-master-srv1 ~]# kubectl get po -n paas NAME READY STATUS RESTARTS AGE demo-hello-perf-dev-v0-5-0-f9f9cd5c9-r27cw 1/1 Running 0 11d [root@kube-master-srv1 ~]# kubectl exec -it demo-hello-perf-dev-v0-5-0-f9f9cd5c9-r27cw -n paas -- bash root@demo-hello-perf-dev-v0-5-0-f9f9cd5c9-r27cw:/# cat /etc/issue Debian GNU/Linux 10 \n \l root@demo-hello-perf-dev-v0-5-0-f9f9cd5c9-r27cw:/# apt -y install dnsutils
接着找到某一个coredns，然后去他所调度到的node节点通过nsenter进入网络名称空间进行抓包分析
# 在k8s-master上查看coredns调度在哪个node # 接着我就选择了第一个coredns [root@kube-master-srv1 ~]# kubectl get po -n kube-system -o wide | grep coredns coredns-69d9b6c494-4nrxt 1/1 Running 0 96d 10.20.246.18 node2.core <none> <none> coredns-69d9b6c494-6vjw4 1/1 Running 0 96d 10.20.240.239 node3.core <none> <none> coredns-69d9b6c494-pw5gx 1/1 Running 0 96d 10.20.240.232 node3.core <none> <none> # 登录到 node2.core 节点，找到coredns的pid # 进入这个pid进入coredns容器的网络名称空间进行抓包过滤分析 [root@kube-node-srv2 ~]# docker ps -a | grep coredns 4d38fd311a78 bfe3a36ebd25 "/coredns -conf /etc…" 3 months ago Up 3 months k8s_coredns_coredns-69d9b6c494-4nrxt_kube-system_803290a5-b4bd-4f2e-81b3-5ce82c9aa57c_0 00722e50786b registry.xx.xx/library/k8s.gcr.io/pause:3.2 "/pause" 3 months ago Up 3 months k8s_POD_coredns-69d9b6c494-4nrxt_kube-system_803290a5-b4bd-4f2e-81b3-5ce82c9aa57c_0 [root@kube-node-srv2 ~]# docker inspect -f {{.State.Pid}} 4d38fd311a78 896949 [root@kube-node-srv2 ~]# nsenter -n -t 896949 [root@kube-node-srv2 ~]# ifconfig eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1380 inet 10.20.246.18 netmask 255.255.255.255 broadcast 10.20.246.18 ether 46:c1:e0:30:b4:9d txqueuelen 0 (Ethernet) RX packets 1489941923 bytes 162419228606 (151.2 GiB) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 1488233127 bytes 297011464372 (276.6 GiB) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0 lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536 inet 127.0.0.1 netmask 255.0.0.0 loop txqueuelen 1000 (Local Loopback) RX packets 83731165 bytes 6681735331 (6.2 GiB) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 83731165 bytes 6681735331 (6.2 GiB) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
解析k8s集群的内部域名
这里说的集群内部域名就是 service的名字。我这里用的是kubernetes这个service来测试 连续解析6次，为了方便查看，我每执行一次解析，下面抓包的终端就敲一次回车。
[root@kube-master-srv1 ~]# kubectl get svc kubernetes NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE kubernetes ClusterIP 10.10.0.1 <none> 443/TCP 57d root@demo-hello-perf-dev-v0-5-0-f9f9cd5c9-r27cw:/# nslookup kubernetes.default Server: 10.10.0.2 Address: 10.10.0.2#53 Name: kubernetes.default.svc.cluster.local Address: 10.10.0.1
抓包分析
以下是抓取kubernetes这个域名的DNS包的结果
[root@kube-node-srv2 ~]# tcpdump -i eth0 port 53 | grep "kubernetes" tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes 16:44:42.712421 IP 10.20.105.252.60020 > qing-core-kube-node-srv2.domain: 7282+ A? kubernetes.default.svc.cluster.local. (54) 16:44:48.883881 IP 10.20.105.252.ndm-agent-port > qing-core-kube-node-srv2.domain: 25500+ AAAA? kubernetes.default.svc.cluster.local. (54) 16:50:15.361021 IP 10.20.105.252.57205 > qing-core-kube-node-srv2.domain: 24061+ A? kubernetes.default.paas.svc.cluster.local. (59) 16:50:22.186723 IP 10.20.105.252.60715 > qing-core-kube-node-srv2.domain: 55799+ AAAA? kubernetes.default.svc.cluster.local. (54) 16:50:27.813477 IP qing-core-kube-node-srv2.domain > 10.20.176.128.8181: 21787*- 1/0/0 PTR kubernetes.default.svc.cluster.local. (112) 16:46:04.429250 IP 10.20.105.252.33895 > qing-core-kube-node-srv2.domain: 37943+ A? kubernetes.default.svc.cluster.local.svc.cluster.local. (72) 16:46:04.441717 IP 10.20.105.252.54502 > qing-core-kube-node-srv2.domain: 45454+ AAAA? kubernetes.default.svc.cluster.local. (54) 16:46:10.771445 IP 10.20.105.252.54594 > qing-core-kube-node-srv2.domain: 16257+ A? kubernetes.default.svc.cluster.local.svc.cluster.local. (72) 16:46:10.783322 IP 10.20.105.252.59768 > qing-core-kube-node-srv2.domain: 60408+ AAAA? kubernetes.default.svc.cluster.local. (54)
通过以上抓包分析得出结论。当解析kubernetes域名的时候，点的个数比ndots的值小，则按照search后面的本地域参数填补了域名后缀，当按照顺序 用 paas.svc.cluster.local 填补的时候解析到了A记录。然后终止dns查询将查询到的A记录返回。
通过host命令对名为kubernetes的service的集群内部域名进行解析
root@demo-hello-pro-master-5474b97bdf-fvbm5:/# host -v kubernetes.default Trying "kubernetes.default.paas.svc.cluster.local" Trying "kubernetes.default.svc.cluster.local" ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 18054 ;; flags: qr aa rd; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0 ;; QUESTION SECTION: ;kubernetes.default.svc.cluster.local. IN A ;; ANSWER SECTION: kubernetes.default.svc.cluster.local. 5 IN A 10.10.0.1 Received 106 bytes from 10.10.0.2#53 in 3 ms Trying "kubernetes.default.svc.cluster.local" ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 58952 ;; flags: qr aa rd; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0 ;; QUESTION SECTION: ;kubernetes.default.svc.cluster.local. IN AAAA ;; AUTHORITY SECTION: cluster.local. 5 IN SOA ns.dns.cluster.local. hostmaster.cluster.local. 1622445553 7200 1800 86400 5 Received 147 bytes from 10.10.0.2#53 in 2 ms Trying "kubernetes.default.svc.cluster.local" ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 37783 ;; flags: qr aa rd; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0 ;; QUESTION SECTION: ;kubernetes.default.svc.cluster.local. IN MX ;; AUTHORITY SECTION: cluster.local. 5 IN SOA ns.dns.cluster.local. hostmaster.cluster.local. 1622445553 7200 1800 86400 5 Received 147 bytes from 10.10.0.2#53 in 2 ms
解析k8s集群外部域名
接下来针对我的 www.ayunw.cn 这个域名进行多次解析。这里我为了测试，发起了6 次解析。我每执行一次解析，下面抓包的终端就敲一次回车。解析的同时去coredns这个容器所在的节点进行抓包分析。
root@demo-hello-perf-dev-v0-5-0-f9f9cd5c9-r27cw:/# nslookup www.ayunw.cn Server: 10.10.0.2 Address: 10.10.0.2#53 Non-authoritative answer: Name: www.ayunw.cn Address: 134.175.123.64
抓包分析
抓包开始，由于我的集群有大量的服务，每秒都有很多内部服务dns解析请求。所以这里我过滤了关键字ayunw。上面的dns每执行一次，我在这个抓包的窗口就敲一下回车，这样的话方便看清楚每一次的解析结果
以下是抓 www.ayunw.cn 的域名DNS包的结果：
[root@kube-node-srv2 ~]# tcpdump -i eth0 port 53 | grep "ayunw" tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes 14:38:07.350640 IP 10.20.105.252.47767 > qing-core-kube-node-srv2.domain: 13102+ A? www.ayunw.cn.cluster.local. (44) 14:38:19.098753 IP 10.20.105.252.47071 > qing-core-kube-node-srv2.domain: 15535+ A? www.ayunw.cn.paas.svc.cluster.local. (53) 14:38:19.111441 IP 10.20.105.252.56968 > qing-core-kube-node-srv2.domain: 62838+ A? www.ayunw.cn. (30) 14:38:19.111720 IP qing-core-kube-node-srv2.35187 > 172.16.0.11.domain: 62838+ A? www.ayunw.cn. (30) 14:38:31.200982 IP 10.20.105.252.50777 > qing-core-kube-node-srv2.domain: 10715+ A? www.ayunw.cn.svc.cluster.local. (48) 14:38:31.214096 IP 10.20.105.252.51233 > qing-core-kube-node-srv2.domain: 37585+ AAAA? www.ayunw.cn. (30) 14:38:31.214299 IP qing-core-kube-node-srv2.35187 > 172.16.0.11.domain: 37585+ AAAA? www.ayunw.cn. (30) 14:39:04.691754 IP 10.20.105.252.34080 > qing-core-kube-node-srv2.domain: 34206+ A? www.ayunw.cn.paas.svc.cluster.local. (53) 14:39:04.704758 IP 10.20.105.252.36478 > qing-core-kube-node-srv2.domain: 64751+ A? www.ayunw.cn. (30) 14:39:04.705068 IP qing-core-kube-node-srv2.48926 > 172.16.0.11.domain: 64751+ A? www.ayunw.cn. (30) 14:39:13.925872 IP 10.20.105.252.59868 > qing-core-kube-node-srv2.domain: 45121+ A? www.ayunw.cn.paas.svc.cluster.local. (53) 14:39:13.937328 IP 10.20.105.252.45290 > qing-core-kube-node-srv2.domain: 27511+ A? www.ayunw.cn. (30) 14:39:13.937576 IP qing-core-kube-node-srv2.48926 > 172.16.0.11.domain: 27511+ A? www.ayunw.cn. (30) 14:39:24.838444 IP 10.20.105.252.37510 > qing-core-kube-node-srv2.domain: 45926+ A? www.ayunw.cn.cluster.local. (44) 14:45:13.438961 IP 10.20.105.252.55462 > qing-core-kube-node-srv2.domain: 60170+ A? www.ayunw.cn.paas.svc.cluster.local. (53) 14:45:13.450865 IP 10.20.105.252.42674 > qing-core-kube-node-srv2.domain: 25680+ A? www.ayunw.cn. (30) 14:45:13.451110 IP qing-core-kube-node-srv2.56396 > 172.16.0.11.domain: 25680+ A? www.ayunw.cn. (30) ^C35952 packets captured 35956 packets received by filter 0 packets dropped by kernel
从上面抓包分析的结果来看， www.ayunw.cn 的这个域名只有两个点，比pod里面 /etc/resolv.conf 文件中的 ndots 配置的值小(ndots的值为5，域名的点为2)。则会按照search的参数填补域名后缀，并且是根据search后面的顺序 paas.svc.cluster.local 、 svc.cluster.local 、 cluster.local 依次来填充的。因为根据search后面的本地域匹配后都没有域名解析的结果，因此他就直接解析了 www.ayunw.cn 这个域名查询到了该域名的A记录并且返回了结果。
通过host命令来进行解析
root@demo-hello-pro-master-5474b97bdf-fvbm5:/# host -v www.ayunw.cn Trying "www.ayunw.cn.paas.svc.cluster.local" Trying "www.ayunw.cn.svc.cluster.local" Trying "www.ayunw.cn.cluster.local" Trying "www.ayunw.cn" ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 8135 ;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 13, ADDITIONAL: 0 ;; QUESTION SECTION: ;www.ayunw.cn. IN A ;; ANSWER SECTION: www.ayunw.cn. 30 IN A 134.175.123.64 ;; AUTHORITY SECTION: . 30 IN NS l.root-servers.net. . 30 IN NS e.root-servers.net. . 30 IN NS h.root-servers.net. . 30 IN NS k.root-servers.net. . 30 IN NS d.root-servers.net. . 30 IN NS b.root-servers.net. . 30 IN NS g.root-servers.net. . 30 IN NS j.root-servers.net. . 30 IN NS m.root-servers.net. . 30 IN NS i.root-servers.net. . 30 IN NS f.root-servers.net. . 30 IN NS c.root-servers.net. . 30 IN NS a.root-servers.net. Received 461 bytes from 10.10.0.2#53 in 94 ms Trying "www.ayunw.cn" ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 11085 ;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0 ;; QUESTION SECTION: ;www.ayunw.cn. IN AAAA ;; AUTHORITY SECTION: ayunw.cn. 5 IN SOA dns17.hichina.com. hostmaster.hichina.com. 2019070911 3600 1200 86400 360 Received 113 bytes from 10.10.0.2#53 in 99 ms Trying "www.ayunw.cn" ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 19432 ;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0 ;; QUESTION SECTION: ;www.ayunw.cn. IN MX ;; AUTHORITY SECTION: ayunw.cn. 5 IN SOA dns17.hichina.com. hostmaster.hichina.com. 2019070911 3600 1200 86400 360 Received 113 bytes from 10.10.0.2#53 in 51 ms
因为我的pod中存在三个本地域：paas.svc.cluster.local 、 svc.cluster.local 、 cluster.local ，通过host命令可以看到，Trying 一共尝试了四次，一次根据我search后面的本地域进行了解析搜索，结果没有搜索到正确的解析，因此通过pod所在的宿主机本地的 /etc/resolv.conf 文件中进行了解析。
本地宿主机的/etc/resolv.conf的解析如下: 我这里公司用了内部bind服务，做了内部dns，然后上游指向了百度的dns。
# cat /etc/resolv.conf options rotate timeout:1 ; generated by /usr/sbin/dhclient-script nameserver 172.16.0.11 nameserver 172.16.0.12
解析 www.jd.com 域名
root@demo-hello-perf-dev-v0-5-0-f9f9cd5c9-r27cw:/# nslookup www.jd.com Server: 10.10.0.2 Address: 10.10.0.2#53 Non-authoritative answer: www.jd.com canonical name = www.jd.com.gslb.qianxun.com. www.jd.com.gslb.qianxun.com canonical name = www.jdcdn.com. www.jdcdn.com canonical name = img20.360buyimg.com.s.galileo.jcloud-cdn.com. img20.360buyimg.com.s.galileo.jcloud-cdn.com canonical name = img2x-sched.jcloud-cdn.com. Name: img2x-sched.jcloud-cdn.com Address: 113.107.249.3
以下是抓www.jd.com的域名DNS包的结果：
[root@kube-node-srv2 ~]# tcpdump -i eth0 port 53 | grep "jd" tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes 16:17:52.935226 IP 10.20.105.252.56775 > qing-core-kube-node-srv2.domain: 17278+ A? www.jd.com.paas.svc.cluster.local. (51) 16:17:52.947890 IP 10.20.105.252.52012 > qing-core-kube-node-srv2.domain: 12806+ A? www.jd.com. (28) 16:17:52.948150 IP qing-core-kube-node-srv2.54626 > 172.16.0.11.domain: 12806+ A? www.jd.com. (28) 16:17:53.054427 IP 172.16.0.11.domain > qing-core-kube-node-srv2.54626: 12806 5/13/0 CNAME www.jd.com.gslb.qianxun.com., CNAME www.jdcdn.com., CNAME img20.360buyimg.com.s.galileo.jcloud-cdn.com., CNAME img2x-sched.jcloud-cdn.com., A 113.107.249.3 (398) 16:17:53.054677 IP qing-core-kube-node-srv2.domain > 10.20.105.252.52012: 12806 5/13/0 CNAME www.jd.com.gslb.qianxun.com., CNAME www.jdcdn.com., CNAME img20.360buyimg.com.s.galileo.jcloud-cdn.com., CNAME img2x-sched.jcloud-cdn.com., A 113.107.249.3 (398)
通过host命令检测www.jd.com和上面www.ayunw.cn一样的
root@demo-hello-pro-master-5474b97bdf-fvbm5:/# host -v www.jd.com Trying "www.jd.com.paas.svc.cluster.local" Trying "www.jd.com.svc.cluster.local" Trying "www.jd.com.cluster.local" Trying "www.jd.com" ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 61910 ;; flags: qr rd ra; QUERY: 1, ANSWER: 5, AUTHORITY: 13, ADDITIONAL: 0 ;; QUESTION SECTION: ;www.jd.com. IN A ;; ANSWER SECTION: www.jd.com. 13 IN CNAME www.jd.com.gslb.qianxun.com. www.jd.com.gslb.qianxun.com. 13 IN CNAME www.jdcdn.com. www.jdcdn.com. 13 IN CNAME img20.360buyimg.com.s.galileo.jcloud-cdn.com. img20.360buyimg.com.s.galileo.jcloud-cdn.com. 13 IN CNAME img2x-sched.jcloud-cdn.com. img2x-sched.jcloud-cdn.com. 13 IN A 113.107.249.3 ;; AUTHORITY SECTION: . 13 IN NS f.root-servers.net. . 13 IN NS i.root-servers.net. . 13 IN NS d.root-servers.net. . 13 IN NS l.root-servers.net. . 13 IN NS j.root-servers.net. . 13 IN NS g.root-servers.net. . 13 IN NS k.root-servers.net. . 13 IN NS m.root-servers.net. . 13 IN NS h.root-servers.net. . 13 IN NS c.root-servers.net. . 13 IN NS a.root-servers.net. . 13 IN NS e.root-servers.net. . 13 IN NS b.root-servers.net. Received 398 bytes from 10.10.0.2#53 in 5 ms Trying "img2x-sched.jcloud-cdn.com" ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 64422 ;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0 ;; QUESTION SECTION: ;img2x-sched.jcloud-cdn.com. IN AAAA ;; AUTHORITY SECTION: jcloud-cdn.com. 5 IN SOA ns1.jdgslb.com. apollo.jdgslb.com. 1622435242 10800 3600 604800 3600 Received 125 bytes from 10.10.0.2#53 in 4 ms Trying "img2x-sched.jcloud-cdn.com" ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 43091 ;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0 ;; QUESTION SECTION: ;img2x-sched.jcloud-cdn.com. IN MX ;; AUTHORITY SECTION: jcloud-cdn.com. 5 IN SOA ns1.jdgslb.com. apollo.jdgslb.com. 1622435242 10800 3600 604800 3600 Received 125 bytes from 10.10.0.2#53 in 40 ms
查看域名的点数等于ndots的值5的域名解析
这里我有一个阿里云上的域名，做了一个dns解析并且测试了4次
root@demo-hello-perf-dev-v0-5-0-f9f9cd5c9-r27cw:/# nslookup x.y.z.v.ayunw.cn Server: 10.10.0.2 Address: 10.10.0.2#53 Non-authoritative answer: Name: x.y.z.v.ayunw.cn Address: 134.175.123.64
抓包分析
[root@kube-node-srv2 ~]# tcpdump -i eth0 port 53 | grep "ayunw" tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes 16:36:49.928116 IP 10.20.105.252.46581 > qing-core-kube-node-srv2.domain: 38769+ A? x.y.z.v.ayunw.cn. (34) 16:36:49.928383 IP qing-core-kube-node-srv2.59801 > 172.16.0.11.domain: 38769+ A? x.y.z.v.ayunw.cn. (34) 16:36:56.901762 IP 10.20.105.252.43844 > qing-core-kube-node-srv2.domain: 3524+ A? x.y.z.v.ayunw.cn. (34) 16:37:01.763743 IP 10.20.105.252.36053 > qing-core-kube-node-srv2.domain: 62952+ AAAA? x.y.z.v.ayunw.cn. (34) 16:37:01.764110 IP qing-core-kube-node-srv2.59801 > 172.16.0.11.domain: 62952+ AAAA? x.y.z.v.ayunw.cn. (34) 16:37:06.851820 IP 10.20.105.252.36305 > qing-core-kube-node-srv2.domain: 58393+ AAAA? x.y.z.v.ayunw.cn. (34) 16:37:06.852118 IP qing-core-kube-node-srv2.59801 > 172.16.0.11.domain: 58393+ AAAA? x.y.z.v.ayunw.cn. (34)
从上述抓包结果可以看到，如果域名中的点等于ndots的值，他会直接解析域名，不会用search后面的本地域来填补的。可能因为我阿里云上这个域名的原因，不支持超过5个点的域名解析。所以超过5个点的域名我无法测试。
结论
如果点的个数小于5个，那么会根据search中配置的本地域列表一次在对应域中先进行搜索。如果没有返回，则最后再查询域名本身。如果说search中配置的本地域列表没有一个匹配的，那么就会走到服务器宿主机的/etc/resolv.conf中去解析。如果你kubelet中clusterdomain配置错了。那么search中没有任何一个匹配的到，直接转发到本地DNS，走正常的递归查询逻辑。
通过以上测试发现ndots的值和请求的域名是相关的。为了避免多次的DNS解析查询，可以将需要进行解析的域名进行相对的优化 尽可能将域名中的点都带上，并且最好是等于ndots的值。比如：kubernetes.paas.svc.cluster.local。这样他就直接解析到了这个域名返回了A记录而不是在通过search后面的本地域去解析多次。如果你解析的域名是kubernetes.paas他就会根据search后面的本地域去进行补全解析多次了 在同一个namespace下可以直接解析service的名称。比如：nslookup kubernetes，他会补全default.svc.cluster.local，但是为了解析失败个人建议最好还是将域名写完整。
