抓包就明白CoreDNS域名解析

時間：2023-02-06 11:32:01 | 來源：建站知識

時間：2023-02-06 11:32:01 來源：建站知識

微信公眾號：運維開發(fā)故事，作者：姜總

DNS 其實就是一個分布式的樹狀命名系統(tǒng)，它就像一個去中心化的分布式數(shù)據(jù)庫，存儲著從域名到 IP 地址的映射。k8s中利用CoreDNS進(jìn)行域名解析。

在進(jìn)行CoreDNS抓包之前先來了解幾個概念

完全限定名稱

完全限定域名(FQDN)就是互聯(lián)網(wǎng)上計算機(jī)或者主機(jī)的完整域名。由主機(jī)名、域名、頂級域組成。FQDN= HostName + DomainName

如：域名 http://www.ayunw.cn ，實際上它應(yīng)該是 www.ayunw.cn. ，而通常最后的點可以不寫。最后的點被稱為根域
www就是主機(jī)名，http://ayunw.cn就是域名，而.cn又被稱為頂級域(一級域名)，ayunw被稱為二級域名，最后的點被稱為 根域。

如：www.allen.ayunw.cn. ，其中最后的點被稱為 根域(TLD)，cn被稱為頂級域(一級域名)，ayunw被稱為二級域名，allen被稱為三級域名，www被稱為主機(jī)名。

k8s中，非完全限定名稱比如：demo-hello.paas.svc.cluster.local

無類域間路由(CIDR)

如：192.168.1.0/24。想要更好的對CIDR了解的可以自行谷歌查詢詳情，這里不展開說。

這里，我自己有一個域名叫 http://www.ayunw.cn ，然后這里我嘗試用一個 paas 名稱空間下的一個pod對 http://www.ayunw.cn 做 nslookup 域名解析。并且對某一個coredns的pod進(jìn)行抓包分析。

為了測試，我這里用一個已經(jīng)發(fā)布好測試的容器。進(jìn)入容器，查看 /etc/resolv.conf 文件內(nèi)容

root@demo-hello-perf-dev-v0-5-0-f9f9cd5c9-r27cw:/# cat /etc/resolv.confnameserver 10.10.0.2search paas.svc.cluster.local svc.cluster.local cluster.localoptions ndots:5在該容器中安裝 nslookup 工具，然后對 http://www.ayunw.cn 域名進(jìn)行解析

[root@kube-master-srv1 ~]# kubectl get po -n paasNAME READY STATUS RESTARTS AGEdemo-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 -- bashroot@demo-hello-perf-dev-v0-5-0-f9f9cd5c9-r27cw:/# cat /etc/issueDebian GNU/Linux 10 /n /lroot@demo-hello-perf-dev-v0-5-0-f9f9cd5c9-r27cw:/# apt -y install dnsutils接著找到某一個coredns，然后去他所調(diào)度到的node節(jié)點通過nsenter進(jìn)入網(wǎng)絡(luò)名稱空間進(jìn)行抓包分析

# 在k8s-master上查看coredns調(diào)度在哪個node# 接著我就選擇了第一個coredns[root@kube-master-srv1 ~]# kubectl get po -n kube-system -o wide | grep corednscoredns-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># 登錄到 node2.core 節(jié)點，找到coredns的pid# 進(jìn)入這個pid進(jìn)入coredns容器的網(wǎng)絡(luò)名稱空間進(jìn)行抓包過濾分析[root@kube-node-srv2 ~]# docker ps -a | grep coredns4d38fd311a78 bfe3a36ebd25 "/coredns -conf /etc…" 3 months ago Up 3 months k8s_coredns_coredns-69d9b6c494-4nrxt_kube-system_803290a5-b4bd-4f2e-81b3-5ce82c9aa57c_000722e50786b 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}} 4d38fd311a78896949[root@kube-node-srv2 ~]# nsenter -n -t 896949[root@kube-node-srv2 ~]# ifconfigeth0: 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 0lo: 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集群的內(nèi)部域名

這里說的集群內(nèi)部域名就是 service的名字。我這里用的是kubernetes這個service來測試
連續(xù)解析6次，為了方便查看，我每執(zhí)行一次解析，下面抓包的終端就敲一次回車。

[root@kube-master-srv1 ~]# kubectl get svc kubernetesNAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGEkubernetes ClusterIP 10.10.0.1 <none> 443/TCP 57droot@demo-hello-perf-dev-v0-5-0-f9f9cd5c9-r27cw:/# nslookup kubernetes.defaultServer: 10.10.0.2Address: 10.10.0.2#53Name: kubernetes.default.svc.cluster.localAddress: 10.10.0.1

抓包分析

以下是抓取kubernetes這個域名的DNS包的結(jié)果

[root@kube-node-srv2 ~]# tcpdump -i eth0 port 53 | grep "kubernetes"tcpdump: verbose output suppressed, use -v or -vv for full protocol decodelistening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes16:44:42.712421 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)通過以上抓包分析得出結(jié)論。當(dāng)解析kubernetes域名的時候，點的個數(shù)比ndots的值小，則按照search后面的本地域參數(shù)填補(bǔ)了域名后綴，當(dāng)按照順序 用 paas.svc.cluster.local 填補(bǔ)的時候解析到了A記錄。然后終止dns查詢將查詢到的A記錄返回。

通過host命令對名為kubernetes的service的集群內(nèi)部域名進(jìn)行解析

root@demo-hello-pro-master-5474b97bdf-fvbm5:/# host -v kubernetes.defaultTrying "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 10.10.0.1Received 106 bytes from 10.10.0.2#53 in 3 msTrying "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 5Received 147 bytes from 10.10.0.2#53 in 2 msTrying "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 5Received 147 bytes from 10.10.0.2#53 in 2 ms

解析k8s集群外部域名

接下來針對我的 http://www.ayunw.cn 這個域名進(jìn)行多次解析。這里我為了測試，發(fā)起了6 次解析。我每執(zhí)行一次解析，下面抓包的終端就敲一次回車。解析的同時去coredns這個容器所在的節(jié)點進(jìn)行抓包分析。

root@demo-hello-perf-dev-v0-5-0-f9f9cd5c9-r27cw:/# nslookup www.ayunw.cnServer: 10.10.0.2Address: 10.10.0.2#53Non-authoritative answer:Name: www.ayunw.cnAddress: 134.175.123.64

抓包分析

抓包開始，由于我的集群有大量的服務(wù)，每秒都有很多內(nèi)部服務(wù)dns解析請求。所以這里我過濾了關(guān)鍵字ayunw。上面的dns每執(zhí)行一次，我在這個抓包的窗口就敲一下回車，這樣的話方便看清楚每一次的解析結(jié)果

以下是抓 http://www.ayunw.cn 的域名DNS包的結(jié)果：

[root@kube-node-srv2 ~]# tcpdump -i eth0 port 53 | grep "ayunw"tcpdump: verbose output suppressed, use -v or -vv for full protocol decodelistening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes14:38:07.350640 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 captured35956 packets received by filter0 packets dropped by kernel從上面抓包分析的結(jié)果來看， http://www.ayunw.cn 的這個域名只有兩個點，比pod里面 /etc/resolv.conf 文件中的 ndots 配置的值小(ndots的值為5，域名的點為2)。則會按照search的參數(shù)填補(bǔ)域名后綴，并且是根據(jù)search后面的順序 paas.svc.cluster.local 、 svc.cluster.local 、 cluster.local 依次來填充的。因為根據(jù)search后面的本地域匹配后都沒有域名解析的結(jié)果，因此他就直接解析了 http://www.ayunw.cn 這個域名查詢到了該域名的A記錄并且返回了結(jié)果。

通過host命令來進(jìn)行解析

root@demo-hello-pro-master-5474b97bdf-fvbm5:/# host -v www.ayunw.cnTrying "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 msTrying "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 360Received 113 bytes from 10.10.0.2#53 in 99 msTrying "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 360Received 113 bytes from 10.10.0.2#53 in 51 ms因為我的pod中存在三個本地域：paas.svc.cluster.local 、 svc.cluster.local 、 cluster.local ，通過host命令可以看到，Trying 一共嘗試了四次，一次根據(jù)我search后面的本地域進(jìn)行了解析搜索，結(jié)果沒有搜索到正確的解析，因此通過pod所在的宿主機(jī)本地的 /etc/resolv.conf 文件中進(jìn)行了解析。

本地宿主機(jī)的/etc/resolv.conf的解析如下:
我這里公司用了內(nèi)部bind服務(wù)，做了內(nèi)部dns，然后上游指向了百度的dns。

# cat /etc/resolv.confoptions rotate timeout:1; generated by /usr/sbin/dhclient-scriptnameserver 172.16.0.11nameserver 172.16.0.12解析 http://www.jd.com 域名

root@demo-hello-perf-dev-v0-5-0-f9f9cd5c9-r27cw:/# nslookup www.jd.comServer: 10.10.0.2Address: 10.10.0.2#53Non-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.comAddress: 113.107.249.3以下是抓http://www.jd.com的域名DNS包的結(jié)果：

[root@kube-node-srv2 ~]# tcpdump -i eth0 port 53 | grep "jd"tcpdump: verbose output suppressed, use -v or -vv for full protocol decodelistening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes16:17:52.935226 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)通過host命令檢測http://www.jd.com和上面http://www.ayunw.cn一樣的

root@demo-hello-pro-master-5474b97bdf-fvbm5:/# host -v www.jd.comTrying "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 msTrying "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 3600Received 125 bytes from 10.10.0.2#53 in 4 msTrying "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 3600Received 125 bytes from 10.10.0.2#53 in 40 ms

查看域名的點數(shù)等于ndots的值5的域名解析

這里我有一個阿里云上的域名，做了一個dns解析并且測試了4次

root@demo-hello-perf-dev-v0-5-0-f9f9cd5c9-r27cw:/# nslookup x.y.z.v.ayunw.cnServer: 10.10.0.2Address: 10.10.0.2#53Non-authoritative answer:Name: x.y.z.v.ayunw.cnAddress: 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 decodelistening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes16:36:49.928116 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)從上述抓包結(jié)果可以看到，如果域名中的點等于ndots的值，他會直接解析域名，不會用search后面的本地域來填補(bǔ)的?？赡芤驗槲野⒗镌粕线@個域名的原因，不支持超過5個點的域名解析。所以超過5個點的域名我無法測試。

結(jié)論

如果點的個數(shù)小于5個，那么會根據(jù)search中配置的本地域列表一次在對應(yīng)域中先進(jìn)行搜索。如果沒有返回，則最后再查詢域名本身。如果說search中配置的本地域列表沒有一個匹配的，那么就會走到服務(wù)器宿主機(jī)的/etc/resolv.conf中去解析。如果你kubelet中clusterdomain配置錯了。那么search中沒有任何一個匹配的到，直接轉(zhuǎn)發(fā)到本地DNS，走正常的遞歸查詢邏輯。

通過以上測試發(fā)現(xiàn)ndots的值和請求的域名是相關(guān)的。為了避免多次的DNS解析查詢，可以將需要進(jìn)行解析的域名進(jìn)行相對的優(yōu)化
盡可能將域名中的點都帶上，并且最好是等于ndots的值。比如：kubernetes.paas.svc.cluster.local。這樣他就直接解析到了這個域名返回了A記錄而不是在通過search后面的本地域去解析多次。如果你解析的域名是kubernetes.paas他就會根據(jù)search后面的本地域去進(jìn)行補(bǔ)全解析多次了
在同一個namespace下可以直接解析service的名稱。比如：nslookup kubernetes，他會補(bǔ)全default.svc.cluster.local，但是為了解析失敗個人建議最好還是將域名寫完整。




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