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前面介绍了Docker管理工具-Swarm部署记录，下面重点说下Swarm基于多主机容器通信的覆盖网络
在Docker版本1.12之后swarm模式原生支持覆盖网络(overlay networks)，可以先创建一个覆盖网络，然后启动容器的时候启用这个覆盖网络，
这样只要是这个覆盖网络内的容器，不管在不在同一个宿主机上都能相互通信，即跨主机通信！不同覆盖网络内的容器组之间是相互隔离的（相互ping不通）。swarm模式的覆盖网络包括以下功能：
1）可以附加多个服务到同一个网络。
2）默认情况下，service discovery为每个swarm服务分配一个虚拟IP地址(vip)和DNS名称，使得在同一个网络中容器之间可以使用服务名称为互相连接。
3）可以配置使用DNS轮循而不使用VIP
4）为了可以使用swarm的覆盖网络，在启用swarm模式之间你需要在swarm节点之间开放以下端口：
5）TCP/UDP端口7946 – 用于容器网络发现
6）UDP端口4789 – 用于容器覆盖网络实例如下：
-----------在Swarm集群中创建overlay网络------------
[root@localhost ~]# docker network create --driver overlay --opt encrypted --subnet 10.10.19.0/16 ngx_net
i5czdn39hrkg0q38crxzrk8g5参数解释：
–opt encrypted 默认情况下swarm中的节点通信是加密的。在不同节点的容器之间，可选的–opt encrypted参数能在它们的vxlan流量启用附加的加密层。
--subnet 命令行参数指定overlay网络使用的子网网段。当不指定一个子网时，swarm管理器自动选择一个子网并分配给网络。[root@localhost ~]# docker network ls
NETWORK ID NAME DRIVER SCOPE
f8fcec5c6ab0 bridge bridge local
5d04342a289b docker_gwbridge bridge local
0cae1d696f88 harbor_harbor bridge local
750da6f52800 host host local
nf8hr7thujhv ingress overlay swarm
i5czdn39hrkg ngx_net overlay swarm
383ddb3c7e8d none null local由上可知，Swarm当中拥有2套覆盖网络。其中"ngx_net"网络正是我们在部署容器时所创建的成果。而"ingress"覆盖网络则为默认提供。
Swarm 管理节点会利用 ingress 负载均衡以将服务公布至集群之外。
在将服务连接到这个创建的网络之前，网络覆盖到manager节点。上面输出的SCOPE为 swarm 表示将服务部署到Swarm时可以使用此网络。
在将服务连接到这个网络后，Swarm只将该网络扩展到特定的worker节点，这个worker节点被swarm调度器分配了运行服务的任务。
在那些没有运行该服务任务的worker节点上，网络并不扩展到该节点。------------------将服务连接到overlay网络-------------------
[root@localhost ~]# docker service create --replicas 5 --network ngx_net --name my-test -p 80:80 nginx
xt7hbixrb0ozxs7d8cyqqj4lq
overall progress: 5 out of 5 tasks
1/5: running [==================================================>]
2/5: running [==================================================>]
3/5: running [==================================================>]
4/5: running [==================================================>]
5/5: running [==================================================>]
verify: Service converged上面名为"my-test"的服务启动了5个task，用于运行每个任务的容器都可以彼此通过overlay网络进行通信。Swarm集群将网络扩展到所有任务处于Running状态的节点上。
[root@localhost ~]# docker service ls
ID NAME MODE REPLICAS IMAGE PORTS
xt7hbixrb0oz my-test replicated 5/5 nginx:latest *:80->80/tcp在manager-node节点上，通过下面的命令查看哪些节点有处于running状态的任务：
[root@localhost ~]# docker service ps my-test
ID NAME IMAGE NODE DESIRED STATE CURRENT STATE ERROR PORTS
uvuon5zlsw56 my-test.1 nginx:latest localhost Running Running 7 minutes ago
wy6up5e0vgyc \_ my-test.1 nginx:latest hsp2 Shutdown Shutdown 7 minutes ago
b4rctcg0bwqo my-test.2 nginx:latest hsp1 Running Running 8 minutes ago
2q2mm1u2woey my-test.3 nginx:latest hsp2 Running Running 7 minutes ago
46c0ycahza3o \_ my-test.3 nginx:latest hsp2 Shutdown Shutdown 7 minutes ago
b09z0ac8lxn5 my-test.4 nginx:latest localhost Running Running 8 minutes ago
krq4k8zcnxu9 my-test.5 nginx:latest hsp1 Running Running 8 minutes ago可见三个节点都有处于running状态的任务，所以my-network网络扩展到三个节点上。可以查询某个节点上关于my-network的详细信息：
[root@localhost ~]# docker network inspect ngx_net
[{"Name": "ngx_net","Id": "i5czdn39hrkg0q38crxzrk8g5","Created": "2020-05-19T17:34:03.810709768+08:00","Scope": "swarm","Driver": "overlay","EnableIPv6": false,"IPAM": {"Driver": "default","Options": null,"Config": [{"Subnet": "10.10.19.0/16","Gateway": "10.10.0.1"}]},"Internal": false,"Attachable": false,"Ingress": false,"ConfigFrom": {"Network": ""},"ConfigOnly": false,"Containers": {"abf3b461762907f5526b9909e6ad2d43a679394a66f311ae78a2ed772e440834": {"Name": "my-test.4.b09z0ac8lxn5fiut07riv9rkn","EndpointID": "6c0caf990b389581afed6112d6cc3f597ef3a753509254b0503f252e47798092","MacAddress": "02:42:0a:0a:00:09","IPv4Address": "10.10.0.9/16","IPv6Address": ""},"c556d0b4800ba7e3748d3b44db094016ce88edc66eb3044e13ae9685b3379104": {"Name": "my-test.1.uvuon5zlsw56c36ksrcjia555","EndpointID": "cd3134a2a278d68febbf07711986c687f5a1f2bbb88262ed8621734bcbf0481d","MacAddress": "02:42:0a:0a:00:0c","IPv4Address": "10.10.0.12/16","IPv6Address": ""},"lb-ngx_net": {"Name": "ngx_net-endpoint","EndpointID": "331805a3d75b2ed3536105be65e3d378f80f01eab7d9f2770e0532c8ab24b64c","MacAddress": "02:42:0a:0a:00:03","IPv4Address": "10.10.0.3/16","IPv6Address": ""}},"Options": {"com.docker.network.driver.overlay.vxlanid_list": "4097","encrypted": ""},"Labels": {},"Peers": [{"Name": "75e8d4c227ad","IP": "192.168.31.10"},{"Name": "d25f52e1f4f2","IP": "192.168.31.136"},{"Name": "b1fc9d6bf4a6","IP": "192.168.31.137"}]}
]从上面的信息可以看出在manager-node节点上，名为my-test的服务有一个名为my-test.1.uvuon5zlsw56c36ksrcjia555和
my-test.4.b09z0ac8lxn5fiut07riv9rkn连接到名为ngx_net的网络上（另外两个节点同样可以用上面命令查看）
[root@hsp2 ~]# docker network inspect ngx_net
[{"Name": "ngx_net","Id": "i5czdn39hrkg0q38crxzrk8g5","Created": "2020-05-19T17:34:03.832827626+08:00","Scope": "swarm","Driver": "overlay","EnableIPv6": false,"IPAM": {"Driver": "default","Options": null,"Config": [{"Subnet": "10.10.19.0/16","Gateway": "10.10.0.1"}]},"Internal": false,"Attachable": false,"Ingress": false,"ConfigFrom": {"Network": ""},"ConfigOnly": false,"Containers": {"27b292006ebb3f18ed83016136c994d42bcd5de2e044d6c087cc9e5f7d802007": {"Name": "my-test.2.55164ssl3hrx4bfum96947ibh","EndpointID": "2a04498496e3448ed0cc39ce1c76777d7f6020b55ee70c5e659b1bce7c3d0df5","MacAddress": "02:42:0a:0a:00:0d","IPv4Address": "10.10.0.13/16","IPv6Address": ""},"5319057785b2ac303597812a6ca6d1906505ffa7df8fb4d5db104e5c0127f48e": {"Name": "my-test.3.2q2mm1u2woeywu1iffamd16xl","EndpointID": "5b941d033b899b63ee0294fac64d79348f4fd3049e1b4ad81381230f2bc55d06","MacAddress": "02:42:0a:0a:00:0b","IPv4Address": "10.10.0.11/16","IPv6Address": ""},"d3e66cdff911d5e63a20e30be010b379176d3eeea085f1b5b9ff28033c0da975": {"Name": "my-test.5.i2slw15fa6aysup6kkkxk9ja7","EndpointID": "ad9a7d3c49e432b92761ada8a18c92894c583f84e27a4ec2cb8cf01a78647126","MacAddress": "02:42:0a:0a:00:0e","IPv4Address": "10.10.0.14/16","IPv6Address": ""},"lb-ngx_net": {"Name": "ngx_net-endpoint","EndpointID": "7d7640449ad265276133aa962288fcb111e6b046bb853197cd8de5234b2c1165","MacAddress": "02:42:0a:0a:00:02","IPv4Address": "10.10.0.2/16","IPv6Address": ""}},"Options": {"com.docker.network.driver.overlay.vxlanid_list": "4097","encrypted": ""},"Labels": {},"Peers": [{"Name": "b1fc9d6bf4a6","IP": "192.168.31.137"},{"Name": "d25f52e1f4f2","IP": "192.168.31.136"},{"Name": "75e8d4c227ad","IP": "192.168.31.10"}]}
]由上结果可知，10.10.0.5其实就是swarm集群内部的vip.
加入ngx_net网络的容器彼此之间可以通过IP地址通信，也可以通过名称通信。```bash
[root@hsp2 ~]# docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
5319057785b2 nginx:latest "nginx -g 'daemon of…" 17 minutes ago Up 16 minutes 80/tcp my-test.3.2q2mm1u2woeywu1iffamd16xl[root@hsp2 ~]# docker exec -it 5319057785b2 /bin/bash
root@5319057785b2:/# ifconfig
bash: ifconfig: command not found
root@5319057785b2:/# apt-get update
root@5319057785b2:/# apt-get install -y net-tools
root@5319057785b2:/# ifconfig
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1450inet 10.255.0.11 netmask 255.255.0.0 broadcast 10.255.255.255ether 02:42:0a:ff:00:0b txqueuelen 0 (Ethernet)RX packets 0 bytes 0 (0.0 B)RX errors 0 dropped 0 overruns 0 frame 0TX packets 0 bytes 0 (0.0 B)TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0eth1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1424inet 10.10.0.11 netmask 255.255.0.0 broadcast 10.10.255.255ether 02:42:0a:0a:00:0b txqueuelen 0 (Ethernet)RX packets 0 bytes 0 (0.0 B)RX errors 0 dropped 0 overruns 0 frame 0TX packets 0 bytes 0 (0.0 B)TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0eth2: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500inet 172.18.0.4 netmask 255.255.0.0 broadcast 172.18.255.255ether 02:42:ac:12:00:04 txqueuelen 0 (Ethernet)RX packets 2369 bytes 8767222 (8.3 MiB)RX errors 0 dropped 0 overruns 0 frame 0TX packets 2200 bytes 120261 (117.4 KiB)TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536inet 127.0.0.1 netmask 255.0.0.0loop txqueuelen 1 (Local Loopback)RX packets 6 bytes 536 (536.0 B)RX errors 0 dropped 0 overruns 0 frame 0TX packets 6 bytes 536 (536.0 B)TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0root@5319057785b2:/# ping 10.10.0.9
bash: ping: command not found
root@5319057785b2:/# apt-get install -y iputils-ping
root@5319057785b2:/# ping 10.10.0.9
PING 10.10.0.9 (10.10.0.9) 56(84) bytes of data.
64 bytes from 10.10.0.9: icmp_seq=1 ttl=64 time=105 ms
64 bytes from 10.10.0.9: icmp_seq=2 ttl=64 time=0.496 ms
^C
--- 10.10.0.9 ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 3ms
rtt min/avg/max/mdev = 0.496/52.642/104.788/52.146 ms
root@5319057785b2:/# ping 10.10.0.12
PING 10.10.0.12 (10.10.0.12) 56(84) bytes of data.
64 bytes from 10.10.0.12: icmp_seq=1 ttl=64 time=26.4 ms
64 bytes from 10.10.0.12: icmp_seq=2 ttl=64 time=0.686 ms
^C
--- 10.10.0.12 ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 2ms
rtt min/avg/max/mdev = 0.686/13.534/26.382/12.848 ms
root@5319057785b2:/#----------------------------使用swarm模式的服务发现--------------------------
默认情况下，当创建了一个服务并连接到某个网络后，swarm会为该服务分配一个VIP。此VIP根据服务名映射到DNS。在网络上的容器共享该服务的DNS映射，
所以网络上的任意容器可以通过服务名访问服务。在同一overlay网络中，不用通过端口映射来使某个服务可以被其它服务访问。Swarm内部的负载均衡器自动将请求发送到服务的VIP上，然后分发到所有的
active的task上。如下示例：
在同一个网络中添加了一个nginx服务，此服务可以通过名称my-test访问前面创建的nginx服务：
[root@localhost ~]# docker service create --network ngx_net --name my-test2 nginx
ufwwkmrkzzwt68abpjogblly7
overall progress: 1 out of 1 tasks
1/1: running [==================================================>]
verify: Service converged
[root@localhost ~]# docker service ls
ID NAME MODE REPLICAS IMAGE PORTS
xt7hbixrb0oz my-test replicated 5/5 nginx:latest *:80->80/tcp
ufwwkmrkzzwt my-test2 replicated 1/1 nginx:latest 查询nginx运行在哪个节点上（上面创建命令执行后，需要一段时间才能完成这个nginx服务的创建）
[root@localhost ~]# docker service ps my-test2
ID NAME IMAGE NODE DESIRED STATE CURRENT STATE ERROR PORTS
t4z8sfmlst04 my-test2.1 nginx:latest hsp1 Running Running 7 minutes ago登录nginx运行的节点（由上可知是hsp1节点），打开nginx的交互shell：
[root@hsp1 ~]# docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
d746be71a2e3 nginx:latest "nginx -g 'daemon of…" 8 minutes ago Up 8 minutes 80/tcp my-test2.1.t4z8sfmlst040oxm0hv70sr09[root@node2 ~]# docker exec -ti my-centos.1.9yk5ie28gwk9mw1h1jovb68ki /bin/bash
root@d746be71a2e3:/# apt-get update
root@d746be71a2e3:/# apt-get install dnsutils
root@d746be71a2e3:/# nslookup my-test
Server: 127.0.0.11
Address: 127.0.0.11#53Non-authoritative answer:
Name: my-test
Address: 10.10.0.5从nginx容器内部，使用特殊查询 查询DNS，来找到my-test服务的所有容器的IP地址：
root@d746be71a2e3:/# nslookup tasks.my-test
Server: 127.0.0.11
Address: 127.0.0.11#53Non-authoritative answer:
Name: tasks.my-test
Address: 10.10.0.13
Name: tasks.my-test
Address: 10.10.0.14
Name: tasks.my-test
Address: 10.10.0.12
Name: tasks.my-test
Address: 10.10.0.9
Name: tasks.my-test
Address: 10.10.0.11从nginx容器内部，通过wget来访问my-test服务中运行的网页服务器
root@d746be71a2e3:/# apt-get install -y wget
root@d746be71a2e3:/# wget -O- my-test
--2020-05-19 10:58:23-- http://my-test/
Resolving my-test (my-test)... 10.10.0.5
Connecting to my-test (my-test)|10.10.0.5|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 612 [text/html]
Saving to: 'STDOUT'- 0%[ ] 0 --.-KB/s <!DOCTYPE html>
<html>
<head>
<title>Welcome to nginx!</title>
<style>body {width: 35em;margin: 0 auto;font-family: Tahoma, Verdana, Arial, sans-serif;}
</style>
</head>
<body>
<h1>Welcome to nginx!</h1>
<p>If you see this page, the nginx web server is successfully installed and
working. Further configuration is required.</p><p>For online documentation and support please refer to
<a href="http://nginx.org/">nginx.org</a>.<br/>
Commercial support is available at
<a href="http://nginx.com/">nginx.com</a>.</p><p><em>Thank you for using nginx.</em></p>
</body>
</html>
- 100%[==============================================>] 612 --.-KB/s in 0.1s 2020-05-19 10:58:25 (5.38 KB/s) - written to stdout [612/612]Swarm的负载均衡器自动将HTTP请求路由到VIP上，然后到一个active的task容器上。它根据round-robin选择算法将后续的请求分发到另一个active的task上。-----------------------------------为服务使用DNS round-robin-----------------------------
在创建服务时，可以配置服务直接使用DNS round-robin而无需使用VIP。这是通过在创建服务时指定 --endpoint-mode dnsrr 命令行参数实现的。
当你想要使用自己的负载均衡器时可以使用这种方式。如下示例（注意：使用DNS round-robin方式创建服务，不能直接在命令里使用-p指定端口）
[root@localhost ~]# docker service create --replicas 3 --name my-dnsrr-nginx --network ngx_net --endpoint-mode dnsrr nginx
o5uyghy9vsh3ilx9745jhuoy7
overall progress: 3 out of 3 tasks
1/3: running [==================================================>]
2/3: running [==================================================>]
3/3: running [==================================================>]
verify: Service converged
[root@localhost ~]# docker service ps my-dnsrr-nginx
ID NAME IMAGE NODE DESIRED STATE CURRENT STATE ERROR PORTS
vtuqgoqri9bh my-dnsrr-nginx.1 nginx:latest hsp1 Running Running about a minute ago
qcdl1bddlzgp my-dnsrr-nginx.2 nginx:latest localhost Running Running 15 seconds ago
w4u0va00d24c my-dnsrr-nginx.3 nginx:latest hsp2 Running Running 39 seconds ago当通过服务名称查询DNS时，DNS服务返回所有任务容器的IP地址：
root@d746be71a2e3:/# nslookup my-dnsrr-nginx
Server: 127.0.0.11
Address: 127.0.0.11#53Non-authoritative answer:
Name: my-dnsrr-nginx
Address: 10.10.0.149
Name: my-dnsrr-nginx
Address: 10.10.0.148
Name: my-dnsrr-nginx
Address: 10.10.0.150需要注意的是：一定要确认VIP的连通性
通常Docker官方推荐使用dig，nslookup或其它DNS查询工具来查询通过DNS对服务名的访问。因为VIP是逻辑IP，ping并不是确认VIP连通性的正确的工具。