刘剑的博客
2014年05月09日

Docker之跨主机容器连接

本文主要分析docker容器跨主机连接实现方式，主要是对自官方文档中的实现方式的过程进行记录和分析。

问题

  1. 主机1上启动一个运行redis-server的docker容器
  2. 从主机2上运行redis-cli的docker容器访问主机1上docker容器中的redis-server

官方方案

(consumer) --> (redis-ambassador) ---network---> (redis-ambassador) --> (redis)

redis-ambassador

官方提供的image，其中运行socat进行redis数据转发。

服务端

redis-server


paas@ubuntu:~$ docker run -d --name redis shipyard/redis
ee76fff375499f5f69060633e3c07edcf92d264d41b6e71ad4123891bb9808ba
paas@ubuntu:~$ docker inspect 5ea7e8765ad4f94e97514abadf730a682a16fe6ed4ef221ea0b659e2381b256b
paas@ubuntu:~$ docker inspect ee76fff375499f5f69060633e3c07edcf92d264d41b6e71ad4123891bb9808ba
[{
    "ID": "ee76fff375499f5f69060633e3c07edcf92d264d41b6e71ad4123891bb9808ba",
    "Created": "2014-05-08T03:15:28.374074044Z",
    "Path": "/usr/local/bin/redis-server",
    "Args": [
        "/etc/redis.conf"
    ],
    "Config": {
        "Hostname": "ee76fff37549",
        "Domainname": "",
        "User": "",
        "Memory": 0,
        "MemorySwap": 0,
        "CpuShares": 0,
        "AttachStdin": false,
        "AttachStdout": false,
        "AttachStderr": false,
        "PortSpecs": null,
        "ExposedPorts": {
            "6379/tcp": {}
        },
        "Tty": false,
        "OpenStdin": false,
        "StdinOnce": false,
        "Env": [
            "HOME=/",
            "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
        ],
        "Cmd": [
            "/etc/redis.conf"
        ],
        "Image": "shipyard/redis",
        "Volumes": {
            "/var/lib/redis": {}
        },
        "WorkingDir": "",
        "Entrypoint": [
            "/usr/local/bin/redis-server"
        ],
        "NetworkDisabled": false,
        "OnBuild": null
    },
    "State": {
        "Running": true,
        "Pid": 3244,
        "ExitCode": 0,
        "StartedAt": "2014-05-08T03:16:16.392818522Z",
        "FinishedAt": "2014-05-08T03:15:42.985608485Z",
        "Ghost": false
    },
    "Image": "b48b681ac98459fdc7efc98c9cce7e25f8b1537a4496aad9d355b3b11ca7144a",
    "NetworkSettings": {
        "IPAddress": "172.17.0.2",
        "IPPrefixLen": 16,
        "Gateway": "172.17.42.1",
        "Bridge": "docker0",
        "PortMapping": null,
        "Ports": {
            "6379/tcp": null
        }
    },
    "ResolvConfPath": "/var/lib/docker/containers/ee76fff375499f5f69060633e3c07edcf92d264d41b6e71ad4123891bb9808ba/resolv.conf",
    "HostnamePath": "/var/lib/docker/containers/ee76fff375499f5f69060633e3c07edcf92d264d41b6e71ad4123891bb9808ba/hostname",
    "HostsPath": "/var/lib/docker/containers/ee76fff375499f5f69060633e3c07edcf92d264d41b6e71ad4123891bb9808ba/hosts",
    "Name": "/redis",
    "Driver": "aufs",
    "ExecDriver": "native-0.1",
    "Volumes": {
        "/var/lib/redis": "/var/lib/docker/vfs/dir/f8a140fcfba06f9fc454d30a11a23c0ed981b712a8b29f913fdbbd4f2d5a9653"
    },
    "VolumesRW": {
        "/var/lib/redis": true
    },
    "HostConfig": {
        "Binds": null,
        "ContainerIDFile": "",
        "LxcConf": [],
        "Privileged": false,
        "PortBindings": {
            "6379/tcp": null
        },
        "Links": null,
        "PublishAllPorts": false,
        "Dns": null,
        "DnsSearch": null,
        "VolumesFrom": null
    }
}]
iptables

paas@ubuntu:~$ sudo iptables -S
-P INPUT ACCEPT
-P FORWARD ACCEPT
-P OUTPUT ACCEPT
-A FORWARD -o docker0 -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -i docker0 ! -o docker0 -j ACCEPT
-A FORWARD -i docker0 -o docker0 -j ACCEPT
paas@ubuntu:~$ sudo iptables -S -t nat
-P PREROUTING ACCEPT
-P INPUT ACCEPT
-P OUTPUT ACCEPT
-P POSTROUTING ACCEPT
-N DOCKER
-A PREROUTING -m addrtype --dst-type LOCAL -j DOCKER
-A OUTPUT ! -d 127.0.0.0/8 -m addrtype --dst-type LOCAL -j DOCKER
-A POSTROUTING -s 172.17.0.0/16 ! -d 172.17.0.0/16 -j MASQUERADE
说明

可以看到，此时redis-server只是运行在容器中。系统中没有配置任何转发规则，外部无法访问到redis-server的服务。

svendowideit/ambassador


paas@ubuntu:~$ docker run -d --link redis:redis --name redis_ambassador -p 6379:6379 svendowideit/ambassador
4480d8897fdedc1f34f0460d3853e4cc696a3751891f5783ebc12c44d985e68e
paas@ubuntu:~$ docker inspect 4480d8897fdedc1f34f0460d3853e4cc696a3751891f5783ebc12c44d985e68e
[{
    "ID": "4480d8897fdedc1f34f0460d3853e4cc696a3751891f5783ebc12c44d985e68e",
    "Created": "2014-05-08T03:19:28.369725238Z",
    "Path": "/bin/sh",
    "Args": [
        "-c",
        "env | grep _TCP= | sed 's/.*_PORT_\\([0-9]*\\)_TCP=tcp:\\/\\/\\(.*\\):\\(.*\\)/socat TCP4-LISTEN:\\1,fork,reuseaddr TCP4:\\2:\\3 \\\u0026/'  | sh \u0026\u0026 top"
    ],
    "Config": {
        "Hostname": "4480d8897fde",
        "Domainname": "",
        "User": "",
        "Memory": 0,
        "MemorySwap": 0,
        "CpuShares": 0,
        "AttachStdin": false,
        "AttachStdout": false,
        "AttachStderr": false,
        "PortSpecs": null,
        "ExposedPorts": {
            "6379/tcp": {}
        },
        "Tty": false,
        "OpenStdin": false,
        "StdinOnce": false,
        "Env": [
            "HOME=/",
            "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
        ],
        "Cmd": [
            "/bin/sh",
            "-c",
            "env | grep _TCP= | sed 's/.*_PORT_\\([0-9]*\\)_TCP=tcp:\\/\\/\\(.*\\):\\(.*\\)/socat TCP4-LISTEN:\\1,fork,reuseaddr TCP4:\\2:\\3 \\\u0026/'  | sh \u0026\u0026 top"
        ],
        "Image": "svendowideit/ambassador",
        "Volumes": null,
        "WorkingDir": "",
        "Entrypoint": null,
        "NetworkDisabled": false,
        "OnBuild": null
    },
    "State": {
        "Running": true,
        "Pid": 3316,
        "ExitCode": 0,
        "StartedAt": "2014-05-08T03:19:28.466496993Z",
        "FinishedAt": "0001-01-01T00:00:00Z",
        "Ghost": false
    },
    "Image": "0d2200edc53e6e8a201d7d42ddcac3235bfb8655223ba1029d3e7765736f2a75",
    "NetworkSettings": {
        "IPAddress": "172.17.0.3",
        "IPPrefixLen": 16,
        "Gateway": "172.17.42.1",
        "Bridge": "docker0",
        "PortMapping": null,
        "Ports": {
            "6379/tcp": [
                {
                    "HostIp": "0.0.0.0",
                    "HostPort": "6379"
                }
            ]
        }
    },
    "ResolvConfPath": "/var/lib/docker/containers/4480d8897fdedc1f34f0460d3853e4cc696a3751891f5783ebc12c44d985e68e/resolv.conf",
    "HostnamePath": "/var/lib/docker/containers/4480d8897fdedc1f34f0460d3853e4cc696a3751891f5783ebc12c44d985e68e/hostname",
    "HostsPath": "/var/lib/docker/containers/4480d8897fdedc1f34f0460d3853e4cc696a3751891f5783ebc12c44d985e68e/hosts",
    "Name": "/redis_ambassador",
    "Driver": "aufs",
    "ExecDriver": "native-0.1",
    "Volumes": {},
    "VolumesRW": {},
    "HostConfig": {
        "Binds": null,
        "ContainerIDFile": "",
        "LxcConf": [],
        "Privileged": false,
        "PortBindings": {
            "6379/tcp": [
                {
                    "HostIp": "0.0.0.0",
                    "HostPort": "6379"
                }
            ]
        },
        "Links": null,
        "PublishAllPorts": false,
        "Dns": null,
        "DnsSearch": null,
        "VolumesFrom": null
    }
}]
iptables

paas@ubuntu:~$ sudo iptables -S
-P INPUT ACCEPT
-P FORWARD ACCEPT
-P OUTPUT ACCEPT
-A FORWARD -s 172.17.0.2/32 -d 172.17.0.3/32 -i docker0 -o docker0 -p tcp -m tcp --sport 6379 -j ACCEPT
-A FORWARD -s 172.17.0.3/32 -d 172.17.0.2/32 -i docker0 -o docker0 -p tcp -m tcp --dport 6379 -j ACCEPT
-A FORWARD -d 172.17.0.3/32 ! -i docker0 -o docker0 -p tcp -m tcp --dport 6379 -j ACCEPT
-A FORWARD -o docker0 -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -i docker0 ! -o docker0 -j ACCEPT
-A FORWARD -i docker0 -o docker0 -j ACCEPT

paas@ubuntu:~$ sudo iptables -S -t nat
-P PREROUTING ACCEPT
-P INPUT ACCEPT
-P OUTPUT ACCEPT
-P POSTROUTING ACCEPT
-N DOCKER
-A PREROUTING -m addrtype --dst-type LOCAL -j DOCKER
-A OUTPUT ! -d 127.0.0.0/8 -m addrtype --dst-type LOCAL -j DOCKER
-A POSTROUTING -s 172.17.0.0/16 ! -d 172.17.0.0/16 -j MASQUERADE
-A DOCKER -p tcp -m tcp --dport 6379 -j DNAT --to-destination 172.17.0.3:6379
socat

paas@ubuntu:~$ ps -ef | grep socat
root      3316   367  0 11:19 ?        00:00:00 /bin/sh -c env | grep _TCP= | sed 's/.*_PORT_\([0-9]*\)_TCP=tcp:\/\/\(.*\):\(.*\)/socat TCP4-LISTEN:\1,fork,reuseaddr TCP4:\2:\3 \&/'  | sh && top
root      3350  3316  0 11:19 ?        00:00:00 socat TCP4-LISTEN:6379,fork,reuseaddr TCP4:172.17.0.2:6379
paas      3376  2952  0 11:23 pts/4    00:00:00 grep --color=auto socat
端口监听

paas@ubuntu:~$ sudo netstat -nalp | grep 6379
tcp6       0      0 :::6379                 :::*                    LISTEN      367/docker
说明

docker run -d --link redis:redis --name redis_ambassador -p 6379:6379 svendowideit/ambassador中的-p 6379:6379导致增加了以下iptables规则

-A FORWARD -d 172.17.0.3/32 ! -i docker0 -o docker0 -p tcp -m tcp --dport 6379 -j ACCEPT
-A DOCKER -p tcp -m tcp --dport 6379 -j DNAT --to-destination 172.17.0.3:6379

本机的docker进程监听6379端口。

将访问本机6379端口的请求转发到172.17.0.3:6379，即socat进程（3350）。

docker run -d --link redis:redis --name redis_ambassador -p 6379:6379 svendowideit/ambassador中的--link redis:redis导致增加了以下iptables规则

-A FORWARD -s 172.17.0.2/32 -d 172.17.0.3/32 -i docker0 -o docker0 -p tcp -m tcp --sport 6379 -j ACCEPT
-A FORWARD -s 172.17.0.3/32 -d 172.17.0.2/32 -i docker0 -o docker0 -p tcp -m tcp --dport 6379 -j ACCEPT

socat TCP4-LISTEN:6379,fork,reuseaddr TCP4:172.17.0.2:6379

表示监听6379端口，从这个端口上收到的数据转发到172.17.0.2:6379

客户端

svendowideit/ambassador

启动容器

paas@ubuntu:~$ sudo docker run -d --name redis_ambassador --expose 6379 -e REDIS_PORT_6379_TCP=tcp://192.168.0.71:6379 svendowideit/ambassador
507652fd48d308ad2f7d3a206dacd899776c2458eb103112843b0419d175a75a
容器信息

paas@ubuntu:~$ sudo docker inspect 507652fd48d308ad2f7d3a206dacd899776c2458eb103112843b0419d175a75a
[{
    "ID": "507652fd48d308ad2f7d3a206dacd899776c2458eb103112843b0419d175a75a",
    "Created": "2014-05-08T05:00:21.137393924Z",
    "Path": "/bin/sh",
    "Args": [
        "-c",
        "env | grep _TCP= | sed 's/.*_PORT_\\([0-9]*\\)_TCP=tcp:\\/\\/\\(.*\\):\\(.*\\)/socat TCP4-LISTEN:\\1,fork,reuseaddr TCP4:\\2:\\3 \\\u0026/'  | sh \u0026\u0026 top"
    ],
    "Config": {
        "Hostname": "507652fd48d3",
        "Domainname": "",
        "User": "",
        "Memory": 0,
        "MemorySwap": 0,
        "CpuShares": 0,
        "AttachStdin": false,
        "AttachStdout": false,
        "AttachStderr": false,
        "PortSpecs": null,
        "ExposedPorts": {
            "6379/tcp": {}
        },
        "Tty": false,
        "OpenStdin": false,
        "StdinOnce": false,
        "Env": [
            "REDIS_PORT_6379_TCP=tcp://192.168.0.71:6379",
            "HOME=/",
            "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
        ],
        "Cmd": [
            "/bin/sh",
            "-c",
            "env | grep _TCP= | sed 's/.*_PORT_\\([0-9]*\\)_TCP=tcp:\\/\\/\\(.*\\):\\(.*\\)/socat TCP4-LISTEN:\\1,fork,reuseaddr TCP4:\\2:\\3 \\\u0026/'  | sh \u0026\u0026 top"
        ],
        "Image": "svendowideit/ambassador",
        "Volumes": null,
        "WorkingDir": "",
        "Entrypoint": null,
        "NetworkDisabled": false,
        "OnBuild": null
    },
    "State": {
        "Running": true,
        "Pid": 17124,
        "ExitCode": 0,
        "StartedAt": "2014-05-08T05:00:21.172505186Z",
        "FinishedAt": "0001-01-01T00:00:00Z",
        "Ghost": false
    },
    "Image": "0d2200edc53e6e8a201d7d42ddcac3235bfb8655223ba1029d3e7765736f2a75",
    "NetworkSettings": {
        "IPAddress": "172.17.0.2",
        "IPPrefixLen": 16,
        "Gateway": "172.17.42.1",
        "Bridge": "docker0",
        "PortMapping": null,
        "Ports": {
            "6379/tcp": null
        }
    },
    "ResolvConfPath": "/etc/resolv.conf",
    "HostnamePath": "/var/lib/docker/containers/507652fd48d308ad2f7d3a206dacd899776c2458eb103112843b0419d175a75a/hostname",
    "HostsPath": "/var/lib/docker/containers/507652fd48d308ad2f7d3a206dacd899776c2458eb103112843b0419d175a75a/hosts",
    "Name": "/redis_ambassador",
    "Driver": "aufs",
    "ExecDriver": "native-0.1",
    "Volumes": {},
    "VolumesRW": {},
    "HostConfig": {
        "Binds": null,
        "ContainerIDFile": "",
        "LxcConf": [],
        "Privileged": false,
        "PortBindings": {
            "6379/tcp": null
        },
        "Links": null,
        "PublishAllPorts": false,
        "Dns": null,
        "DnsSearch": null,
        "VolumesFrom": null
    }
}]
iptables
paas@ubuntu:~$ sudo iptables -S
-P INPUT ACCEPT
-P FORWARD ACCEPT
-P OUTPUT ACCEPT
-A FORWARD -o docker0 -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -i docker0 ! -o docker0 -j ACCEPT
-A FORWARD -i docker0 -o docker0 -j ACCEPT
paas@ubuntu:~$ sudo iptables -S -t nat
-P PREROUTING ACCEPT
-P INPUT ACCEPT
-P OUTPUT ACCEPT
-P POSTROUTING ACCEPT
-N DOCKER
-A PREROUTING -m addrtype --dst-type LOCAL -j DOCKER
-A OUTPUT ! -d 127.0.0.0/8 -m addrtype --dst-type LOCAL -j DOCKER
-A POSTROUTING -s 172.17.0.0/16 ! -d 172.17.0.0/16 -j MASQUERADE
socat
paas@ubuntu:~$ ps -ef | grep socat
root     17124  1789  0 13:00 ?        00:00:00 /bin/sh -c env | grep _TCP= | sed 's/.*_PORT_\([0-9]*\)_TCP=tcp:\/\/\(.*\):\(.*\)/socat TCP4-LISTEN:\1,fork,reuseaddr TCP4:\2:\3 \&/'  | sh && top
root     17146 17124  0 13:00 ?        00:00:00 socat TCP4-LISTEN:6379,fork,reuseaddr TCP4:192.168.0.71:6379
paas     17164 16719  0 13:03 pts/1    00:00:00 grep --color=auto socat

进程环境变量


sudo cat /proc/17146/environ
HOSTNAME=507652fd48d3
HOME=/
REDIS_PORT_6379_TCP=tcp://192.168.0.71:6379
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
PWD=/
说明

socat TCP4-LISTEN:6379,fork,reuseaddr TCP4:192.168.0.71:6379 socat监听6379端口，将收到的数据转发到192.168.0.71:6379。是在容器中监听，在本机上是看不到有进程监听6379端口的，需要后续的iptables的nat规则配合，socat才能收到数据。

sudo docker run -d --name redis_ambassador --expose 6379 -e REDIS_PORT_6379_TCP=tcp://192.168.0.71:6379 svendowideit/ambassador中，--expose 6379表示允许link到此容器的容器访问6379端口，-e REDIS_PORT_6379_TCP=tcp://192.168.0.71:6379为设置访问redis的环境变量。

REDIS_PORT_6379_TCP这个环境变量的格式是固定的，socat命令需要从中获取监听端口，如果改成例如REDIS_PORT_1234_TCP，那么此容器中的socat命令将会变成socat TCP4-LISTEN:1234,fork,reuseaddr TCP4:192.168.0.71:6379

redis-cli

启动容器
paas@ubuntu:~$ sudo docker run -i -t --rm --link redis_ambassador:redis relateiq/redis-cli
redis 172.17.0.2:6379>
容器信息
paas@ubuntu:~$ sudo docker inspect 043ab6f80052
[{
    "ID": "043ab6f80052b14fad50ec4d01f3b550b4b8a1aa053394dc314c86b50443b6e3",
    "Created": "2014-05-08T05:48:31.872337998Z",
    "Path": "/start.sh",
    "Args": [],
    "Config": {
        "Hostname": "043ab6f80052",
        "Domainname": "",
        "User": "",
        "Memory": 0,
        "MemorySwap": 0,
        "CpuShares": 0,
        "AttachStdin": true,
        "AttachStdout": true,
        "AttachStderr": true,
        "PortSpecs": null,
        "ExposedPorts": {},
        "Tty": true,
        "OpenStdin": true,
        "StdinOnce": true,
        "Env": [
            "HOME=/",
            "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
        ],
        "Cmd": null,
        "Image": "relateiq/redis-cli",
        "Volumes": {},
        "WorkingDir": "",
        "Entrypoint": [
            "/start.sh"
        ],
        "NetworkDisabled": false,
        "OnBuild": null
    },
    "State": {
        "Running": true,
        "Pid": 17373,
        "ExitCode": 0,
        "StartedAt": "2014-05-08T05:48:31.965979461Z",
        "FinishedAt": "0001-01-01T00:00:00Z",
        "Ghost": false
    },
    "Image": "616b2d3dc69c2b655e2a0b98dca40bbc629c5c812eb5b62afcd8311dace6feb0",
    "NetworkSettings": {
        "IPAddress": "172.17.0.3",
        "IPPrefixLen": 16,
        "Gateway": "172.17.42.1",
        "Bridge": "docker0",
        "PortMapping": null,
        "Ports": {}
    },
    "ResolvConfPath": "/etc/resolv.conf",
    "HostnamePath": "/var/lib/docker/containers/043ab6f80052b14fad50ec4d01f3b550b4b8a1aa053394dc314c86b50443b6e3/hostname",
    "HostsPath": "/var/lib/docker/containers/043ab6f80052b14fad50ec4d01f3b550b4b8a1aa053394dc314c86b50443b6e3/hosts",
    "Name": "/mad_hoover",
    "Driver": "aufs",
    "ExecDriver": "native-0.1",
    "Volumes": {},
    "VolumesRW": {},
    "HostConfig": {
        "Binds": null,
        "ContainerIDFile": "",
        "LxcConf": [],
        "Privileged": false,
        "PortBindings": {},
        "Links": null,
        "PublishAllPorts": false,
        "Dns": null,
        "DnsSearch": null,
        "VolumesFrom": null
    }
}]

可以看到这个容器中执行的是start.sh。start.sh内容如下


#!/bin/bash

if [ $# -eq 0 ]; then
        /redis/src/redis-cli -h $REDIS_PORT_6379_TCP_ADDR -p $REDIS_PORT_6379_TCP_PORT
else
        /redis/src/redis-cli "$@"
fi

start.sh进程的环境变量

sudo cat /proc/17373/environ
HOME=/
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
HOSTNAME=043ab6f80052
TERM=xterm
REDIS_PORT=tcp://172.17.0.2:6379
REDIS_PORT_6379_TCP=tcp://172.17.0.2:6379
REDIS_PORT_6379_TCP_ADDR=172.17.0.2
REDIS_PORT_6379_TCP_PORT=6379
REDIS_PORT_6379_TCP_PROTO=tcp
REDIS_NAME=/mad_hoover/redis
REDIS_ENV_REDIS_PORT_6379_TCP=tcp://192.168.0.71:6379
iptables
paas@ubuntu:~$ sudo iptables -S
-P INPUT ACCEPT
-P FORWARD ACCEPT
-P OUTPUT ACCEPT
-A FORWARD -s 172.17.0.2/32 -d 172.17.0.3/32 -i docker0 -o docker0 -p tcp -m tcp --sport 6379 -j ACCEPT
-A FORWARD -s 172.17.0.3/32 -d 172.17.0.2/32 -i docker0 -o docker0 -p tcp -m tcp --dport 6379 -j ACCEPT
-A FORWARD -o docker0 -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -i docker0 ! -o docker0 -j ACCEPT
-A FORWARD -i docker0 -o docker0 -j ACCEPT
paas@ubuntu:~$ sudo iptables -S -t nat
-P PREROUTING ACCEPT
-P INPUT ACCEPT
-P OUTPUT ACCEPT
-P POSTROUTING ACCEPT
-N DOCKER
-A PREROUTING -m addrtype --dst-type LOCAL -j DOCKER
-A OUTPUT ! -d 127.0.0.0/8 -m addrtype --dst-type LOCAL -j DOCKER
-A POSTROUTING -s 172.17.0.0/16 ! -d 172.17.0.0/16 -j MASQUERADE
说明

以下环境变量的存在是sudo docker run -i -t --rm --link redis_ambassador:redis relateiq/redis-cli命令中的--link redis_ambassador:redis部分与sudo docker run -d --name redis_ambassador --expose 6379 -e REDIS_PORT_6379_TCP=tcp://192.168.0.71:6379 svendowideit/ambassador命令中的--expose 6379部分功能导致的，本质上是因为link存在expose的端口。详情可以参考docker之容器连接实现中关于links的代码。如果在sudo docker run -d --name redis_ambassador --expose 6379 -e REDIS_PORT_6379_TCP=tcp://192.168.0.71:6379 svendowideit/ambassador命令中去掉--expose 6379，则redis-cli进程会因为没有相应的环境变量而无法连接服务器。

REDIS_PORT=tcp://172.17.0.2:6379
REDIS_PORT_6379_TCP=tcp://172.17.0.2:6379
REDIS_PORT_6379_TCP_ADDR=172.17.0.2
REDIS_PORT_6379_TCP_PORT=6379
REDIS_PORT_6379_TCP_PROTO=tcp

总结

从上面的过程跟踪可以了解以redis为例的docker容克跨主机连接实现方式，如果要解决其他服务的跨主机连接可以仿照此方式实现。主要是制作相应的ambassador容器（使用socat），确定环境变量规则，以及编写简单的客户端脚本。

其他

docker之容器连接实现

官方文档

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