Docker Daemon生产环境配置提到了MTU设置，但是这只是针对于名为bridge的docker bridge network，对于overlay network是无效的。

如果docker host machine的网卡MTU为1500，则不需要此步骤

设置ingress和docker_gwbridge的MTU

以下步骤得在swarm init或join之前做

假设你有三个机器，manager、worker-1、worker-2，准备搞一个Docker swarm集群

1) [manager] docker swarm init

2) [manager] 获得docker_gwbridge的参数，注意Subnet

$ docker network inspect docker_gwbridge
[
    {
        "Name": "docker_gwbridge",
        ...
        "IPAM": {
            ...
            "Config": [
                {
                    "Subnet": "172.18.0.0/16",
                    ...
                }
            ]
        },
        ...
    }
]

3) [manager] docker swarm leave --force

4) [manager] 停掉docker sudo systemctl stop docker.service

5) [manager] 删掉虚拟网卡docker_gwbridge

$ sudo ip link set docker_gwbridge down
$ sudo ip link del dev docker_gwbridge

6) [manager] 启动docker sudo systemctl start docker.service

7) [manager] 重建docker_gwbridge，

记得设置之前得到的Subnet参数和正确的MTU值

$ docker network rm docker_gwbridge
$ docker network create \
  --subnet 172.18.0.0/16 \
  --opt com.docker.network.bridge.name=docker_gwbridge \
  --opt com.docker.network.bridge.enable_icc=false \
  --opt com.docker.network.bridge.enable_ip_masquerade=true \
  --opt com.docker.network.driver.mtu=1450 \
  docker_gwbridge

再到worker-1和worker-2上执行相同的命令。

8) [manager] docker swarm init

9) [manager] 先观察ingress network的参数，注意Subnet和Gateway：

$ docker network inspect ingress
[
    {
        "Name": "ingress",
        ...
        "IPAM": {
            ...
            "Config": [
                {
                    "Subnet": "10.255.0.0/16",
                    "Gateway": "10.255.0.1"
                }
            ]
        },
        ...
    }
]

10) [manager] 删除ingress network，docker network rm ingress。

11) [manager] 重新创建ingress network，记得填写之前得到的Subnet和Gateway，以及正确的MTU值：

$ docker network create \
  --driver overlay \
  --ingress \
  --subnet=10.255.0.0/16 \
  --gateway=10.255.0.1 \
  --opt com.docker.network.driver.mtu=1450 \
  ingress

12) [worker-1] [worker-2] join docker swarm join ...

注意：新机器在join到swarm之前，得先执行第7步

验证：

1) 启动一个swarm service，docker service create -td --name busybox busybox

2) 观察虚拟网卡

发现MTU都是1450：

$ ip link
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: ens3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1450 qdisc pfifo_fast state UP mode DEFAULT group default qlen 1000
    link/ether fa:16:3e:71:09:f5 brd ff:ff:ff:ff:ff:ff
3: docker0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1450 qdisc noqueue state UP mode DEFAULT group default
    link/ether 02:42:6b:de:95:71 brd ff:ff:ff:ff:ff:ff
298: docker_gwbridge: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1450 qdisc noqueue state UP mode DEFAULT group default
    link/ether 02:42:ae:7b:cd:b4 brd ff:ff:ff:ff:ff:ff
309: veth7e0f9e5@if308: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1450 qdisc noqueue master docker_gwbridge state UP mode DEFAULT group default
    link/ether 16:ca:8f:c7:d3:7f brd ff:ff:ff:ff:ff:ff link-netnsid 1
311: vethcb94fec@if310: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1450 qdisc noqueue master docker0 state UP mode DEFAULT group default
    link/ether 9a:aa:de:7b:4f:d4 brd ff:ff:ff:ff:ff:ff link-netnsid 2

3) 观察容器内网卡

网卡MTU也是1450：

$ docker exec b.1.pdsdgghzyy5rhqkk5et59qa3o ip link
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue qlen 1
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
310: eth0@if311: <BROADCAST,MULTICAST,UP,LOWER_UP,M-DOWN> mtu 1450 qdisc noqueue
    link/ether 02:42:ac:11:00:02 brd ff:ff:ff:ff:ff:ff

自建overlay network的MTU

方法一：在docker compose file设置

...

networks:                                
  my-overlay:                               
    driver: bridge                       
    driver_opts:                         
      com.docker.network.driver.mtu: 1450

不过这样不好，因为这样就把docker compose file的内容和生产环境绑定了，换了个环境这个MTU值未必合适。

方法二：外部创建时设置

docker network create \
  -d overlay \
  --opt com.docker.network.driver.mtu=1450 \
  --attachable \
  my-overlay

用法：

  1. 在docker compose file里这样用：

    ...
    
networks:
  app-net:
    external: true
    name: my-overlay

  2. docker run --network my-overlay ...

  3. docker service create --network my-overlay ...

参考资料