# 扁平网络 Flannel 如果你使用 [kubernetes-vagrant-centos-cluster](https://github.com/rootsongjc/kubernetes-vagrant-centos-cluster) 安装了拥有三个节点的 Kubernetes 集群，节点的状态如下所述。 ```bash [root@node1 ~]# kubectl get nodes -o wide NAME STATUS ROLES AGE VERSION EXTERNAL-IP OS-IMAGE KERNEL-VERSION CONTAINER-RUNTIME node1 Ready 2d v1.9.1 CentOS Linux 7 (Core) 3.10.0-693.11.6.el7.x86_64 docker://1.12.6 node2 Ready 2d v1.9.1 CentOS Linux 7 (Core) 3.10.0-693.11.6.el7.x86_64 docker://1.12.6 node3 Ready 2d v1.9.1 CentOS Linux 7 (Core) 3.10.0-693.11.6.el7.x86_64 docker://1.12.6 ``` 当前 Kubernetes 集群中运行的所有 Pod 信息： ```bash [root@node1 ~]# kubectl get pods --all-namespaces -o wide NAMESPACE NAME READY STATUS RESTARTS AGE IP NODE kube-system coredns-5984fb8cbb-sjqv9 1/1 Running 0 1h 172.33.68.2 node1 kube-system coredns-5984fb8cbb-tkfrc 1/1 Running 1 1h 172.33.96.3 node3 kube-system heapster-v1.5.0-684c7f9488-z6sdz 4/4 Running 0 1h 172.33.31.3 node2 kube-system kubernetes-dashboard-6b66b8b96c-mnm2c 1/1 Running 0 1h 172.33.31.2 node2 kube-system monitoring-influxdb-grafana-v4-54b7854697-tw9cd 2/2 Running 2 1h 172.33.96.2 node3 ``` 当前 etcd 中的注册的宿主机的 pod 地址网段信息： ```bash [root@node1 ~]# etcdctl ls /kube-centos/network/subnets /kube-centos/network/subnets/172.33.68.0-24 /kube-centos/network/subnets/172.33.31.0-24 /kube-centos/network/subnets/172.33.96.0-24 ``` 而每个 node 上的 Pod 子网是根据我们在安装 flannel 时配置来划分的，在 etcd 中查看该配置： ```bash [root@node1 ~]# etcdctl get /kube-centos/network/config {"Network":"172.33.0.0/16","SubnetLen":24,"Backend":{"Type":"host-gw"}} ``` 我们知道 Kubernetes 集群内部存在三类 IP，分别是： - Node IP：宿主机的 IP 地址 - Pod IP：使用网络插件创建的 IP（如 flannel），使跨主机的 Pod 可以互通 - Cluster IP：虚拟 IP，通过 iptables 规则访问服务 在安装 node 节点的时候，节点上的进程是按照 flannel -> docker -> kubelet -> kube-proxy 的顺序启动的，我们下面也会按照该顺序来讲解，flannel 的网络划分和如何与 docker 交互，如何通过 iptables 访问 service。 ### Flannel Flannel 是作为一个二进制文件的方式部署在每个 node 上，主要实现两个功能： - 为每个 node 分配 subnet，容器将自动从该子网中获取 IP 地址 - 当有 node 加入到网络中时，为每个 node 增加路由配置 下面是使用 `host-gw` backend 的 flannel 网络架构图： ![flannel 网络架构（图片来自 openshift）](../images/flannel-networking.png) **注意**：以上 IP 非本示例中的 IP，但是不影响读者理解。 Node1 上的 flannel 配置如下: ```bash [root@node1 ~]# cat /usr/lib/systemd/system/flanneld.service [Unit] Description=Flanneld overlay address etcd agent After=network.target After=network-online.target Wants=network-online.target After=etcd.service Before=docker.service [Service] Type=notify EnvironmentFile=/etc/sysconfig/flanneld EnvironmentFile=-/etc/sysconfig/docker-network ExecStart=/usr/bin/flanneld-start $FLANNEL_OPTIONS ExecStartPost=/usr/libexec/flannel/mk-docker-opts.sh -k DOCKER_NETWORK_OPTIONS -d /run/flannel/docker Restart=on-failure [Install] WantedBy=multi-user.target RequiredBy=docker.service ``` 其中有两个环境变量文件的配置如下： ```bash [root@node1 ~]# cat /etc/sysconfig/flanneld # Flanneld configuration options FLANNEL_ETCD_ENDPOINTS="http://172.17.8.101:2379" FLANNEL_ETCD_PREFIX="/kube-centos/network" FLANNEL_OPTIONS="-iface=eth2" ``` 上面的配置文件仅供 flanneld 使用。 ```bash [root@node1 ~]# cat /etc/sysconfig/docker-network # /etc/sysconfig/docker-network DOCKER_NETWORK_OPTIONS= ``` 还有一个`ExecStartPost=/usr/libexec/flannel/mk-docker-opts.sh -k DOCKER_NETWORK_OPTIONS -d /run/flannel/docker`，其中的`/usr/libexec/flannel/mk-docker-opts.sh` 脚本是在 flanneld 启动后运行，将会生成两个环境变量配置文件： - /run/flannel/docker - /run/flannel/subnet.env 我们再来看下 `/run/flannel/docker` 的配置。 ```bash [root@node1 ~]# cat /run/flannel/docker DOCKER_OPT_BIP="--bip=172.33.68.1/24" DOCKER_OPT_IPMASQ="--ip-masq=true" DOCKER_OPT_MTU="--mtu=1500" DOCKER_NETWORK_OPTIONS="--bip=172.33.68.1/24 --ip-masq=true --mtu=1500" ``` 如果你使用`systemctl` 命令先启动 flannel 后启动 docker 的话，docker 将会读取以上环境变量。 我们再来看下 `/run/flannel/subnet.env` 的配置。 ```bash [root@node1 ~]# cat /run/flannel/subnet.env FLANNEL_NETWORK=172.33.0.0/16 FLANNEL_SUBNET=172.33.68.1/24 FLANNEL_MTU=1500 FLANNEL_IPMASQ=false ``` 以上环境变量是 flannel 向 etcd 中注册的。 ### Docker Node1 的 docker 配置如下： ```bash [root@node1 ~]# cat /usr/lib/systemd/system/docker.service [Unit] Description=Docker Application Container Engine Documentation=http://docs.docker.com After=network.target rhel-push-plugin.socket registries.service Wants=docker-storage-setup.service Requires=docker-cleanup.timer [Service] Type=notify NotifyAccess=all EnvironmentFile=-/run/containers/registries.conf EnvironmentFile=-/etc/sysconfig/docker EnvironmentFile=-/etc/sysconfig/docker-storage EnvironmentFile=-/etc/sysconfig/docker-network Environment=GOTRACEBACK=crash Environment=DOCKER_HTTP_HOST_COMPAT=1 Environment=PATH=/usr/libexec/docker:/usr/bin:/usr/sbin ExecStart=/usr/bin/dockerd-current \ --add-runtime docker-runc=/usr/libexec/docker/docker-runc-current \ --default-runtime=docker-runc \ --exec-opt native.cgroupdriver=systemd \ --userland-proxy-path=/usr/libexec/docker/docker-proxy-current \ $OPTIONS \ $DOCKER_STORAGE_OPTIONS \ $DOCKER_NETWORK_OPTIONS \ $ADD_REGISTRY \ $BLOCK_REGISTRY \ $INSECURE_REGISTRY\ $REGISTRIES ExecReload=/bin/kill -s HUP $MAINPID LimitNOFILE=1048576 LimitNPROC=1048576 LimitCORE=infinity TimeoutStartSec=0 Restart=on-abnormal MountFlags=slave KillMode=process [Install] WantedBy=multi-user.target ``` 查看 Node1 上的 docker 启动参数： ```bash [root@node1 ~]# systemctl status -l docker ● docker.service - Docker Application Container Engine Loaded: loaded (/usr/lib/systemd/system/docker.service; enabled; vendor preset: disabled) Drop-In: /usr/lib/systemd/system/docker.service.d └─flannel.conf Active: active (running) since Fri 2018-02-02 22:52:43 CST; 2h 28min ago Docs: http://docs.docker.com Main PID: 4334 (dockerd-current) CGroup: /system.slice/docker.service ‣ 4334 /usr/bin/dockerd-current --add-runtime docker-runc=/usr/libexec/docker/docker-runc-current --default-runtime=docker-runc --exec-opt native.cgroupdriver=systemd --userland-proxy-path=/usr/libexec/docker/docker-proxy-current --selinux-enabled --log-driver=journald --signature-verification=false --bip=172.33.68.1/24 --ip-masq=true --mtu=1500 ``` 我们可以看到在 docker 在启动时有如下参数：`--bip=172.33.68.1/24 --ip-masq=true --mtu=1500`。上述参数 flannel 启动时运行的脚本生成的，通过环境变量传递过来的。 我们查看下 node1 宿主机上的网络接口： ```bash [root@node1 ~]# ip addr 1: lo: mtu 65536 qdisc noqueue state UNKNOWN qlen 1 link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00 inet 127.0.0.1/8 scope host lo valid_lft forever preferred_lft forever inet6 ::1/128 scope host valid_lft forever preferred_lft forever 2: eth0: mtu 1500 qdisc pfifo_fast state UP qlen 1000 link/ether 52:54:00:00:57:32 brd ff:ff:ff:ff:ff:ff inet 10.0.2.15/24 brd 10.0.2.255 scope global dynamic eth0 valid_lft 85095sec preferred_lft 85095sec inet6 fe80::5054:ff:fe00:5732/64 scope link valid_lft forever preferred_lft forever 3: eth1: mtu 1500 qdisc pfifo_fast state UP qlen 1000 link/ether 08:00:27:7b:0f:b1 brd ff:ff:ff:ff:ff:ff inet 172.17.8.101/24 brd 172.17.8.255 scope global eth1 valid_lft forever preferred_lft forever 4: eth2: mtu 1500 qdisc pfifo_fast state UP qlen 1000 link/ether 08:00:27:ef:25:06 brd ff:ff:ff:ff:ff:ff inet 172.30.113.231/21 brd 172.30.119.255 scope global dynamic eth2 valid_lft 85096sec preferred_lft 85096sec inet6 fe80::a00:27ff:feef:2506/64 scope link valid_lft forever preferred_lft forever 5: docker0: mtu 1500 qdisc noqueue state UP link/ether 02:42:d0:ae:80:ea brd ff:ff:ff:ff:ff:ff inet 172.33.68.1/24 scope global docker0 valid_lft forever preferred_lft forever inet6 fe80::42:d0ff:feae:80ea/64 scope link valid_lft forever preferred_lft forever 7: veth295bef2@if6: mtu 1500 qdisc noqueue master docker0 state UP link/ether 6a:72:d7:9f:29:19 brd ff:ff:ff:ff:ff:ff link-netnsid 0 inet6 fe80::6872:d7ff:fe9f:2919/64 scope link valid_lft forever preferred_lft forever ``` 我们分类来解释下该虚拟机中的网络接口。 - lo：回环网络，127.0.0.1 - eth0：NAT 网络，虚拟机创建时自动分配，仅可以在几台虚拟机之间访问 - eth1：bridge 网络，使用 vagrant 分配给虚拟机的地址，虚拟机之间和本地电脑都可以访问 - eth2：bridge 网络，使用 DHCP 分配，用于访问互联网的网卡 - docker0：bridge 网络，docker 默认使用的网卡，作为该节点上所有容器的虚拟交换机 - veth295bef2@if6：veth pair，连接 docker0 和 Pod 中的容器。veth pair 可以理解为使用网线连接好的两个接口，把两个端口放到两个 namespace 中，那么这两个 namespace 就能打通。参考 [linux 网络虚拟化： network namespace 简介](http://cizixs.com/2017/02/10/network-virtualization-network-namespace)。 我们再看下该节点的 docker 上有哪些网络。 ```bash [root@node1 ~]# docker network ls NETWORK ID NAME DRIVER SCOPE 940bb75e653b bridge bridge local d94c046e105d host host local 2db7597fd546 none null local ``` 再检查下 bridge 网络`940bb75e653b`的信息。 ```bash [root@node1 ~]# docker network inspect 940bb75e653b [ { "Name": "bridge", "Id": "940bb75e653bfa10dab4cce8813c2b3ce17501e4e4935f7dc13805a61b732d2c", "Scope": "local", "Driver": "bridge", "EnableIPv6": false, "IPAM": { "Driver": "default", "Options": null, "Config": [ { "Subnet": "172.33.68.1/24", "Gateway": "172.33.68.1" } ] }, "Internal": false, "Containers": { "944d4aa660e30e1be9a18d30c9dcfa3b0504d1e5dbd00f3004b76582f1c9a85b": { "Name": "k8s_POD_coredns-5984fb8cbb-sjqv9_kube-system_c5a2e959-082a-11e8-b4cd-525400005732_0", "EndpointID": "7397d7282e464fc4ec5756d6b328df889cdf46134dbbe3753517e175d3844a85", "MacAddress": "02:42:ac:21:44:02", "IPv4Address": "172.33.68.2/24", "IPv6Address": "" } }, "Options": { "com.docker.network.bridge.default_bridge": "true", "com.docker.network.bridge.enable_icc": "true", "com.docker.network.bridge.enable_ip_masquerade": "true", "com.docker.network.bridge.host_binding_ipv4": "0.0.0.0", "com.docker.network.bridge.name": "docker0", "com.docker.network.driver.mtu": "1500" }, "Labels": {}} ] ``` 我们可以看到该网络中的`Config` 与 docker 的启动配置相符。 Node1 上运行的容器： ```bash [root@node1 ~]# docker ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES a37407a234dd docker.io/coredns/coredns@sha256:adf2e5b4504ef9ffa43f16010bd064273338759e92f6f616dd159115748799bc "/coredns -conf /etc/" About an hour ago Up About an hour k8s_coredns_coredns-5984fb8cbb-sjqv9_kube-system_c5a2e959-082a-11e8-b4cd-525400005732_0 944d4aa660e3 docker.io/openshift/origin-pod "/usr/bin/pod" About an hour ago Up About an hour k8s_POD_coredns-5984fb8cbb-sjqv9_kube-system_c5a2e959-082a-11e8-b4cd-525400005732_0 ``` 我们可以看到当前已经有 2 个容器在运行。 Node1 上的路由信息： ```bash [root@node1 ~]# route -n Kernel IP routing table Destination Gateway Genmask Flags Metric Ref Use Iface 0.0.0.0 10.0.2.2 0.0.0.0 UG 100 0 0 eth0 0.0.0.0 172.30.116.1 0.0.0.0 UG 101 0 0 eth2 10.0.2.0 0.0.0.0 255.255.255.0 U 100 0 0 eth0 172.17.8.0 0.0.0.0 255.255.255.0 U 100 0 0 eth1 172.30.112.0 0.0.0.0 255.255.248.0 U 100 0 0 eth2 172.33.68.0 0.0.0.0 255.255.255.0 U 0 0 0 docker0 172.33.96.0 172.30.118.65 255.255.255.0 UG 0 0 0 eth2 ``` 以上路由信息是由 flannel 添加的，当有新的节点加入到 Kubernetes 集群中后，每个节点上的路由表都将增加。 我们在 node 上来 `traceroute` 下 node3 上的 `coredns-5984fb8cbb-tkfrc` 容器，其 IP 地址是 `172.33.96.3`，看看其路由信息。 ```bash [root@node1 ~]# traceroute 172.33.96.3 traceroute to 172.33.96.3 (172.33.96.3), 30 hops max, 60 byte packets 1 172.30.118.65 (172.30.118.65) 0.518 ms 0.367 ms 0.398 ms 2 172.33.96.3 (172.33.96.3) 0.451 ms 0.352 ms 0.223 ms ``` 我们看到路由直接经过 node3 的公网 IP 后就到达了 node3 节点上的 Pod。 Node1 的 iptables 信息： ```bash [root@node1 ~]# iptables -L Chain INPUT (policy ACCEPT) target prot opt source destination KUBE-FIREWALL all -- anywhere anywhere KUBE-SERVICES all -- anywhere anywhere /* kubernetes service portals */ Chain FORWARD (policy ACCEPT) target prot opt source destination KUBE-FORWARD all -- anywhere anywhere /* kubernetes forward rules */ DOCKER-ISOLATION all -- anywhere anywhere DOCKER all -- anywhere anywhere ACCEPT all -- anywhere anywhere ctstate RELATED,ESTABLISHED ACCEPT all -- anywhere anywhere ACCEPT all -- anywhere anywhere Chain OUTPUT (policy ACCEPT) target prot opt source destination KUBE-FIREWALL all -- anywhere anywhere KUBE-SERVICES all -- anywhere anywhere /* kubernetes service portals */ Chain DOCKER (1 references) target prot opt source destination Chain DOCKER-ISOLATION (1 references) target prot opt source destination RETURN all -- anywhere anywhere Chain KUBE-FIREWALL (2 references) target prot opt source destination DROP all -- anywhere anywhere /* kubernetes firewall for dropping marked packets */mark match 0x8000/0x8000 Chain KUBE-FORWARD (1 references) target prot opt source destination ACCEPT all -- anywhere anywhere /* kubernetes forwarding rules */mark match 0x4000/0x4000 ACCEPT all -- 10.254.0.0/16 anywhere /* kubernetes forwarding conntrack pod source rule */ctstate RELATED,ESTABLISHED ACCEPT all -- anywhere 10.254.0.0/16 /* kubernetes forwarding conntrack pod destination rule */ctstate RELATED,ESTABLISHED Chain KUBE-SERVICES (2 references) target prot opt source destination ``` 从上面的 iptables 中可以看到注入了很多 Kuberentes service 的规则。 ## 参考 - [coreos/flannel - github.com](https://github.com/coreos/flannel) - [Linux 网络虚拟化：network namespace 简介 - cizixs.com](http://cizixs.com/2017/02/10/network-virtualization-network-namespace) - [Linux 虚拟网络设备之 veth - segmentfault.com](https://segmentfault.com/a/1190000009251098) - [flannel host-gw network - hustcat.github.io](http://hustcat.github.io/flannel-host-gw-network/) - [flannel - openshift.com](https://docs.openshift.com/container-platform/3.4/architecture/additional_concepts/flannel.html)