我已经在 centos 机器之间配置了 GRE 隧道，并在各个 centos 机器上配置了相应的路由表，如下图所示：
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路由器 1-------gre1---------中转路由器---------gre2--------路由器 2
10.2.32.0/24--路由器1--10.0.0.1---gre1---10.0.0.2--中转路由器--11.0.0.2---gre2--11.0.0.1--路由器2--10.4.32.0/24
我能够从 Router-1 Ping 到 gre1 隧道的另一端：
worker]# ping 10.0.0.2
PING 10.0.0.2 (10.0.0.2) 56(84) bytes of data.
64 bytes from 10.0.0.2: icmp_seq=1 ttl=64 time=1.43 ms
64 bytes from 10.0.0.2: icmp_seq=2 ttl=64 time=0.472 ms
64 bytes from 10.0.0.2: icmp_seq=3 ttl=64 time=0.291 ms
64 bytes from 10.0.0.2: icmp_seq=4 ttl=64 time=0.319 ms
流量通过 GRE 隧道到达中转路由器（这已通过 tcpdump proto gre 验证）
从 Router-2 对 gre2 隧道的另一端执行 ping 操作：
worker]# ping 11.0.0.2
PING 11.0.0.2 (11.0.0.2) 56(84) bytes of data.
64 bytes from 11.0.0.2: icmp_seq=1 ttl=64 time=1.10 ms
64 bytes from 11.0.0.2: icmp_seq=2 ttl=64 time=0.392 ms
64 bytes from 11.0.0.2: icmp_seq=3 ttl=64 time=0.369 ms
64 bytes from 11.0.0.2: icmp_seq=4 ttl=64 time=0.258 ms
这条交通也在隧道上流动
在中转路由器上，添加路由条目后，我可以 ping 通路由器 1 和路由器 2 的私有地址：中转路由器：
[root@vmc-centos conf]# ping 10.2.32.1
PING 10.2.32.1 (10.2.32.1) 56(84) bytes of data.
64 bytes from 10.2.32.1: icmp_seq=1 ttl=64 time=0.589 ms
64 bytes from 10.2.32.1: icmp_seq=2 ttl=64 time=0.380 ms
64 bytes from 10.2.32.1: icmp_seq=3 ttl=64 time=0.383 ms
路由器 1：
worker]# tcpdump -i any proto gre -n
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on any, link-type LINUX_SLL (Linux cooked), capture size 65535 bytes
04:54:36.684864 IP 10.206.83.3 > 10.206.90.103: GREv0, length 88: IP 10.0.0.2 > 10.2.32.1: ICMP echo request, id 20445, seq 34, length 64
04:54:36.684951 IP 10.206.90.103 > 10.206.83.3: GREv0, length 88: IP 10.2.32.1 > 10.0.0.2: ICMP echo reply, id 20445, seq 34, length 64
04:54:37.684776 IP 10.206.83.3 > 10.206.90.103: GREv0, length 88: IP 10.0.0.2 > 10.2.32.1: ICMP echo request, id 20445, seq 35, length 64
中转路由器：
[root@vmc-centos conf]# ping 10.4.32.1
PING 10.4.32.1 (10.4.32.1) 56(84) bytes of data.
64 bytes from 10.4.32.1: icmp_seq=1 ttl=64 time=0.553 ms
64 bytes from 10.4.32.1: icmp_seq=2 ttl=64 time=0.325 ms
64 bytes from 10.4.32.1: icmp_seq=3 ttl=64 time=0.354 ms
路由器 2：
worker]# sudo tcpdump -i any proto gre -n
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on any, link-type LINUX_SLL (Linux cooked), capture size 65535 bytes
04:56:57.549823 IP 10.206.83.3 > 10.206.86.199: GREv0, length 88: IP 11.0.0.2 > 10.4.32.1: ICMP echo request, id 20690, seq 24, length 64
04:56:57.549896 IP 10.206.86.199 > 10.206.83.3: GREv0, length 88: IP 10.4.32.1 > 11.0.0.2: ICMP echo reply, id 20690, seq 24, length 64
但是现在，当我尝试从 Router-1 访问 Router-2（10.4.32.1）的专用网络时，数据包到达中转路由器，但不会从那里转发到 Router-2：Router-1：
worker]# ping 10.4.32.1
PING 10.4.32.1 (10.4.32.1) 56(84) bytes of data.
中转路由器：
[root@vmc-centos conf]# tcpdump -i any proto gre -n
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on any, link-type LINUX_SLL (Linux cooked), capture size 262144 bytes
04:59:06.382024 IP 10.206.90.103 > 10.206.83.3: GREv0, length 88: IP 10.0.0.1 > 10.4.32.1: ICMP echo request, id 36131, seq 40, length 64
04:59:07.382007 IP 10.206.90.103 > 10.206.83.3: GREv0, length 88: IP 10.0.0.1 > 10.4.32.1: ICMP echo request, id 36131, seq 41, length 64
路由器 2：
[root@wdc-10-206-86-199 worker]# sudo tcpdump -i any proto gre -n
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on any, link-type LINUX_SLL (Linux cooked), capture size 65535 bytes
所有机器上都启用了路由转发：
[root@vmc-centos conf]# sudo sysctl -p
net.ipv4.ip_forward = 1
中转路由器上的 iptables：
[root@vmc-centos ~]# iptables -L
Chain INPUT (policy ACCEPT)
target prot opt source destination
ACCEPT gre -- anywhere anywhere
ACCEPT gre -- anywhere anywhere
Chain FORWARD (policy DROP)
target prot opt source destination
DOCKER-USER all -- anywhere anywhere
DOCKER-ISOLATION-STAGE-1 all -- anywhere anywhere
ACCEPT all -- anywhere anywhere ctstate RELATED,ESTABLISHED
DOCKER all -- anywhere anywhere
ACCEPT all -- anywhere anywhere
ACCEPT all -- anywhere anywhere
Chain OUTPUT (policy ACCEPT)
target prot opt source destination
ACCEPT gre -- anywhere anywhere
Chain DOCKER (1 references)
target prot opt source destination
Chain DOCKER-ISOLATION-STAGE-1 (1 references)
target prot opt source destination
DOCKER-ISOLATION-STAGE-2 all -- anywhere anywhere
RETURN all -- anywhere anywhere
Chain DOCKER-ISOLATION-STAGE-2 (1 references)
target prot opt source destination
DROP all -- anywhere anywhere
RETURN all -- anywhere anywhere
Chain DOCKER-USER (1 references)
target prot opt source destination
RETURN all -- anywhere anywhere
注意：我之前尝试过这个，数据包到达了另一个专用网络。现在我尝试另一个设置，我遗漏了一些配置。
答案1
Docker 守护进程似乎在转发机器上运行。默认情况下，为了隔离不同网桥和主机上的容器，Docker 会在 iptables 中的转发链上安装默认的 DROP 策略。Docker 守护进程中有一个设置可以不这样做。在 中将 iptables 设置为 false /etc/docker/daemon.json。请参阅Docker 和 iptables
如果您将默认策略更改为接受，那么它将会起作用。
iptables --policy FORWARD ACCEPT
但是，如果您没有更改 docker 守护进程的设置，当您（或 docker 的包升级或重新启动）重新启动 Docker 守护进程时，默认策略将再次更改为 DROP。