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vi /etc/sysctl.conf
把
net.ipv4.ip_forward = 0
改成
net.ipv4.ip_forward = 1
如果此文件中没有这个选项则将其添加上就行。
然后执行命令：#sysctl -p
使其生效。

[root@liuyazhuang145 ~]# sysctl -p
net.ipv4.ip_forward = 1
net.ipv4.conf.default.rp_filter = 2
net.ipv4.conf.default.accept_source_route = 0
kernel.sysrq = 0
kernel.core_uses_pid = 1
net.ipv4.tcp_syncookies = 1
kernel.msgmnb = 65536
kernel.msgmax = 65536
kernel.shmmax = 68719476736
kernel.shmall = 4294967296
vm.min_free_kbytes = 65536
kernel.panic_on_oops = 1
kernel.panic = 60

原来的0就变为1了。