6to4 Tunnel
6to4隧道定义了一个网络前缀2002::/16用于表达这是一个6to4网络整体，
地址格式2002:a.b.c.d:xxxx:xxxx:xxxx:xxxx:xxxx，
任何一个公共IPv4地址将地址的十六进制值加在6to4网络前缀之后，从而产生一个前缀数为48的相应IPv4的6to4子网的网络前缀，而且其仍然可以继续分割至最小前缀数为64的子网段用于区分出这个6to4子网的子网。
两台主机ipv4地址是192.168.1.104和192.168.1.105，实现6to4隧道，前缀是2002::/16
192.168.1.104配置：
[root@sed ~]# ip tunnel add 6to4 mode sit remote any local 192.168.1.104
[root@sed ~]# ip -6 addr add 2002:c0a8:0168:2019::1/64 dev 6to4
[root@sed ~]# ip link set 6to4 up
[root@sed ~]# route -6 add 2002::/16 dev 6to4
192.168.1.105配置：
[root@sm ~]# ip tunnel add 6to4 mode sit remote any local 192.168.1.105
[root@sm ~]# ip -6 addr add 2002:c0a8:0168:2019::1/64 dev 6to4
[root@sm ~]# ip link set 6to4 up
[root@sm ~]# route -6 add 2002::/16 dev 6to4
[root@sed ~]# ping6 2002:c0a8:169:2019::1
PING 2002:c0a8:169:2019::1(2002:c0a8:169:2019::1) 56 data bytes
64 bytes from 2002:c0a8:169:2019::1: icmp_seq=1 ttl=64 time=0.482 ms
64 bytes from 2002:c0a8:169:2019::1: icmp_seq=2 ttl=64 time=0.528 ms
64 bytes from 2002:c0a8:169:2019::1: icmp_seq=3 ttl=64 time=0.582 ms
64 bytes from 2002:c0a8:169:2019::1: icmp_seq=4 ttl=64 time=0.570 ms
64 bytes from 2002:c0a8:169:2019::1: icmp_seq=5 ttl=64 time=0.512 ms
64 bytes from 2002:c0a8:169:2019::1: icmp_seq=6 ttl=64 time=0.499 ms
64 bytes from 2002:c0a8:169:2019::1: icmp_seq=7 ttl=64 time=0.553 ms
--------------------------------------------------------------------------
[root@sm ~]# tcpdump -i 6to4 icmp6 -n -vv
tcpdump: listening on 6to4, link-type RAW (Raw IP), capture size 262144 bytes
02:07:05.596632 IP6 (hlim 64, next-header ICMPv6 (58) payload length: 64) 2002:c0a8:168:2019::1 > 2002:c0a8:169:2019::1: [icmp6 sum ok] ICMP6, echo request, seq 1
02:07:05.596683 IP6 (hlim 64, next-header ICMPv6 (58) payload length: 64) 2002:c0a8:169:2019::1 > 2002:c0a8:168:2019::1: [icmp6 sum ok] ICMP6, echo reply, seq 1
02:07:06.597324 IP6 (hlim 64, next-header ICMPv6 (58) payload length: 64) 2002:c0a8:168:2019::1 > 2002:c0a8:169:2019::1: [icmp6 sum ok] ICMP6, echo request, seq 2
02:07:06.597378 IP6 (hlim 64, next-header ICMPv6 (58) payload length: 64) 2002:c0a8:169:2019::1 > 2002:c0a8:168:2019::1: [icmp6 sum ok] ICMP6, echo reply, seq 2
02:07:07.598455 IP6 (hlim 64, next-header ICMPv6 (58) payload length: 64) 2002:c0a8:168:2019::1 > 2002:c0a8:169:2019::1: [icmp6 sum ok] ICMP6, echo request, seq 3
02:07:07.598512 IP6 (hlim 64, next-header ICMPv6 (58) payload length: 64) 2002:c0a8:169:2019::1 > 2002:c0a8:168:2019::1: [icmp6 sum ok] ICMP6, echo reply, seq 3
02:07:08.599894 IP6 (hlim 64, next-header ICMPv6 (58) payload length: 64) 2002:c0a8:168:2019::1 > 2002:c0a8:169:2019::1: [icmp6 sum ok] ICMP6, echo request, seq 4
02:07:08.599952 IP6 (hlim 64, next-header ICMPv6 (58) payload length: 64) 2002:c0a8:169:2019::1 > 2002:c0a8:168:2019::1: [icmp6 sum ok] ICMP6, echo reply, seq 4
