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201 lines
9.5 KiB
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检查 Linux 系统上的网络连接
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============================================================
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> Linux 系统提供了许多有用的命令来检查网络配置和连接。下面来看几个，包括 `ifquery`、`ifup`、`ifdown` 和 `ifconfig`。
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Linux 上有许多可用于查看网络设置和连接的命令。在今天的文章中，我们将会通过一些非常方便的命令来看看它们是如何工作的。
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### ifquery 命令
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一个非常有用的命令是 `ifquery`。这个命令应该会显示一个网络接口列表。但是，你可能只会看到类似这样的内容 - 仅显示回环接口：
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```
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$ ifquery --list
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lo
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```
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如果是这种情况，那说明你的 `/etc/network/interfaces` 不包括除了回环接口之外的网络接口信息。在下面的例子中，假设你使用 DHCP 来分配地址，且如果你希望它更有用的话，你可以添加例子最后的两行。
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```
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# interfaces(5) file used by ifup(8) and ifdown(8)
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auto lo
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iface lo inet loopback
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auto eth0
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iface eth0 inet dhcp
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```
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### ifup 和 ifdown 命令
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可以使用相关的 `ifup` 和 `ifdown` 命令来打开网络连接并根据需要将其关闭，只要该文件具有所需的描述性数据即可。请记住，“if” 在这里意思是<ruby>接口<rt>interface</rt></ruby>，这与 `ifconfig` 命令中的一样，而不是<ruby>如果我只有一个大脑<rt>if I only had a brain</rt></ruby> 中的 “if”。
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### ifconfig 命令
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另外，`ifconfig` 命令完全不读取 `/etc/network/interfaces`，但是仍然提供了网络接口相当多的有用信息 —— 配置数据以及可以告诉你每个接口有多忙的数据包计数。`ifconfig` 命令也可用于关闭和重新启动网络接口（例如：`ifconfig eth0 down`）。
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```
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$ ifconfig eth0
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eth0 Link encap:Ethernet HWaddr 00:1e:4f:c8:43:fc
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inet addr:192.168.0.6 Bcast:192.168.0.255 Mask:255.255.255.0
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inet6 addr: fe80::b44b:bdb6:2527:6ae9/64 Scope:Link
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UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
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RX packets:60474 errors:0 dropped:0 overruns:0 frame:0
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TX packets:33463 errors:0 dropped:0 overruns:0 carrier:0
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collisions:0 txqueuelen:1000
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RX bytes:43922053 (43.9 MB) TX bytes:4000460 (4.0 MB)
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Interrupt:21 Memory:fe9e0000-fea00000
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```
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输出中的 RX 和 TX 数据包计数很低。此外，没有报告错误或数据包冲突。或许可以用 `uptime` 命令确认此系统最近才重新启动。
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上面显示的广播 （Bcast） 和网络掩码 （Mask） 地址表明系统运行在 C 类等效网络（默认）上，所以本地地址范围从 `192.168.0.1` 到 `192.168.0.254`。
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### netstat 命令
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`netstat` 命令提供有关路由和网络连接的信息。`netstat -rn` 命令显示系统的路由表。192.168.0.1 是本地网关 （Flags=UG)。
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```
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$ netstat -rn
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Kernel IP routing table
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Destination Gateway Genmask Flags MSS Window irtt Iface
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0.0.0.0 192.168.0.1 0.0.0.0 UG 0 0 0 eth0
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169.254.0.0 0.0.0.0 255.255.0.0 U 0 0 0 eth0
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192.168.0.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
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```
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上面输出中的 `169.254.0.0` 条目仅在你正在使用或计划使用本地链路通信时才有必要。如果不是这样的话，你可以在 `/etc/network/if-up.d/avahi-autoipd` 中注释掉相关的行：
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```
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$ tail -12 /etc/network/if-up.d/avahi-autoipd
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#if [ -x /bin/ip ]; then
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# # route already present?
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# ip route show | grep -q '^169.254.0.0/16[[:space:]]' && exit 0
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#
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# /bin/ip route add 169.254.0.0/16 dev $IFACE metric 1000 scope link
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#elif [ -x /sbin/route ]; then
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# # route already present?
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# /sbin/route -n | egrep -q "^169.254.0.0[[:space:]]" && exit 0
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#
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# /sbin/route add -net 169.254.0.0 netmask 255.255.0.0 dev $IFACE metric 1000
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#fi
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```
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### netstat -a 命令
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`netstat -a` 命令将显示“所有”网络连接。为了将其限制为显示正在监听和已建立的连接（通常更有用），请改用 `netstat -at` 命令。
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```
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$ netstat -at
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Active Internet connections (servers and established)
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Proto Recv-Q Send-Q Local Address Foreign Address State
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tcp 0 0 *:ssh *:* LISTEN
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tcp 0 0 localhost:ipp *:* LISTEN
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tcp 0 0 localhost:smtp *:* LISTEN
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tcp 0 256 192.168.0.6:ssh 192.168.0.32:53550 ESTABLISHED
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tcp6 0 0 [::]:http [::]:* LISTEN
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tcp6 0 0 [::]:ssh [::]:* LISTEN
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tcp6 0 0 ip6-localhost:ipp [::]:* LISTEN
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tcp6 0 0 ip6-localhost:smtp [::]:* LISTEN
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```
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### host 命令
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`host` 命令就像 `nslookup` 一样，用来查询远程系统的 IP 地址，但是还提供系统的邮箱处理地址。
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```
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$ host world.std.com
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world.std.com has address 192.74.137.5
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world.std.com mail is handled by 10 smtp.theworld.com.
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```
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### nslookup 命令
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`nslookup` 还提供系统中（本例中是本地系统）提供 DNS 查询服务的信息。
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```
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$ nslookup world.std.com
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Server: 127.0.1.1
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Address: 127.0.1.1#53
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Non-authoritative answer:
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Name: world.std.com
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Address: 192.74.137.5
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```
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### dig 命令
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`dig` 命令提供了很多有关连接到远程系统的信息 - 包括与我们通信的名称服务器以及查询需要多长时间进行响应，并经常用于故障排除。
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```
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$ dig world.std.com
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; <<>> DiG 9.10.3-P4-Ubuntu <<>> world.std.com
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;; global options: +cmd
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;; Got answer:
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;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 28679
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;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1
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;; OPT PSEUDOSECTION:
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; EDNS: version: 0, flags:; udp: 512
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;; QUESTION SECTION:
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;world.std.com. IN A
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;; ANSWER SECTION:
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world.std.com. 78146 IN A 192.74.137.5
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;; Query time: 37 msec
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;; SERVER: 127.0.1.1#53(127.0.1.1)
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;; WHEN: Mon Oct 09 13:26:46 EDT 2017
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;; MSG SIZE rcvd: 58
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```
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### nmap 命令
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`nmap` 经常用于探查远程系统，但是同样也用于报告本地系统提供的服务。在下面的输出中，我们可以看到登录可以使用 ssh、smtp 用于电子邮箱、web 站点也是启用的，并且 ipp 打印服务正在运行。
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```
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$ nmap localhost
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Starting Nmap 7.01 ( https://nmap.org ) at 2017-10-09 15:01 EDT
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Nmap scan report for localhost (127.0.0.1)
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Host is up (0.00016s latency).
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Not shown: 996 closed ports
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PORT STATE SERVICE
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22/tcp open ssh
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25/tcp open smtp
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80/tcp open http
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631/tcp open ipp
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Nmap done: 1 IP address (1 host up) scanned in 0.09 seconds
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```
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Linux 系统提供了很多有用的命令用于查看网络配置和连接。如果你都探索完了，请记住 `apropos network` 或许会让你了解更多。
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--------------------------------------------------------------------------------
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via: https://www.networkworld.com/article/3230519/linux/examining-network-connections-on-linux-systems.html
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作者：[Sandra Henry-Stocker][a]
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译者：[geekpi](https://github.com/geekpi)
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校对：[wxy](https://github.com/wxy)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]:https://www.networkworld.com/author/Sandra-Henry_Stocker/
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[1]:https://www.networkworld.com/article/3221393/linux/review-considering-oracle-linux-is-a-no-brainer-if-you-re-an-oracle-shop.html
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[2]:https://www.networkworld.com/article/3221393/linux/review-considering-oracle-linux-is-a-no-brainer-if-you-re-an-oracle-shop.html#tk.nww_nsdr_ndxprmomod
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[3]:https://www.networkworld.com/article/3221423/linux/review-suse-linux-enterprise-server-12-sp2-scales-well-supports-3rd-party-virtualization.html
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[4]:https://www.networkworld.com/article/3221423/linux/review-suse-linux-enterprise-server-12-sp2-scales-well-supports-3rd-party-virtualization.html#tk.nww_nsdr_ndxprmomod
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[5]:https://www.networkworld.com/article/3221476/linux/review-free-linux-fedora-server-offers-upgrades-as-they-become-available-no-wait.html
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[6]:https://www.networkworld.com/article/3221476/linux/review-free-linux-fedora-server-offers-upgrades-as-they-become-available-no-wait.html#tk.nww_nsdr_ndxprmomod
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[7]:https://www.networkworld.com/article/3227929/linux/making-good-use-of-the-files-in-proc.html
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[8]:https://www.networkworld.com/article/3221415/linux/linux-commands-for-managing-partitioning-troubleshooting.html
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[9]:https://www.networkworld.com/article/2225768/cisco-subnet/dual-protocol-routing-with-raspberry-pi.html
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[10]:https://www.networkworld.com/video/51206/solo-drone-has-linux-smarts-gopro-mount
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[11]:https://www.networkworld.com/insider
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[12]:https://www.networkworld.com/article/3227929/linux/making-good-use-of-the-files-in-proc.html
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[13]:https://www.networkworld.com/article/3221415/linux/linux-commands-for-managing-partitioning-troubleshooting.html
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[14]:https://www.networkworld.com/video/51206/solo-drone-has-linux-smarts-gopro-mount
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[15]:https://www.networkworld.com/video/51206/solo-drone-has-linux-smarts-gopro-mount
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[16]:https://www.flickr.com/photos/cogdog/4317096083/in/photolist-7zufg6-8JS2ym-bmDGsu-cnYW2C-mnrvP-a1s6VU-4ThA5-33B4ME-7GHEod-ERKLhX-5iPi6m-dTZAW6-UC6wyi-dRCJAZ-dq4wxW-peQyWU-8AGfjw-8wGAqs-4oLjd2-4T6pXM-dQua38-UKngxR-5kQwHN-ejjXMo-q4YvvL-7AUF3h-39ya27-7HiWfp-TosWda-6L3BZn-uST4Hi-TkRW8U-H7zBu-oDkNvU-6T2pZg-dQEbs9-39hxfS-5pBhQL-eR6iKT-7dgDwk-W15qVn-nVQHN3-mdRj8-75tqVh-RajJsC-7gympc-7dwxjt-9EadYN-p1qH1G-6rZhh6
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[17]:https://creativecommons.org/licenses/by/2.0/legalcode
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