幫 ubuntu 16.04 換 static ip
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Ref:
- [Ubuntu解決RTNETLINK answers: File exists_兜兜空蕩蕩的博客-CSDN博客](https://blog.csdn.net/w18mc0431/article/details/79661869)
- [Linux下常用的配置IP及route的命令匯編_李如磊的技術博客_51CTO博客](https://blog.51cto.com/lee90/1690606)
- [How to Set Static IP Address and Configure Network in Linux](https://www.tecmint.com/set-add-static-ip-address-in-linux/)
- [How to Set a Static IP on a Linux Machine | IT Support Blog](https://sites.cns.utexas.edu/oit-blog/blog/how-set-static-ip-linux-machine)

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1. 打開這個檔案

    # vim /etc/network/interfaces

1. Default 只會有這個

    # The loopback network interface
    auto lo
    iface lo inet loopback

> Ubuntu desktop system 是用 [NetworkManager](https://en.wikipedia.org/wiki/NetworkManager) 去管理網路連線 (automatically and dynamically)

Ref: [networking - /etc/network/interfaces file almost empty - Server Fault](https://serverfault.com/questions/816064/etc-network-interfaces-file-almost-empty)
1. 在後面加上類似這樣的東西

    auto eth0
    iface eth0 inet static 
      address 192.168.0.100
      netmask 255.255.255.0
      gateway 192.168.0.1
      dns-nameservers 4.4.4.4
      dns-nameservers 8.8.8.8

1. eth0 要換成自己的 network interface 的名字，how to check:

    ip link

1. Restart network

    sudo ifdown -a # 關閉所有網路
    sudo ifup -a # 打開所有網路

1. 如果出現這種 error

    RTNETLINK answers: File exists
    Failed to bring up eth2.

    ip addr flush dev eth0 # 清空 eth0 網卡的配置
    ifdown eth0 && ifup eth0

> 如果用 ssh， `ip addr flush` 會斷線 （會一直 hang 住的感覺）
> 好像又可以回來？

Debian 也差不多
[How to set up static IP address on Debian Linux 10/11 - nixCraft](https://www.cyberciti.biz/faq/add-configure-set-up-static-ip-address-on-debianlinux/)

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### 掃沒有在用的 ip
Ref:
- [How to Find Unused IP Addresses in Network - Fedingo](https://fedingo.com/how-to-find-unused-ip-addresses-in-network/)
- [Nmap Cheat Sheet](https://www.stationx.net/nmap-cheat-sheet/)

$ sudo nmap -sn 192.168.1.0/24 -v

`-sn`: Disable port scanning. Host discovery only.