Ở mức tối thiểu, đây là tập tin cấu hình đăng nhập cung cấp một công cụ đồ họa cho cấu hình xác thực hệ thống (system-config-auth, được hiển thị trong hình 1-1), bạn sẽ cần phải tự sửa đổi các tập tin cấu hình PAM cho các dịch vụ sẽ xác thực Tập hai, một bộ sưu tập thứ hai của lời khuyên vô cùng hữu ích và thủ thuật cho việc tìm kiếm | 63 Cheap IP Takeover After missing 3 pings in a row Pinky will leap into action bringing up ethO brain up as the public IP that Brain is supposed to be serving. It will then continue to watch Brain s real IP address and relinquish control when it is back online. The ping -c 1 -w 1 means send one ping packet and time out after one second no matter what happens. ping will return non-zero if the packet didn t come back in the one second time limit. But this isn t quite the entire solution. Although Pinky is now answering for Brain any machines on the same network as the two servers notably the router just upstream at your ISP will have the wrong MAC address cached for . With the wrong MAC address cached no traffic will flow to Pinky since it will only respond to packets that bear its own MAC address. How can we tell all of the machines on the network that the MAC address for has been updated One way is to use the send_arp utility from the 1 ligh Availability Linux project. This very handy and tiny utility will craft an ARP packet to your specifications and send it to a MAC address of your choice on the local network. If we specify all ones . ff ff ff for the destination then it effectively becomes a broadcast ARP packet. While most routers won t update their ARP tables when they see unrequested ARP broadcasts such a packet will signal them to resend an ARP request to which Pinky will obligingly reply. The advantage of using broadcast is that it will signal all machines on the subnet simultaneously instead of having to track all of the MAC addresses of machines that need updating. The syntax of send_arp is send_arp Source IP Source MAC Target IP Target MAC For example our simple monitoring script above should run the following when it detects that Brain is down send_arp OO ll 22 aa bb cc fffffffffff Where OO ll 22 aa bb cc is the hardware MAC address of Pinky s etho. The script
