Loopback 接口
什么接口是loopback？先看一个linux下面的配置：
[root@ZhouTianzuo ~]# ifconfig lo
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:2584 errors:0 dropped:0 overruns:0 frame:0
TX packets:2584 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:129200 (126.1 KiB) TX bytes:129200 (126.1 KiB
从配置可以看出，lo的mask仅为8bit，也就是说，只要127开头，任何网络主机号都可以，也就是说环回接口可以理解为一个网络号。
验证一下，ping 127.0.0.1 127.0.1.1 127.1.1.1 126.1.1.1
linux:~ # ping 127.0.0.1 PING 127.0.0.1 (127.0.0.1) 56(84) bytes of data. 64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.019 ms 64 bytes from 127.0.0.1: icmp_seq=2 ttl=64 time=0.012 ms ^C --- 127.0.0.1 ping statistics --- 2 packets transmitted, 2 received, 0% packet loss, time 999ms rtt min/avg/max/mdev = 0.012/0.015/0.019/0.005 ms linux:~ # linux:~ # linux:~ # ping 127.0.1.1 PING 127.0.1.1 (127.0.1.1) 56(84) bytes of data. 64 bytes from 127.0.1.1: icmp_seq=1 ttl=64 time=0.015 ms 64 bytes from 127.0.1.1: icmp_seq=2 ttl=64 time=0.010 ms ^C --- 127.0.1.1 ping statistics --- 2 packets transmitted, 2 received, 0% packet loss, time 999ms rtt min/avg/max/mdev = 0.010/0.012/0.015/0.004 ms linux:~ # linux:~ # ping 127.1.1.1 PING 127.1.1.1 (127.1.1.1) 56(84) bytes of data. 64 bytes from 127.1.1.1: icmp_seq=1 ttl=64 time=0.014 ms 64 bytes from 127.1.1.1: icmp_seq=2 ttl=64 time=0.009 ms 64 bytes from 127.1.1.1: icmp_seq=3 ttl=64 time=0.010 ms ^C --- 127.1.1.1 ping statistics --- 3 packets transmitted, 3 received, 0% packet loss, time 1998ms rtt min/avg/max/mdev = 0.009/0.011/0.014/0.002 ms linux:~ # linux:~ # ping 126.1.1.1 PING 126.1.1.1 (126.1.1.1) 56(84) bytes of data. ^C --- 126.1.1.1 ping statistics --- 31 packets transmitted, 0 received, 100% packet loss, time 30238ms linux:~ #
可以，127这网络号下面的主机编址都是loopback。
继续这个，我们修改一下lo的地址，也是可以的：
[root@ZhouTianzuo ~]# ifconfig lo 130.0.0.1
[root@ZhouTianzuo ~]# ifconfig lo
lo Link encap:Local Loopback
inet addr:130.0.0.1 Mask:255.255.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:3363 errors:0 dropped:0 overruns:0 frame:0
TX packets:3363 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:169619 (165.6 KiB) TX bytes:169619 (165.6 KiB)
[root@ZhouTianzuo ~]#
这个时候，默认的掩码变成了16bits，也就是网络130.0的下面的所有的主机都是loopback了，再ping之前的127.0.0.1也就不可能通了：
[root@ZhouTianzuo ~]# ping 127.0.0.1 PING 127.0.0.1 (127.0.0.1) 56(84) bytes of data.
无法通，这里就不抓包了。
再看配置为32bits的掩码，会有什么情况：
[root@ZhouTianzuo ~]# ifconfig lo 130.0.0.1 netmask 255.255.255.255
[root@ZhouTianzuo ~]#
[root@ZhouTianzuo ~]# ifconfig lo
lo Link encap:Local Loopback
inet addr:130.0.0.1 Mask:255.255.255.255
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:3363 errors:0 dropped:0 overruns:0 frame:0
TX packets:3363 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:169619 (165.6 KiB) TX bytes:169619 (165.6 KiB)
[root@ZhouTianzuo ~]#
[root@ZhouTianzuo ~]# ping 130.0.0.1
PING 130.0.0.1 (130.0.0.1) 56(84) bytes of data.
64 bytes from 130.0.0.1: icmp_seq=1 ttl=64 time=0.186 ms
^C
--- 130.0.0.1 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 925ms
rtt min/avg/max/mdev = 0.186/0.186/0.186/0.000 ms
[root@ZhouTianzuo ~]# ping 130.0.0.2
PING 130.0.0.2 (130.0.0.2) 56(84) bytes of data.
明显ping 130.0.0.2就没有这个ip了。
总结：环回接口可以配置，而且是一个网络号，并非主机号，除非把掩码配置为32bits。在tcp ip详解里面有提及，就是，net/2的实现，会把到不是完全匹配ip的到lo的报文转发，比如lo的配置为127.0.0.1，到lo 127.0.0.2的报文就会转发，如此就不对了。