我有一个Linux机器负责处理我的网络的路由。
以下是一些相关配置（公共 IP 地址屏蔽为 192.0.0.x）：
IP 配置：
[root@gw ~]# ip addr
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
valid_lft forever preferred_lft forever
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
link/ether 00:0d:b9:32:fa:d0 brd ff:ff:ff:ff:ff:ff
inet 192.168.1.1/24 brd 192.168.1.255 scope global eth0
valid_lft forever preferred_lft forever
inet6 fe80::20d:b9ff:fe32:fad0/64 scope link
valid_lft forever preferred_lft forever
3: eth1: <NO-CARRIER,BROADCAST,MULTICAST,PROMISC,UP> mtu 1500 qdisc fq_codel master br0 state DOWN group default qlen 1000
link/ether 00:0d:b9:32:fa:d1 brd ff:ff:ff:ff:ff:ff
4: eth2: <BROADCAST,MULTICAST,PROMISC,UP,LOWER_UP> mtu 1500 qdisc fq_codel master br0 state UP group default qlen 1000
link/ether 00:0d:b9:32:fa:d2 brd ff:ff:ff:ff:ff:ff
inet6 fe80::20d:b9ff:fe32:fad2/64 scope link
valid_lft forever preferred_lft forever
5: br0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default
link/ether 00:0d:b9:32:fa:d1 brd ff:ff:ff:ff:ff:ff
inet 192.0.0.2/30 brd 192.0.0.3 scope global br0
valid_lft forever preferred_lft forever
inet6 fe80::20d:b9ff:fe32:fad1/64 scope link
valid_lft forever preferred_lft forever
（是和br0之间的桥梁。目前没有到 的连接。）eth1eth2eth1
路由表：
[root@gw ~]# ip route
default via 192.0.0.1 dev br0
192.0.0.0/30 dev br0 proto kernel scope link src 192.0.0.2
192.168.1.0/24 dev eth0 proto kernel scope link src 192.168.1.1
测试路线：
[root@gw ~]# ip route get 192.168.1.2
192.168.1.2 dev eth0 src 192.168.1.1
cache
PING 的数据包捕获：
(as seen from PING)
[root@gw ~]# ping 192.168.1.2
PING 192.168.1.2 (192.168.1.2) 56(84) bytes of data.
64 bytes from 192.168.1.2: icmp_seq=1 ttl=64 time=0.476 ms
64 bytes from 192.168.1.2: icmp_seq=2 ttl=64 time=0.701 ms
64 bytes from 192.168.1.2: icmp_seq=3 ttl=64 time=0.466 ms
64 bytes from 192.168.1.2: icmp_seq=4 ttl=64 time=0.701 ms
^C
--- 192.168.1.2 ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 3000ms
rtt min/avg/max/mdev = 0.466/0.586/0.701/0.115 ms
(as seen from tcpdump on same machine)
[root@gw ~]# tcpdump -i eth0 -n host 192.168.1.2
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes
12:48:58.187862 IP 192.0.0.2 > 192.168.1.2: ICMP echo request, id 1451, seq 1, length 64
12:48:58.188242 IP 192.168.1.2 > 192.0.0.2: ICMP echo reply, id 1451, seq 1, length 64
12:48:59.187086 IP 192.0.0.2 > 192.168.1.2: ICMP echo request, id 1451, seq 2, length 64
12:48:59.187696 IP 192.168.1.2 > 192.0.0.2: ICMP echo reply, id 1451, seq 2, length 64
12:49:00.188837 IP 192.0.0.2 > 192.168.1.2: ICMP echo request, id 1451, seq 3, length 64
12:49:00.189214 IP 192.168.1.2 > 192.0.0.2: ICMP echo reply, id 1451, seq 3, length 64
12:49:01.187834 IP 192.0.0.2 > 192.168.1.2: ICMP echo request, id 1451, seq 4, length 64
12:49:01.188450 IP 192.168.1.2 > 192.0.0.2: ICMP echo reply, id 1451, seq 4, length 64
^C
8 packets captured
8 packets received by filter
0 packets dropped by kernel
在机器 192.168.1.2 上捕获 PING 信息：
[admin@raspberry ~]$ sudo tcpdump -i eth0 -n icmp
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes
12:51:32.586016 IP 192.0.0.2 > 192.168.1.2: ICMP echo request, id 1451, seq 1, length 64
12:51:32.586822 IP 192.168.1.2 > 192.0.0.2: ICMP echo reply, id 1451, seq 1, length 64
12:51:33.587715 IP 192.0.0.2 > 192.168.1.2: ICMP echo request, id 1451, seq 2, length 64
12:51:33.588446 IP 192.168.1.2 > 192.0.0.2: ICMP echo reply, id 1451, seq 2, length 64
12:51:34.589160 IP 192.0.0.2 > 192.168.1.2: ICMP echo request, id 1451, seq 3, length 64
12:51:34.589885 IP 192.168.1.2 > 192.0.0.2: ICMP echo reply, id 1451, seq 3, length 64
12:51:35.590523 IP 192.0.0.2 > 192.168.1.2: ICMP echo request, id 1451, seq 4, length 64
12:51:35.591254 IP 192.168.1.2 > 192.0.0.2: ICMP echo reply, id 1451, seq 4, length 64
^C
8 packets captured
8 packets received by filter
0 packets dropped by kernel
[admin@raspberry ~]$
好的，PING 确实有效，但我的问题仍然存在：为什么 Linux 将面向公共接口的源 IP 分配给发往内部接口的 ping？
当我在内部局域网上设置一台故意没有设置默认网关的测试机时，问题就变得很明显了。因此，PING 会发往发送数据的机器192.0.0.2，因此该机器的回复永远不会到达服务器（因为它没有配置默认网关）。
PING 正在进行进入网关盒确实正确返回，很可能是因为它们的源 IP 设置正确，因此来自网关的 PING 回复使用正确的源 IP 进行回复。
假设路由表显示正确的源，有人能解释为什么源 IP 是错误的，以及如何修复它吗？
顺便提一句：
[root@gw ~]# uname -a
Linux gw 4.0.4-2-ARCH #1 SMP PREEMPT Fri May 22 03:19:32 UTC 2015 i686 GNU/Linux