无法在非默认接口上 ping 多宿主 Linux 机器

Question

我有一个多宿主 Ubuntu 服务器，其中包含一组接口：

eth2: 10.10.0.131/24
eth3: 10.20.0.2/24

默认接口为 eth2，网关为 10.10.0.1。下面是路由表的样子：

root@c220-1:~# netstat -rn
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
0.0.0.0         10.10.0.1       0.0.0.0         UG        0 0          0 eth2
10.10.0.0       0.0.0.0         255.255.255.0   U         0 0          0 eth2
10.20.0.0       0.0.0.0         255.255.255.0   U         0 0          0 eth3
10.30.0.0       0.0.0.0         255.255.255.0   U         0 0          0 eth0
10.40.0.0       0.0.0.0         255.255.0.0     U         0 0          0 eth1

从单独的网络 ( 192.168.3.5/24) 我可以通过 eth2 接口（具有默认网关的接口）访问这台机器，但不能通过 eth3 接口访问。我可以毫无问题地从同一网络 (10.20.0.1) 上的路由器 ping eth3 接口。

如果我从 192.168.3.5 ping 10.10.0.131，数据包到达机器，但它不发送任何回复：

c4:c8:80:90:22:eb > 73:10:73:e4:10:06, IPv4, length 98: 192.168.3.5 > 10.20.0.2: ICMP echo request, id 5451, seq 0, length 64
c4:c8:80:90:22:eb > 73:10:73:e4:10:06, IPv4, length 98: 192.168.3.5 > 10.20.0.2: ICMP echo request, id 5451, seq 1, length 64
c4:c8:80:90:22:eb > 73:10:73:e4:10:06, IPv4, length 98: 192.168.3.5 > 10.20.0.2: ICMP echo request, id 5451, seq 2, length 64
c4:c8:80:90:22:eb > 73:10:73:e4:10:06, IPv4, length 98: 192.168.3.5 > 10.20.0.2: ICMP echo request, id 5451, seq 3, length 64
c4:c8:80:90:22:eb > 73:10:73:e4:10:06, IPv4, length 98: 192.168.3.5 > 10.20.0.2: ICMP echo request, id 5451, seq 4, length 64
c4:c8:80:90:22:eb > 73:10:73:e4:10:06, IPv4, length 98: 192.168.3.5 > 10.20.0.2: ICMP echo request, id 5451, seq 5, length 64
c4:c8:80:90:22:eb > 73:10:73:e4:10:06, IPv4, length 98: 192.168.3.5 > 10.20.0.2: ICMP echo request, id 5451, seq 6, length 64
c4:c8:80:90:22:eb > 73:10:73:e4:10:06, IPv4, length 98: 192.168.3.5 > 10.20.0.2: ICMP echo request, id 5451, seq 7, length 64

如果我从同一网络上的路由器 (10.20.0.1) ping，服务器会正​​确回复：

c4:c8:80:90:22:eb > 73:10:73:e4:10:06, IPv4, length 114: 10.20.0.1 > 10.20.0.2: ICMP echo request, id 28899, seq 2932, length 80
73:10:73:e4:10:06 > c4:c8:80:90:22:eb, IPv4, length 114: 10.20.0.2 > 10.20.0.1: ICMP echo reply, id 28899, seq 2932, length 80
c4:c8:80:90:22:eb > 73:10:73:e4:10:06, IPv4, length 114: 10.20.0.1 > 10.20.0.2: ICMP echo request, id 28899, seq 2932, length 80
73:10:73:e4:10:06 > c4:c8:80:90:22:eb, IPv4, length 114: 10.20.0.2 > 10.20.0.1: ICMP echo reply, id 28899, seq 2932, length 80
c4:c8:80:90:22:eb > 73:10:73:e4:10:06, IPv4, length 114: 10.20.0.1 > 10.20.0.2: ICMP echo request, id 28899, seq 2932, length 80
73:10:73:e4:10:06 > c4:c8:80:90:22:eb, IPv4, length 114: 10.20.0.2 > 10.20.0.1: ICMP echo reply, id 28899, seq 2932, length 80
c4:c8:80:90:22:eb > 73:10:73:e4:10:06, IPv4, length 114: 10.20.0.1 > 10.20.0.2: ICMP echo request, id 28899, seq 2932, length 80
73:10:73:e4:10:06 > c4:c8:80:90:22:eb, IPv4, length 114: 10.20.0.2 > 10.20.0.1: ICMP echo reply, id 28899, seq 2932, length 80
c4:c8:80:90:22:eb > 73:10:73:e4:10:06, IPv4, length 114: 10.20.0.1 > 10.20.0.2: ICMP echo request, id 28899, seq 2932, length 80
73:10:73:e4:10:06 > c4:c8:80:90:22:eb, IPv4, length 114: 10.20.0.2 > 10.20.0.1: ICMP echo reply, id 28899, seq 2932, length 80

请注意，根据这个类似问题中的答案，我在所有接口上都关闭了 rp_filter，但它并没有解决问题：

$ for i in eth0 eth1 eth2 eth3 all default
> do
> cat /proc/sys/net/ipv4/conf/$i/rp_filter
> done
0
0
0
0
0
0

Answer 1

问题是，由于默认路由是通过 eth2，即使请求是在 eth3 上收到的，ping 响应也会通过 eth2 发送。（如果你 tcpdump eth2 你应该看到正在发送的响应。）然后可能有一些设备正在丢弃数据包，因为它们对于它们所在的网络具有无效的源 IP。您需要一些源策略路由，以便将响应发送到接收它们的接口。

1. 创建一个新的路由表（只需要做一次）：

   echo 13 eth3 >> /etc/iproute2/rt_tables
   

2. 添加一个默认路由到这个从 eth3 出去的新表：

   ip route add default via 10.20.0.1 table eth3
   

3. 添加一个策略规则来使用这个新表来处理源地址为 eth3 的 IP 的数据包：

   ip rule add from 10.20.0.2 lookup eth3