Multiple Network Interface Not Working Properly

dkregen

4/2/23, 4:50 AM

I have two network interfaces on my Linux CentOS. When I tried to access the web server from outside network via eth0 it works properly. But when I tried to access via eth1 it gives me time out.

Here is the route:

[root@fe fe]# ip route list
default via 192.168.10.1 dev eth0 proto dhcp metric 101
default via 192.168.18.1 dev eth1 proto dhcp metric 102
192.168.10.0/24 dev eth0 proto kernel scope link src 192.168.10.237 metric 101
192.168.18.0/24 dev eth1 proto kernel scope link src 192.168.18.14 metric 102

Here is the weird thing: When I change the metric for eth1 to 100, eth1 is working, but eth0 is timeout.

This is the ip a command produce

[fe@fe ~]$ ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    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 mq state UP group default qlen 1000
    link/ether d2:ce:a3:6d:a2:f4 brd ff:ff:ff:ff:ff:ff
    inet 192.168.10.237/24 brd 192.168.10.255 scope global dynamic noprefixroute eth0
       valid_lft 416sec preferred_lft 416sec
    inet6 fe80::d0ce:a3ff:fe6d:a2f4/64 scope link noprefixroute
       valid_lft forever preferred_lft forever
3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
    link/ether 8a:be:5d:40:8b:34 brd ff:ff:ff:ff:ff:ff
    inet 192.168.18.14/24 brd 192.168.18.255 scope global dynamic noprefixroute eth1
       valid_lft 3584sec preferred_lft 3584sec
    inet6 fe80::88be:5dff:fe40:8b34/64 scope link noprefixroute
       valid_lft forever preferred_lft forever

I can confirm the ETH1 receive the request using tcpdump.

[root@fe fe]# tcpdump -i eth1 port 80
dropped privs to tcpdump
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth1, link-type EN10MB (Ethernet), capture size 262144 bytes
22:04:57.763317 IP 43.224.168.199.50770 > fe.lan.http: Flags [S], seq 2609591301, win 64240, options [mss 1412,nop,wscale 8,nop,nop,sackOK], length 0
22:04:57.764527 IP 43.224.168.199.50771 > fe.lan.http: Flags [S], seq 821872254, win 64240, options [mss 1412,nop,wscale 8,nop,nop,sackOK], length 0
22:04:58.021494 IP 43.224.168.199.50772 > fe.lan.http: Flags [S], seq 3220097528, win 64240, options [mss 1412,nop,wscale 8,nop,nop,sackOK], length 0

0 + 0

routing

centos

vidarlo

4/2/23, 6:08 AM

Why do you want two default routes?

NiKiZe

4/2/23, 6:46 AM

This is multihoming and requires some special handling in regards to which way traffic should go out. You need to add IP rules and have different tables based on the source address.

dkregen

4/2/23, 6:46 AM

Because both IP produced from different router. What should I do? Should I add the third router?

dkregen

4/2/23, 6:49 AM

@NiKiZe ah, I see. I just don't know how to config the tables you mention. Btw, I only need to access eth0 from IP 192.168.3.0/24.

Score:0

Server

Tero Kilkanen

4/2/23, 6:58 AM

The reason the packets are not forwarded correctly is because the server sends the response packets via other interface than they arrive from.

If you only need to use the other interface for one subnet, you should configure only that subnet traffic to be forwarded via eth0.

Steps you need to take:

Disable DHCP on eth0
Assign static IP address on eth0
Add route to 192.168.3.0/24 via 192.168.10.1

After these steps, your routing table should look something like this:

[root@fe fe]# ip route list
default via 192.168.18.1 dev eth1 proto dhcp metric 102
192.168.10.0/24 dev eth0 proto kernel scope link src 192.168.10.237 metric 101
192.168.18.0/24 dev eth1 proto kernel scope link src 192.168.18.14 metric 102
192.168.3.0/24 via 192.168.10.1 dev eth0 metric 101

0 + 0

Elon Musk

I sit in a Tesla and translated this thread with Ai:

EN: Multiple Network Interface Not Working Properly

TH: หลายอินเทอร์เฟซเครือข่ายทำงานไม่ถูกต้อง

RO: Interfața de rețea multiplă nu funcționează corect

RU: Несколько сетевых интерфейсов не работают должным образом

VI: Nhiều giao diện mạng không hoạt động bình thường

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