Multiple IPs on Multiple NICs same subnet

linux networking router ip routing

5,684

I think you should read up about bonding interfaces.

If you want to load-balance using two interfaces the system you are connecting to has to "speak" exactly the same LB-protocol as you do.

Else you will loose packets.

5,684

Hugh

Updated on September 18, 2022

Comments

Hugh over 1 year
So, I've got two NICs:
```
eth0
eth1
```
and I have multiple IP addresses assigned to each:
```
auto lo
iface lo inet loopback
allow-hotplug eth0
iface eth0 inet static
    address 10.0.0.194
    netmask 255.255.255.0
    gateway 10.0.0.1
    network 10.0.0.0

auto eth0:0
allow-hotplug eth0:0
iface eth0:0 inet static
    address 10.0.0.253
    netmask 255.255.255.0

auto eth0:1
allow-hotplug eth0:1
iface eth0:1 inet static
    address 10.0.0.252
    netmask 255.255.255.0

auto eth0:2
allow-hotplug eth0:2
iface eth0:2 inet static
    address 10.0.0.251
    netmask 255.255.255.0

auto eth1
allow-hotplug eth1
iface eth1 inet static
    address 10.0.0.74
    netmask 255.255.255.0
    gateway 10.0.0.1
    network 10.0.0.0

auto eth1:0
allow-hotplug eth1:0
iface eth1:0 inet static
    address 10.0.0.105
    netmask 255.255.255.0

auto eth1:1
allow-hotplug eth1:1
iface eth1:1 inet static
    address 10.0.0.104
    netmask 255.255.255.0

auto eth1:2
allow-hotplug eth1:2
iface eth1:2 inet static
    address 10.0.0.106
    netmask 255.255.255.0
```
I've also setup ip route like so:
```
sudo ip route add 10.0.0.0/24 dev eth0 table eth0
sudo ip route add default via 10.0.0.1 dev eth0 table eth0

sudo ip route add 10.0.0.0/24 dev eth1 table eth1
sudo ip route add default via 10.0.0.1 dev eth1 table eth1

sudo ip rule add from 10.0.0.194 table eth0
sudo ip rule add from 10.0.0.74 table eth1
```
Now when I execute the following commands, everything works fine I get back my external IP:
```
curl --interface eth0  http://ipecho.net/plain ; echo
curl --interface eth0:0  http://ipecho.net/plain ; echo
curl --interface eth0:1  http://ipecho.net/plain ; echo
curl --interface eth0:2  http://ipecho.net/plain ; echo
curl --interface eth1  http://ipecho.net/plain ; echo
```
However, when I run:
```
curl --interface eth1:0  http://ipecho.net/plain ; echo
```
Nothing happens. I've obviously messed up my routes or something. Can anyone help me out? Thanks.

ip route
```
default via 10.0.0.1 dev eth0 
10.0.0.0/24 dev eth0  proto kernel  scope link  src 10.0.0.194 
10.0.0.0/24 dev eth1  proto kernel  scope link  src 10.0.0.74
```
route -n
```
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         10.0.0.1        0.0.0.0         UG    0      0        0 eth0
10.0.0.0        0.0.0.0         255.255.255.0   U     0      0        0 eth0
10.0.0.0        0.0.0.0         255.255.255.0   U     0      0        0 eth1
```
EDIT:

So I found this, which is what I'm trying to do however, I don't want to specify any subnets. I just want it to round-robin the requests.

https://unix.stackexchange.com/questions/111293/load-balancing-among-multiple-virtual-network-interfaces
- jason about 10 years
  
  I'm guessing you are statically assigning your IP addresses, no? If you are, wouldn't you agree that posting the contents of /etc/network/interfaces is more informative than the output of ifconfig? I'd like to see how you actually have it manually configured.
- Ankit Sagar about 10 years
  
  Wich distribution are you using?
- Daniel B about 10 years
  
  What happens when you add explicit routing rules for all outgoing IP addresses?
- jason about 10 years
  
  Where is your initial connection(s) to the Internet? Which interface(s)? What are configuration details for such interface(s).
- Ankit Sagar about 10 years
  
  Please explain why you have those IPs assigned. BTW: ip route get $IP will show you which interface will be used for that target IP.
- Hugh about 10 years
  
  Because its on AWS VPC
- David Schwartz about 10 years
  
  Whatever it is you are trying to do, this is not the way to do it. Linux fundamentally follows the weak end system model (IP addresses belong to machines) and your configuration only makes sense under a strong end system model (IP addresses belong to interfaces).
- Ankit Sagar about 10 years
  
  Follow the bonding link in my answer. Everything is explained there...
- davidgo over 8 years
  
  Have you set reverse path filtering off ? Its possible that traffic for eth1:0 is leaving one interface and coming back on the other.
- David Schwartz over 8 years
  
  It sounds like you want a strong host model, which Linux just doesn't do.
- LawrenceC almost 8 years
  
  Read my answer here: superuser.com/questions/1056054/… - if you are not interested in bonding the two interfaces (making them look like 1 to the OS and also the receiving end), then it should explain things.
Hugh about 10 years

Ok, I'll trying putting the interfaces on different subnets. Once I've done that how can I load-balance outbound requests over those two interfaces?
Daniel B about 10 years

I don't see how having multiple NICs on the same network would cause any problems at all. Packet collisions don't exist on a fully switched network. Address collisions won't occur with the OPs configuration.
Hugh about 10 years

Can I still choose which IPs are used for outbound requests over that bond for each interface? Thanks.
Ankit Sagar about 10 years

@James Why would you want to do that?