A question about virtual interface

Jason.nix · 07-24-2023, 05:24 AM

Hello,
I created a virtual interface like the below:

Code:

# touch /etc/network/interfaces.d/ifcfg-enp0s3:0
# nano /etc/network/interfaces.d/ifcfg-enp0s3:0

auto enp0s3:0
iface enp0s3:0 inet static
address 10.0.5.20
netmask 255.255.255.0

Then:

Code:

# systemctl restart networking
#
# ifconfig
enp0s3: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 10.0.2.15  netmask 255.255.255.0  broadcast 10.0.2.255
        inet6 fe80::a00:27ff:feed:b47c  prefixlen 64  scopeid 0x20<link>
        ether 08:00:27:ed:b4:7c  txqueuelen 1000  (Ethernet)
        RX packets 7624  bytes 2522366 (2.4 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 5147  bytes 869088 (848.7 KiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

enp0s3:0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 10.0.5.20  netmask 255.255.255.0  broadcast 10.0.5.255
        ether 08:00:27:ed:b4:7c  txqueuelen 1000  (Ethernet)
...

But, why I can't use it for some common network tasks:

Code:

# ping -I enp0s3 google.com
PING google.com (216.239.38.120) from 10.0.2.15 enp0s3: 56(84) bytes of data.
64 bytes from any-in-2678.1e100.net (216.239.38.120): icmp_seq=1 ttl=63 time=35.3 ms
64 bytes from any-in-2678.1e100.net (216.239.38.120): icmp_seq=2 ttl=63 time=71.7 ms
^C
--- google.com ping statistics ---
3 packets transmitted, 2 received, 33.3333% packet loss, time 2100ms
rtt min/avg/max/mdev = 35.292/53.511/71.730/18.219 ms
#
# ping -I enp0s3:0 google.com
ping: invalid source address: enp0s3:0

Thank you.

Jason.nix · 07-26-2023, 08:24 AM

Hello,
Any idea?
Suppose you have a NIC on a server and you want to set five public IP addresses on it, then you should be able to communicate with these other IP addresses and use them.

yvesjv · 07-26-2023, 01:36 PM

Did you create vlans for your sub-interfaces?

elgrandeperro · 07-26-2023, 01:57 PM

try ping -I enp0s3:0 google.com

Jason.nix · 07-26-2023, 03:17 PM

Quote:

Originally Posted by yvesjv

Did you create vlans for your sub-interfaces?

Hello,
Thanks.
Not really. Why a VLAN?

Jason.nix · 07-26-2023, 03:19 PM

Quote:

Originally Posted by elgrandeperro

try ping -I enp0s3:0 google.com

Hello,
Thank you.
I will test it. What does this command do?

elgrandeperro · 07-26-2023, 11:10 PM

When you don't specify a source binding, then the system is going to use the default route interface. I believe if you use -S IP to be the source of your ping OR -I interface, then it will use the alternate interface. So ping -s 10.0.5.20 google.com will use the alternate ip as well -I interface.

Jason.nix · 07-27-2023, 06:20 AM

Quote:

Originally Posted by elgrandeperro

When you don't specify a source binding, then the system is going to use the default route interface. I believe if you use -S IP to be the source of your ping OR -I interface, then it will use the alternate interface. So ping -s 10.0.5.20 google.com will use the alternate ip as well -I interface.

Hello,
Thank you so much for your reply.
I have done this before and I created this thread because of it:

Code:

# ping -I enp0s3:0 google.com
ping: invalid source address: enp0s3:0

elgrandeperro · 07-27-2023, 08:45 AM

Yeah, so you just use the -I or -S with your source ip address. When people test (and have multiple ips) they often forget to set the source and it gives them the wrong results. This is often when pinging from a router where there might be lots of paths out or routing instances.

Most tools where the source makes a difference have a way to bind the source.

yvesjv · 07-27-2023, 03:53 PM

Quote:

Originally Posted by Jason.nix

Hello,
Thanks.
Not really. Why a VLAN?

Vlans are used to transport different networks across a single ethernet cable.
Linux support it: https://www.baeldung.com/linux/vlans-create

The switch/router the ethernet cable will connect to from your server must also support it plus creating Switch Virtual Interfaces (SVI) to route the different subnets.

Jason.nix · 07-27-2023, 04:23 PM

Quote:

Originally Posted by elgrandeperro

Yeah, so you just use the -I or -S with your source ip address. When people test (and have multiple ips) they often forget to set the source and it gives them the wrong results. This is often when pinging from a router where there might be lots of paths out or routing instances.

Most tools where the source makes a difference have a way to bind the source.

Hello,
Thank you for your reply.
You mean multipath routing?

elgrandeperro · 07-27-2023, 09:23 PM

That would be one case, you have if you 2 paths out, you need to bind to the appropriate source to route to the right interface. In your case (simple added ip) I would not consider that to be multipath because your are using the same interface for both paths, hardly redundant. Multi Path refers to connecting to 2 separate networks and interfaces to provide redundancy.

KatrinAlec · 07-28-2023, 08:30 AM

Have you tried instead of -I enpx -I 10.0.5.20?

You can also use SNAT to change the source address.

Jason.nix · 07-28-2023, 01:48 PM

Quote:

Originally Posted by elgrandeperro

That would be one case, you have if you 2 paths out, you need to bind to the appropriate source to route to the right interface. In your case (simple added ip) I would not consider that to be multipath because your are using the same interface for both paths, hardly redundant. Multi Path refers to connecting to 2 separate networks and interfaces to provide redundancy.

Hello,
Thanks again.
You said:

Quote:

In your case (simple added ip)...

What do you mean?
Same interface? Can you tell me more?

Jason.nix · 07-28-2023, 01:51 PM

Quote:

Originally Posted by KatrinAlec

Have you tried instead of -I enpx -I 10.0.5.20?

You can also use SNAT to change the source address.

Hello,
Thank you for your reply.
Is you mean:

Code:

# ping -I 10.0.5.20 google.com

SNAT? How?