[Resolved] CentOS 7 VM can't communicate to host

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aik099
Posts: 9
Joined: 15. Sep 2019, 20:51

[Resolved] CentOS 7 VM can't communicate to host

Post by aik099 »

Hi,

My goal: for both VMs and host to communicate and VMs to have NAT-ed internet access through host

My setup:

- host os: macOS
- the DHCP inside Host-only Network configuration inside VirtualBox is disabled (I'm using static IP on every VM to access them from host OS via SSH/Apache later)
- Host-only Network: 10.0.2.0/255.255.255.0

- vm1:
-- OS: Slackware 13.2 or Slackware 14.2
-- NIC1: NAT (IP: returned by DHCP; always 10.0.2.15)
-- NIC2: Host-only Network (IP: static inside VM; always 10.0.2.4)

- vm2:
-- OS: Centos 7.6
-- NIC1: NAT (IP: returned by DHCP; always 10.0.2.15)
-- NIC2: Host-only Network (IP: static inside VM; always 10.0.2.6)

Here is what happens:

The vm1 (Slackware) works perfectly:
- eth0 interface uses NIC1 (NAT)
- eth1 interface uses NIC2 (Host-only Network)
- the VM can: use internet
- the VM can: connect back to host via 10.0.2.1 IP (ping returns: "64 bytes from 10.0.2.1: icmp_req=1 ttl=64 time=0.139 ms")
- the host OS can: connect to VM via 10.0.2.4 IP address

The vm1 (Slackware) works partially and very unusual:
- enp0s3 interface uses NIC1 (NAT)
- enp0s8 interface uses NIC2 (Host-only Network)
- the VM can: use internet
- the VM can't connect back to host via 10.0.2.1 IP (ping returns: "From 10.0.2.15 icmp_seq=1 Destination Host Unreachable")
- the host OS can't: connect to VM via 10.0.2.4 IP address
- if I attempt to ssh to 10.0.2.4 IP address, then I end up connecting to host (which isn't accessible by 10.0.2.1)

I'm not sure why it happened, but I've never faced such an issue with Slackware OS.

So here is again what I consider odd/non-working in CentOS 7.6 VM:

1. inability to access other VMs
2. inability to access host os
3. when pinging host OS I'm getting responses from NAT IP: 10.0.2.15

This does sound like a network configuration error on CentOS, but I have no idea where.

Thanks.
Last edited by socratis on 16. Sep 2019, 19:52, edited 2 times in total.
Reason: Marked as [Resolved].
Martin
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Re: CentOS 7 VM can't communicate to host

Post by Martin »

You cannot use the same IP subnet for two different network interfaces.
aik099
Posts: 9
Joined: 15. Sep 2019, 20:51

Re: CentOS 7 VM can't communicate to host

Post by aik099 »

I'm using "Attached to: NAT", where you can't specify subnet. I have no idea, why ".15" IP from subnet of "Host-only Network" subnet is being used. As I've said before it worked perfectly for other OSes, but not CentOS.

You can specify subnet for "Attached: NAT Network" only. To experiment I've tried to use "NAT Network" as well, but for unknown reasons "NAT Network" + "Host-only Network" result in 10.0.2.4 IP being used on both interfaces via DHCP.
Martin
Volunteer
Posts: 2560
Joined: 30. May 2007, 18:05
Primary OS: Fedora other
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Guest OSses: XP, Win7, Win10, Linux, OS/2

Re: CentOS 7 VM can't communicate to host

Post by Martin »

You need to use another subnet for the host-only network, for example 10.0.3.x instead of the 10.0.2.x of the NAT network.
Also make sure that your host-only network adapters use a /24 = 255.255.255.0 netmask.
aik099
Posts: 9
Joined: 15. Sep 2019, 20:51

Re: CentOS 7 VM can't communicate to host

Post by aik099 »

Ah, so that NAT network is using 10.0.2.0/255.255.255.0 all the time and I was unlucky to pick same subnet for my Host-only Network as well?

I'll try changing Host-only Network from 10.0.2.0/255.255.255.0 to 10.0.3.0/255.255.255.0 later today and see how it goes.
fth0
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Re: CentOS 7 VM can't communicate to host

Post by fth0 »

aik099 wrote:Ah, so that NAT network is using 10.0.2.0/255.255.255.0 all the time and I was unlucky to pick same subnet for my Host-only Network as well?
Yes, you were unlucky (or not reading the User Manual ;)).

Note that the NAT networking may automatically use 10.0.2.0/255.255.255.0 up to 10.0.9.0/255.255.255.0 (for up to 8 NAT networks). So as not to be unlucky again in the future, I would choose yet another network like 10.1.2.0/255.255.255.0. ;)
aik099
Posts: 9
Joined: 15. Sep 2019, 20:51

Re: CentOS 7 VM can't communicate to host

Post by aik099 »

Totally missed that part from the docs (Chapter 6. Virtual Networking). After you've told me I was able to find a link to "9.10.1. Configuring the Address of a NAT Network Interface", where that all was explained.

Thank you.

---

Note for VirtualBox developers:

When creating "Host-only Network" please inform user, that chosen submit will conflict with NAT subnet (both default ones in 10.0.x.0 and user defined in any of VMs already created).
aik099
Posts: 9
Joined: 15. Sep 2019, 20:51

Re: CentOS 7 VM can't communicate to host

Post by aik099 »

It worked perfectly. Thank you once again.

If I would have known, that I could post on these forums before I've spent 2 days on solving this problem myself :(
socratis
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Primary OS: Mac OS X other
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Guest OSses: Win(*>98), Linux*, OSX>10.5
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Re: CentOS 7 VM can't communicate to host

Post by socratis »

aik099 wrote:If I would have known, that I could post on these forums before I've spent 2 days on solving this problem myself
Or read the User Manual! :)
You should have a searchable PDF included with your installation. If not, there's the on-line manual in PDF format, again searchable. Take advantage of that fact, and use it to search the PDF for the terms that interest you.
aik099 wrote:It worked perfectly.
Marking as [Resolved].
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