Creating a dummy adapter in RHEL7

Latest response

Hi,

We use RHEL7.6, and are trying to configure a dummy adapter. We use a minimal install, and apply Centre for Internet Security Benchmark hardening.

As part of the build, we stick with using network manager, but change the host based firewall back from firewalld to iptables. Network manager is cumbersome, but understand the functionality it provides to other services, and the fact that the raw config files cannot be edited once they are network manager managed.

herein lies the problem.

Unexpectedly, when using nmtui, you cant use it to create a new interface of type "dummy". To do this, you need to use nmcli. This isn't a big deal, and I've followed, among others, the instructions laid out in https://access.redhat.com/solutions/4878391.

The problem is though, that the /etc/sysconfig/network-scripts/ifconfig-dummy file never gets created - even if forcing the "save" option (which is default behaviour anyway) in the mncli command.

If I reboot the server, then the configuration is back OK, but the adapter is down, and no longer network manager managed - I need to manually claim the device by network manager again and it then springs into life.

What am I doing wrong? as this doesn't seem to be working as documented.

Responses

Hi R Bland-Ward,

When you say you need a dummy adapter, do you just need an additional temporary interface? There's more than one approach to this. What will this dummy adapter do? I get the idea you need a temporary interface? Is that correct, or can you clarify? Thanks

Regards,
RJ

Hi RJ, we use them for adding IP addresses to hosts which are routing services through an F5 load balancer. It makes sure that the IP's are bound and the host is listening for them but they cannot be used to route traffic (I believe).

In solaris, I would have plumbed the IP's to the physical NIC and then marked it as deprecated. I think that's what I'm required to achieve here, but I will clarify with networking...

The steps linked above create a config file at /etc/NetworkManager/system-connections/dummy-dummy0 however I also need to manually activate the connection.

RHEL8 does activate a dummy created in this way, so it looks like that's an old bug which has since been fixed.

One possible workaround could be to activate the dummy in rc.local with nmcli con up dummy-dummy0 or write a systemd unit which does the same.

Another alternative is to forget dummies and add the IPs onto the loopback interface lo. Using the loopback for local bind IPs is a common practice on Linux.

Thanks Jamie, your ability to reproduce the need to up the interface is useful and means I can stop looking for reasons and go straight to workaround. Is this something we can expect to see fixed in the future for RHEL7 (and hence a change of behavior requiring workaround back-out), or would you expect it left as-is? I would ask support, but we only have self-support entitlements.

Hi R Bland-Ward,

With self-support you can open a bugzilla report.

Point to consider: There might be a fix in already in 7.7 or 7.8, so there is a slight change the bugzilla will be closed as will not fix,

but at least you will get a reason why the bugzilla will not be fixed.

Regards,

Jan Gerrit Kootstra