[CentOS] disable/mask NetworkManager leads to unit startup fails

Tue Jan 17 10:28:00 UTC 2017
Oberdorfer Patrick <patrick.oberdorfer at a1telekom.at>

Thank you very much James for your deep explaining answer!
That cleared everything up for me and yes that helped a lot.

Have a nice day :)

-----Ursprüngliche Nachricht-----
Von: CentOS [mailto:centos-bounces at centos.org] Im Auftrag von James Hogarth
Gesendet: Montag, 16. Jänner 2017 19:37
An: CentOS mailing list <centos at centos.org>
Betreff: Re: [CentOS] disable/mask NetworkManager leads to unit startup

On 16 January 2017 at 15:24, Oberdorfer Patrick
<patrick.oberdorfer at a1telekom.at> wrote:
> Hello!
> For me it was best practice to disable "NetworkManager" on headless 
> installations.
> Now suddenly I ran into an problem with several programs not starting 
> correctly upon boot anymore.
> The problem seems to be that their unit files contain
> but network.target wont wait till network is up and working, just 
> waits for some low level network stuff.
> Now I read that it would be good practice to edit this and use 
> "After=network-online.target" instead. But I don't want to touch 
> several systemd unit files shipped via rpm packages.
> Software like unbound, postfix and so on which needs to bind to 
> ip+port on startup fails on my systems now since I disabled
> What solved the problem for me was to install "systemd-networkd" 
> instead and enable it upon boot.
> It's system unit file pulls in "network.target" as "Wants" this way 
> software that says "After=network.target" will start after network 
> cards are configured. Why there is this behavior I haven't found out yet.
> Whats your best practice to archieve stable networking upon reboot? Do 
> you use NetworkManager or systemd-networkd or maybe something exotic

Use NetworkManager ... it's the supported RH configuration tool.

This article is a little old (7.3 had a NM rebase and I've not revisited it
yet ... the prime gain in 7.3 though is arbitrary layering of
vlan/bridge/bond/etc negating the caveat at the end for edge case).


As for the targets ... man systemd.special to get a better understanding:


By default services you install will listen on all interfaces, usually a
configuration with something like ListenAddress :: to indicate this (there
is an internal ipv6->ipv4 translation going on so you only should listen on
:: and not as well) ... when a service binds to a port on all
addresses then the address does not need to exist yet
- the kernel just handles it.

If you configure your service to bind on a specific IP though then they way
the binding system call works it already needs to exist (by
default) which is why with the default configuration if the service tries to
start before the IPs are applied (quite likely given parallel
startup) you have a nice race condition and a fairly high likelihood of

There's two ways to handle this - and it depends on the service.

You can order it via after (and potential requires if you want no attempt to
start the service if the target fails to activate) network-online.target
(see referenced man page) which won't be complete until at least one IP is
on any interface.

If it's your own application and you can set the flags on the actual bind
then you can use so_freebind on the socket opened, some applications may
have this as a compile time or configurable option.

Another option is to enable net.ipv4.ip_nonlocal_bind and
net.ipv6.ip_nonlocal_bind which allows the kernel to bind a socket on an IP
it doesn't have (do note the caveat it may break some things).


If it's a systemd socket being used by the service then you can enable the
freebind option via FreeBind=true


Overall you may be interested in this:


I really wouldn't use systemd-network on CentOS as a general thing as it's
basically undocumented/unsupported by Red Hat at this time, with NM being
their actual supported solution.

https://access.redhat.com/solutions/783533 <-- network Vs NetworkManager

https://access.redhat.com/solutions/876183 <-- no systemd alternative for NM
in EL7

Hope that helps and explains things!
CentOS mailing list
CentOS at centos.org