mjhjmtu

When I do systemctl status network I see its status as failed, but it can still request external URLs. Can someone explain, what does this service do exactly? Is it an interface for physical connections?

grep desc -A 1 /etc/rc.d/init.d/network
# description: Activates/Deactivates all network interfaces configured to 
# start at boot time.

DHCP etc

Background

First off, this is not technically network.service, but rather network.target. Notice when we look for either of these files on a CentOS 7 box that we only find the network.target file.

$ locate network.service
$

$ locate network.target
/usr/lib/systemd/system/network.target
$

You can get systemd unit file descriptions directly from systemd via the systemctl command. For eg:

$ systemctl show network | grep -i description
Description=LSB: Bring up/down networking

If you look at the output of systemctl status ... notice this description shows up there:

$ systemctl status network
● network.service - LSB: Bring up/down networking
 Loaded: loaded (/etc/rc.d/init.d/network; bad; vendor preset: disabled)
 Active: failed (Result: exit-code) since Mon 2018-09-03 21:49:05 EDT; 2 days ago
 Docs: man:systemd-sysv-generator(8)
 Process: 809 ExecStart=/etc/rc.d/init.d/network start (code=exited, status=1/FAILURE)

Warning: Journal has been rotated since unit was started. Log output is incomplete or unavailable.

Also notice that there's a reference to documentation in the form of a man page.

 systemd-sysv-generator is a generator that creates wrapper .service units for 
 SysV init[1] scripts in /etc/init.d/* at boot and when configuration of the 
 system manager is reloaded. This will allow systemd(1) to support them 
 similarly to native units.

 LSB headers[2] in SysV init scripts are interpreted, and the ordering specified 
 in the header is turned into dependencies between the generated unit and other 
 units. LSB facilities "$remote_fs", "$network", "$named", "$portmap", "$time" 
 are supported and will be turned into dependencies on specific native systemd 
 targets. See systemd.special(5) for more details.

 SysV runlevels have corresponding systemd targets (runlevelX.target). Wrapper 
 unit that is generated will be wanted by those targets which correspond to 
 runlevels for which the script is enabled.

 systemd does not supports SysV scripts as part of early boot, so all wrapper 
 units are ordered after basic.target.

 systemd-sysv-generator implements systemd.generator(7).

systemd-sysv-generator constructs targets & services based on existing SysV init scripts. In this case the init script that systemd is "wrapping" is this file - /etc/rc.d/init.d/network.

Purpose

In terms of what this unit file does, it's what brings up the network interfaces which are specified under /etc/sysconfig/network. This is a previously used directory that most of hte Red Hat based distros used before NetworkManager and systemd existed.

Systemd is bringing up any network interfaces defined in this directory as a way to stay backward compatible with how previous versions of Red Hat (CentOS, Fedora, & RHEL) behaved.