Description of problem:
I simply restarted, and the error appeared.
SELinux is preventing nm-dhcp-helper from 'connectto' accesses on the unix_stream_socket /run/NetworkManager/private-dhcp.

*****  Plugin catchall (100. confidence) suggests   **************************

If you believe that nm-dhcp-helper should be allowed connectto access on the private-dhcp unix_stream_socket by default.
Then you should report this as a bug.
You can generate a local policy module to allow this access.
Do
allow this access for now by executing:
# ausearch -c 'nm-dhcp-helper' --raw | audit2allow -M my-nmdhcphelper
# semodule -X 300 -i my-nmdhcphelper.pp

Additional Information:
Source Context                unconfined_u:system_r:dhcpc_t:s0-s0:c0.c1023
Target Context                unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1
                              023
Target Objects                /run/NetworkManager/private-dhcp [
                              unix_stream_socket ]
Source                        nm-dhcp-helper
Source Path                   nm-dhcp-helper
Port                          <Unknown>
Host                          (removed)
Source RPM Packages           
Target RPM Packages           
Policy RPM                    selinux-policy-3.13.1-283.14.fc27.noarch
Selinux Enabled               True
Policy Type                   targeted
Enforcing Mode                Enforcing
Host Name                     (removed)
Platform                      Linux (removed) 4.13.9-300.fc27.x86_64 #1 SMP Mon
                              Oct 23 13:41:58 UTC 2017 x86_64 x86_64
Alert Count                   5
First Seen                    2017-11-09 06:10:32 CST
Last Seen                     2017-11-10 07:28:20 CST
Local ID                      1a027249-5722-4827-90a0-908b12d84e76

Raw Audit Messages
type=AVC msg=audit(1510320500.741:170): avc:  denied  { connectto } for  pid=2230 comm="nm-dhcp-helper" path="/run/NetworkManager/private-dhcp" scontext=unconfined_u:system_r:dhcpc_t:s0-s0:c0.c1023 tcontext=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 tclass=unix_stream_socket permissive=0


Hash: nm-dhcp-helper,dhcpc_t,unconfined_t,unix_stream_socket,connectto

Version-Release number of selected component:
selinux-policy-3.13.1-283.14.fc27.noarch

Additional info:
component:      selinux-policy
reporter:       libreport-2.9.2
hashmarkername: setroubleshoot
kernel:         4.13.9-300.fc27.x86_64
type:           libreport

Potential duplicate: bug 1015756