Description of problem:
on FC26, with strongswan-5.5.3, while starting strongswan
Tried both with the same effect:
1. under regular user with
sudo systemctl start strongswan
the user has sudoers rights to run systemctl

2. under root
systemctl start strongswan

Running strongswan from command line as root works well.
SELinux is preventing starter from 'execute_no_trans' accesses on the file /usr/libexec/strongswan/charon.

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

If you believe that starter should be allowed execute_no_trans access on the charon file 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 'starter' --raw | audit2allow -M my-starter
# semodule -X 300 -i my-starter.pp

Additional Information:
Source Context                system_u:system_r:ipsec_t:s0
Target Context                system_u:object_r:ipsec_exec_t:s0
Target Objects                /usr/libexec/strongswan/charon [ file ]
Source                        starter
Source Path                   starter
Port                          <Unknown>
Host                          (removed)
Source RPM Packages           
Target RPM Packages           strongswan-5.5.3-1.fc26.x86_64
Policy RPM                    selinux-policy-3.13.1-259.fc26.noarch
Selinux Enabled               True
Policy Type                   targeted
Enforcing Mode                Enforcing
Host Name                     (removed)
Platform                      Linux (removed) 4.11.8-300.fc26.x86_64 #1 SMP Thu
                              Jun 29 20:09:48 UTC 2017 x86_64 x86_64
Alert Count                   77
First Seen                    2017-07-05 16:38:16 CEST
Last Seen                     2017-07-05 16:45:28 CEST
Local ID                      4024b33c-c595-4bbe-9a0f-df454836f4e5

Raw Audit Messages
type=AVC msg=audit(1499265928.672:653): avc:  denied  { execute_no_trans } for  pid=27715 comm="starter" path="/usr/libexec/strongswan/charon" dev="dm-2" ino=202306635 scontext=system_u:system_r:ipsec_t:s0 tcontext=system_u:object_r:ipsec_exec_t:s0 tclass=file permissive=0


Hash: starter,ipsec_t,ipsec_exec_t,file,execute_no_trans

Version-Release number of selected component:
selinux-policy-3.13.1-259.fc26.noarch

Additional info:
component:      selinux-policy
reporter:       libreport-2.9.1
hashmarkername: setroubleshoot
kernel:         4.11.8-300.fc26.x86_64
type:           libreport