Setup Dedicated server (installed by steamcmd preferably). Then install another copy of the game via steam CLIENT.
Issue with authentication through scmd explained again here: http://feedback.arma3.com/view.php?id=13857

Then attempt to connect, if the cleint 'HC' does connect confirm that EXTERNAL clients can connect to the DS and that the session is not "locked".

I found that setting localClient[]={81.19.212.50}; to localClient[]={127.0.0.1}; results in HC being unable to connect.

Server test startup batch.
start "" "C:\Games\ArmA3\server\arma3server.exe" -ip=172.19.0.2 -port=2502 "-config=C:\Games\ArmA3\server\config\server.cfg" "-cfg=C:\Games\ArmA3\server\config\basic.cfg" "-profiles=C:\Games\ArmA3\server\config" -name=development

SERVER.CFG
// STEAM

steamport =2500;
steamqueryport =2501;

GLOBAL SETTINGS
hostname = "InchTest"; The name of the server that shall be displayed in the public server list
password = "123"; Password for joining, eg connecting to the server
passwordAdmin = "ef554fd54gff4fd"; Password to become server admin.
reportingIP = "arma3pc.master.gamespy.com";
logFile = "server_console.log";

WELCOME MESSAGE ("message of the day")
It can be several lines, separated by comma
// Empty messages "" will not be displayed at all but are only for increasing the interval

motd[] = {

"POTATO",
"POTATO DONT LEAVE",
"VOIP IN GLOBAL WILL RESULT IN BEING KICKED",
"POTATO",
"ANOTHER POTATO"

};
motdInterval = 0; // Time interval (in seconds) between each message
BattlEye=0;
localClient[]={81.19.212.50};

JOINING RULES
kickDuplicate = 1;
verifySignatures = 2;
maxPlayers = 60; Maximum amount of players. Civilians and watchers, beholder, bystanders and so on also count as player.

VOTING
voteMissionPlayers = 1; Tells the server how many people must connect so that it displays the mission selection screen.
voteThreshold = 0.33; // 33% or more players need to vote for something, for example an admin or a new map, to become effective

INGAME SETTINGS
disableVoN = 0; If set to 1, Voice over Net will not be available
vonCodecQuality = 8; Quality from 1 to 10
persistent = 1; If 1, missions still run on even after the last player disconnected.

class Missions
{

 class coop
{
     template="Altis Sightseeing Tour v4.2.Altis";
     difficulty="Regular";
 };

};

BASIC.CFG,
language="English";
adapter=2;
MinBandwidth=800000;
MaxBandwidth=25000000;
MaxMsgSend=384;
MaxSizeGuaranteed=512;
MaxSizeNonguaranteed=256;
MinErrorToSend=0.003;
MaxCustomFileSize=100000;
Windowed=0;
serverLongitude=0;
serverLatitude=52;
serverLongitudeAuto=0;
serverLatitudeAuto=52;

HC startup batch.
start "" "C:\Games\ArmA3\HeadlessClient\arma3.exe" -localhost=81.19.212.50 -connect=81.19.212.52 -port=2502 -password=123 -client -nosound "-profiles=C:\Users\Administrator\Documents\Arma 3" -name=HC

HC Profile.
version=1;
blood=1;
singleVoice=0;
gamma=1;
brightness=1;
maxSamplesPlayed=96;
battleyeLicense=1;
sceneComplexity=1000000;
shadowZDistance=100;
viewDistance=3800;
preferredObjectViewDistance=3200;
terrainGrid=3.125;
volumeCD=10;
volumeFX=10;
volumeSpeech=10;
volumeVoN=10;
vonRecThreshold=0.029999999;

Routing Table info;

Interface List
13...00 15 17 9f 5f fc ......Intel(R) PRO/1000 EB Network Connection with I/O Acceleration 0000002
11...00 15 17 9f 5f fd ......Intel(R) PRO/1000 EB Network Connection with I/O Acceleration

1...........................Software Loopback Interface 1

12...00 00 00 00 00 00 00 e0 Microsoft ISATAP Adapter
14...00 00 00 00 00 00 00 e0 Microsoft ISATAP Adapter 0000002
15...00 00 00 00 00 00 00 e0 Teredo Tunneling Pseudo-Interface

16...00 00 00 00 00 00 00 e0 Microsoft 6to4 Adapter

IPv4 Route Table

Active Routes:
Network Destination Netmask Gateway Interface Metric

        0.0.0.0 0.0.0.0 81.19.212.1 81.19.212.50 276
        0.0.0.0 0.0.0.0 172.19.0.1 172.19.0.2 266
    81.19.212.0 255.255.255.0 On-link 81.19.212.50 276
   81.19.212.50 255.255.255.255 On-link 81.19.212.50 276
  81.19.212.255 255.255.255.255 On-link 81.19.212.50 276
      127.0.0.0 255.0.0.0 On-link 127.0.0.1 306
      127.0.0.1 255.255.255.255 On-link 127.0.0.1 306
127.255.255.255 255.255.255.255 On-link 127.0.0.1 306
     172.19.0.0 255.255.255.0 On-link 172.19.0.2 266
     172.19.0.2 255.255.255.255 On-link 172.19.0.2 266
   172.19.0.255 255.255.255.255 On-link 172.19.0.2 266
      224.0.0.0 240.0.0.0 On-link 127.0.0.1 306
      224.0.0.0 240.0.0.0 On-link 81.19.212.50 276
      224.0.0.0 240.0.0.0 On-link 172.19.0.2 266
255.255.255.255 255.255.255.255 On-link 127.0.0.1 306
255.255.255.255 255.255.255.255 On-link 81.19.212.50 276
255.255.255.255 255.255.255.255 On-link 172.19.0.2 266

Persistent Routes:

Network Address Netmask Gateway Address Metric
        0.0.0.0 0.0.0.0 81.19.212.1 Default 
        0.0.0.0 0.0.0.0 172.19.0.1 Default

IPv6 Route Table

Active Routes:
If Metric Network Destination Gateway
16 1040 ::/0 2002:c058:6301::1
16 1026 ::/0 2002:c058:6301::c058:6301

1 306 ::1/128 On-link

15 58 2001::/32 On-link
15 306 2001:0:4137:9e76:38d9:3472:53ec:fffd/128

On-link

16 1025 2002::/16 On-link
16 281 2002:5113:d432::5113:d432/128

On-link

11 276 fe80::/64 On-link
13 266 fe80::/64 On-link
15 306 fe80::/64 On-link
11 276 fe80::870:9e36:809e:657d/128

On-link

15 306 fe80::38d9:3472:53ec:fffd/128

On-link

13 266 fe80::b5e0:eb54:f9b9:66ca/128

                                  On-link
1 306 ff00::/8 On-link

15 306 ff00::/8 On-link
11 276 ff00::/8 On-link

13 266 ff00::/8 On-link