Ports

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Ports
Ports are communication endpoints for an application. A port will either listen or transmit network traffic on specific ports. 
There are port standards set by IANA for common protocols such as HTTP (port 80), SSH (port 22), SMTP (port 25) etc. Game servers tend to use standard ports depending upon the game engine. For example source engine games by default use port 27015 as the port it listens on.
Here are the most common ones:
When you want to run Multiple Game Servers on the same machine
When you have a strict firewall and you need to open the right ports in order to let people connect to your server (see Firewalls)
When you need to forward ports on a NAT to the desired local IP in order for your server to be reachable from outside the local network (likely from the internet)
Vocabulary
Port listening is what a service like a game server does in order to receive packets from incoming connections: by listening to a port, the program waiting for incoming packets on a given port.
Port opening happens on a firewall, it consists of allowing traffic to a port.
Port redirection is part of NAT and happens on a router or firewall. It consists of forwarding incoming traffic on a given port to a specific local IP.
Network Address Translation (NAT)  is a method of remapping an IP address into another by modifying network address information.
Game Server Ports
Game Servers typicaliy have ports for the following functions.
Game port: The port that players connect to.
Query port: Used by software to gather information from the server such as server name, map and number of players.
Rcon port
Web port: used for game server that have web admin panels 
Depending upon which game server being used, you can set game server ports within your start parameters or game server config file.
LinuxGSM allows you to see the ports your game server is using with the ./gameserver details command.
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Useful port diagnostic command:
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netstat -atunp | grep srcds_linux
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​
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DESCRIPTION  DIRECTION  PORT   PROTOCOL
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> Game/RCON  INBOUND    27027  tcp/udp
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> SourceTV   INBOUND    27052  udp
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< Client     OUTBOUND   27002  udp
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Changing Default Ports
Default ports are set in either the start parameters or game config. 
LinuxGSMYou can use ./gameserver details to find out where to edit port settings
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# Ports
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# =====================================
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# Change ports by editing the parameters in:
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# /home/lgsm/qlserver/serverfiles/baseq3/ql-server.cfg
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To alter them you will need to edit the file using nano.
Example: for source games, you need to alter the following:
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# Start Variables
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port="27015"
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sourcetvport="27020"
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clientport="27005"
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ip="0.0.0.0"
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Which ports can be used?
You can use any port as long as it is not already in use and it is up to you to set up port allocation. It is recommended that ports are close together and sequential i.e 27015, 27016, 27017 to save confusion. Some servers ports by default are not sequential, however, there is nothing stopping you from changing this. Example for source engine game servers; you could set the port 27015, source tv port 27016, client port 27018. The next game server instance could simply follow on from this.
If you're running multiple game servers, make sure you're using different ports on all of your game servers to avoid a port conflict.
Multiple IP addresses
Should a server have multiple dedicated IP addresses allocated, it is possible for game servers to have the same ports but bound to the different IP addresses. Admins will need to set the specific IP address in the LinuxGSM config or game config.
Port Allocation Scheme
Each type of server has a default set of ports. This is fine to use if only one server is being set up. However, if multiple servers are being used a port scheme needs to be considered. There is no requirement to stick to specific ports but it is recommended they are in a logical order.
Source Default Ports
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port="27015"
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sourcetvport="27020"
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clientport="27005"
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Incremental (recommended)
You can allocate ports incrementally, one after the other. With each server following on from the last.
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port="27015"
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sourcetvport="27016"
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clientport="27017"
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This scheme is the simplest option to avoid confusion when managing ports. Plus this is the most efficient scheme reducing unused ports.
Standard Port Increment
This method increments each type of port keeping the scheme closer to the default. It is not as efficient but keeps ports more "standard".
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port="27016"
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sourcetvport="27021"
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clientport="27006"
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Then increment those port by 1 for every new server. 25 servers are using 75 ports. After making 25 servers, if you need more it is a good idea is to add +100 to this 27000 range.
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port="27125"
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sourcetvport="27150"
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clientport="27100"
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Separate IP Addresses
You can get new IP addresses for your dedicated server, and assign each server a new IP. This scheme is quite inefficient.
Track your ports
If you are running several game servers it is a good idea to create a spreadsheet of the ports you have used. Allowing you to keep track of what you have already used.
Diagnosing server accessibility
Check port are bound
Ensure the game server ports are bound and listening before anything else, you need to know if your server is actually listening. Input : ./gameserver details and look for this kind of diagnose command:
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Useful port diagnostic command:
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netstat -atunp | grep srcds_linux
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Running the command will bring up any ports that are listening. If not, the game server has not started or not correctly binding to its allocated ports. 
Check that ports are not already in use by something else.
Check that you are trying to listen to an actual interface IP.
Check that ports are not already in use by another application.
check that the server does not crash upon start by checking console logs or try starting the server with ./gameserver debug
Check the Firewall
​Firewalls are a regular source of connectivity issues. When diagnosing connection issues temporarily disabling a firewall will help identify if it is the source of the problem.
Check Port Forwarding (Local Networks)
Ensure that the router correctly redirects incoming traffic to the correct ports and local IP displayed with ./gameserver details with the correct protocol (TCP and/or UDP).