Forwarding With iptables
The default Jenkins installation runs on ports 8080 and 8443. Typically, HTTP/HTTPS servers run on ports 80 and 443, respectively. But these ports are considered privileged on Unix/Linux systems, and the process using them must be owned by root. Running Jenkins as root is not recommended - it should be run as its own user. One solution is to front Jenkins with a web server such as Apache, and let it proxy requests to Jenkins, but this requires maintaining the Apache installation as well. In situations where you are wanting to run Jenkins on port 80 or 443 (i.e. HTTP/HTTPS), but you do not want to setup a proxy server you can use iptables on Linux to forward traffic.
The article Installing Jenkins on Ubuntu includes a similar procedure that has been independently verified to work on Ubuntu 16 LTS, whereas the procedures shown below did not.
In order to forward traffic from 80/443 to 8080/8443, first you must ensure that iptables has allowed traffic on all 4 of these ports. Use the following command to list the current iptables configuration:
You should should see in the output entries for 80, 443, 8080,and 8443. Here is an example output for comparison.
If you don't see entries for these ports, then you need to run commands (as root or with sudo) to add those ports. For example, if you see none of these and need to add them all, you would need to issue the following commands:
Note that I used -I INPUT 1. In a lot of iptables documentation/examples, you will see -A INPUT. The difference is that -A appends to the list of rules, while -I INPUT 1 inserts before the first entry. Usually when adding new accept ports to iptables configuration, you want to put them at the beginning of the ruleset, not the end. Run iptables -L -n again and you should now see entries for these 4 ports.
Once the before and after forwarding ports are allowed, you can now run the command to forward port 80 traffic to 8080, and port 443 traffic to 8443. The commands look like this:
Once these rules are set and confirmed with iptables -L -n, and once your Jenkins instance is up and running on port 8080, attempt to access your Jenkins instance on port 80 instead of 8080. It should work and your URL should stay on port 80 - in other words, it should not get redirected to 8080. The fact that forwarding from 80 to 8080 (or 443 to 8443) should remain hidden from the client.
Saving iptables Configuration
Using the iptables command to change port configuration and routing rules only changes the current, in-memory configuration. It does not persist between restarts of the iptables service. So, you need to make sure you save the configuration to make the changes permanent.
Saving the configuration is slightly different between RedHat-based and Debian-based systems. On a RedHat-based system (Fedora, CentOS, RHEL, etc), issue the following command:
On a Debian-based system (Debian, Ubuntu, Mint, etc), issue the following command:
The iptables-restore command will need to be executed manually, or your system configured to automatically run it on boot, against the /etc/iptables.rules file you have created, in order for your iptables configuration to be retained across reboots. On Ubuntu fastest way is to install iptables-persistent after configuring iptables - it will automatically create necessery files from current configuration and load them on boot.
See https://help.ubuntu.com/community/IptablesHowTo for other Ubuntu options. There are many other resources describing this; please consult your system's documentation or search on the internet for information specific to your flavor of Linux.
If you are unsure at all about what kind of system you have, consult that system's documentation on how to update iptables configuration.
Note that an earlier version of this documentation had different iptables commands for forwarding ports. I found that these did not work for me, and so I researched and came up with the information above. I do not believe that DNAT is required, as it is typically used to publish a service from an internal network to a publicly accessible IP. But, if the above forwarding commands do not work for you, you might want to consider trying the commands below. Keep in mind that the rules to allow incoming traffic are still required in order for these rules to work:
iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 -j DNAT --to-destination 127.0.0.1:8080
The first rule uses -j DNAT target of the PREROUTING chain in NAT to specify a destination IP address and port where incoming packets requesting a connection to your Jenkins service can be forwarded.
If you have a default policy of DROP in your FORWARD chain, you will need to add a rule to forward all incoming traffic.
iptables -A FORWARD -i eth0 -m state --state NEW -m tcp -p tcp -d 127.0.0.1 --dport 8080 -j ACCEPT
For HTTPS you will need to repeat the above steps but specify port 443 instead of port 80.
Newer Linux distributions (CentOS 7, RHEL 7, etc.) ship with firewalld which serves as a front-end for iptables. Configuration thru firewalld is done via the firewall-cmd command. Instead of using any of the iptables commands mentioned above, all you should need to do is something like:
With the above commands, jenkins can be configured to run on localhost:8080 and/or localhost:8443 (depending if you need or want to do SSL or not)
firewalld will then create the required iptables rules so that incoming connections on port 80 are forwarded to jenkins on 8080 (and 443 is forwarded to 8443).