Add privileged ports post

content/posts/allow-non-root-processes-to-bind-to-privileged-ports.md

@@ -0,0 +1,68 @@
++++
+title = "Allow Non Root Processes to Bind to Privileged Ports"
+date = "2021-03-28T20:03:16-05:00"
+author = "bbaovanc"
+tags = ["guide", "linux", "systemd"]
+keywords = ["linux", "privileged", "ports", "tutorial", "guide", "gitea",
+"systemd"]
+
+description = """
+In Linux, processes cannot bind to privileged ports (<=1024) unless they are
+running as root. Here's how to allow any process to bind to privileged ports.
+"""
+
+showFullContent = false
+toc = true
++++
+
+## Introduction
+
+In Linux, processes cannot bind to privileged ports (<=1024) unless they are
+running as root. I learned about this when I was trying to add SSH cloning to my
+[Gitea](https://gitea.io) instance. This can be bypassed by giving
+`CAP_NET_BIND_SERVICE` capabilities to either the systemd service, or the
+executable itself.
+
+## Giving `CAP_NET_BIND_SERVICE` capabilities
+
+### Using systemd (preferred)
+
+The best way is to tell systemd to give `CAP_NET_BIND_SERVICE`
+capabilities to the service. In fact, the Gitea systemd service has two
+lines[^1] that are commented out:
+
+```systemd
+CapabilityBoundingSet=CAP_NET_BIND_SERVICE
+AmbientCapabilities=CAP_NET_BIND_SERVICE
+```
+
+Uncommenting these two lines was all I had to do for Gitea.
+
+### Using `setcap`
+
+You can add `CAP_NET_BIND_SERVICE` to the executable directly using `setcap`,
+allowing it to bind to any port. Run the following command[^2]:
+
+```bash
+setcap 'cap_net_bind_service=+ep' /path/to/program
+```
+
+Note that this means that anyone with permission to run this program will be
+able to run it and bind to any privileged ports.
+
+Other caveats[^2]:
+
+> 1. You will need at least a 2.6.24 kernel
+> 2. This won't work if your file is a script. (ie, uses a #! line to launch an
+>    interpreter). In this case, as far I as understand, you'd have to apply the
+>    capability to the interpreter executable itself, which of course is a
+>    security nightmare, since any program using that interpreter will have the
+>    capability. I wasn't able to find any clean, easy way to work around this
+>    problem.
+> 3. Linux will disable LD\_LIBRARY\_PATH on any program that has elevated
+>    privileges like setcap or suid. So if your program uses its own .../lib/,
+>    you might have to look into another option like port forwarding.
+
+[^1]: https://github.com/go-gitea/gitea/blob/3416e2a82586fca4cd452b93237b979300f55d62/contrib/systemd/gitea.service#L69
+      and https://stackoverflow.com/a/47065825
+[^2]: https://stackoverflow.com/a/414258