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+++ title = "Allow Non Root Processes to Bind to Privileged Ports" date = "2021-03-28" aliases = [ "posts/allow-non-root-processes-to-bind-to-privileged-ports/", ]
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. """
tags = [ "guide", "linux", ] +++
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 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
lines1 that are commented out:
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 command2:
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 caveats2:
- You will need at least a 2.6.24 kernel
- 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.
- 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.