I need to allow a user (different from root) to run a server listening on port 80.
Is there any way to do this?
setcap 'cap_net_bind_service=+ep' /path/to/program
this will work for specific processes. But to allow a particular user to bind to ports below 1024 you will have to add him to sudoers.
Have a look at this discussion for more.
(Some of these methods have been mentioned in other answers; I'm giving several possible choices in rough order of preference.)
You can redirect the low port to a high port and listen on the high port.
iptables -t nat -A PREROUTING -p tcp --dport 80 -j REDIRECT --to-ports 1080
You can start your server as root and drop privileges after it's started listening on the privileged port. Preferably, rather than coding that yourself, start your server from a wrapper that does the job for you. If your server starts one instance per connection, start it from
inetd (or a similar program such as
inetd, use a line like this in
http stream tcp nowait username:groupname /path/to/server/executable argv argv…
If your server listens in a single instance, start it from a program such as
authbind. Either create an empty file
/etc/authbind/byport/80 and make it executable to the user running the server; or create
/etc/authbind/byuid/1234, where 1234 is the UID running the server, containing the line
If your server executable is stored on a filesystem that supports capabilities, you can give it the
cap_net_bind_service capability. Beware that capabilities are still relatively new and still have a few kinks.
setcap cap_net_bind_service=ep /path/to/server/executable
The short answer is that this is not possible by design.
The long answer is that in the open source worlds there are lots of people playing with the design and coming up with alternate methods. In general it is widely accepted practice that this should not be possible. The fact that you are trying probably means you have another design fault in your system and should reconsider your whole system architecture in light of *nix best practices and security implications.
Lastly, if you want specific users to run specific programs as root and what you really need is just to allow a user to restart apache or something like that,
sudo is your friend!
You could use netcat or xinetd or iptables port forwarding, or use apache as a front end proxy and run the process on a non-privileged port.
Authbind, @Gilles already mentioned it, but I'd like to expand on it a bit.
It has convenient access control (details in man page): you can filter access by port, interface address, uid, ranges of address or port and combination of these.
It has very useful parameter
Causes authbind to affect programs which are levels deep in the calling graph. The default is 1.
"Levels deep" means when a script (or program), runs another script it descends a level. So if you have
--depth 5 it means on levels 1 (or is it 0?) through 5 you have permission to bind, whereas on level 6 and on, you don't. Useful when you want a script to have access, but not programs that it runs with or without your knowledge.
To illustrate, you could have something like this: for sake of security, you have a user
java that is meant to only run java and you want to give him access to port 80:
sudo echo > /etc/authbind/byport/80 && chown root:java /etc/authbind/byport/80 chmod 710 /etc/authbind/byport/80
I've created the
../byport/80 file, given it to
java users group (each user has it's own group), and made it executable by group, which means it's executable by
java user. If you're giving access by port, the file has to be executable by the user that should have access, so we did that.
This might be enough for the average Joe, but because you know how to use the
--depth parameter, you run (as
authbind --depth [depth] my_web_app's_start_script starting at
--depth 1 and working your way up until you find the smallest depth that works and use that.
I tried the iptables PREROUTING REDIRECT method, but found that it also affects forwarded packets. That is, if the machine is also forwarding packets between interfaces (e.g. if it's acting as a Wi-Fi access point connected to an Ethernet network), then the iptables rule will also catch connected clients' connections to Internet destinations, and redirect them to the machine. That's not what I wanted—I only wanted to redirect connections that were directed to the machine itself.
One possibility is to use TCP port forwarding. E.g. using
socat TCP4-LISTEN:www,reuseaddr,fork TCP4:localhost:8080
However one disadvantage with that method is, the application that is listening on port 8080 then doesn't know the source address of incoming connections (e.g. for logging or other identification purposes).