[Howto] Run programs as non-root user on privileged ports via Systemd

TuxRunning programs as a non-root user is must in security sensitive environments. However, these programs sometimes need to publish their service on privileged ports like port 80 – which cannot be used by local users. Systemd offers a simple way to solve this problem.

Background

Running services as non-root users is a quite obvious: if it is attacked and a malicious user gets control of the service, the rest of the system should still be secure in terms of access rights.

At the same time plenty programs need to publish their service at ports like 80 or 443 since these are the default ports for http communication and thus for interfaces like REST. But these ports are not available to non-root users.

Problem shown at the example gitea

To show how to solve this with systemd, we take the self hosted git service gitea as an example. Currently there are hardly any available packages, so most people end up installing it from source, for example as the user git. A proper sysmted unit file for such an installation in a local path, running the service as a local user, is:

$ cat /etc/systemd/system/gitea.service
[Unit]
Description=Gitea (Git with a cup of tea)
After=syslog.target
After=network.target
After=postgresql.service

[Service]
RestartSec=2s
Type=simple
User=git
Group=git
WorkingDirectory=/home/git/go/src/code.gitea.io/gitea
ExecStart=/home/git/go/src/code.gitea.io/gitea/gitea web
Restart=always
Environment=USER=git HOME=/home/git

[Install]
WantedBy=multi-user.target

If this service is started, and the application configuration is set to port 80, it fails during the startup with a bind error:

Jan 04 09:12:47 gitea.qxyz.de gitea[8216]: 2018/01/04 09:12:47 [I] Listen: http://0.0.0.0:80
Jan 04 09:12:47 gitea.qxyz.de gitea[8216]: 2018/01/04 09:12:47 [....io/gitea/cmd/web.go:179 runWeb()] [E] Failed to start server: listen tcp 0.0.0.0:80: bind: permission denied

Solution

One way to tackle this would be a reverse proxy, running on port 80 and forwarding traffic to a non-privileged port like 8080. However, it is much more simple to add an additional systemd socket which listens on port 80:

$ cat /etc/systemd/system/gitea.socket
[Unit]
Description=Gitea socket

[Socket]
ListenStream=80
NoDelay=true

As shown above, the definition of a socket is straight forward, and hardly needs any special configuration. We use NoDelay here since this is a default for Go on sockets it opens, and we want to imitate that.

Given this socket definition, we add the socket as requirement to the service definition:

[Unit]
Description=Gitea (Git with a cup of tea)
Requires=gitea.socket
After=syslog.target
After=network.target
After=postgresql.service

[Service]
RestartSec=2s
Type=simple
User=git
Group=git
WorkingDirectory=/home/git/go/src/code.gitea.io/gitea
ExecStart=/home/git/go/src/code.gitea.io/gitea/gitea web
Restart=always
Environment=USER=git HOME=/home/git
NonBlocking=true

[Install]
WantedBy=multi-user.target

As seen above, the unit definition hardly changes, only the requirement for the socket is added – and NonBlocking as well, to imitate Go behavior.

That’s it! Now the service starts up properly and everything is fine:

[...]
Jan 04 09:21:02 gitea.qxyz.de gitea[8327]: 2018/01/04 09:21:02 Listening on init activated [::]:80
Jan 04 09:21:02 gitea.qxyz.de gitea[8327]: 2018/01/04 09:21:02 [I] Listen: http://0.0.0.0:80
Jan 04 09:21:08 gitea.qxyz.de gitea[8327]: [Macaron] 2018-01-04 09:21:08: Started GET / for 192.168.122.1
[...]

Sources, further reading

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[Howto] Workaround failing MongoDB on RHEL/CentOS 7

Ansible LogoMongoDB is often installed right from upstream provided repositories. In such cases with recent updates the service might fail to start via systemctl. A workaround requires some SELinux work.

Ansible Tower collects system data inside a MongoDB. Since MongoDB is not part of RHEL/CentOS, it is installed directly form the upstream MongoDB repositories. However, with recent versions of MongoDB the database might not come up via systemctl:

[root@ansible-demo-tower init.d]# systemctl start mongod
Job for mongod.service failed because the control process exited with error code. See "systemctl status mongod.service" and "journalctl -xe" for details.
[root@ansible-demo-tower init.d]# journalctl -xe
May 03 08:26:00 ansible-demo-tower systemd[1]: Starting SYSV: Mongo is a scalable, document-oriented database....
-- Subject: Unit mongod.service has begun start-up
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit mongod.service has begun starting up.
May 03 08:26:00 ansible-demo-tower runuser[7266]: pam_unix(runuser:session): session opened for user mongod by (uid=0)
May 03 08:26:00 ansible-demo-tower runuser[7266]: pam_unix(runuser:session): session closed for user mongod
May 03 08:26:00 ansible-demo-tower mongod[7259]: Starting mongod: [FAILED]
May 03 08:26:00 ansible-demo-tower systemd[1]: mongod.service: control process exited, code=exited status=1
May 03 08:26:00 ansible-demo-tower systemd[1]: Failed to start SYSV: Mongo is a scalable, document-oriented database..
-- Subject: Unit mongod.service has failed
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit mongod.service has failed.
-- 
-- The result is failed.
May 03 08:26:00 ansible-demo-tower systemd[1]: Unit mongod.service entered failed state.
May 03 08:26:00 ansible-demo-tower systemd[1]: mongod.service failed.
May 03 08:26:00 ansible-demo-tower polkitd[11436]: Unregistered Authentication Agent for unix-process:7254:1405622 (system bus name :1.184, object path /org/freedesktop/PolicyKit1/AuthenticationAgent, locale en_

The root cause of the problem is that the MongoDB developers do not provide a proper SELinux</a configuration with their packages, see the corresponding bug report.

A short workaround is to create a proper (more or less) SELinux rule and install it to the system:

[root@ansible-demo-tower ~]# grep mongod /var/log/audit/audit.log | audit2allow -m mongod > mongod.te
[root@ansible-demo-tower ~]# cat mongod.te 

module mongod 1.0;

require {
	type locale_t;
	type mongod_t;
	type ld_so_cache_t;
	class file execute;
}

#============= mongod_t ==============
allow mongod_t ld_so_cache_t:file execute;
allow mongod_t locale_t:file execute;
[root@ansible-demo-tower ~]# grep mongod /var/log/audit/audit.log | audit2allow -M mongod
******************** IMPORTANT ***********************
To make this policy package active, execute:

semodule -i mongod.pp

[root@ansible-demo-tower ~]# semodule -i mongod.pp 
[root@ansible-demo-tower ~]# sudo service mongod start
                                                           [  OK  ]

Keep in mind that audit2allow generated rule sets are not to be used on production systems. The generated SELinux rules need to be analyzed manually to verify that it covers nothing but the problematic use case.