Podman: Issues with multiple users running containers simultaneously

Submitted ⁨⁨2⁩ ⁨weeks⁩ ago⁩ by ⁨MoogleMaestro@lemmy.zip⁩ to ⁨selfhosted@lemmy.world⁩

Self cross-posting from: lemmy.zip/post/70909658

Intention to have slightly better visibility from the self-hosted crowd and I’m interested in more general feedback on this too.

Hey everyone! I’m trying to find a solution to a really confusing problem…

I have the following simple nginx docker compose configuration on my Fedora home server that I can run without issue on my uid 1000 user, lets call this user “userA”.
services:
  nginx:
    container_name: nginx-alt
    image: docker.io/library/nginx
    restart: unless-stopped
    ports:
      - 8181:80
This exposes internal port 80 as 8181 and can be accessed in a lan in the expected matter.

However, for security reasons, I want to actually host this service eventually on a completely different user with less permissions. Let’s call this user “userB” who has a very limited scope of the file system. This is to prevent potential escaping of the rootless container causing major file system havoc (i.e. reduce the scope of the user to a very limited network of containers.)

The problem is really simple: For some reason, when userB runs this service (uid 1001), the nginx service suddenly complains about privileges. As a result, I get a “Forbidden 403” error when hosting. Turning off selinux has no affect (so setenforce 0 does nothing, meaning I can rule out secure linux interruption.)

The errors look like the following:
nginx-alt  | 2026/09/04 20:03:34 [error] 25#25: *1 "/usr/share/nginx/html/index.html" is forbidden (13: Permission denied), client: xx.xx.x.x, server: localhost, request: "GET / HTTP/1.1", host: "xxx.xxx.xxx.xxx:8181"
nginx-alt  | 10.89.0.2 - - [04/Sep/2026:20:03:34 +0000] "GET / HTTP/1.1" 403 153 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:155.0) Gecko/20100101 Firefox/155.0" "-"
For what it’s worth, both users should be relatively vanilla and all ports are appropriately exported. There shouldn’t be anything, for example, that is making userA run as “privileged” over the other users and podman should be running rootless in both containers.

I did see a note on the nginx image about running in rootless that I might try, but it doesn’t solve my bigger issue here which is the lack of consistency between the two users. Additionally, userns_mode: keep-ids only caused the container to fail to boot for other reason entirely.

There must be something fundamentally wrong with my configuration of my system. Has anyone had any experience running two podman containers on two different users simultaneously that can provide feedback?

Obviously, I’m not trying to run just an nginx server, but I found this to be the easiest configuration to reproduce.

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glizzyguzzler@piefed.blahaj.zone ⁨2⁩ ⁨weeks⁩ ago
The best way to run Podman is root with UserNS to dole out UID/GID protection. Running Podman as root allows you to share networks between containers while having the containers run under different users. If you go rootless, you’d need to run under one user to share the user’s network space with all the containers you want.

As for your issue, I can’t really divine what the problem is from the errors. I avoid nginx because it’s coded to not play well with user abstraction and changing the user with the files it wants to write to etc. Gotta write into a ton of random folders! So not sure exactly what is up. But with Podman root it is easy to run as root 0 internally and make nginx think it has all the control it could ever want.

Try this setup (it is in Podman Quadlet format, apologies I don’t know the compose versions). It runs the container as root 0 internally, externally it runs as some random UID/GID - secure! It uses Volume idmap to map the internal root 0 user to 1001 for write access to the Volume.

Note that in Debian 13 symlinks are broken and won’t work with idmap, just point to the original source. If you need symlinks, I have an alternate UserNS that maps internal user root 0 to external user 1001 directly. You’d drop the idmap in Volume then and use that. You lose some extra security - now the container is running as external user 1001 instead of some random UID/GID - but that’s a pretty minor hit as long as your external user doesn’t have access to tons of things.

# Volumes to mount -> the @ is essential for saying "1001 is absolute and external" basically. 0 is internal. size of 1. You can map 1001 to 0 and 1002 to 1 with @1001-0-2, etc., etc., etc. Volume=/mnt/something:/etc/nginx/wants/to/write/here:rw,noexec,nosuid,nodev,Z,idmap=uids=@1001-0-1;gids=@1001-0-1 # Run as user running the container UserNS=auto # [use this if req symlink b/c idmap does NOT work with symlinks] -> I tested and it is fixed in at least Podman v5.8.3, so Debian 14 will work with idmap and symlinks directly ! drop the idmap if using ! # UserNS=auto:uidmapping=0:@1001:1,gidmapping=0:@1001:1 # Security time NoNewPrivileges=true # https://man7.org/linux/man-pages/man7/capabilities.7.html DropCapability=all ReadOnly=true ReadOnlyTmpfs=True # These capabilities are needed for linuxserver's s6 "launcher" thing #AddCapability=CAP_CHOWN #AddCapability=CAP_DAC_OVERRIDE #AddCapability=CAP_FOWNER #AddCapability=CAP_SETGID #AddCapability=CAP_SETUID # Needs this if the container tries to bind below port 1024. I'm not sure if it is needed if it only tries to bind internally. #AddCapability=CAP_NET_BIND_SERVICE

Root Podman and UserNS=auto needs a containers user to pull uid/gid from.

# Root Podman needs a `containers` "user" (not really a user, just a reserved uid/gid space) sudo echo "containers:2147483647:2147483648" >> /etc/subuid sudo echo "containers:2147483647:2147483648" >> /etc/subgid

The documentation for Podman is critically lacking in the “hobbyist” space. Hope this helps.
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- forbiddenlake@lemmy.world ⁨2⁩ ⁨weeks⁩ ago
  Note that sudo echo does not work as non root because the shell redirection is attempted before the command runs. You want echo foo | sudo tee -a /bar instead
  
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  - glizzyguzzler@piefed.blahaj.zone ⁨2⁩ ⁨weeks⁩ ago
    So the sudo echo does echo as sudo but doesn’t carry over? Makes sense damn I hate bash! Any way to keep the >>? Or do you need to tee? Cause the >> is pretty cool
    
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in_my_honest_opinion@piefed.social ⁨2⁩ ⁨weeks⁩ ago
You might want to switch to rootless quadlets with subuids

https://www.golinuxcloud.com/rootless-podman-quadlet/

You’ll get unified logging through systemctl and journalctl, you’ll also get segmented user space and if you need persistent storage that you can gate at subuid.

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robber@lemmy.ml ⁨2⁩ ⁨weeks⁩ ago
You use podman-compose with the Podman backend?

And there are no volumes mounted, just to be sure?

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- MoogleMaestro@lemmy.zip ⁨2⁩ ⁨weeks⁩ ago
  No volumes mounted and yes, it’s using the podman backend. Just double checked it wasn’t using the docker one.
  
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  - robber@lemmy.ml ⁨2⁩ ⁨weeks⁩ ago
    The service starts as it should right? The errors appear only on request, right?
    
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rhymepurple@lemmy.ml ⁨2⁩ ⁨weeks⁩ ago
This is due to a security design deicison of Podman. Each user’s network(s) is only available to that user. This is great for most services, but can cause issues for some services - especially reverse proxies. Unfortunately, I’m not aware of an ideal solution. The only solution I’ve seen is moving the reverse proxy to another host and exposing the services’ ports on the localhost. I hope someone can provide a better solution!

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- robber@lemmy.ml ⁨2⁩ ⁨weeks⁩ ago
  No expert but don’t the logs look like the requests are coming through?
  
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  - rhymepurple@lemmy.ml ⁨2⁩ ⁨weeks⁩ ago
    I didn’t see the logs when I originally posted. However, I’m not sure what the logs indicate. It could be that nginx successfully received the request and received an authorization error from Podman’s networking stack then returned a 503 error to the client (or logged the 503 response that was returned to nginx).
    
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- MoogleMaestro@lemmy.zip ⁨2⁩ ⁨weeks⁩ ago
  OK now this would make some sense but would definitely be a bit of a show stopper for me. I’ll give it a try though.
  
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  - rhymepurple@lemmy.ml ⁨2⁩ ⁨weeks⁩ ago
    I’m not trying to convince you of this solution (I personally don’t like it), but I am curious what didn’t work for you. Were you unable to get the reverse proxy to serve each service? Were you unable to have the services behind the reverse proxy to talk to each other?
    
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  - Overspark@piefed.social ⁨2⁩ ⁨weeks⁩ ago
    The solution to this can be multiple reverse proxies. Each user runs it’s own reverse proxy (if you actually need one, otherwise you let the container bind directly to a port on the host system). Then you run one main reverse proxy on port 80/443 that proxies to those other ports based on hostname.
    
    The upside of this construction is that containers running as different users can’t directly access each others internal networks, which is much better for security, as those networks often contain barely protected services, which is why you’re using a reverse proxy in the first place.
    
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chameleon@fedia.io ⁨2⁩ ⁨weeks⁩ ago
Complete shot in the dark, but I would start by comparing podman system info between the two user accounts, with the most importance on the idMappings. Podman is usually configured to use rootless by default, and this can result in wildly different behavior based on whether /etc/subuid and /etc/subgid are configured to give your user IDs or not. See tutorial if your user 1001 is missing.

Modern distro installers tend to give the user created during setup 65536 IDs, but useradd usually doesn't do that by default. This is really distro-specific stuff though and I don't know how modern Fedora handles it, so I might be grasping at nothing here.

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- MoogleMaestro@lemmy.zip ⁨2⁩ ⁨weeks⁩ ago
  I think I have uids set up, but I’m not positive they’re set up correctly.
  
  Here’s an example of /etc/subuid for example, with it a bit anonymised for my specific user setup:
  
  core:524288:65536 syncthing:589824:65536 userA:655360:65536 userB:720896:65536 userC:786432:65536
  
  Syncthing in this case isn’t running any podman containers, all sharing is done bare-metal. userA is working, but userB and userC do look like they’re configured “correctly” at first glance. podman system info seems to be outputting the results above
  
  idMappings: gidmap: - container_id: 0 host_id: 1101 size: 1 - container_id: 1 host_id: 786432 size: 65536 uidmap: - container_id: 0 host_id: 1101 size: 1 - container_id: 1 host_id: 786432 size: 65536
  
  Note: The user in question here is uid 1101, another new user I created for testing purposes.
  
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  - chameleon@fedia.io ⁨2⁩ ⁨weeks⁩ ago
    That looks correct to me if uid 1101 == userC. Sorry, don't really know what it might be then.
    
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frongt@lemmy.zip ⁨2⁩ ⁨weeks⁩ ago
“/usr/share/nginx/html/index.html” is forbidden (13: Permission denied)

Does the user have permission to that file?

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- MoogleMaestro@lemmy.zip ⁨2⁩ ⁨weeks⁩ ago
  As far as I understand, it should as it’s embedded inside the actual image file and not mounted.
  
  I’m having other strange behaviour on non-1000 uid accounts. For example, running docker.io/library/httpd using the podman examples page on an account that isn’t userid 1000 will create an issue where it complains that libgcc_0 isn’t installed. This works flawlessly on the uid 1000 user.
  
  It does make me think that something needs to be configured for non-uid 1000 users on podman. This is not replicable on arch linux on my raspberry pi, fwiw.
  
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  - frongt@lemmy.zip ⁨2⁩ ⁨weeks⁩ ago
    Shell into the container and find out.
    
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InnerScientist@lemmy.world ⁨2⁩ ⁨weeks⁩ ago
What are the assigned subuids and subgids for userA and userB?

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Svinhufvud@sopuli.xyz ⁨2⁩ ⁨weeks⁩ ago
Why not just use subuids for separation instead?

podman run -d --restart unless-stopped \ --uidmap "+0:@100001:1" \ --name nginx-1 \ --port 8081:80 \ docker.io/library/nginx

podman run -d --restart unless-stopped \ --uidmap "+0:@100002:1" \ --name nginx-2 \ --port 8082:80 \ docker.io/library/nginx
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- MoogleMaestro@lemmy.zip ⁨2⁩ ⁨weeks⁩ ago
  I mean, I might be wrong, but this only changes the mapping right? In the case that user broke containment, I would assume that they would have the user id of the user that’s running the process?
  
  Granted this is all in theory so I’m not sure exactly – that might be a perfectly reasonable way to do it.
  
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