Better document rootless containers

Need to return an error pointing user in right direction if rootless podman
fails, because of no /etc/subuid or /etc/subgid files.

Also fix up man pages to better describe rootless podman.

Signed-off-by: Daniel J Walsh <dwalsh@redhat.com>

2 files changed, 8 insertions, 4 deletions

diff --git a/docs/podman.1.md b/docs/podman.1.md
index c66a9cf05..b7433d850 100644
--- a/docs/podman.1.md
+++ b/docs/podman.1.md
@@ -192,7 +192,7 @@ the exit codes follow the `chroot` standard, see below:
     When Podman runs in rootless mode, the file `$HOME/.config/containers/storage.conf` is also loaded.
 
 ## Rootless mode
-Podman can also be used as non-root user.  When podman runs in rootless mode, an user namespace is automatically created.
+Podman can also be used as non-root user.  When podman runs in rootless mode, a user namespace is automatically created for the user, defined in /etc/subuid and /etc/subgid.
 
 Containers created by a non-root user are not visible to other users and are not seen or managed by podman running as root.
 
@@ -209,13 +209,14 @@ Or just add the content manually.
 	$ echo USERNAME:10000:65536 >> /etc/subuid
 	$ echo USERNAME:10000:65536 >> /etc/subgid
 
+See the `subuid(5)` and `subgid(5)` man pages for more information.
+
 Images are pulled under `XDG_DATA_HOME` when specified, otherwise in the home directory of the user under `.local/share/containers/storage`.
 
-Currently it is not possible to create a network device, so rootless containers need to run in the host network namespace.  If a rootless container creates a network namespace,
-then only the loopback device will be available.
+Currently the slirp4netns package is required to be installed to create a network device, otherwise rootless containers need to run in the network namespace of the host.
 
 ## SEE ALSO
-`containers-mounts.conf(5)`, `containers-registries.conf(5)`, `containers-storage.conf(5)`, `crio(8)`, `libpod.conf(5)`, `oci-hooks(5)`, `policy.json(5)`
+`containers-mounts.conf(5)`, `containers-registries.conf(5)`, `containers-storage.conf(5)`, `crio(8)`, `libpod.conf(5)`, `oci-hooks(5)`, `policy.json(5)`, `subuid(5)`, `subgid(5)`, `slirp4netns(1)`
 
 ## HISTORY
 Dec 2016, Originally compiled by Dan Walsh <dwalsh@redhat.com>
diff --git a/pkg/rootless/rootless_linux.go b/pkg/rootless/rootless_linux.go
index 5c45f2694..ff8c8fe34 100644
--- a/pkg/rootless/rootless_linux.go
+++ b/pkg/rootless/rootless_linux.go
@@ -187,6 +187,9 @@ func BecomeRootInUserNS() (bool, int, error) {
 	if username == "" {
 		user, err := user.LookupId(fmt.Sprintf("%d", os.Getuid()))
 		if err != nil && os.Getenv("PODMAN_ALLOW_SINGLE_ID_MAPPING_IN_USERNS") == "" {
+			if os.IsNotExist(err) {
+				return false, 0, errors.Wrapf(err, "/etc/subuid or /etc/subgid does not exist, see subuid/subgid man pages for information on these files")
+			}
 			return false, 0, errors.Wrapf(err, "could not find user by UID nor USER env was set")
 		}
 		if err == nil {