Ensure that hostname is added to hosts with net=host

When a container uses --net=host the default hostname is set to
the host's hostname. However, we were not creating any entries
in `/etc/hosts` despite having a hostname, which is incorrect.
This hostname, for Docker compat, will always be the hostname of
the host system, not the container, and will be assigned to IP
127.0.1.1 (not the standard localhost address).

Also, when `--hostname` and `--net=host` are both passed, still
use the hostname from `--hostname`, not the host's hostname (we
still use the host's hostname by default in this case if the
`--hostname` flag is not passed).

Fixes #8054

Signed-off-by: Matthew Heon <matthew.heon@pm.me>

3 files changed, 40 insertions, 5 deletions

diff --git a/libpod/container_internal_linux.go b/libpod/container_internal_linux.go
index eff390e46..ffb2f5b73 100644
--- a/libpod/container_internal_linux.go
+++ b/libpod/container_internal_linux.go
@@ -1541,11 +1541,31 @@ func (c *Container) getHosts() string {
 		}
 	}
 
-	if c.config.NetMode.IsSlirp4netns() {
-		// When using slirp4netns, the interface gets a static IP
-		hosts += fmt.Sprintf("# used by slirp4netns\n%s\t%s %s\n", "10.0.2.100", c.Hostname(), c.Config().Name)
-	}
 	hosts += c.cniHosts()
+
+	// If not making a network namespace, add our own hostname.
+	if c.Hostname() != "" {
+		if c.config.NetMode.IsSlirp4netns() {
+			// When using slirp4netns, the interface gets a static IP
+			hosts += fmt.Sprintf("# used by slirp4netns\n%s\t%s %s\n", "10.0.2.100", c.Hostname(), c.config.Name)
+		} else {
+			hasNetNS := false
+			for _, ns := range c.config.Spec.Linux.Namespaces {
+				if ns.Type == spec.NetworkNamespace {
+					hasNetNS = true
+					break
+				}
+			}
+			if !hasNetNS {
+				// 127.0.1.1 and host's hostname to match Docker
+				osHostname, err := os.Hostname()
+				if err != nil {
+					osHostname = c.Hostname()
+				}
+				hosts += fmt.Sprintf("127.0.1.1 %s\n", osHostname)
+			}
+		}
+	}
 	return hosts
 }
 
diff --git a/pkg/specgen/generate/namespaces.go b/pkg/specgen/generate/namespaces.go
index 7adb8be6a..7e4f09dc4 100644
--- a/pkg/specgen/generate/namespaces.go
+++ b/pkg/specgen/generate/namespaces.go
@@ -342,7 +342,7 @@ func specConfigureNamespaces(s *specgen.SpecGenerator, g *generate.Generator, rt
 				return errors.Wrapf(err, "error looking up container to share uts namespace with")
 			}
 			hostname = utsCtr.Hostname()
-		case s.NetNS.NSMode == specgen.Host || s.UtsNS.NSMode == specgen.Host:
+		case (s.NetNS.NSMode == specgen.Host && hostname == "") || s.UtsNS.NSMode == specgen.Host:
 			tmpHostname, err := os.Hostname()
 			if err != nil {
 				return errors.Wrap(err, "unable to retrieve hostname of the host")
diff --git a/test/e2e/run_networking_test.go b/test/e2e/run_networking_test.go
index e14482db7..540ac5409 100644
--- a/test/e2e/run_networking_test.go
+++ b/test/e2e/run_networking_test.go
@@ -571,4 +571,19 @@ var _ = Describe("Podman run networking", func() {
 		podrm.WaitWithDefaultTimeout()
 		Expect(podrm.ExitCode()).To(BeZero())
 	})
+
+	It("podman run net=host adds entry to /etc/hosts", func() {
+		run := podmanTest.Podman([]string{"run", "--net=host", ALPINE, "cat", "/etc/hosts"})
+		run.WaitWithDefaultTimeout()
+		Expect(run.ExitCode()).To(BeZero())
+		Expect(strings.Contains(run.OutputToString(), "127.0.1.1")).To(BeTrue())
+	})
+
+	It("podman run with --net=host and --hostname sets correct hostname", func() {
+		hostname := "testctr"
+		run := podmanTest.Podman([]string{"run", "--net=host", "--hostname", hostname, ALPINE, "hostname"})
+		run.WaitWithDefaultTimeout()
+		Expect(run.ExitCode()).To(BeZero())
+		Expect(strings.Contains(run.OutputToString(), "testctr")).To(BeTrue())
+	})
 })