Fix rootlesskit port forwarder with custom slirp cidr

The source ip for the rootlesskit port forwarder was hardcoded to the
standard slirp4netns ip. This is incorrect since users can change the
subnet used by slirp4netns with `--network slirp4netns:cidr=10.5.0.0/24`.
The container interface ip is always the .100 in the subnet. Only when
the rootlesskit port forwarder child ip matches the container interface
ip the port forwarding will work.

Fixes #9828

Signed-off-by: Paul Holzinger <paul.holzinger@web.de>

libpod/networking_linux.go

@@ -577,7 +577,7 @@ func (r *Runtime) setupRootlessNetNS(ctr *Container) error {
 		// set up port forwarder for CNI-in-slirp4netns
 		netnsPath := ctr.state.NetNS.Path()
 		// TODO: support slirp4netns port forwarder as well
-		return r.setupRootlessPortMappingViaRLK(ctr, netnsPath)
+		return r.setupRootlessPortMappingViaRLK(ctr, netnsPath, "")
 	}
 	return nil
 }

libpod/networking_slirp4netns.go

@@ -312,7 +312,7 @@ func (r *Runtime) setupSlirp4netns(ctr *Container) error {
 		if netOptions.isSlirpHostForward {
 			return r.setupRootlessPortMappingViaSlirp(ctr, cmd, apiSocket)
 		}
-		return r.setupRootlessPortMappingViaRLK(ctr, netnsPath)
+		return r.setupRootlessPortMappingViaRLK(ctr, netnsPath, netOptions.cidr)
 	}
 	return nil
 }
@@ -363,7 +363,7 @@ func waitForSync(syncR *os.File, cmd *exec.Cmd, logFile io.ReadSeeker, timeout t
 	return nil
 }
 
-func (r *Runtime) setupRootlessPortMappingViaRLK(ctr *Container, netnsPath string) error {
+func (r *Runtime) setupRootlessPortMappingViaRLK(ctr *Container, netnsPath, slirp4CIDR string) error {
 	syncR, syncW, err := os.Pipe()
 	if err != nil {
 		return errors.Wrapf(err, "failed to open pipe")
@@ -391,6 +391,16 @@ func (r *Runtime) setupRootlessPortMappingViaRLK(ctr *Container, netnsPath strin
 	}
 
 	childIP := slirp4netnsIP
+	// set the correct childIP when a custom cidr is set
+	if slirp4CIDR != "" {
+		_, cidr, err := net.ParseCIDR(slirp4CIDR)
+		if err != nil {
+			return errors.Wrap(err, "failed to parse slirp4netns cidr")
+		}
+		// the slirp container ip is always the hundredth ip in the subnet
+		cidr.IP[len(cidr.IP)-1] = cidr.IP[len(cidr.IP)-1] + 100
+		childIP = cidr.IP.String()
+	}
 outer:
 	for _, r := range ctr.state.NetworkStatus {
 		for _, i := range r.IPs {

test/system/500-networking.bats

@@ -77,13 +77,25 @@ load helpers
     # FIXME: randomize port, and create second random host port
     myport=54321
 
+    for cidr in "" "$(random_rfc1918_subnet).0/24"; do
+        myport=$(( myport + 1 ))
+        if [[ -z $cidr ]]; then
+            # regex to match that we are in 10.X subnet
+            match="10\..*"
+        else
+            # Issue #9828 make sure a custom slir4netns cidr also works
+            network_arg="--network slirp4netns:cidr=$cidr"
+            # slirp4netns interface ip is always .100
+            match="${cidr%.*}.100"
+        fi
+
         # Container will exit as soon as 'nc' receives input
         # We use '-n -v' to give us log messages showing an incoming connection
         # and its IP address; the purpose of that is guaranteeing that the
         # remote IP is not 127.0.0.1 (podman PR #9052).
         # We could get more parseable output by using $NCAT_REMOTE_ADDR,
         # but busybox nc doesn't support that.
-    run_podman run -d --userns=keep-id -p 127.0.0.1:$myport:$myport \
+        run_podman run -d --userns=keep-id $network_arg -p 127.0.0.1:$myport:$myport \
                    $IMAGE nc -l -n -v -p $myport
         cid="$output"
 
@@ -97,15 +109,15 @@ load helpers
         # string, but sometimes we get it after. So, just do substring checks.
         is "$output" ".*listening on \[::\]:$myport .*" "nc -v shows right port"
 
-    # This is the truly important check: make sure the remote IP is
+        # This is the truly important check: make sure the remote IP is not 127.X.
-    # in the 10.X range, not 127.X.
         is "$output" \
-       ".*connect to \[::ffff:10\..*\]:$myport from \[::ffff:10\..*\]:.*" \
+           ".*connect to \[::ffff:$match*\]:$myport from \[::ffff:$match\]:.*" \
-       "nc -v shows remote IP address in 10.X space (not 127.0.0.1)"
+           "nc -v shows remote IP address is not 127.0.0.1"
         is "$output" ".*${teststring}.*" "test string received on container"
 
         # Clean up
         run_podman rm $cid
+    done
 }
 
 # "network create" now works rootless, with the help of a special container