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>

2 files changed, 13 insertions, 3 deletions

diff --git a/libpod/networking_linux.go b/libpod/networking_linux.go
index 6e2c2880f..cfed5a1f2 100644
--- a/libpod/networking_linux.go
+++ b/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
 }
diff --git a/libpod/networking_slirp4netns.go b/libpod/networking_slirp4netns.go
index 72ab3c919..c46dc6972 100644
--- a/libpod/networking_slirp4netns.go
+++ b/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 {