qemu.rst: slirp port forwarding details

Explain the default behavior of `runqemu slirp`.
Explain how to forward ports from the guest to the host machine.

(From yocto-docs rev: 239314da5998f27d0e3a0f2e538216e852bb19d8)

Signed-off-by: Atanas Bunchev <atanas.bunchev@konsulko.com>
Reviewed-by: Quentin Schulz <foss+yocto@0leil.net>
Reviewed-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

1 files changed, 24 insertions, 1 deletions

diff --git a/documentation/dev-manual/qemu.rst b/documentation/dev-manual/qemu.rst
index d9b9f69ab3..504fcbdd8d 100644
--- a/documentation/dev-manual/qemu.rst
+++ b/documentation/dev-manual/qemu.rst
@@ -329,7 +329,7 @@ Following is the command-line help output for the ``runqemu`` command::
      Simplified QEMU command-line options can be passed with:
        nographic - disable video console
        serial - enable a serial console on /dev/ttyS0
-       slirp - enable user networking, no root privileges is required
+       slirp - enable user networking, no root privileges required
        kvm - enable KVM when running x86/x86_64 (VT-capable CPU required)
        kvm-vhost - enable KVM with vhost when running x86/x86_64 (VT-capable CPU required)
        publicvnc - enable a VNC server open to all hosts
@@ -427,6 +427,29 @@ command line:
    networking that does not need root access but also is not as easy to
    use or comprehensive as the default.
 
+   Using ``slirp`` by default will forward the guest machine's
+   22 and 23 TCP ports to host machine's 2222 and 2323 ports
+   (or the next free ports). Specific forwarding rules can be configured
+   by setting ``QB_SLIRP_OPT`` as environment variable or in ``qemuboot.conf``
+   in the :term:`Build Directory` ``deploy/image`` directory.
+   Examples::
+
+      QB_SLIRP_OPT="-netdev user,id=net0,hostfwd=tcp::8080-:80"
+
+      QB_SLIRP_OPT="-netdev user,id=net0,hostfwd=tcp::8080-:80,hostfwd=tcp::2222-:22"
+
+   The first example forwards TCP port 80 from the emulated system to
+   port 8080 (or the next free port) on the host system,
+   allowing access to an http server running in QEMU from
+   ``http://<host ip>:8080/``.
+
+   The second example does the same, but also forwards TCP port 22 on the
+   guest system to 2222 (or the next free port) on the host system,
+   allowing ssh access to the emulated system using
+   ``ssh -P 2222 <user>@<host ip>``.
+
+   Keep in mind that proper configuration of firewall software is required.
+
 -  ``kvm``: Enables KVM when running "qemux86" or "qemux86-64" QEMU
    architectures. For KVM to work, all the following conditions must be
    met: