Merge pull request #2 from trishnaguha/networkd

Networkd docs added. 

Thank you @trishnaguha

.gitignore

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+*.swp
+_build/

docs/index.rst

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    :maxdepth: 2
 
    whatis
+   networkd
    atomic
    base
    docker

docs/networkd.rst

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+This Documenation is created with the help of `Major Hayden <https://major.io/>`_. Thanks to Major Hayden :).
+
+Networkd
+=========
+
+This chapter shows how to convert Network manger to Networkd and various use cases of Networkd
+
+Comparing systemd-networkd and NetworkManager
+---------------------------------------------
+
+- NetworkManager has been around for quite some time and systemd-networkd is relatively new.
+- Re-configuring existing network interfaces is challenging for both NetworkManager and systemd-networkd
+- Accessing raw systemd-networkd configuration files is more straightforward than NetworkManager
+- systemd-networkd's memory and CPU usage is extremely low (good for containerized environments)
+- systemd-networkd can handle various sysctl activities automatically, like IP forwarding
+- Tunnels and Overlays
+  - NetworkManager has more options for advanced tunnels, like vpnc, openconnect, and openvpn
+  - systemd-networkd makes gre, vlan, vxlan, and other overlay technologies much easier to implement
+- NetworkManager's logging is more verbose by default, which can be good for troubleshooting
+- systemd-networkd is meant to be configured without a GUI
+
+Configuration
+-------------
+
+Fedora Cloud Base Image. Download it from `here <https://getfedora.org/en/cloud/download/>`_.
+
+Switching from NetworkManager to systemd-networkd
+-------------------------------------------------
+
+Start by ensuring that NetworkManager and network scripts don't start on reboot:
+
+::
+
+    # systemctl disable NetworkManager
+    # systemctl disable network
+
+Ensure that systemd-networkd starts on the next boot:
+
+::
+
+    # systemctl enable systemd-networkd
+
+Enable the resolver and make a symlink:
+
+::
+
+    # systemctl enable systemd-resolved
+    # systemctl start systemd-resolved
+    # rm -f /etc/resolv.conf
+    # ln -s /run/systemd/resolve/resolv.conf /etc/resolv.conf
+
+Be sure to configure your network interfaces in ``/etc/systemd/network`` and then reboot.
+
+systemd-networkd use cases
+--------------------------
+
+Here are some sample use cases for systemd-networkd and example configurations.
+
+**Simple DHCP on a single interface**
+
+For an interface ``eth0``, a single ``.network`` file is needed:
+
+::
+
+    # cat /etc/systemd/network/eth0.network
+    [Match]
+    Name=eth0
+
+    [Network]
+    DHCP=yes
+
+**Static address on a single interface**
+
+For an interface ``eth0``, a single ``.network`` file is needed:
+
+::
+
+    # cat /etc/systemd/network/eth0.network
+    [Match]
+    Name=eth0
+
+    [Network]
+    Address=192.168.0.50/24
+    Address=2001:db8:dead:beef::/64
+
+    # These are optional but worth mentioning
+    DNS=8.8.8.8
+    DNS=8.8.4.4
+    NTP=pool.ntp.org
+
+You can also split up the addresses into separate blocks:
+
+::
+
+    # cat /etc/systemd/network/eth0.network
+    [Match]
+    Name=eth0
+
+    [Network]
+    DNS=8.8.8.8
+    DNS=8.8.4.4
+    NTP=pool.ntp.org
+
+    [Address]
+    Address=192.168.0.50/24
+
+    [Address]
+    Address=2001:db8:dead:beef::/64
+
+Or add static routes:
+
+::
+
+    # cat /etc/systemd/network/eth0.network
+    [Match]
+    Name=eth0
+
+    [Network]
+    DNS=8.8.8.8
+    DNS=8.8.4.4
+    NTP=pool.ntp.org
+
+    [Address]
+    Address=192.168.0.50/24
+
+    [Address]
+    Address=2001:db8:dead:beef::/64
+
+    [Route]
+    Destination=10.0.10.0/24
+    Gateway=192.168.50.1
+
+    [Route]
+    Destination=10.0.20.0/24
+    Gateway=192.168.50.1
+
+**Do DHCP on all network devices**
+
+You can use wildcards almost anywhere in the ``[Match]`` block. For example, this will cause systemd-networkd to do DHCP on all interfaces:
+
+::
+
+    [Match]
+    Name=eth*
+
+    [Network]
+    DHCP=yes
+
+**Bridging**
+
+Let's consider an example where we have ``eth0`` and we want to add it to a bridge. This could be handy for servers where you want to build containers or virtual machines and attach them to the network bridge.
+
+Start by setting up our bridge interface, ``br0``:
+
+::
+
+    # cat /etc/systemd/network/br0.netdev
+    [NetDev]
+    Name=br0
+    Kind=bridge
+
+Now that we have a bridge device, let's configure the network for the bridge:
+
+::
+
+    # cat /etc/systemd/network/br0.network
+    [Match]
+    Name=br0
+
+    [Network]
+    IPForward=yes
+    DHCP=yes
+
+The ``IPForward=yes`` will take care of the sysctl forwarding setting for us ``(net.ipv4.conf.br0.forwarding = 1)`` automatically when the interface comes up.
+
+Now, let's take the ethernet adapter and add it to the bridge:
+
+::
+
+    # cat /etc/systemd/network/eth0.network
+    [Match]
+    Name=eth0
+
+    [Network]
+    Bridge=br0
+
+Simply reboot the system and it will come up with ``eth0`` as a port on ``br0``.
+
+**Bonding**
+
+Configuring a bonded interface is very similar to configuring a bridge. Start by setting up the individual network adapters:
+
+::
+
+    # /etc/systemd/network/ens9f0.network
+    [Match]
+    Name=ens9f0
+
+    [Network]
+    Bond=bond1
+
+::
+
+    # /etc/systemd/network/ens9f1.network
+    [Match]
+    Name=ens9f1
+
+    [Network]
+    Bond=bond1
+
+Now we can create the network device for the bond:
+
+::
+
+    # /etc/systemd/network/bond1.netdev
+    [NetDev]
+    Name=bond1
+    Kind=bond
+
+    [Bond]
+    Mode=802.3ad
+    TransmitHashPolicy=layer3+4
+    MIIMonitorSec=1s
+    LACPTransmitRate=fast
+
+Once the device is defined, let's add some networking to it:
+
+::
+
+    # /etc/systemd/network/bond1.network
+    [Match]
+    Name=bond1
+
+    [Network]
+    DHCP=yes
+    BindCarrier=ens9f0 ens9f1
+
+The **BindCarrier** is optional but recommended. It gives systemd-networkd the hint that if both bonded interfaces are offline, it should remove the bond configuration until one of the interfaces comes up again.
+
+Status & Diagnostics
+--------------------
+
+All of the output from systemd-networkd will appear in your system journal. Any errors when setting up interfaces or configuring routes will be printed there. The ``networkctl`` command allows you to check your network devices at a glance. Here's an example of a fairly complicated network setup:
+
+::
+
+    # networkctl
+    IDX LINK             TYPE               OPERATIONAL SETUP
+      1 lo               loopback           carrier     unmanaged
+      2 enp3s0           ether              off         unmanaged
+      3 enp1s0f0         ether              degraded    configured
+      4 enp1s0f1         ether              degraded    configured
+      5 br1              ether              routable    configured
+      6 br0              ether              routable    configured
+      7 gre0             ipgre              off         unmanaged
+      8 gretap0          ether              off         unmanaged
+      9 gre-colocation   ipgre              routable    configured
+     12 vlan100          ether              routable    configured
+     13 tun1             none               routable    unmanaged
+     14 tun0             none               routable    unmanaged
+     15 vlan200          ether              routable    configured
+
+You'll find two physical network cards (``enp1s0f0`` and ``enp1s0f1``) each attached to a bridge (``br0`` and ``br1``, respectively). The physical network adapters show up as degraded because they don't have network addresses directly assigned -- that assignment is done on the bridge. The ``gre0`` and ``gretap0`` devices are created automatically to handle the gre tunnel ``gre-colocation``. There are also two VLANs configured within systemd and attached to a bridge. The ``tun`` interfaces are OpenVPN interfaces and they are not configured by systemd-networkd (hence the unmanaged setup).
+
+**Further Reading**
+
+- `ArchLinux systemd-networkd documentation <https://wiki.archlinux.org/index.php/Systemd-networkd/>`_
+- `Upstream systemd-networkd documentation <https://www.freedesktop.org/software/systemd/man/systemd-networkd.service.html/>`_