build-vyos: ease build instructions

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+.. _upstream_packages:
+
+Upstream packages
+-----------------
+
+Many base system packages are pulled straight from Debian's main and contrib
+repositories, but there are exceptions.
+
+This chapter lists those exceptions and gives you a brief overview what we
+have done on those packages. If you only want to build yourself a fresh ISO
+you can completely skip this chapter. It may become interesting once you have
+a VyOS deep dive.
+
+vyos-netplug
+^^^^^^^^^^^^
+
+Due to issues in the upstream version that sometimes set interfaces down, a
+modified version is used.
+
+The source is located at https://github.com/vyos/vyos-netplug
+
+In the future, we may switch to using systemd infrastructure instead. Building
+it doesn't require a special procedure.
+
+keepalived
+^^^^^^^^^^
+
+Keepalived normally isn't updated to newer feature releases between Debian
+versions, so we are building it from source.
+
+Debian does keep their package in git, but it's upstream tarball imported into
+git without its original commit history. To be able to merge new tags in, we
+keep a fork of the upstream repository with packaging files imported from
+Debian at https://github.com/vyos/keepalived-upstream
+
+strongswan
+^^^^^^^^^^
+
+Our StrongSWAN build differs from the upstream:
+
+- strongswan-nm package build is disabled since we don't use NetworkManager
+- Patches for DMVPN are merged in
+
+The source is at https://github.com/vyos/vyos-strongswan
+
+DMVPN patches are added by this commit:
+https://github.com/vyos/vyos-strongswan/commit/1cf12b0f2f921bfc51affa3b81226
+
+Our op mode scripts use the python-vici module, which is not included in
+Debian's build, and isn't quite easy to integrate in that build. For this
+reason we debianize that module by hand now, using this procedure:
+
+0. Install https://pypi.org/project/stdeb/
+1. `cd vyos-strongswan`
+2. `./configure --enable-python-eggs`
+3. `cd src/libcharon/plugins/vici/python`
+4. `make`
+5. `python3 setup.py --command-packages=stdeb.command bdist_deb`
+
+The package ends up in deb_dist dir.
+
+ppp
+^^^
+
+Properly renaming PPTP and L2TP interfaces to pptpX and l2tpX from generic and
+non-informative pppX requires a patch that is neither in the upstream nor in
+Debian.
+
+We keep a fork of Debian's repo at https://github.com/vyos/ppp-debian
+
+The patches for pre-up renaming are:
+
+* https://github.com/vyos/ppp-debian/commit/e728180026a051d2a96396276e7e4ae
+* https://github.com/vyos/ppp-debian/commit/f29ba8d9ebb043335a096d70bcd07e9
+
+Additionally, there's a patch for reopening the log file to better support
+logging to files, even though it's less essential:
+https://github.com/vyos/ppp-debian/commit/dd2ebd5cdcddb40230dc4cc43d374055f
+
+The patches were written by Stephen Hemminger back in the Vyatta times.
+
+mdns-repeater
+^^^^^^^^^^^^^
+
+This package doesn't exist in Debian. A debianized fork is kept at
+https://github.com/vyos/mdns-repeater
+
+No special build procedure is required.
+
+udp-broadcast-relay
+^^^^^^^^^^^^^^^^^^^
+
+This package doesn't exist in Debian. A debianized fork is kept at
+https://github.com/vyos/udp-broadcast-relay
+
+No special build procedure is required.
+
+Linux kernel
+^^^^^^^^^^^^
+
+In the past a fork of the Kernel source code was kept at the well-known
+location of https://github.com/vyos/vyos-kernel - where it is kept for history.
+
+Nowadays the Kernel we use is the upstream source code which is patched
+with two additional patches from the good old Vyatta times which never made it
+into the mainstream Kernel. The patches can be found here:
+https://github.com/vyos/vyos-build-kernel/tree/master/patches/kernel and are
+automatically applied to the Kernel by the Jenkins Pipeline which is used to
+generate the Kernel binaries.
+
+The Pipeline script not only builds the Kernel with the configuration named
+``x86_64_vyos_defconfig`` which is located in the vyos-build-kernel repository,
+too - but in addition also builds some Intel out-of-tree drivers, WireGuard
+(as long it is not upstreamed) and Accel-PPP.
+
+The ``Jenkinsfile`` tries to be as verbose as possible on each individual build
+step.
+
+Linux Firmware
+^^^^^^^^^^^^^^
+
+More and more hardware cards require an additional firmware which is not open
+source. The Kernel community hosts a special linux-firmware Git repository
+with all available binary files which can be loaded by the Kernel.
+
+The ``vyos-build`` repository fetches a specific commit of the linux-firmware
+repository and embeds those binaries into the resulting ISO image. This step is
+done in the ``data/live-build-config/hooks/live/40-linux-firmware.chroot`` file.
+
+If the firmware needs to be updated it is sufficient to just exchange the Git
+commit id we reference in our build.
+
+Intel NIC drivers
+^^^^^^^^^^^^^^^^^
+
+We do not make use of the building Intel NIC drivers except for e1000e. Main
+reason is that the out of tree Intel drivers seem be perform a bit better,
+e.q. have proper receive-side-scaling and multi-queue support.
+
+Drivers are build as part of the Kernel Pipeline - read above.
+
+Accel-PPP
+^^^^^^^^^
+
+Accel-PPP used to be an upstream fork for quite some time but now has been
+converted to make use of the upstream source code and build system.
+
+It is build as part of the Kernel Pipeline - read above.
+
+hvinfo
+^^^^^^
+
+A fork with packaging changes for VyOS is kept at https://github.com/vyos/hvinfo
+
+The original repo is at https://github.com/dmbaturin/hvinfo
+
+It's an Ada program and requires GNAT and gprbuild for building, dependencies
+are properly specified so just follow debuild's suggestions.