[PATCH] doc: Describe offline building

[Baurzhan Ismagulov <ibr@radix50.net>]

From: Baurzhan Ismagulov <ibr@ilbers.de>

Describes building Isar offline using a local partial Debian mirror.

Signed-off-by: Baurzhan Ismagulov <ibr@ilbers.de>
---
 doc/offline.md | 111 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 111 insertions(+)
 create mode 100644 doc/offline.md

diff --git a/doc/offline.md b/doc/offline.md
new file mode 100644
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--- /dev/null
+++ b/doc/offline.md
@@ -0,0 +1,111 @@
+= Working Offline
+
+== Prepare Partial Debian Mirror
+
+```
+HOST=ftp.de.debian.org
+WEB_HOME=/mnt/sdc1/w
+ARCHES=amd64,arm64,armhf,i386
+mkdir -p $WEB_HOME
+```
+
+```
+DST=$WEB_HOME/debian
+DISTROS=jessie,jessie-updates,stretch,stretch-updates,buster,buster-updates
+time debmirror -p --getcontents -e http -h $HOST -d $DISTROS -a $ARCHES $DST
+```
+
+```
+DST=$WEB_HOME/debian-security
+DISTROS=jessie/updates,stretch/updates,buster/updates
+time debmirror -p --getcontents -e http -h $HOST -r /debian-security \
+    -d $DISTROS -a $ARCHES $DST
+```
+
+== Prepare Git Mirror
+
+```
+DST=$WEB_HOME/git
+```
+
+```
+PKG=hello
+mkdir -p $DST/ilbers/$PKG.git
+cd $DST/ilbers/$PKG.git
+git init --bare
+cp hooks/post-update.sample hooks/post-update
+cd -
+git clone https://github.com/ilbers/$PKG.git
+cd $PKG
+git remote add local $DST/ilbers/$PKG.git
+git push local master
+```
+
+Repeat for `libhello`.
+
+== Set Up Apache
+
+```
+sudo apt-get install apache2
+sudo vi /etc/apache2/sites-available/000-default.conf
+```
+
+```
+<VirtualHost *:80>
+	...
+	Alias /debian /mnt/sdc1/w/debian
+	Alias /debian-security /mnt/sdc1/w/debian-security
+	Alias /git /mnt/sdc1/w/git
+	<Directory /mnt/sdc1/w>
+		Options +Indexes
+		AllowOverride None
+		Require all granted
+	</Directory>
+</VirtualHost>
+```
+
+Repeat for `/etc/apache2/sites-available/default-ssl.conf`.
+
+`sudo systemctl reload apache2`
+
+== Set Up BIND
+
+A DNS server is necessary to resolve `localhost` from `buildchroot-target`.
+
+```
+sudo apt-get install bind9
+sudo systemctl start bind9
+sudo vi /etc/resolv.conf
+```
+
+```
+nameserver 127.0.0.1
+```
+
+== Use Offline Files
+
+```
+git clone https://github.com/ilbers/isar.git
+cd isar
+. isar-init-build-env ../build
+vi conf/local.conf
+```
+
+```
+DISTRO_APT_PREMIRRORS = "\
+    ftp\.de\.debian\.org localhost\n\
+    security\.debian\.org localhost/debian-security\n"
+
+PREMIRRORS_prepend = "git://github.com/ git://localhost/git/\n"
+
+FETCHCMD_git = "GIT_SSL_NO_VERIFY=1 git -c core.fsyncobjectfiles=0"
+```
+
+Setting `GIT_SSL_NO_VERIFY` is necessary if SSL certificate is self-signed or
+its CN isn't `localhost`.
+
+== TODO
+
+* Build without BIND
+* Change `protocol=https` to `protocol=http` in `PREMIRRORS`, drop FETCHCMD_git
+* Tool for mirroring git repos
-- 
2.11.0