Editing How does the Synology Linux implementation work, e.g. bootup, device management etc

==What is this page about==
This page is intended to pull together lots of bits of information on just how Synology have implemented linux on their NASs. This needs to be a factual resource, it is '''NOT''' a document to discuss the pros or cons of the way Synology have implemented Linux.

If you are adding information to this page please '''always''' state the model and firmware version for which you are commenting as some of the information is likely to vary from model to model and firmware to firmware.

If you are unsure but have a hunch at how something works then also please state it making it clear that you don't know for certain by stating 'I think'..... If you as a reader know for sure then please remove the 'I think's. 

===Bootup===
For CS407 fw598 'I think' the system reads;

#File "/etc/defaults/rc.conf" to set initial parameters and the path to find other startup scripts which is set as... local_startup="/etc/rc.d /usr/syno/etc/rc.d /usr/local/etc/rc.d"
#So the script "/etc/rc" is run. It does many things including a check for (and then execute if it exists) the file "/etc/rc.local" which 'I think' is an ipkg installed script to load and run any installed optware (ipkg programs).

===How are Alias's set===
For CS407 fw597 they are set in "/etc/profile", you can add your own to it.

===How are ports/services defined===
For CS407 fw731 the linux standard services file "/etc/services" is used, see http://linux.about.com/library/cmd/blcmdl5_services.htm

===How is PATH set===
For CS407 fw597 'I think' this is first set by "/etc/rc" then by "/etc/rc.local" (if you have ipkg) then by "/etc/profile" and then by the ".profile" file in the users HOME directory (i.e. "/root/.profile" for user root).

===What happens when you log in to the Command Line Interface===
For CS407 fw598 when you log in to the Comand Line Interface (via either SSH or Telnet) as a user (usually root) the NAS creates a ptty ash shell for you and drops you into that users "home" directory as defined in the file "/etc/passwd". It then runs the file "/etc/profile". If the home directory of the logging in user contains a file ".profile" (synology use one for the user "root" it is "/root/.profile") this is read and also actioned setting environment variables, such as path, alias's, prompt etc. You can add to this file to run scripts or set parameters automatically when you log in using the CLI.

It appears: For the DS210j, a newly created user is blocked from logging in.   To change this block, edit /etc/passwd to /bin/ash (instead of /sbin/nologin) for the appropriate user.   It is good practice to login as root/admin only when necessary and general fiddling with program configs is safer done as a non-root user.