Merge pull request opsschool#209 from mynamewastaken/master

Added some text to cron/troubleshooting articles, fleshing out DHCP

cron_101.rst

@@ -131,6 +131,9 @@ Knowing this, we can see that this "crontab" means:
   At 12:05 every Sunday, every month, regardless of the day of the month, run the
   command ``copy-to-partition`` in the ``/home/jdoe/jobs`` directory.
 
+**Caveat**: entries in your crontab must be one long line; if you try to split it up
+(with the linebreak ``\`` character for example) you will get an error!
+
 Let's take another example and create a cron job that checks disk space
 available every minute, every hour, every day of the month, every month, for
 every day of the week, and outputs it to a file named :file:``disk_space.txt``.
@@ -209,6 +212,8 @@ Notes on composing good "cron expressions"
   variables that come from your shell initialization files (like ``.bashrc`` or
   ``.zshrc`` or such). In particular, make sure to specify the full path to your
   program if it's not in a commonly-used location like ``/usr/bin``.
+- If you have problems with the syntax, or something isn't working properly, there
+  are websites [#]_ that will translate crontab to English, and vice versa. [#]_ 
 
 Modify a specific user's crontab
 --------------------------------
@@ -241,4 +246,8 @@ Footnotes
 
 .. [#] `"Examples" in cron - Wikipedia, a free encyclopedia <http://en.wikipedia.org/wiki/Cron#Examples_2>`_
 
-.. [#] `Where can I set environment variables that crontab will use?, <http://stackoverflow.com/questions/2229825/where-can-i-set-environment-variables-that-crontab-will-use/10657111#10657111>`_
+.. [#] `Crontab to plain English <http://cronchecker.net>`_
+
+.. [#] `English to crontab <http://corntab.com/>`_
+
+.. [#] `Where can I set environment variables that crontab will use? <http://stackoverflow.com/questions/2229825/where-can-i-set-environment-variables-that-crontab-will-use/10657111#10657111>`_

dhcp.rst

@@ -1,14 +1,32 @@
 DHCP
 ****
+DHCP, or, Dynamic Host Control Protocol is a standard that allows one central 
+source, i.e. a server or router, to automatically assign requesting hosts an IP
+address.
+
+The most common DHCP servers on Linux platforms are the ISC's
+(Internet System Consortium) ``dhcpd``, and ``dnsmasq``. On Windows platforms
+there are a variety of DHCP servers, though the Microsoft DHCP server is
+arguably the best in terms of support and integration.
+
 
 Tools: ISC dhcpd
 ================
+``dhcpd`` is the most common DHCP server, and it offers excellent integration
+with BIND (DNS server).
 
 Protocol
 ========
 
 dhcp helper
 ===========
+DHCP helpers are sometimes referred to as DHCP relayers. The basic idea is that
+a relay agent will forward DHCP requests to the appropriate server. This is
+necessary because a host that comes online on a subnet with no DHCP server
+has no way of finding the correct server; it needs a DHCP assigned address to 
+find a route to the correct DHCP server, but can't get there because it has no
+IP address! DHCP relaying solves this chicken and egg problem by acting as an
+intermediary. See this wikipedia [#]_ article for more details.
 
 Defining classes, leases
 ========================
@@ -23,5 +41,5 @@ DNS server(s)
 -------------
 (Tie in previous chapters re: TFTP, PXE with related options?)
 
-
+.. [#] `DHCP relaying explained <http://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol#DHCP_relaying>`_
 

troubleshooting_101.rst

@@ -33,8 +33,8 @@ problem. Over time you will expand what is in your toolbelt, but to start with
 you must know how to use each of these:
 
 * ``top``, ``vmstat``, ``iostat``, ``systat``, ``sar``, ``mpstat``
-  These help you see the current state of the system - what is running, what is
-  using cpu, memory? Is the disk being heavily used? There is a lot of
+  help you see the current state of the system - what is running, what is
+  using cpu, memory? Is the disk being heavily used? There is a great deal of
   information, and knowing how these tools work will help you pick out the bits
   you should focus on.
 * ``tcpdump``, ``ngrep``
@@ -49,7 +49,7 @@ Walk through of a diagnosis
  * What changed recently?
  * Could any of the symptoms be red herrings?
 
-* Common culprits (is it plugged in?)
+* Common culprits (is it plugged in? is it network accessible?)
 * Look through your logs
 * Communicating during an outage
 * 'Talking Out-Loud' (IRC/GroupChat)
@@ -72,9 +72,31 @@ identify and fix them.
 Cannot bind to socket
 ^^^^^^^^^^^^^^^^^^^^^
 
+There are two common reasons that you can't bind to a socket: the port is
+already in use, or you don't have permission.
+As an example, you can see what happens when I try to start a Python 
+SimpleHTTPServer on a port that is already in use:
+
+.. code-block:: console
+    
+    user@opsschool ~$ python -m SimpleHTTPServer 8080
+    ...
+    socket.error: [Errno 98] Address already in use
+
+Here's an example of what happens when I try to bind to a privileged port 
+without proper permissions (in Linux, ports < 1024 are privileged):
+
+.. code-block:: console
+
+    user@opsschool ~$ python -m SimpleHTTPServer 80
+    ...
+    socket.error: [Errno 13] Permission denied
+    
 Permission denied reading to / writing from disk
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+
+
 Out of disk space
 ^^^^^^^^^^^^^^^^^
 (finding large files, and also finding deleted-but-open files)