Linux

OK, we are *almost* done getting our network set up properly for VDI, but we’ve got a few more things to do.  Specifically, we need to address:
1. Handling our external, Internet facing, dynamic IP address.
2. DHCP and DNS
3. External VPN access

Dynamic External IP

Most ISP’s (not all) provide a single, dynamic IP address for consumer grade service (i.e. home use).  But when we’re trying to connect to our virtual desktops from somewhere out on the public Internet, how do we know which IP address to connect to?

Generally speaking, connecting to an IP address is bad practice because it’s inflexible.  Instead, we should connect to a Fully Qualified Domain Name (FQDN).  OK fine, so we’ll set up a DNS entry and use a FQDN to connect to our desktops.  But what happens when our dynamic IP address changes and the DNS entry is still mapped to the old IP address?

What we need is an external Dynamic DNS (aka DDNS) service which will allow us to programmatically update our IP address whenever it changes.  There are a number of both free and paid-for DNS providers out there that can deliver DDNS services.  Personally, I use EditDNS (www.editdns.net).  They have a ton of functionality and they’ve been rock solid for the past few years I’ve been using their services, so I’m quite happy with them.

Now, many home use routers these days have the capability to update a DDNS provider.  But in my experience, the functionality is somewhat limited.  What if, for example, I want aaron.sweemer.com and desktop.sweemer.com to be dynamic entries and www.sweemer.com to be a static entry, pointing to my blog server hosted somewhere else?  In reality, I’ve got about 20 FQDN’s that I need to be dynamically updated and about 100 that I want static.  So instead, I created a script that will:

1. Query my external IP address (check this out, a free tool from Whatismyip.com)
2. Compare the result of the query with the IP obtained from the previous query
3. If the IP is the same, or contains something other than an IP (e.g. HTTP error), the scirpt exits.
4. If the IP is different, the script updates my DDNS entries via the EditDNS API, then updates a log file documenting the change, and finally adds the new IP to the last line of a file called previous_ips.

If you’d like to use the script I wrote, you’ll first need to do the following:

1. If don’t already have one, set up an account with EditDNS and make sure you have properly configured the domain name(s) you own.
2. Verify your linux distro has lynx (a command line, text only, www client)
3. Verify your linux distro has curl (a tool to transfer data using HTTP)
4. Create a directory (anywhere you have rwx access is fine) for the script and its files to live
5. In this directory, create a text file called editdns.sh.  Paste the content (below) into it.
6. Replace XXXXXXXX with your EditDNS password.
7. Make editdns.sh executable (chmod +x /path/to/editdns/editdns.sh)
8. Create another text file called records and, one per line, enter the FQDN’s of the DDNS entries you wanted updated (e.g. mydesktop.mydomain.com)
9. Add the editdns.sh script to your crontab to run at regular intervals (e.g. mine runs every five minutes and the entry in cron looks like this:    */5 * * * * cd /usr/local/editdns; ./editdns.sh)
   And here’s a copy of the actual script …

#!/bin/bash

EDITDNSPASS=”XXXXXXXX”
LYNX=`which lynx`
TIME=`date`
CIP=`curl -s http://www.whatismyip.com/automation/n09230945.asp | awk –re-interval ‘$1 ~ /^([0-9]{1,3}\.){3}[0-9]{1,3}$/ {print}’`
PIP=`tail -1 ./previous_ips`

if [ "$CIP" != "$PIP" && –n "$CIP" ]; then
cat ./records | while read FQDN; do
$LYNX -source “http://DynDNS.EditDNS.net/api/dynLinux.php?p=$EDITDNSPASS&r=$FQDN”
done
echo “IP Change!  New IP is $CIP.  Editdns.net was updated at $TIME.” >> ./editdns.log
echo $CIP >> ./previous_ips
else
exit 0
fi

If you have any issues or questions, feel free to email me.  Also, keep in mind, this a quick and dirty script that accomplishes what I want it to accomplish.  Feel free to make it more robust (e.g. error handling or better logging) to suit your needs.

DNS and DHCP

It’s quite possible you’ll want to skip this section and opt for setting up DHCP and DNS via Microsoft’s built in DHCP and DNS services that come out of the box with their server products.  To properly set up VMware View, we’ll need to set up Active Directory anyway, and quite frankly, it’s far easier to set up a Microsoft server with DHCP and DNS than it is to set up a Linux server.  So feel free to skip this section and leverage Microsoft for these services.  If, however, you’re a gluten for punishment then by all means, read on.

Let’s first start with DNS.  Here too we need Dynamic DNS because as we’re handing out IP addresses via DHCP, we want our DNS server to properly reflect current information as IP addresses change.  So, if you don’t already have bind9 (the DNS server), go ahead and install it (sudo apt-get install bind9 should work on Ubuntu / Debian distros).

The default configuration for bind9 is to act as a caching server, so the first thing we need to do is configure our DNS to forward all unknown DNS requests to another DNS server.  These should be provided to you from your ISP.  Edit the forwarders {} section of your named.conf.options file (usually located in /etc/bind/) to look like this …

asweemer@cincylab-rtr1:/etc/bind$ more named.conf.options
options {
directory “/var/cache/bind”;

forwarders {
1.2.3.4;
5.6.7.8;
};

auth-nxdomain no;    # conform to RFC1035
listen-on-v6 { any; };
};

Obviously, you’ll need to change 1.2.3.4 and 5.6.7.8 to the IP addresses given to you by your ISP.

Next, we need to modify our master named.conf to allow dynamic updates to DNS.  Add the following entry to the bottom of your named.conf file.

controls {
inet 127.0.0.1 allow {127.0.0.1; 192.168.9.25; 10.10.7.1; 10.10.7.2; } keys {“rndc-key”;};
};

This tells the DNS server to allow updates from the IP address located between the {}.  Notice the first three IP addresses are local IP addresses.  The fourth IP address is a slave DNS server, which I have yet to set up.  The rndc-key is the default key generated during installation of bind9 and it’s used to authorize the updating of DNS records.  If you’re using Ubuntu, then you’ll likely find the key in the file /etc/bind/rndc.key …

asweemer@cincylab-rtr1:/etc/bind$ sudo cat rndc.key
key “rndc-key” {
algorithm hmac-md5;
secret “QZ5jOmcr/OW3nzksR5q0Hw==”;
};
asweemer@cincylab-rtr1:/etc/bind$

Note the file is a text file named rndc.key, and the actual key is called rndc-key located within the text file.

OK, next we need to define our zones in the named.conf.local file.  For each domain you’re using (probably just one), you’ll need two entries:  one for the domain and one for the reverse lookup of the domain.  I have two domains I’ll be updating, so my named.conf.local file looks like this …

asweemer@cincylab-rtr1:/etc/bind$ cat named.conf.local
//
// Do any local configuration here
//

include “/etc/bind/rndc.key”;

zone “mydomain.com” {
type master;
file “/etc/bind/zones/mydomain.com.db”;
allow-update { key “rndc-key”; };
allow-transfer {10.10.7/24; };
};

zone “7.10.10.in-addr.arpa” {
type master;
file “/etc/bind/zones/rev.7.10.10.in-addr.arpa”;
allow-update { key “rndc-key”; };
allow-transfer {10.10.7/24; };
};

zone “dmz.mydomain.com” {
type master;
file “/etc/bind/zones/dmz.mydomain.com.db”;
allow-update { key “rndc-key”; };
allow-transfer {192.168.9/24; };
};

zone “9.168.192.in-addr.arpa” {
type master;
file “/etc/bind/zones/rev.9.168.192.in-addr.arpa”;
allow-update { key “rndc-key”; };
allow-transfer {192.168.9/24; };
};

A couple points to note here:

• I created a subdirectory called “zones” under /etc/bind/ where I put all my zone files.  This isn’t the default location, and in addition, this isn’t necessary as the zone files can be located anywhere you’d like.  But be aware the configuration file above reflects the location of my files.
• Notice the include “/etc/bind/rndc.key” on the first line and the all-update directive within each zone definition?  This should be self explanatory at this point.
• The allow-transfer directive within each zone definition explicitly limits zone transfers (copy) to the IP(s) defined.  This is an important security feature since, by default, DNS allows transfers to anyone, and the info contained within a DNS zone file can really give hackers visibility into your network.

Now we need to create the zone files we just defined above, which will contain our actual DNS records.  Here is the zone file for our dmz.mydomain.com …

asweemer@cincylab-rtr1:/etc/bind/zones$ cat dmz.mydomain.com.db
$TTL 3600       ; 1 hour
dmz.mydomain.com             IN SOA  master.dmz.mydomain.com. root.master.dmz.mydomain.com. (
2009060514 ; serial
86400      ; refresh (1 day)
86400      ; retry (1 day)
2419200    ; expire (4 weeks)
3600       ; minimum (1 hour)
)
NS      master.dmz.mydomain.com.
NS      slave.dmz.mydomain.com.
A       192.168.9.25
MX      10 mail.dmz.mydomain.com.
MX      20 mail-spool.dmz.mydomain.com.
computer-1              A       192.168.9.247
TXT     “317bf41a2c5b70fd9ca4e283d364dcddd5″
computer-2              A       192.168.9.250
TXT     “00cf6242f693ebbf1d545159548e44ab81″
computer-3              A       192.168.9.243
TXT     “31a0cb7e096a96c63dc998d2db3be6e450″
mail                    A       192.168.9.25
mail-spool              A       192.168.9.26
master                  A       192.168.9.25
www                     CNAME   master

A couple important things to point out here:

• The entries for computer-1, 2 and 3 are dynamic entries there were generated by the DHCP server.  The TXT record that follows these entries is a unique identifier which is also generated by the DHCP server and is used to ensure it won’t overwrite existing DNS records that were generated by another process/server.
• You’ll obviously need to change the domain names and IP addresses to match your environment.
• If you haven’t worked with bind9 before, this file probably looks pretty cryptic to you.  If so, I would recommend taking a look at http://www.zytrax.com/books/dns/ch8/soa.html, which gives a pretty good overview of the SOA (defined in the first part of the file).  The balance of the file (i.e. the record definitions) is pretty straight forward.

The reverse zone should look like this …

asweemer@cincylab-rtr1:/etc/bind/zones$ more rev.9.168.192.in-addr.arpa
$ORIGIN 9.168.192.IN-ADDR.ARPA.
$TTL    1h;
IN      SOA     master.dmz.mydomain.com. root.master.dmz.mydomain.com. (
2009060501      ;
1d              ;
1d              ;
4w              ;
1h              ;
)
IN      NS      master.dmz.mydomain.com.
IN      NS      slave.dmz.mydomain.com.
25      IN      PTR     master.dmz.mydomain.com.
26      IN      PTR     slave.dmz.mydomain.com.

I mixed it up just a bit in this file to point out a few different ways to configure a zone file.  In this file, notice the following differences:

• The $ORIGIN directive sets the domain name to be appended to any unqualified records.  If the $ORIGIN directive doesn’t exist (as it doesn’t in the first config file), then it is implicitly defined by the zone name.
• The time variables can be defined with d (day), w (week), h (hour), etc.

That’s about it for DNS.  Once you’ve got your bind9 server configured, restart your bind9 server (sudo /etc/init.d/bind9 restart).  And of course, be sure to test your configurations by using the standard DNS tools (e.g. dig, nslookup).  If you get errors, pay careful attention to your local syslog file (probably located at /var/log/syslog) as that’s where DNS and DHCP errors typically write their error messages.

OK, next up is configuring our DHCP server.  And once again, this post is starting to get way to long, so it looks like I’ll need a fourth and (hopefully) final post to this section.