This post is about NATing an ESX VM, but first, why do I need NAT:

The SIP protocol is not NAT oblivious. To traverse NAT our application has to replace the DNS in the SIP message contact header to the external FQDN that the message receiver will be sending responses to (A NAT with static routing configured).
Therefore I needed to test our software in a NAT topology.

In the past, when we used VMWare player/workstation, it had a build-in NAT network. But, unfortunately, the ESX hypervisor does not provide a NATed network option.
Seeking alternatives at VMWare’s appliance marketplace, I found and downloaded the Vyatta’s community edition (VC5) router appliance (also downladble from sourceforge), and comes under the GPL license.
After 3-4 hours – guided by the official quick start guide -  I had a working NAT configuration in the ESX. Hurray!
Overall, not a hard nut to crack ;) , though I wish VMWare will wise up and just add an build-in NAT option to vSphere.

Left to do:
Obtain some static IPs, so the config won’t break each time the vm reboots and the DHCP lease expires.
Tip #1:
If you want want to access your NATed VM by RDP/VNC, without setting up extra NAT routing rules, consider adding the VM an additional un-NATed NIC, but when doing so, make sure that the OS routing tables are set to route through the NIC that is NATed.
Tip #2:
This short vyatta user installation report also helped me a bit.

Here’s the complete configuration script I ended up feeding to the appliance console (network topology is similar to the one presented in the Vyatta’s getting stated guide):
Where:
1.2.3.4 is your department’s DNS server
192.168.1.199 is the VMs NATed private IP address (provided by the DHCP).
The script contains a NAT forward rule for VNC (port 5900)


configure
set system host-name vyatta-nat
set interfaces ethernet eth0 address dhcp
set service ssh
set service https
commit;
save;
# restart the appliance to switch from console remote desktop to SSH:

#login with user and password
configure
show interfaces

set interfaces ethernet eth1 address 192.168.1.254/24

commit;

delete service dhcp-server
set service dhcp-server shared-network-name ETH1_POOL subnet 192.168.1.0/24 start 192.168.1.100 stop 192.168.1.199
set service dhcp-server shared-network-name ETH1_POOL subnet 192.168.1.0/24 default-router 192.168.1.254
set service dhcp-server shared-network-name ETH1_POOL subnet 192.168.1.0/24 dns-server 1.2.3.4
commit;
show service dhcp-server

set service nat rule 1 source address 192.168.1.0/24
set service nat rule 1 outbound-interface eth0
set service nat rule 1 type masquerade
commit;
show service nat
save;
exit
show nat rules
configure
set service nat rule 20 type destination
set service nat rule 20 inbound-interface eth0
# use a negative fake address to so that all incoming communication will be nated
#set service nat rule 20 destination address !192.168.50.0
#Forward traffic to address 192.168.1.199
set service nat rule 20 inside-address address 192.168.1.199
set service nat rule 20 protocol tcp
set service nat rule 20 destination port 5900
commit;
save;
exit