When running Asterisk on Amazon EC2 with an EIP, what are the NAT configurations for Asterisk?
[general] nat=yes externip=xxx.yyy.zzz.vvv localnet=192.168.0.0/255.255.0.0 ; RFC 1918 addresses localnet=10.0.0.0/255.0.0.0 ; Also RFC1918 localnet=172.16.0.0/12 ; Another RFC1918 with CIDR notation localnet=169.254.0.0/255.255.0.0 ; Zero conf local network
Apparently, EIC uses NAT:
If your EC2 instance is in a private subnet in your VPC, then it can use your NAT to make outbound connections. The outside world would see it's IP address as your NAT's IP address, but the NAT's IP address would never "be" the Public IP address of the instance.
Overview of how EIP works:
Before going into an example, let's review how the Elastic IPs work:
Remember that each instance has an internal IP address and an external (public) one, which is translated to the internal one. If two external IPs were translated to the same internal IP then inbound packets would arrive fine, but sorting out outgoing packets (i.e. determining which external IP address to assign to outgoing packets) would be very difficult. Hence, the limitation of a single external IP address per instance at any given point in time.
wikipedia (for my reference):
Elastic IP addresses Amazon Elastic IP
Amazon's elastic IP address feature is similar to static IP address in traditional data centers, with one key difference. A user can programmatically map an elastic IP address to any virtual machine instance without a network administrator's help and without having to wait for DNS to propagate the binding. In this sense an Elastic IP Address belongs to the account and not to a virtual machine instance. It exists until it is explicitly removed, and remains associated with the account even while it is associated with no instance.
Getting down to nuts and bolts, in the context of SIP and Asterisk:
1.3. Different types of NATs and firewalls.
There are several ways UDP might be handled by a specific NAT or firewall implementations, these are categorized into:
1.3.1 Full Cone NAT
A full cone NAT is one where all requests from the same internal IP address and port are mapped to the same external IP address and port. Furthermore, any external host can send a packet to the internal host, by sending a packet to the mapped external address.
1.3.2 Restricted Cone:
A restricted cone NAT is one where all requests from the same internal IP address and port are mapped to the same external IP address and port. Unlike a full cone NAT, an external host (with IP address X) can send a packet to the internal host only if the internal host had previously sent a packet to IP address X.
1.3.3 Port Restricted Cone:
A port restricted cone NAT is like a restricted cone NAT, but the restriction includes port numbers.
Specifically, an external host can send a packet, with source IP address X and source port P, to the internal host only if the internal host had previously sent a packet to IP address X and port P.
1.3.4 Symmetric Nat:
A symmetric NAT is one where all requests from the same internal IP address and port, to a specific destination IP address and port, are mapped to the same external IP address and port. If the same host sends a packet with the same source address and port, but to a different destination, a different mapping is used. Furthermore, only the external host that receives a packet can send a UDP packet back to the internal host.
It sounds like EIC uses "full cone" NAT. What would be the Asterisk settings for NAT, then?