RoR: Securing a Closed API - ruby-on-rails

Question

I have two rails apps on separate virtual servers, but in the same facility. Both apps can communicate via local ip addresses.
This is a two part question:
1) How do I check where the request is originating and limit requests only to those from that location?
2) Do you think this would be secure enough?
My gut is telling me this isn't secure enough because of IP spoofing, but I'm thinking OAuth or similar is a little too hardcore for my needs. Though, maybe not.
This is the first time I've approached something like this and I'm looking for anyone that can push me in the right direction here.
Thanks.

Answer 1

Depending on who's hosting you, the local network (to which your local addresses belong) could be a private network only accessible to your instances or, more likely, it would be shared with other virtual machines that do not belong to you. You would not be open to direct external attacks, but any compromised virtual machine sharing the same local network as you can be a springboard for attack, so your concerns are absolutely valid.
Answering, in order, your two concerns:
Configure iptables for the local interfaces to only accept requests coming on specific ports from specific local IPs (read a tutorial for a better understanding of iptables configuration.) All other virtual machines on the local network should not be able to probe you, although they might be able to intercept your traffic (addressed below.)
No; you should use SSL over all intra-node connections. This will protect you in two ways: firstly it will protect you from spoofing (an attacker will be rejected if he does not have your certificate, even if he bypasses iptables by spoofing his address, or because your iptables config gets overwritten by an admin), and secondly it will protect your data from prying eyes (e.g. an attacker will not be able to snoop traffic for your passwords.) Some applications (e.g. most database engines, net-snmpd set up in v3 mode, etc.) support SSL natively. Alternatively, establish and use ssh tunnels, or use stunnel
Sample base iptables configuration allowing basic services (HTTP, SSH etc.) on the public (internet) interface, as well as allowing www1 and www2 to connect to this node's MySQL on port 3306 on the eth0 interface (www1 and www2 are defined in /etc/hosts so they resolve to the appropriate IP addresses.):
# * raw
#
# Allows internal traffic without loading conntrack
# -A PREROUTING -i lo -d 127.0.0.0/8 -j NOTRACK
*filter
# Allows all loopback (lo0) traffic and drop all traffic to 127/8 that doesn't use lo0
-A INPUT -i lo -j ACCEPT
-A INPUT -i ! lo -d 127.0.0.0/8 -j DROP
# Accepts all established inbound connections (TCP, UDP, ICMP incl. "network unreachable" etc.)
-A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
# Allows all outbound traffic
# You can modify this to only allow certain traffic
-A OUTPUT -j ACCEPT
# Allows HTTP and HTTPS connections from anywhere (the normal ports for websites)
-A INPUT -p tcp --dport 80 -j ACCEPT
-A INPUT -p tcp --dport 443 -j ACCEPT
# Allows SSH connections
-A INPUT -p tcp -m state --state NEW --dport 22 -j ACCEPT
# Allow ping
-A INPUT -p icmp -m icmp --icmp-type echo-request -j ACCEPT
# log iptables denied calls
-A INPUT -m limit --limit 5/min -j LOG --log-prefix "iptables denied: " --log-level debug
# Reject all other inbound - default deny unless explicitly allowed policy
-A INPUT -j REJECT
-A FORWARD -j REJECT
# Allows MySQL within our cluster ONLY
-A INPUT -p tcp -s www1 -i eth0 --dport 3306 -j ACCEPT
-A INPUT -p udp -s www1 -i eth0 --dport 3306 -j ACCEPT
-A INPUT -p tcp -s www2 -i eth0 --dport 3306 -j ACCEPT
-A INPUT -p udp -s www2 -i eth0 --dport 3306 -j ACCEPT
COMMIT

Answer 2

This doesn't really sound like a Rails question, it's more of a question about web architecture. I'm assuming that both machines are accessible to the outside world via HTTP. If that's the case, you may want to consider putting a firewall in front of both machines to create a local network that the two machines are on.
Once you've done that, you should be able to configure the firewall to disallow requests based on any criteria you specify. Given that this is a Rails application I'm going to assume that the API is a set of resources. If that's the case you could configure your firewall to filter requests to the private API.
This way, the machines on the local network can communicate freely as their requests to one another aren't going through the firewall.