authbind vs iptables on AWS

Here is a short description of the scenario I was working on. I am using a standard AWS AMI to run tomcat (tomcat7, to be specific.) The default configuration of AWS AMIs (and many other off-the-shelf unix based servers) is such that tomcat (or any other program that runs with a non-superuser credentials) can’t bind to privileged ports. However, tomcat needs to use these privileged ports (443 for TLS and 80 for standard HTTP) to serve public facing pages.

Making tomcat run as superuser is really a bad idea (the why question is beyond this article.) So there are a few tricks to make tomcat work on privileged ports.

authbind

There is lot of mindshare around authbind when it comes to hosted environments. The manpage of authbind describes how authbind can be used to make a program bind to sockets on privileged ports. However, if you are using a standard AWS AMI, you may have some challenges using authbind. Also, for automated environments (read Chef) in AWS, I felt authbind is more complicated to work with.

iptables

Port redirection using NAT features of iptables is very simple and straight forward. However, it requires an additional configuration on tomcat to use proxy mode on privileged ports.

Here is the NAT configuration using iptables.

sudo /sbin/iptables -t nat -I PREROUTING -p tcp --dport 80 -j REDIRECT --to-port 8080
sudo /sbin/iptables -t nat -I PREROUTING -p tcp --dport 443 -j REDIRECT --to-port 8443
sudo service iptables save

Once this is done, all inbound traffic on 80 will be redirected to 8080. The same is the case with port pair 8443 and 443. This way, tomcat can still bind to port 8080 for HTTP and 8443 for TLS while serving incoming connections on 80 and 443 respectively.

When a client program queries the port information from tomcat, it should respond with port 80 and 443 instead of 8080 and 8443. To ensure that, one can use the proxy support feature of tomcat. Here is the additional configuration in tomcat connector settings in server.xml

<Connector port="8443" proxyPort="443" .../>
<Connector port="8080" proxyPort="80" .../>

Other Considerations

There are better ways to handle this port redirection when you have front-ending loadbalancers and/or proxy servers in place. Having proxy/loadbalancers solves helps mitigate more issues than just solving the redirection problems. However, the iptables approach is better than authbind approach when you are using a single server on AWS without lot of additional infrastructure and configurations in place.

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