Spam Mail FilterJeff Rupp & Frank Watson1 Implement Spam Mail Filtration inside Linux kernel Jeff Rupp and Frank Watson

Spam Mail FilterJeff Rupp & Frank Watson2 Introduction Our original goals: 1.To figure out how User Mode Linux works 2.To modify the kernel to filter out spam mail and send the appropriate packets to both the receiver and server. 3.Then to make further modifications to send information packets to the sender to see if the user and sender existed (which is common for spammer) Modified proposal: getting the User Mode Linux (UML) to work and modify the Linux source to change the user name to all x’s if the sender is on a spam list. senderreceiver Sends mail Checks to see if sender exists

Spam Mail FilterJeff Rupp & Frank Watson3 What is User Mode Linux? User-Mode Linux is a virtual machine that runs entirely inside a single file on the physical machine. It has internet,and xwindow support and allows users to install and test applications on this virtual machine Two main advantages of using UML are: 1.If the kernel crashes because of test applications or kernel modifications, the machine does not go with it. 2.Has a very handy debugger that allows you to debug the actual linux kernel. A good three-fifths of our time was spent on this project trying to get UML to work. This was not a trivial process.

Spam Mail FilterJeff Rupp & Frank Watson4 How to install UML 1.Download 3 main files Kernel source codeKernel source code, UML kernel patch, and Root file systemUML kernel patchRoot file system 2.Installation Unpack the kernel, Apply the UML patch, and Build the "linux“ executable from the linux source 3.Running UML execute uml with the following command line: “ linux mem=128M udb=root_fs_slackware_7.0_big udb2=swap debug=go eth0=ethertap,tap0,fd:fe:0:0:0:1, 4.Setting up the internet connection Use the following commands: Ifconfig eth0 Route del –net dev eth0 netmask Route add –host dev eth0 Route add default gw

Spam Mail FilterJeff Rupp & Frank Watson5 How UML works UML is composed of two major pieces. A root_fs (a root file system) and a linux executable. The linux executable is the linux kernel. The kernel information on the root_fs is not used for booting, so it is possible to boot a root_fs (made with Redhat 7.1 and a kernel) with a UML linux exectuable which boots the kernel into The internet access is achieved by using a driver called ethertap. Ethertap is an older driver and intercepts packets at the ethernet card and sends them up to the UML code

Spam Mail FilterJeff Rupp & Frank Watson6 Source Code Modifications Checks to see if the packet is for port 25 by checking sk->num We look for two things in the pay load, text stating “MAIL FROM:” (case insensitive) sets the flag wasMailfrom and “RCPT TO:” sets the flag wasRcpt using if statements. IP TCP (port #) sk->num IP TCP (port #) Payload sk->num sk->data

Spam Mail FilterJeff Rupp & Frank Watson7 Source Code Modifications (cont.) If either flag is set, a pointer searches the payload to find the start of the name. If the MAILfrom flag is set, it searches to see if it is on the spam sender’s list. If so, the code waits for the RCPT TO: in the packet and replaces the RCPT TO: with x’s. For example: would become which does not exist IP TCP (port #) Payload sk-num sk->data Searches for the user name

Spam Mail FilterJeff Rupp & Frank Watson8 Results Have a working prototype which catches a spammer and changes the “RCPT TO:” field in the SMTP protocol to an unknown address. Below is a sample of an returned to the user: The following addresses had permanent fatal errors (reason: User unknown) Known bug: the sender’s mail server keeps a cache of the addresses and on occasion does not send a “RCPT TO:” in the payload to see if the user exists. In these situations, the mail does go through and is not modified or blocked.

Spam Mail FilterJeff Rupp & Frank Watson9 Conclusion/Future work Because of time constrains, we only touched the surface of what can be done with routing and manipulating packets at the TCP level. This project is just a simple demonstration of the power and changeability of the linux kernel to suite to a specific need and purpose. In the future, a packet can be sent back and fully responds to the SMTP command of the receiver and sender. The spam list can be inserted through a module oppose to hard coded into the kernel (as for this project). This project can be integrated into UCCS’ Linux Content Switch and be used as a front end switch, filtering unwanted spam mails before the mails even hit the back end servers.

Spam Mail FilterJeff Rupp & Frank Watson10 References The user mode linux webpagehttp://user-mode-linux.sourceforge.org An extremely helpful article about setting up UML with a step by step examplehttp:// A comprehensive webpage about setting up a linux kernel from scratch. This webpage helped with setting up the networking files. ng.html An extremely valuable document about the linux IP networking layerhttp://kernelnewbies.org/documents/ipnetworking/linuxipnetworki ng.html The RFC 821 for the SMTP protocolhttp://