Netfilter home assignment

Delivered Files

Readme.md - This file.
net_filt.c - Source file of the loadable kernel module.
Makefile - Makefile of the loadable kernel module.

Please note that the readme file is in Markdown format.
Online MD file viewer like https://dillinger.io/ can be used.
Just copy the content of the file there.

Usage commands

# Compile module
make 

# Install module
sudo make i

# Remove module
sudo make r

Design notes

Cache entry structure

Each cache entry is represented by the following structure:

struct FilterCacheEntry
{
    char mac_address[MAC_ADDRESS_SIZE];

    /**
     * @brief: String containing the network device of the ARP inbound request.
     *         IFNAMSIZ defined in linux/if.h. Its max device name in netdevice.h.
     */
    char network_device_name[IFNAMSIZ];
    
    long arrival_time_secs;
    
    struct list_head list;
};

Note that since the netfilter hook in on ARP protocol (MAC layer), the sk_buff has an empty IP header.
From my observation this field indeed is always zero.

Cache TTL implementation

Cache TTL is managed by timer. Each second, the timer runs a routine that
iterates over the cache, and removes all the expired entries.

If cache is reached its maximum size and a new entry is inserted,
the oldest entry will be removed to make space for the new one.

Synchronization

Since the access to the cache can be done from multiple threads, synchronization is needed.
Spin lock is used to sync all cache accesses.

Proc file

The file /proc/netfilter_proc_file is created by the net_filt module.
This file can be used to interact with the ARP cache that is stored in the
kernel space.

Reading from proc file

Reading from /proc/netfilter_proc_file, will print all the cache content in human readable format.

Usage example:

$ sudo cat /proc/netfilter_proc_file
MAC: [4c:27:3e:c9:65:b5], DEV: [ens160]
MAC: [8c:22:3e:c9:65:b5], DEV: [ens160]
MAC: [8c:2c:3e:c9:65:b5], DEV: [ens160]
MAC: [0c:2b:3e:c9:65:b5], DEV: [ens160]
MAC: [4c:2d:3e:c9:65:b5], DEV: [ens160]
MAC: [8c:21:3e:c9:65:b5], DEV: [ens160]
MAC: [8c:2f:3e:c9:65:b5], DEV: [ens160]
MAC: [cc:21:3e:c9:65:b5], DEV: [ens160]

Writing to proc file

Writing number N to the /proc/netfilter_proc_file, will cause the module
deleting N cache entries.

Non numerical inputs will be ignored.
If N is larger that the current cache size, all cache entries will be removed.