A simple python function to return `iptables -m multiport` compatible shortest representation of a given list on integers.

multiport_represent_ports.py

# Author: Beren Oguz <beren@berkeley.edu>

# function: multiport_represent_ports
# input: arr, a list of port numbers, their type must be int
# output: shortest representation that can be passed to `iptables -m multiport`

def multiport_represent_ports(arr):
    min_max = [[i, i] for i in range(len(arr))] # we keep track of min and max of ranges
    port2id = dict() # port number -> array index
    id2port = dict() # array index -> port number
    arr = sorted(arr)
    
    i = 0
    for port in arr:
        if port - 1 in port2id:
            min = min_max[port2id[port - 1]][0] # min of range
            min_max[min][1] = i # make min's max this port
            min_max[i][0] = min # make my min this min
        port2id[port] = i
        id2port[i] = port
        i += 1

    output = []
    total_ports = 15 # we start from 15 so that if len(arr) == 0, output is []

    i = 0
    while True:
        if i >= len(arr): # we're done
            return output
        current = min_max[i] # current port
        if total_ports % 15 == 0: # maximum amount of ports reached
            output.append('')
        if current[0] == current[1]: # not a range
            current_str = str(id2port[current[0]])
            if total_ports % 15 == 0:
                output[-1] = output[-1] + current_str
            else:
                output[-1] = output[-1] + "," + current_str
        else: # range
            current_str = str(id2port[current[0]]) + ":" + str(id2port[current[1]])
            if total_ports % 15 == 0:
                output[-1] = output[-1] + current_str
            else:
                output[-1] = output[-1] + "," + current_str
            i = current[1]
            total_ports += 1
        total_ports += 1
        i += 1