ipfw divert filter for IPv4 geo-blocking

[Dr. Rolf Jansen]

> Am 29.07.2016 um 10:23 schrieb Dr. Rolf Jansen <rj at obsigna.com>:
>> Am 29.07.2016 um 06:50 schrieb Julian Elischer <julian at freebsd.org>:
>> On 29/07/2016 5:22 PM, Julian Elischer wrote:
>>> On 29/07/2016 4:53 PM, Dr. Rolf Jansen wrote:
>>>>> Am 28.07.2016 um 23:48 schrieb Lee Brown <leeb at ratnaling.org>:
>>>>> 
>>>>> That makes sense to me.  Your /20 range encompasses 201.222.16.0 -
>>>>> 201.222.31.255.
>>>>> If you want 201.222.20.0-201.222.31.255, you'll need 3 ranges:
>>>>> 
>>>>> 201.222.20.0/22 (201.222.20.0-201.222.23.255)
>>>>> 201.222.24.0/22 (201.222.24.0-201.222.27.255)
>>>>> 201.222.28.0/22 (201.222.28.0-201.222.31.255)
>>>> 
>>>> Ian, Julian and Lee,
>>>> 
>>>> Thank you vary much for your responses. In order not bloat the thread, I answer only to one message.
>>>> 
>>>> I found the problem. As a matter of fact, the respective IP ranges in the LACNIC delegation statistics file are 3 adjacent blocks with 1024 addresses, i.e. those that you listed in your message above:
>>>> 
>>>> $grep 201.222.2 /usr/local/etc/ipdb/IPRanges/lacnic.dat
>>>> lacnic|BR|ipv4|201.222.20.0|1024|20140710|allocated|164725
>>>> lacnic|BR|ipv4|201.222.24.0|1024|20140630|allocated|138376
>>>> lacnic|BR|ipv4|201.222.28.0|1024|20140701|allocated|129095
>>>> 
>>>> However, my database compilation combines adjacent blocks with the same country code, and the ranges above turn into one block of 3072 addresses, which obviously doesn't have a valid netmask - log(3072) = 11,5849625.
>>>> ...
>>>> ..., it is not sufficient to forget about optimization but I need to check also whether, the delegation files contain already some non-CIDR ranges, which need to be broken down.
>>> 
>>> there is code to take ranges and produce cidr sets.
>>> 
>>> We used to have exactly that code in the appletalk code before we took it out. Appletalk uses ranges.
>>> https://svnweb.freebsd.org/base/release/3.2.0/sys/netatalk/at_control.c?view=annotate#l703 
>> 
>> though htat uassumes input in the form af an appletak sockaddr..
>> there is also this python module
>> https://pythonhosted.org/netaddr/tutorial_01.html#support-for-non-standard-address-ranges
>> 
>>> maybe you can find other versions on the net.
>>> however if you fully populate the table, you will get the correct result because more specific entries will
>>> override less specific entries. To do that you would have to have a way to describe to your program what
>>> value each table entry should output.
>>> If you did what you do now, then you would specify the value for the required countries, and give a default falue for "all others".
>>> aggregation of adjacent ranges with same value would be an optimisation.
> 
> Don't worry, breaking down an arbitrary IP-range into a CIDR conforming set of ranges, doesn't seem too difficult. ...
> ...
> Once I come to a conclusion, I will post it to this mailing list.

I finished the work on CIDR conformity of the IP ranges tables generated by the tool geoip. The main constraint is that the start and end address of an IP block given by the delegation files MUST BE PRESERVED during the transformation to a set of CIDR records. This target is achieved by:

 1. Finding the largest common netmask boundary of the start address utilizing
    int(log2(addr_count)); then iteration like Euclid's algorithm in computing
    a GCD.

 2. Output the CIDR with the given start address and the masklen belonging
    to the found netmask.

 3. If the CIDR does not match the whole original IP range then set the start
    address of the next CIDR block to the next boundary of the common netmask,
    and loop over starting at 1. until the original range has been satisfied.

I carefully tested the algorithm and a table that I pipe by the new geoip tool into ipfw is 100 % identical to the output of the ipfw command 'table N list'.

It is worth to note, that already the original RIR delegation files contain 457 non CIDR conforming IPv4 ranges in a total of 165815 original records. I guess that this number will increase in the future because the RIR's ran empty on new IPv4 ranges and are urged to subdivide returned old ranges for new delegations. The above algorithm is ready for this.

Generally, CIDR conforming tables are more than twice as large as optimized (joined adjacencies) IP range tables. All said changes have been pushed to GitHup already.

I am still a little bit amazed how ipfw come to accept incorrect CIDR ranges and arbitrarily moves the start/end addresses in order to achieve CIDR conformity, and that without any further notice, and that given that ipfw can be considered as being quite relevant to system security. Or, may I assume that ipfw knows always better than the user what should be allowed or denied. Otherwise, perhaps I am the only one ever who input incorrect CIDR ranges for processing by ipfw.

Best regards

Rolf