Friday, January 29, 2010


Recommended rfc networks to consider as filters

There are many ways to help protect your network from attack, one of the simpilist and most effective is actually to filter incoming and outgoing traffic from your network. An excellent place to start is to utilize the rfc's to define IPv4 addresses that are not or never will be in use on the public Internet and not allowing that traffic inbound. In the same vein, you can use the same information to limit what is allowed to leave your network, such as only IPv4 addresses that are legitimately routable on the public Internet.

This is not a new or unique solution but it is more commonly done at the service provider and larger enterprise level because those type of operations pay attention to the rfc's but also because they recieve much higher traffic loads on average traditionally. I believe that this technique is still useful for much smaller operations to use and is relatively simple to set up and maintain.

Here is a short list of rfc's to put in your firewall or edge router of addresses you should not be seeing from the Internet and ones that you should consider filtering out before sending traffic out to the Internet.

network RFC 1112
description - Host Extensions for IP Multicasting - in RFC 1700 also

network RFC 1700
description - assigned numbers - multicast, current, host, and reserved

network RFC 1797
description - Class A Subnet Experiment - may get reallocated - use the bogon list instead

network RFC 1918
description - reserved private IPv4 addresses

network RFC 2544
description - Benchmarking Methodology for Network Interconnect Devices

network RFC 3068
description - IPv4 reserved 6to4 IPv6 gateway services

network RFC 3171
description - IANA Guidelines for IPv4 Multicast Address Assignments (covers through

network RFC 3927
description - Dynamic Configuration of IPv4 Link-Local Addresses - in 5735 above

network RFC 5735 (update of RFC 3330)
description - this rfc really collects all the other rfc with special use (reserved and limited IPv4 blocks) in a single doc, these is only a partial listing (may be reallocated - use the bogon list instead just in case)

network RFC 5736
description - IPv4 Special Purpose Address Registry

network RFC 5737
description - reserved for test net

Here are some reference URL's to get you started to determine what you should apply for your needs.

In addition to using IP address list filters there are other protections you can take at the edge. You should consider putting more aggressive ICMP filters in and also filter specific IP protocol numbers from coming in or going out. I'll post more about that another time.
- Ed

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Tuesday, January 26, 2010


Update your IPv4 Bogon list

Everyone should update their IPv4 bogon lists a bit faster now that we are below 10% of IPv4 addresses left and it seems that IANA is handing them out faster to force folks to update their lists a bit quicker and I imagine to put pressure downstream for the adoption of IPv6.

January 2010 the following IP blocks were allocated to APNIC: 1/8 and 27/8. This one has some significance because a lot of network engineers in labs or to test something use or or some other variation within 1/8 or 2/8. Neither should be used now due to the new APNIC allocation and also because in September of 2009 2/8 and 46/8 were allocated to RIPE and should have been removed from your bogon list!

In addition, the IETF obsoleted RFC 3330 with RFC 5735 and as a result the following IP blocks have been reserved for documentation purposes: and That means they should be added to your bogon list.

If you don't like search around and keeping this stuff up to date yourself the best resource out there is The Team Cymru Bogon List. This list is kept up to date and is super useful as it provides the list in all sorts of useful formats. Highly recommended.
- Ed


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