Threat Level: green Handler on Duty: Brad Duncan

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DDoS and BCP 38

Quite often on many lists we will hear the term Best Current Practice (BCP) 38 bandied about and further recommendations to implement [1] [2][3][4] (See NANOG Mailing list archive) . Some will say ‘it will aid in DDoS mitigation’ and even others will even state ‘All Internet Service Providers (ISP) should implement this." Now before the philosophical discussions ensue in the comments, it might be a good idea to discuss, technically, what it is? And perhaps what it can do?

BCP 38 A.K.A. RFC 2267 [5] is a best practice methodology around ingress traffic filtering. The specific purpose as stated in the RFC abstract “to prohibit DoS attacks which use forged IP addresses to be propagated    from 'behind' an Internet Service Provider's (ISP) aggregation point.” [5]

The BCP 38 outlines the concept of “restricting transit traffic” that comes from a “downstream network to know, and internally advertised prefixes” [5, p.4]. In an overly simplified diagram (My interpretation of the RFC, comments and corrections welcome), it means the ISP says:




Let us know if you are using or have implemented BCP38? We recommend it and do feel that it has technical merit and can help reduce risk!












Richard Porter

--- ISC Handler on Duty

Twitter: @packetalien


"Got Packets?"


173 Posts
ISC Handler
Feb 27th 2014

Members of NANOG are putting some efforts to make BCP38 information more available to a wider audience, in the hope that it could create some customers pressure to start cleaning up that mess.

PS: Let moderator "hat" know if they can incorporate the graphic in that wiki.
Posted on NANOG for off-list contact. Permission definitely granted!

173 Posts
ISC Handler
Just out of curiosity, how come you reference RFC2267 as the definition of BCP38, when RFC2827 is the official definition of BCP38? They're almost identical in terms of content, but RFC2827 states in its header that it both obsoletes 2267 and defines BCP38.


1 Posts
Likely because I was not paying attention, thank you for pointing it out! Ill correct it~

173 Posts
ISC Handler
Hi Richard,

Looking into deploy BCP38 on a customer facing interface. I came across some suggestion about deploying something like this.Looking for any input from you guys.


ip verify unicast source reachable-via rx allow-default 171

access-list 171 deny icmp any any echo log-input
access-list 171 deny icmp any any echo-reply log-input
access-list 171 deny udp any any eq echo log-input
access-list 171 deny udp any eq echo any log-input
access-list 171 deny tcp any any established log-input
access-list 171 deny tcp any any log-input
access-list 171 deny ip any any log-input
2 Posts

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