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IAB concerns against permanent deployment of edge-based port filtering, October 2003

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18 October 2003

IAB concerns against permanent deployment of edge-based filtering

The IAB notes that there ISPs/ASes undertaking permanent deployment of edge-based protocol number/port number packet filtering on traffic received from eBGP peers.

As a short term response to security incidents this is a prudent operational measure that limits the spread of various forms of attack, and also mitigates some level of risk associated with network vulnerabilities. For example, many ISPs installed temporary filters in response to a July 2003 security advisory for CISCO routers ( In the case of this incident PIM (protocol #103) and mobile-ip4 (55) packets could trigger the vulnerability. The operational community responded with widespread deployment of filters at AS borders for these protocol numbers. Because of this, PIM and mobile-ip4 no longer work across such AS borders.

The IAB is concerned about the practice of the permanent deployment of such traffic filters, since this could block the operation of certain applications in current use, as well as limiting the potential for deployment of future applications. Such filters ultimately limit extensibility of the Internet protocol as well as the Internet itself.

It is an entirely appropriate and operationally prudent response to filter at the AS border as a short term mitigation of various network vulnerabilities. However, filters at AS borders do not provide any more than a relatively short term mitigation, and certainly do not solve the real problem of eliminating all forms of exploitation of such vulnerabilities. Over time knowledge of a vulnerability spreads across the network and potential exploiters of a vulnerability will be within an ISP/AS as well as being on the outside. The only stable and appropriate longer term operational response is to upgrade network equipment to eliminate the vulnerability, rather than attempting to configure packet filters intended to prevent externally located third parties from exploiting it.

While short term traffic filters are deployed, the appropriate recommended longer term action is to:

  • Install filters to detect packets that are directed to the router itself to protect the router (do not filter traffic that goes through the routers).
  • Update router firmware to a version known to eliminate the vulnerability


Jun-ichiro itojun Hagino,
on behalf of IAB (