Security Considerations for IP Fragment Filtering (RFC1858)
Original Publication Date: 1995-Oct-01
Included in the Prior Art Database: 2019-Feb-12
Internet Society Requests For Comment (RFCs)
G. Ziemba: AUTHOR [+2]
IP fragmentation can be used to disguise TCP packets from IP filters used in routers and hosts. This document describes two methods of attack as well as remedies to prevent them. This memo provides information for the Internet community. This memo does not specify an Internet standard of any kind.
Network Working Group G. Ziemba Request for Comments: 1858 Alantec Category: Informational D. Reed Cybersource P. Traina cisco Systems October 1995
Security Considerations for IP Fragment Filtering
Status of This Memo
This memo provides information for the Internet community. This memo does not specify an Internet standard of any kind. Distribution of this memo is unlimited.
IP fragmentation can be used to disguise TCP packets from IP filters used in routers and hosts. This document describes two methods of attack as well as remedies to prevent them.
System administrators rely on manufacturers of networking equipment to provide them with packet filters; these filters are used for keeping attackers from accessing private systems and information, while permitting friendly agents to transfer data between private nets and the Internet. For this reason, it is important for network equipment vendors to anticipate possible attacks against their equipment and to implement robust mechanisms to deflect such attacks.
The growth of the global Internet has brought with it an increase in "undesirable elements" manifested in antisocial behavior. Recent months have seen the use of novel attacks on Internet hosts, which have in some cases led to the compromise of sensitive data.
Increasingly sophisticated attackers have begun to exploit the more subtle aspects of the Internet Protocol; fragmentation of IP packets, an important feature in heterogeneous internetworks, poses several potential problems which we explore here.
Ziemba, Reed & Traina Informational [Page 1]
RFC 1858 Security Considerations - IP Fragment Filtering October 1995
2. Filtering IP Fragments
IP packet filters on routers are designed with a user interface that hides packet fragmentation from the administrator; conceptually, an IP filter is applied to each IP packet as a complete entity.
One approach to fragment filtering, described by Mogul , involves keeping track of the results of applying filter rules to the first fragment (FO==0) and applying them to subsequent fragments of the same packet. The filtering module would maintain a list of packets indexed by the source address, destination address, protocol, and IP ID. When the initial (FO==0) fragment is seen, if the MF bit is set, a list item would be allocated to hold the result of filter access checks. When packets with a non-zero FO come in, look up the list element with a matching SA/DA/PROT/ID and apply the stored result (pass or block). When a fragment with a zero MF bit is seen, free the list element.
Although this method (or some refinement of it) might successfully remove any trace of the offending whole packet, it has some difficulties. Fragments that arrive out of order, possibly because they traveled over different paths, violate one of the design assumptions, and undesired fragments can leak through as a result. Furthermore, if the filtering router lies on one of several parallel paths, the filtering m...