8.3. Proxy ARP
If Proxy ARP is to be used on a VRRP router, then the VRRP router
must advertise the Virtual Router MAC address in the Proxy ARP
message. Doing otherwise could cause hosts to learn the real MAC
address of the VRRP router.
8.4. Potential Forwarding Loop
A VRRP router SHOULD not forward packets addressed to the IP
Address(es) it becomes Master for if it is not the owner. Forwarding
these packets would result in unnecessary traffic. Also in the case
of LANs that receive packets they transmit (e.g., token ring) this
can result in a forwarding loop that is only terminated when the IP
TTL expires.
One such mechanism for VRRP routers is to add/delete a reject host
route for each adopted IP address when transitioning to/from MASTER
state.
9. Operation over FDDI, Token Ring, and ATM LANE
9.1. Operation over FDDI
FDDI interfaces remove from the FDDI ring frames that have a source
MAC address matching the device’s hardware address. Under some
conditions, such as router isolations, ring failures, protocol
transitions, etc., VRRP may cause there to be more than one Master
router. If a Master router installs the virtual router MAC address
as the hardware address on a FDDI device, then other Masters’
ADVERTISEMENTS will be removed from the ring during the Master
convergence, and convergence will fail.
To avoid this an implementation SHOULD configure the virtual router
MAC address by adding a unicast MAC filter in the FDDI device, rather
than changing its hardware MAC address. This will prevent a Master
router from removing any ADVERTISEMENTS it did not originate.
9.2. Operation over Token Ring
Token ring has several characteristics that make running VRRP
difficult. These include:
- In order to switch to a new master located on a different bridge
token ring segment from the previous master when using source
route bridges, a mechanism is required to update cached source
route information.
- No general multicast mechanism supported across old and new token
ring adapter implementations. While many newer token ring
adapters support group addresses, token ring functional address
support is the only generally available multicast mechanism. Due
to the limited number of token ring functional addresses these may
collide with other usage of the same token ring functional
addresses.
Due to these difficulties, the preferred mode of operation over token
ring will be to use a token ring functional address for the VRID
virtual MAC address. Token ring functional addresses have the two
high order bits in the first MAC address octet set to B’1’. They
range from 03-00-00-00-00-80 to 03-00-02-00-00-00 (canonical format).
However, unlike multicast addresses, there is only one unique
functional address per bit position. The functional addresses
03-00-00-10-00-00 through 03-00-02-00-00-00 are reserved by the Token
Ring Architecture [TKARCH] for user-defined applications. However,
since there are only 12 user-defined token ring functional addresses,
there may be other non-IP protocols using the same functional
address. Since the Novell IPX [IPX] protocol uses the
03-00-00-10-00-00 functional address, operation of VRRP over token
ring will avoid use of this functional address. In general, token
ring VRRP users will be responsible for resolution of other user-
defined token ring functional address conflicts.
VRIDs are mapped directly to token ring functional addresses. In
order to decrease the likelihood of functional address conflicts,
allocation will begin with the largest functional address. Most
non-IP protocols use the first or first couple user-defined
functional addresses and it is expected that VRRP users will choose
VRIDs sequentially starting with 1.
VRID Token Ring Functional Address
---- -----------------------------
1 03-00-02-00-00-00
2 03-00-04-00-00-00
3 03-00-08-00-00-00
4 03-00-10-00-00-00
5 03-00-20-00-00-00
6 03-00-40-00-00-00
7 03-00-80-00-00-00
8 03-00-00-01-00-00
9 03-00-00-02-00-00
10 03-00-00-04-00-00
11 03-00-00-08-00-00
Or more succinctly, octets 3 and 4 of the functional address are
equal to (0x4000 >> (VRID - 1)) in non-canonical format.
Since a functional address cannot be used as a MAC level source
address, the real MAC address is used as the MAC source address in
VRRP advertisements. This is not a problem for bridges since packets
addressed to functional addresses will be sent on the spanning-tree
explorer path [802.1D].
The functional address mode of operation MUST be implemented by
routers supporting VRRP on token ring.
Additionally, routers MAY support unicast mode of operation to take
advantage of newer token ring adapter implementations that support
non-promiscuous reception for multiple unicast MAC addresses and to
avoid both the multicast traffic and usage conflicts associated with
the use of token ring functional addresses. Unicast mode uses the
same mapping of VRIDs to virtual MAC addresses as Ethernet. However,
one important difference exists. ARP request/reply packets contain
the virtual MAC address as the source MAC address. The reason for
this is that some token ring driver implementations keep a cache of
MAC address/source routing information independent of the ARP cache.
Hence, these implementations need to receive a packet with the
virtual MAC address as the source address in order to transmit to
that MAC address in a source-route bridged network.
Unicast mode on token ring has one limitation that should be
considered. If there are VRID routers on different source-route
bridge segments and there are host implementations that keep their
source-route information in the ARP cache and do not listen to
gratuitous ARPs, these hosts will not update their ARP source-route
information correctly when a switch-over occurs. The only possible
solution is to put all routers with the same VRID on the same
source-bridge segment and use techniques to prevent that bridge
segment from being a single point of failure. These techniques are
beyond the scope this document.
For both the multicast and unicast mode of operation, VRRP
advertisements sent to 224.0.0.18 should be encapsulated as described
in [RFC1469].
9.3. Operation over ATM LANE
Operation of VRRP over ATM LANE on routers with ATM LANE interfaces
and/or routers behind proxy LEC’s are beyond the scope of this
document.
10. Security Considerations
VRRP does not currently include any type of authentication. Earlier
versions of the VRRP specification included several types of
authentication ranging from none to strong. Operational experience
and further analysis determined that these did not provide any real
measure of security. Due to the nature of the VRRP protocol, even if
VRRP messages are cryptographically protected, it does not prevent
hostile routers from behaving as if they are a VRRP master, creating
multiple masters. Authentication of VRRP messages could have
prevented a hostile router from causing all properly functioning
routers from going into backup state. However, having multiple
masters can cause as much disruption as no routers, which
authentication cannot prevent. Also, even if a hostile router could
not disrupt VRRP, it can disrupt ARP and create the same effect as
having all routers go into backup.
It should be noted that these attacks are not worse and are a subset
of the attacks that any node attached to a LAN can do independently
of VRRP. The kind of attacks a malicious node on a LAN can do
include promiscuously receiving packets for any routers MAC address,
sending packets with the routers MAC address as the source MAC
addresses in the L2 header to tell the L2 switches to send packets
addressed to the router to the malicious node instead of the router,
send redirects to tell the hosts to send their traffic somewhere
else, send unsolicited ARP replies, answer ARP requests, etc., etc.
All of this can be done independently of implementing VRRP. VRRP
does not add to these vulnerabilities.
Independent of any authentication type VRRP includes a mechanism
(setting TTL=255, checking on receipt) that protects against VRRP
packets being injected from another remote network. This limits most
vulnerabilities to local attacks.
VRRP does not provide any confidentiality. Confidentiality is not
necessary for the correct operation of VRRP and there is no
information in the VRRP messages that must be kept secret from other
nodes on the LAN.
11. Acknowledgements
The authors would like to thank Glen Zorn, and Michael Lane, Clark
Bremer, Hal Peterson, Tony Li, Barbara Denny, Joel Halpern, Steve
Bellovin, Thomas Narten, Rob Montgomery, Rob Coltun, Radia Perlman,
Russ Housley, Harald Alvestrand, Steve Bellovin, Ned Freed, Ted
Hardie, Russ Housley, Bert Wijnen, Bill Fenner, and Alex Zinin for
their comments and suggestions.
12. References
12.1. Normative References
[802.1D] International Standard ISO/IEC 10038: 1993, ANSI/IEEE Std
802.1D, 1993 edition.
[CKSM] Braden, R., Borman, D. and C. Partridge, "Computing the
Internet checksum", RFC 1071, September 1988.
[HSRP] Li, T., Cole, B., Morton, P. and D. Li, "Cisco Hot Standby
Router Protocol (HSRP)", RFC 2281, March 1998.
[IPSTB] Higginson, P. and M. Shand, "Development of Router Clusters
to Provide Fast Failover in IP Networks", Digital Technical
Journal, Volume 9 Number 3, Winter 1997.
[IPX] Novell Incorporated., "IPX Router Specification", Version
1.10, October 1992.
[RFC1469] Pusateri, T., "IP Multicast over Token Ring Local Area
Networks", RFC 1469, June 1993.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2338] Knight, S., Weaver, D., Whipple, D., Hinden, R., Mitzel,
D., Hunt, P., Higginson, P., Shand, M. and A. Lindem,
"Virtual Router Redundancy Protocol", RFC 2338, April 1998.
[TKARCH] IBM Token-Ring Network, Architecture Reference, Publication
SC30-3374-02, Third Edition, (September, 1989).
12.2. Informative References
[DISC] Deering, S., Ed., "ICMP Router Discovery Messages", RFC
1256, September 1991.
[DHCP] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[OSPF] Moy, J., "OSPF version 2", STD 54, RFC 2328, April 1998.
[RIP] Malkin, G., "RIP Version 2", STD 56, RFC 2453, November
1998.
13. Changes from RFC 2338
- Moved authors of RFC 2338 to new Contributers section to comply
with RFC editor policy and listed R. Hinden as Editor.
- Removed authentication methods from VRRP. Changes included:
o Removed the values for password and IPSEC based authentication.
The fields and values are retained to keep backwards
compatibility with RFC 2338.
o Removed section on extensible security
o Updated security consideration section to remove discussion of
different authentication methods and added new text explaining
motivation for change and describe vulnerabilities.
- Revised the section 4 examples text with a clearer description of
mapping of IP address owner, priorities, etc.
- Clarify the section 7.1 text describing address list validation.
- Corrected text in Preempt_Mode definition.
- Changed authentication to be per Virtual Router instead of per
Interface.
- Added new subsection (9.3) stating that VRRP over ATM LANE is
beyond the scope of this document.
- Clarified text describing received packet length check.
- Clarified text describing received authentication check.
- Clarified text describing VRID verification check.
- Added new subsection (8.4) describing need to not forward packets
for adopted IP addresses.
- Added clarification to the security considerations section.
- Added reference for computing the internet checksum.
- Updated references and author information.
- Various small editorial changes.
14. Editor’s Address
Robert Hinden
Nokia
313 Fairchild Drive
Mountain View, CA 94043
US
Phone: +1 650 625-2004
EMail: bob.hinden@nokia.com
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