RFC 3442
The Classless Static Route Option for Dynamic Host Configuration Protocol (DHCP) version 4
The Classless Static Route Option for Dynamic Host Configuration Protocol (DHCP) version 4 🎀
Okay so bestie, this is RFC 3442 💖. It's a Standards Track doc from the Network Working Group, dated December 2002. The authors are T. Lemon (Nominum, Inc.), S. Cheshire (Apple Computer, Inc.), and B. Volz (Ericsson). It updates RFC 2132, and it's basically the glow-up for DHCP routing 💅✨
Status of This Memo 🌸
This document specifies an Internet standards track protocol for the Internet community, and it literally asks for discussion and suggestions for improvements 🫶 For the standardization state and status of this protocol, check the current edition of the "Internet Official Protocol Standards" (STD 1). Distribution of this memo is unlimited 🦋
Copyright Notice 🔒
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract ✨
This document defines a new Dynamic Host Configuration Protocol (DHCP) option 🎀 It gets passed from the DHCP Server to the DHCP Client so the client can configure a list of static routes. The network destinations in these routes are classless, which means each routing table entry includes a subnet mask 😭
Introduction 🧚
This option obsoletes the Static Route option (option 33) defined in RFC 2132 [4] 💀
Okay so the IP protocol [1] uses routers to transmit packets from hosts connected to one IP subnet to hosts connected to a different IP subnet 🌈 When an IP host (the source host) wants to send a packet to another IP host (the destination), it checks its routing table to find the IP address of the router that should forward the packet to the destination host 💅
The routing table on an IP host can be maintained in a few ways. You can use a routing information protocol such as RIP [8], ICMP router discovery [6,9], or the DHCP Router option, defined in RFC 2132 [4] ✨
In a network that already provides DHCP service, using DHCP to update the routing table on a DHCP client has several perks 🍓 It's efficient, since it uses messages that would've been sent anyway. It's convenient too, because the DHCP server configuration is already being maintained, so keeping routing information up to date needs very little extra work, at least on a relatively stable network 🫶 If DHCP service is already in use, no additional infrastructure needs to be deployed. Iconic behavior 👑
The DHCP protocol as defined in RFC 2131 [3] and the options defined in RFC 2132 [4] only provide a mechanism for installing a default route or a table of classful routes 🙈 Classful routes are routes whose subnet mask is implicit in the subnet number. See section 3.2 of STD 5, RFC 791 [1] for details on classful routing 📚
Classful routing isn't in common use anymore, so the DHCP Static Route option isn't really useful now 😭 Right now, classless routing [7, 10] is the most commonly deployed form of routing on the Internet 🌍 In classless routing, IP addresses consist of a network number (the combination of the network number and subnet number described in RFC 950 [7]) and a host number 🧁
In classful IP, the network number and host number are derived from the IP address using a bitmask. The value of that bitmask is determined by the first few bits of the IP address. In classless IP, the network number and host number are derived from the IP address using a separate quantity, the subnet mask 💜 So to figure out which network a given route applies to, an IP host must know both the network number AND the subnet mask for that network. That's the whole vibe 🎯
The Static Routes option (option 33) doesn't provide a subnet mask for each route. It's assumed that the subnet mask is implicit in whatever network number is specified in each route entry 🙃 The Classless Static Routes option does provide a subnet mask for each entry, so the subnet mask can be different from what would be determined using the algorithm specified in STD 5, RFC 791 [1] and STD 5, RFC 950 [7] 💅✨
Definitions 📖
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY" and "OPTIONAL" in this document are to be interpreted as described in BCP 14, RFC 2119 [2] 🔥
This document also uses the following terms 🌷
- "DHCP client": A DHCP client, or "client," is an Internet host using DHCP to obtain configuration parameters such as a network address 🦋
- "DHCP server": A DHCP server, or "server," is an Internet host that returns configuration parameters to DHCP clients 🧚
- "link": Any set of network attachment points that will all receive a link-layer broadcast sent on any one of the attachment points. This term is used in DHCP because in some cases more than one IP subnet may be configured on a link. DHCP uses a local-network (all-ones) broadcast, which is not subnet-specific, and will therefore reach all nodes connected to the link, regardless of the IP subnet or subnets on which they are configured. A "link" is sometimes called a broadcast domain or physical network segment 🌈
Classless Route Option Format 🎀
The code for this option is 121, and its minimum length is 5 bytes 💖 This option can contain one or more static routes. Each route consists of a destination descriptor and the IP address of the router that should be used to reach that destination 🍓
Code Len Destination 1 Router 1
+-----+---+----+-----+----+----+----+----+----+
| 121 | n | d1 | ... | dN | r1 | r2 | r3 | r4 |
+-----+---+----+-----+----+----+----+----+----+
Destination 2 Router 2
+----+-----+----+----+----+----+----+
| d1 | ... | dN | r1 | r2 | r3 | r4 |
+----+-----+----+----+----+----+----+In the above example, two static routes are specified 👯
Destination descriptors describe the IP subnet number and subnet mask of a particular destination using a compact encoding 🧠 This encoding is one octet describing the width of the subnet mask, followed by all the significant octets of the subnet number 🫧
The width of the subnet mask is the number of one bits in the mask. For example, a subnet with a subnet number of 10.0.127.0 and a netmask of 255.255.255.0 would have a subnet mask width of 24 💅
The significant portion of the subnet number is all of the octets of the subnet number where the corresponding octet in the subnet mask is non-zero. The number of significant octets is the width of the subnet mask divided by eight, rounding up, as shown in this table 📊
Width of subnet mask Number of significant octets
0 0
1- 8 1
9-16 2
17-24 3
25-32 4Here are some examples of how various subnet number/mask combinations can be encoded 🌸
Subnet number Subnet mask Destination descriptor
0 0 0
10.0.0.0 255.0.0.0 8.10
10.0.0.0 255.255.255.0 24.10.0.0
10.17.0.0 255.255.0.0 16.10.17
10.27.129.0 255.255.255.0 24.10.27.129
10.229.0.128 255.255.255.128 25.10.229.0.128
10.198.122.47 255.255.255.255 32.10.198.122.47Local Subnet Routes 🏡
In some cases more than one IP subnet may be configured on a link 🌈 When that happens, a host whose IP address is in one IP subnet on the link could talk directly to a host whose IP address is in a different IP subnet on the same link 🫶 When a client is being assigned an IP address on an IP subnet on such a link, then for each IP subnet on the link other than the one the client was assigned to, the DHCP server MAY be configured to specify a router IP address of 0.0.0.0 ✨
Let's look at an example 🧁 Say there are three IP subnets configured on a link: 10.0.0/24, 192.168.0/24, and 10.0.21/24. If the client is assigned the IP address 10.0.21.17, then the server could include a route with a destination of 10.0.0/24 and a router address of 0.0.0.0, and also a route with a destination of 192.168.0/24 and a router address of 0.0.0.0 💖
A DHCP client whose underlying TCP/IP stack doesn't provide this capability MUST ignore routes in the Classless Static Routes option whose router IP address is 0.0.0.0. Please note that the behavior described here only applies to the Classless Static Routes option, not to the Static Routes option or the Router option ⚠️
DHCP Client Behavior 💅
DHCP clients that don't support this option MUST ignore it if it's received from a DHCP server 🙈 DHCP clients that support this option MUST install the routes specified in the option, except as specified in the Local Subnet Routes section 👑 DHCP clients that support this option MUST NOT install the routes specified in the Static Routes option (option code 33) if both a Static Routes option and the Classless Static Routes option are provided 🔥
DHCP clients that support this option and that send a DHCP Parameter Request List option MUST request both this option and the Router option [4] in the DHCP Parameter Request List 🎀
DHCP clients that support this option and send a parameter request list MAY also request the Static Routes option, for compatibility with older servers that don't support Classless Static Routes 🦋 The Classless Static Routes option code MUST appear in the parameter request list prior to both the Router option code and the Static Routes option code, if present 🧚
If the DHCP server returns both a Classless Static Routes option and a Router option, the DHCP client MUST ignore the Router option 🌸
Similarly, if the DHCP server returns both a Classless Static Routes option and a Static Routes option, the DHCP client MUST ignore the Static Routes option 💜
After deriving a subnet number and subnet mask from each destination descriptor, the DHCP client MUST zero any bits in the subnet number where the corresponding bit in the mask is zero. In other words, the subnet number installed in the routing table is the logical AND of the subnet number and subnet mask given in the Classless Static Routes option 🧠 For example, if the server sends a route with a destination of 129.210.177.132 (hexadecimal 81D4B184) and a subnet mask of 255.255.255.128 (hexadecimal FFFFFF80), the client will install a route with a destination of 129.210.177.128 (hexadecimal 81D4B180) 🎯
Requirements to Avoid Sizing Constraints ✨
okay so a full routing table can get lowkey huge 😭, which means the standard 576 octet maximum size for a DHCP message might be too short to fit some legit Classless Static Route options 💀. so clients implementing the Classless Static Route option SHOULD send a Maximum DHCP Message Size [4] option if their TCP/IP stack can receive larger IP datagrams 🫶. in that case the client SHOULD set this option to at least the MTU of the interface it's configuring. the client MAY set it higher, up to the size of the largest UDP packet it's prepared to accept 💅. (quick reminder: the Maximum DHCP Message Size value is the total max packet size, IP and UDP headers included 🎀)
DHCP clients requesting this option, and DHCP servers sending this option, MUST implement DHCP option concatenation [5] 🔥. in RFC 3396 [5] terms, the Classless Static Route Option is a concatenation-requiring option 🦋.
DHCP Server Administrator Responsibilities 🧚
bestie, many clients may not implement the Classless Static Routes option 🙈. so DHCP server admins should configure their servers to send both a Router option and a Classless Static Routes option 💜. the default router(s) should be specified in both the Router option and the Classless Static Routes option, so nobody gets left out 🌸.
when a DHCP client requests the Classless Static Routes option, and also requests either or both of the Router option and the Static Routes option, and the DHCP server is sending Classless Static Routes options to that client, the server SHOULD NOT include the Router or Static Routes options 👑. no double-dipping, periodt 💅.
Security Considerations 💀
okay this part is seriously dramatic, so read carefully 🚨.
potential exposures to attack in the DHCP protocol are discussed in section 7 of the DHCP protocol specification [3] and in Authentication for DHCP Messages [11] 🔐.
the Classless Static Routes option can be used to misdirect network traffic by providing incorrect IP addresses for routers 😱. this can be either a Denial of Service attack, where the router IP address given is simply invalid, or it can be used to set up a man-in-the-middle attack by providing the IP address of a potential snooper 🕵️♀️. this is not a new problem, though, because the existing Router and Static Routes options defined in RFC 2132 [4] have the same vulnerability 🥲.
IANA Considerations 🌈
this DHCP option has been allocated the option code 121 in the list of DHCP option codes that the IANA maintains ⭐.
Normative References 📚
- [1] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981.
- [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
- [3] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997.
- [4] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor Extensions", RFC 2132, March 1997.
- [5] Lemon, T. and S. Cheshire, "Encoding Long Options in the Dynamic Host Configuration Protocol (DHCPv4)", RFC 3396, November 2002.
Informative References 🧠
- [6] Postel, J., "Internet Control Message Protocol", STD 5, RFC 792, September 1981.
- [7] Mogul, J. and J. Postel, "Internet Standard Subnetting Procedure", STD 5, RFC 950, August 1985.
- [8] Hedrick, C., "Routing Information Protocol", RFC 1058, June 1988.
- [9] Deering, S., "ICMP Router Discovery Messages", RFC 1256, September 1991.
- [10] Pummill, T. and B. Manning, "Variable Length Subnet Table For IPv4", RFC 1878, December 1995.
- [11] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages", RFC 3118, June 2001.
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Authors' Addresses 🎀
Ted Lemon
Nominum, Inc.
2385 Bay Road
Redwood City, CA 94063
EMail: Ted.Lemon@nominum.com
Stuart Cheshire
Apple Computer, Inc.
1 Infinite Loop
Cupertino
California 95014
USA
Phone: +1 408 974 3207
EMail: rfc@stuartcheshire.org
Bernie Volz
Ericsson
959 Concord Street
Framingham, MA, 01701
Phone: +1 508 875 3162
EMail: bernie.volz@ericsson.comFull Copyright Statement ©️
Copyright (C) The Internet Society (2002). All Rights Reserved.
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Acknowledgement 🫶
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