What is SLAAC?
IPv6 devices can autonomously configure their own unique IP addresses without the use of a DHCP server, a technique called stateless address autoconfiguration (SLAAC). Compared to DHCPv6, it offers an easier, more direct method for IPv6 auto-addressing.
Core Concept and Purpose
By fusing the device’s own interface identity with information from routers, SLAAC allows a host to create a unique IPv6 address. A “stateless” system is one in which no server maintains a record of which addresses have been assigned or remain available. In contrast, “stateful” assignment involves a server (such as DHCP) keeping track of lease durations, MAC address mappings, and issued addresses. Providing the bare minimum of network parameters required for communication, a global unicast IPv6 address, prefix length, and, if desired, a default router is the main objective of SLAAC. Plug-and-play networking is supported, enabling fast device joining and configuration.
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How SLAAC Works (Step-by-Step)
A node must complete a number of steps in the SLAAC process in order to receive a distinct global unicast IPv6 address:
Link-Local Address Creation
A device first creates a link-local address (using the FE80::/10 or FE80::/64 prefix) when it joins an IPv6 network. The node can communicate at Layer 3 inside its local segment this address. Usually, an interface identifier, which is frequently obtained from the device’s MAC address (e.g., EUI-64) is combined with the link-local prefix to generate it. They may be generated at random by certain operating systems, such as Windows. Because it facilitates the Layer 3 communication required for the process, this step is a precondition for SLAAC.
Duplicate Address Detection (DAD) for Link-Local Address
The device uses Duplicate Address Detection (DAD) to make sure the address is unique on the local link after configuring its link-local address. The node joins a unique solicited-node multicast group (FF02::1:FFxx:xxxx) based on its possible IPv6 address as part of DAD. With its possible address in the Target Address field, the host then sends this multicast group an ICMPv6 Neighbor Solicitation message. The address is deemed unique and accessible if no other device responds. The host tries to select an alternative link-local address if a duplication is found.
Router Solicitation (RS)
The device sends a Router Solicitation (RS) message to the all-routers multicast address (FF02::2) after the link-local address is unique. This message is intended to ask any linked IPv6 routers for information about the network setup, specifically the global unicast prefix that is used on the segment.
Router Advertisement (RA)
Router Advertisement (RA) messages are sent by routers in response to the RS message. Periodically, RAs are sent as well. Important network details are included in this message, such as:
- The prefix length and network prefix (e.g., 2001:db8:1::/64 or 2001:1234:A:B::/64).
- Configuration options that specify if DHCPv6 or SLAAC should be utilized for more details.
- The address of the router by default, usually the link-local address of the router. The all-nodes multicast address (FF02::1) receives the RA message from the router’s link-local address.
Global Unicast Address Configuration
The device creates a full 128-bit global unicast IPv6 address by combining its own 64-bit interface identification (IID) with the advertised network prefix (64 bits) from the RA message. There are two methods for generating the IID:
- Modified EUI-64: This conventional technique generates a 64-bit IID by flipping the 7th bit of the first byte and inserting FFFE in the middle of the device’s 48-bit MAC address.
- RFC 7217 (Stable Privacy Addresses): A more recent method, RFC 7217 (Stable Privacy Addresses), uses an interface identification, a secret key, and the network prefix to create a randomized, stable IID. By preventing the address from being directly linked to the hardware, this allays privacy worries.
- Random Generation: The IID may also be generated at random by hosts.
Duplicate Address Detection (DAD) for Global Unicast Address
The device conducts DAD once more to guarantee its uniqueness on the link before it may actively use its newly created global unicast IPv6 address. The DAD carried out for the link-local address is the same procedure as this one. The host can begin communicating with the address if DAD passes.
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Configuration Options and RA Flags
Basic addressing is provided by SLAAC, but for more configuration information, it can be coupled with other techniques. Messages from router advertisements contain flags that tell hosts where to find more information. These are the flags:
Managed (M) flag: When the managed (M) flag is set to 1, it means that DHCPv6 addresses are accessible and that the router is telling nodes to request addresses and DNS information from a DHCPv6 server. The O flag is disregarded if M is 1.
Other (O) flag: This indicates that DNS information is accessible using DHCPv6 if M is 0 and O is set to 1. In this case, hosts use SLAAC to automatically configure their address, but they ask a DHCPv6 server for DNS and other non-address configuration information.
Default Router Preference (Prf) flag: When multiple RAs are received, nodes can choose which router to use as their default gateway by using the Default Router Preference (Prf) option, which can be Low, Medium, or High.
RDNSS (Recursive DNS Server option): In certain situations, modern operating systems can use RDNSS (RFC 8106) to obtain DNS server information directly via RA messages, doing away with the need for DHCPv6.
If neither the M nor the O flags are set, SLAAC is the only DHCPv6 server accessible for either address or other configuration.
Advantages of SLAAC
Simple and Automatic Configuration: Without any administrator assistance, devices can get IP connectivity on their own.
Reduces Administrative Overhead: DHCP servers and manual configuration are no longer required.
Scalability: Excellent for networks with a large number of devices, like consumer electronics, smart cities, and Internet of Things scenarios, where managing DHCPv6 servers or manual configuration would be prohibitively expensive.
Ensures Unique Addresses: By using the DAD procedure, unique addresses are ensured.
Simplified Network Renumbering: As nodes phase out old prefixes and acquire new ones, networks can be renumbered smoothly.
Limitations of SLAAC
Limited Configuration Information: SLAAC does not offer DNS server information or other extra configuration details on its own. For conventional networks using PCs, laptops, and cellphones, this is a major disadvantage.
Reliance on Routers: For SLAAC to work, a router needs to be operational and transmitting RA messages.
Less Control for Administrators: Administrators have less centralized control over address assignments than they would with DHCPv6.
Security Considerations: Techniques such as SLAAC spying can help reduce the risk of security flaws in SLAAC.
IPv6 Address Lifetimes
SLAAC-generated IPv6 addresses are assigned two timers:
Preferred Lifetime: In this stage, the address is completely operational and suitable for all forms of communication, even those involving new contacts.
Valid Lifetime: The address moves into the deprecated phase when the preferred lifespan ends. While it is discouraged for new connections, it can still be used for those that already exist. The address is erased after the valid lifetime has passed.
Cisco Router Configuration
When IPv6 unicast routing is enabled on Cisco routers, the router usually sends RA messages to interfaces that have a global IPv6 unicast address set up. SLAAC is enabled via the IPv6 address autoconfig command on an interface, which generates a routable unicast address and a link-local address, frequently using EUI-64 rules for the IID. The O-flag or M-flag in RA messages can be set using the ipv6 nd other-config-flag or ipv6 nd managed-config-flag commands, respectively.
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