In a scenario where multiple devices serve as gateway roles on-link, and we aim to access their managed subnets seamlessly without configuration, a typical approach is designating one as the default gateway and configuring a static route to the other device on this gateway. Subsequently, a DHCP daemon is established on the default gateway to distribute a single route to itself, seemingly resolving the issue.
However, this solution presents a drawback: all traffic, including that targeted at the other gateway, is forced to traverse through the default gateway, even when the other gateway is on-link. This limitation becomes particularly critical when dealing with high-volume traffic or when link quality is a significant concern.
We want to distribute multiple routes rather than the single one to the default, so the clients can automatically and directly connect to the corresponding gateway without relaying.
It depends. Since the protocol for IPv4 cannot be used for IPV6 and vice versa, we need to configure respectively.
TLDR: We need to specify DHCP Options 121 (and 249 in some case).
In IPv4, the duty of route distributions falls to the DHCP and more specifically, DHCP Options.
And here I will mention some related options.
This option specifies a list of IP addresses for routers on the client's subnet which is not our case. This is used for providing multiple gateway addresses for a single subnet, not two or more.
This option specifies a list of static routes that the client should configure in its routing table. However, it is nearly useless nowadays because it used classful address instead of classless address, which is the current standard, and it lacks of flexibility.
Similar to the above one, but using classless address.
Classless Static Route Option for Dynamic Host Configuration Protocol (DHCP) version 4
Nearly same to the option 121, but the code changed to 249. This option is not a standardized option but a private option created by Microsoft. For the most modern operating systems, they can understand option 121 and treated it with higher priority. But Windows XP and Windows Server 2003 only recognize option 249. That's the case we need it.
Microsoft Classless Static Route Option
We will not cover the specific configuration for a specific software like odhcpd or ISC DHCP Server. Read the friendly manual please.
TLDR: We need to specify ICMPv6 Option 24.
As IPv6 implementation progressed, we encountered a roadblock due to the lack of IPv6 support in the outdated DHCP Option 121. Fortunately, we found a way forward. In the modern era, the responsibilities of DHCP have been integrated into ICMPv6, addressing our concerns. This integration includes what we're focusing on. Renamed as the Route Information Option, it enables the distribution of IPv6 static routes, though it lacks compatibility with IPv4.
And the audience are expected to read the manual again. FYI, most ICMPv6 server implmentations support this but the clients leave unknown. Your mileage may vary.
As for 2024/March/07, Android does not support DHCP Option 121 nor ICMP Redirect. The audience can read these threads to see how Google ignore this issue.
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