DHCPv6 with slaac doesn't work

[viraniac]

Prerequisites

• I have checked the Wiki and Discussions and found no answer

• I have searched other issues and found no duplicates

• I want to report a bug and not ask a question or ask for help

• I have set up AdGuard Home correctly and configured clients to use it. (Use the Discussions for help with installing and configuring clients.)

Platform (OS and CPU architecture)

Linux, ARM64

Installation

GitHub releases or script from README

Setup

On one machine

AdGuard Home version

v0.107.55

Action

Enabled DHCP server and set ra_allow_slaac as well as ra_allow_slaac to true. Checked ss -lnp | grep -i adguard and output shows the following confirming that option is enabled correctly

icmp6 UNCONN 213376 0 [2405:201:1005:b071:21e:6ff:fe43:ccab]:58 *:* users:(("AdGuardHome",pid=4660,fd=13))

Expected result

Devices should be able to configure IPv6 address based on router advertisements

Actual result

All devices ignored the router advertisements received from adguard and configured link local address instead

Additional information and/or screenshots

Captured icmp packets using tcpdump shows that icmp packet is invalid. This seems to be broken since bfc7e16 as it left the length field in source link layer option section hard coded to 1.

[viraniac]

I have made changes that fixes this issue. Please find the same here - master...viraniac:AdGuardHome:master

I was going to open a PR with the same, but PR descriptions says that changes needs to be discussed first, Hence sharing them here.

The first commit fixes this issue by dynamically calculating the source link layer address option size using the same algorithm as used by radvd.

While working on the fix, I noticed that the comments doesn't really matches the implementation. The second commit fixes the same.