[Bug 252121] [patch] rtadvd should not put an upper limit on rdnssltime and dnsslltime

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=252121

            Bug ID: 252121
           Summary: [patch] rtadvd should not put an upper limit on
                    rdnssltime and dnsslltime
           Product: Base System
           Version: 12.2-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: bin
          Assignee: bugs at FreeBSD.org
          Reporter: ta0kira at gmail.com

Created attachment 220903
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=220903&action=edit
Removes upper limit on RDNSS and DNSSL lifetimes.

rtadvd (in config.c) limits both rdnssltime and dnsslltime to 2*maxinterval. If
this limit is exceeded, rtadvd completely discards the corresponding rdnss or
dnssl and sends router advertisements without it. This means that sending RAs
every 15s places an upper limit on rdnssltime of 30s.

Specifically, this short lifetime seems to prevent Android 11 (maybe others)
from accepting the RDNSS option. (It's unclear what its minimum is, but 600s
seems to be fine.)

I've attached a patch to effectively remove the upper limits. To make it
minimally intrusive, I just set the upper limit to USHRT_MAX. (Potentially
making it 32-bit would be a separate issue.)

Note that RFC 8106 (https://tools.ietf.org/html/rfc8106) recommends a default
value of 3*maxinterval for both parameters, and doesn't mention any sort of
upper limit.

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