Re: [PATCH v5 7/7] dhcp: Handle FQDN option with RFC 3396 concatenation

[Anshu Kumari <anskuma@redhat.com>]

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On Mon, Jul 20, 2026 at 9:54 AM David Gibson <david@gibson.dropbear.id.au>
wrote:

> On Fri, Jul 17, 2026 at 11:26:44PM +0530, Anshu Kumari wrote:
> > Mark Client FQDN (option 81) as concatenation-requiring per
> > RFC 4702, Section 2.  When the encoded FQDN exceeds 255 bytes,
> > fill() now splits it across DHCP message fields using the RFC 3396
> > splitting infrastructure instead of silently dropping it.
> >
> > Link: https://bugs.passt.top/show_bug.cgi?id=192
> > Signed-off-by: Anshu Kumari <anskuma@redhat.com>
> > ---
> > v5:
> >   - New patch: mark Client FQDN (option 81) as concatenation-requiring
> per RFC 4702
> > ---
> >  dhcp.c | 12 +++++++++---
> >  1 file changed, 9 insertions(+), 3 deletions(-)
> >
> > diff --git a/dhcp.c b/dhcp.c
> > index 6bebb5f..a54ac58 100644
> > --- a/dhcp.c
> > +++ b/dhcp.c
> > @@ -484,9 +484,15 @@ enum dhcp_overload {
> >   */
> >  static bool is_concat_opt(int o)
> >  {
> > -     if ((size_t)o >= ARRAY_SIZE(dhcp_opt_types))
> > -             return false;
> > -     return dhcp_opt_types[o] == DHCP_OPT_STR_CONCAT;
> > +     if ((size_t)o < ARRAY_SIZE(dhcp_opt_types) &&
> > +         dhcp_opt_types[o] == DHCP_OPT_STR_CONCAT)
> > +             return true;
> > +
> > +     /* RFC 4702, Section 2: Client FQDN option requires concatenation
> */
> > +     if (o == 81)
> > +             return true;
>
> Why is this explicitly special cased, rather than setting the type in
> the table?  Come to that, does this do anything, since we don't
> appear to currently allow option 81 anyway.
>
> As option 81 is internally supported inside passt, that's why I have not
included inside table
as parsing option 81 has a special format of 3 flag bytes + DNS-encoded
domain name.

Special condition is mentioned for option 81 in  is_concat_opt() function
because FQDN length can exceeds
255 bytes for a very long domain name. Without this condition, such FQDN
are dropped in current approach.
With this condition, they are split across DHCP fields per RFC 3396.

> +
> > +     return false;
> >  }
> >
> >  /**
> > --
> > 2.54.0
> >
>
> --
> David Gibson (he or they)       | I'll have my music baroque, and my code
> david AT gibson.dropbear.id.au  | minimalist, thank you, not the other way
>                                 | around.
> http://www.ozlabs.org/~dgibson
>


-- 
Anshu

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