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

[David Gibson <david@gibson.dropbear.id.au>]

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On Mon, Jul 20, 2026 at 01:08:56PM +0530, Anshu Kumari wrote:
> 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

Ah, sorry, I missed that.

> 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.

I see.

To me this seems like another argument for putting the "is_concat"
flag separately from the data type.  If you can do that, you can
define option 81 as "is_concat" in the table, while still having type
NONE, so it can't be user overwritten.

-- 
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

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