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

20 Jul 2026

On Mon, Jul 20, 2026 at 9:54 AM David Gibson 
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 
---
v5:
  - New patch: mark Client FQDN (option 81) as concatenation-requiring
On Fri, Jul 17, 2026 at 11:26:44PM +0530, Anshu Kumari wrote:
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

    