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Maniphest T9093

dhcp-server: option interface-mtu capped at 9000 — allow full u16 range (jumbo fabrics use 9216)
Closed, ResolvedPublicFEATURE REQUEST
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Description

service dhcp-server shared-network-name <name> subnet <net> option interface-mtu is constrained to 576-9000 (interface-definitions/include/dhcp/option-v4.xml.i, added with T7723).

The 9000 upper bound is arbitrary:

  • DHCP option 26 (Interface MTU, RFC 2132 s5.1) is a 16-bit unsigned value; Kea accepts up to 65535.
  • VyOS itself allows interfaces ethernet <ethN> mtu up to 16000.
  • Datacenter jumbo fabrics commonly run 9216 (typical switch jumbo maximum), so a router whose LAN interfaces run MTU 9216 cannot hand DHCP clients a matching interface-mtu.

Observed on VyOS 1.5 rolling: set service dhcp-server shared-network-name LAN subnet 192.0.2.0/24 option interface-mtu 9216 is rejected by the CLI validator.

Proposed fix: relax the constraint to 576-65535 (the option's full width; Kea validates the wire format). PR follows.

Details

Version
1.5-rolling
Is it a breaking change?
Perfectly compatible
Issue type
Feature (new functionality)

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Event Timeline

bradkollmyer created this task.Jul 15 2026, 7:16 PM
bradkollmyer created this object in space S1 VyOS Public.
bradkollmyer added a comment.Jul 15 2026, 7:18 PM

PR: https://github.com/vyos/vyos-1x/pull/5334

Viacheslav changed the task status from Open to In progress.Jul 16 2026, 10:45 AM
Viacheslav assigned this task to bradkollmyer.
Viacheslav changed the subtype of this task from "Task" to "Feature Request".
pasik subscribed.Jul 17 2026, 9:25 AM
Viacheslav closed this task as Resolved.Jul 24 2026, 11:47 AM
Viacheslav moved this task from Need Triage to Completed on the VyOS Rolling board.