[Kea-users] Minimum packet size compliance issue (RFC 1542)

[Matthew Kassawara]

Hi,

I'm evaluating Kea in my lab and found my OpenBSD DHCP relay (using
dhcrelay) discarding replies from Kea due to invalid size. The same DHCP
relay operates normally with the conventional ISC DHCP server. Further
investigation reveals replies from the Kea server lacking pad options to
make the minimum packet size greater than or equal to 300 bytes as per RFC
1542 [1]. The RFC describes BOOTP which Kea doesn't support, but I'm
guessing a number of DHCP relays use it to validate packet length. I can
add bogus options to subnets as a workaround, but I prefer a cleaner
solution.

DHCP offer from Kea in my lab:

10.30.20.21.67 > 10.30.12.1.67: BOOTP/DHCP, Reply, length 288, xid
0x19b3d107, Flags [none] (0x0000)
  Your-IP 10.30.12.51
  Server-IP 10.30.20.21
  Gateway-IP 10.30.12.1
  Client-Ethernet-Address 00:50:56:b4:59:03
  Vendor-rfc1048 Extensions
    Magic Cookie 0x63825363
    Subnet-Mask Option 1, length 4: 255.255.254.0
    Default-Gateway Option 3, length 4: 10.30.12.1
    Domain-Name-Server Option 6, length 8: 10.30.20.151,10.30.20.152
    NTP Option 42, length 8: 10.30.20.151,10.30.20.152
    Lease-Time Option 51, length 4: 4000
    DHCP-Message Option 53, length 1: Offer
    Server-ID Option 54, length 4: 10.30.20.21
    END Option 255, length 0

DHCP offer from the conventional ISC DHCP server in my lab:

10.30.20.21.67 > 10.30.12.1.67: BOOTP/DHCP, Reply, length 300, xid
0xf0a07b6c, Flags [none] (0x0000)
 Your-IP 10.30.12.51
 Server-IP 10.30.20.21
 Gateway-IP 10.30.12.1
 Client-Ethernet-Address 00:50:56:b4:59:03
 Vendor-rfc1048 Extensions
   Magic Cookie 0x63825363
   DHCP-Message Option 53, length 1: Offer
   Server-ID Option 54, length 4: 10.30.20.21
   Lease-Time Option 51, length 4: 4000
   Subnet-Mask Option 1, length 4: 255.255.254.0
   Default-Gateway Option 3, length 4: 10.30.12.1
   Domain-Name-Server Option 6, length 8: 10.30.20.151,10.30.20.152
   END Option 255, length 0
   PAD Option 0, length 0, occurs 22

Thanks,
Matt

[1] https://tools.ietf.org/html/rfc1542#section-2.1
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