Suspected disable_dgaf bug in hostapd 2.6 (corrupting UDP checksums for broadcast packets)

[Brian Demsky]

When we enable disable_dgaf, our DHCP server stops working with
certain devices.

When we use tcpdump to monitor the traffic, we see the following:

First we see the initial broadcasted DHCP server response.

05:10:24.975465 74:da:38:68:72:84 (oui Unknown) > Broadcast, ethertype IPv4 (0x0
800), length 342: (tos 0xc0, ttl 64, id 33762, offset 0, flags [none], proto UDP
(17), length 328)
 192.168.2.1.67 > 255.255.255.255.68: [udp sum ok] BOOTP/DHCP, Reply, length
300, xid 0xb901c70e, Flags [Broadcast] (0x8000)
       Your-IP 192.168.2.232
       Server-IP 192.168.2.1
       Client-Ethernet-Address d0:73:d5:02:41:da (oui Unknown)
       Vendor-rfc1048 Extensions
         Magic Cookie 0x63825363
         DHCP-Message Option 53, length 1: Offer
         Server-ID Option 54, length 4: 192.168.2.1
         Lease-Time Option 51, length 4: 172800
         RN Option 58, length 4: 86400
         RB Option 59, length 4: 151200
         Subnet-Mask Option 1, length 4: 255.255.255.0
         BR Option 28, length 4: 192.168.2.255
         Default-Gateway Option 3, length 4: 192.168.2.1
         Domain-Name-Server Option 6, length 4: 192.168.2.1
         END Option 255, length 0
         PAD Option 0, length 0, occurs 8


Next we see the conversion to unicast that is done in the hostapd
driver.  The interesting thing is that the UDP checksum is now
invalid.  It has been changed to c3ee.

05:10:24.975564 74:da:38:68:72:84 (oui Unknown) > d0:73:d5:02:41:da (oui Unknown
), ethertype IPv4 (0x0800), length 342: (tos 0xc0, ttl 64, id 33762, offset 0, f
lags [none], proto UDP (17), length 328)
 192.168.2.1.67 > 255.255.255.255.68: [bad udp cksum 0xc3ee -> 0xe880!] BOOTP
/DHCP, Reply, length 300, xid 0xb901c70e, Flags [Broadcast] (0x8000)
       Your-IP 192.168.2.232
       Server-IP 192.168.2.1
       Client-Ethernet-Address d0:73:d5:02:41:da (oui Unknown)
       Vendor-rfc1048 Extensions
         Magic Cookie 0x63825363
         DHCP-Message Option 53, length 1: Offer
         Server-ID Option 54, length 4: 192.168.2.1
         Lease-Time Option 51, length 4: 172800
         RN Option 58, length 4: 86400
         RB Option 59, length 4: 151200
         Subnet-Mask Option 1, length 4: 255.255.255.0
         BR Option 28, length 4: 192.168.2.255
         Default-Gateway Option 3, length 4: 192.168.2.1
         Domain-Name-Server Option 6, length 4: 192.168.2.1
         END Option 255, length 0
         PAD Option 0, length 0, occurs 8


Any suggestions on fixing this?

Thanks,
Brian