Talat, 

I was able to set the Broadcast flag in a DHCP negotiation when using IB interface. 

Trusty patch is possibly ready. 

TCPDUMP from the OFFER and ACK:

$ sudo tcpdump -i ib0 -nn -vvv -S
tcpdump: listening on ib0, link-type LINUX_SLL (Linux cooked), capture size 65535 bytes

04:38:01.051682 IP (tos 0x10, ttl 128, id 0, offset 0, flags [none], proto UDP (17), length 328)
    0.0.0.0.68 > 255.255.255.255.67: [udp sum ok] BOOTP/DHCP, Request, length 300, htype 32, hlen 0, xid 0xb48b8735, Flags [Broadcast] (0x8000)
	  Vendor-rfc1048 Extensions
	    Magic Cookie 0x63825363
	    DHCP-Message Option 53, length 1: Discover
	    Hostname Option 12, length 5: "dixie"
	    Parameter-Request Option 55, length 13:
	      Subnet-Mask, BR, Time-Zone, Default-Gateway
	      Domain-Name, Domain-Name-Server, Option 119, Hostname
	      Netbios-Name-Server, Netbios-Scope, MTU, Classless-Static-Route
	      NTP
	    Client-ID Option 61, length 17: hardware-type 56, 30:3a:30:30:3a:30:30:3a:34:38:3a:46:45:3a:38:30
	    END Option 255, length 0
	    PAD Option 0, length 0, occurs 15


04:38:01.051936 IP (tos 0x10, ttl 128, id 0, offset 0, flags [none], proto UDP (17), length 328)
    172.16.0.1.67 > 255.255.255.255.68: [udp sum ok] BOOTP/DHCP, Reply, length 300, htype 32, hlen 0, xid 0xb48b8735, Flags [Broadcast] (0x8000)
	  Your-IP 172.16.0.19
	  Server-IP 172.16.0.1
	  Vendor-rfc1048 Extensions
	    Magic Cookie 0x63825363
	    DHCP-Message Option 53, length 1: Offer
	    Server-ID Option 54, length 4: 172.16.0.1
	    Lease-Time Option 51, length 4: 600
	    Subnet-Mask Option 1, length 4: 255.255.255.0
	    BR Option 28, length 4: 172.16.0.255
	    Domain-Name Option 15, length 7: "myib.cu"
	    END Option 255, length 0
	    PAD Option 0, length 0, occurs 23


04:38:01.052215 IP (tos 0x10, ttl 128, id 0, offset 0, flags [none], proto UDP (17), length 328)
    0.0.0.0.68 > 255.255.255.255.67: [udp sum ok] BOOTP/DHCP, Request, length 300, htype 32, hlen 0, xid 0xb48b8735, Flags [Broadcast] (0x8000)
	  Vendor-rfc1048 Extensions
	    Magic Cookie 0x63825363
	    DHCP-Message Option 53, length 1: Request
	    Server-ID Option 54, length 4: 172.16.0.1
	    Requested-IP Option 50, length 4: 172.16.0.19
	    Hostname Option 12, length 5: "dixie"
	    Parameter-Request Option 55, length 13:
	      Subnet-Mask, BR, Time-Zone, Default-Gateway
	      Domain-Name, Domain-Name-Server, Option 119, Hostname
	      Netbios-Name-Server, Netbios-Scope, MTU, Classless-Static-Route
	      NTP
	    Client-ID Option 61, length 17: hardware-type 56, 30:3a:30:30:3a:30:30:3a:34:38:3a:46:45:3a:38:30
	    END Option 255, length 0
	    PAD Option 0, length 0, occurs 3


04:38:01.052342 IP (tos 0x10, ttl 128, id 0, offset 0, flags [none], proto UDP (17), length 328)
    172.16.0.1.67 > 255.255.255.255.68: [udp sum ok] BOOTP/DHCP, Reply, length 300, htype 32, hlen 0, xid 0xb48b8735, Flags [Broadcast] (0x8000)
	  Your-IP 172.16.0.19
	  Server-IP 172.16.0.1
	  Vendor-rfc1048 Extensions
	    Magic Cookie 0x63825363
	    DHCP-Message Option 53, length 1: ACK
	    Server-ID Option 54, length 4: 172.16.0.1
	    Lease-Time Option 51, length 4: 600
	    Subnet-Mask Option 1, length 4: 255.255.255.0
	    BR Option 28, length 4: 172.16.0.255
	    Domain-Name Option 15, length 7: "myib.cu"
	    END Option 255, length 0
	    PAD Option 0, length 0, occurs 23


I'll review this tomorrow and release the PPA for you to test.

I have also to review isc-dhcp IB support for Vivid/Wily/Xenial.