[Dnsmasq-discuss] Pushing DNS server for a specific client on EdgeRouter

[Daniele Riccucci]

Hello,
I'm trying to push a different DNS server via DHCP to a specific client 
(a VoIP thingie from Grandstream, model HT802) from an EdgeRouter; the 
current firmware is running Dnsmasq version 2.79-1-2-geff17ee.

Some options are already generated by the OS unfortunately, namely:

     dhcp-range=set:home,10.0.0.20,10.0.0.254,255.255.255.0,86400
     domain=home.domain.com,10.0.0.0/24,local
     dhcp-option=tag:home,option:domain-name,home.domain.com
     dhcp-option=tag:home,option:router,10.0.0.1
     dhcp-option=tag:home,option:dns-server,10.0.0.1
     dhcp-host=c0:74:ad:0c:0c:b7,set:home,10.0.0.5
     host-record=voipgw.home.domain.com,10.0.0.5,86400

which are contained in /etc/dnsmasq.d/dnsmasq-dhcp-config.conf.
I can only set options above this point (/etc/dnsmasq.conf), and I tried 
with:

options dhcp-option=vendor:HT8XX,option:dns-server,85.38.28.0,85.38.28.1
options 
dhcp-option-force=vendor:HT8XX,option:dns-server,85.38.28.0,85.38.28.1

because from tcpdump I saw that the client was sending a Vendor-Class 
string:

c0:74:ad:0c:0c:b7 > ff:ff:ff:ff:ff:ff, ethertype IPv4 (0x0800), length 
590: (tos 0x0, ttl 64, id 0, offset 0, flags [none], proto UDP (17), 
length 576)
     0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from 
c0:74:ad:0c:0c:b7, length 548, xid 0xbed7bb5e, Flags [none]
       Client-Ethernet-Address c0:74:ad:0c:0c:b7
       Vendor-rfc1048 Extensions
         Magic Cookie 0x63825363
         DHCP-Message Option 53, length 1: Discover
         Client-ID Option 61, length 7: ether c0:74:ad:0c:0c:b7
         Hostname Option 12, length 6: "voipgw"
         Vendor-Class Option 60, length 18: "HT8XX dslforum.org"
         Parameter-Request Option 55, length 17:
           Subnet-Mask, Time-Zone, Default-Gateway, Domain-Name-Server
           Hostname, Domain-Name, TTL, BR
           Static-Route, NTP, Vendor-Option, Lease-Time
           Server-ID, TFTP, Classless-Static-Route, Option 125
           Option 160

However, the reply from dnsmasq is sending 10.0.0.1 as DNS server:

20:19:01.081957 24:a4:3c:b3:c2:8d > c0:74:ad:0c:0c:b7, ethertype IPv4 
(0x0800), length 390: (tos 0xc0, ttl 64, id 53937, offset 0, flags 
[none], proto UDP (17), length 376)
     10.0.0.1.67 > 10.0.0.5.68: BOOTP/DHCP, Reply, length 348, xid 
0xbed7bb5e, Flags [none]
       Client-IP 10.0.0.5
       Server-IP 10.0.0.1
       Client-Ethernet-Address c0:74:ad:0c:0c:b7
       Vendor-rfc1048 Extensions
         Magic Cookie 0x63825363
         DHCP-Message Option 53, length 1: ACK
         Server-ID Option 54, length 4: 10.0.0.1
         Lease-Time Option 51, length 4: 86358
         Subnet-Mask Option 1, length 4: 255.255.255.0
         BR Option 28, length 4: 10.0.0.255
         Hostname Option 12, length 6: "voipgw"
         Domain-Name-Server Option 6, length 4: 10.0.0.1
         Default-Gateway Option 3, length 4: 10.0.0.1
         Domain-Name Option 15, length 15: "home.domain.com"
         Vendor-Option Option 43, length 21: 
6.8.85.38.28.0.85.38.28.1.6.8.85.38.28.0.85.38.28.1.255
         Vendor-Class Option 60, length 18: "HT8XX dslforum.org"

Also shown from the device:

Product Model: HT802
MAC Address: c0:74:ad:0c:0c:b7
Network:
     IPv4 Address -- 10.0.0.5
     IPv6 Address --
     Netmask    -- 255.255.255.0
     Gateway    -- 10.0.0.1
     DNS Server -- 10.0.0.1

How could I solve this? Is there a way to override the dns-server option 
for this device?
I can send further tcpdumps if required.
Thank you.

Daniele