Greetings!
    I'm trying to diagnose a problem with DHCP relaying via DNSMasq.  I'm hoping someone can help, or at least point to what to investigate next.  I have a router running DNSMasq as a DHCP server.  I have a second router connected to the first, running DNSMasq as a relay.


[Main Router] 192.168.1.1 <--WAN-Link--> 192.168.1.2 [Remote Router] 192.168.10.1 
<--Client-LAN--> DHCP client.

    When a DHCP client comes on to the Client LAN, it sends a DHCP request.  I see this in the Remote Router Log:

Feb 12 12:32:22 yew daemon.info dnsmasq-dhcp[7855]: DHCP relay 192.168.10.1 -> 
192.168.1.1

    Remote Router forwards it the Main Router:

[Packets seen at Remote Router]

12:32:22.057611 Out 00:24:9b:29:81:f3 ethertype IPv4 (0x0800), length 344: (tos 0x0, ttl 128, id 22793, offset 0, flags [none], proto UDP (17), length 328) 0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request [|bootp] 12:32:22.058858 Out bc:ae:c5:c3:00:4d ethertype IPv4 (0x0800), length 344: (tos 0x0, ttl 64, id 38438, offset 0, flags [none], proto UDP (17), length 328) 192.168.10.1.67 > 192.168.1.1.67: BOOTP/DHCP, Request [|bootp]
[Packets seen at Main Router]
12:32:22.213469  In 68:72:51:88:69:b4 ethertype IPv4 (0x0800), length 344: (tos 0x0, ttl 64, id 38438, offset 0, flags [none], proto UDP (17), length 328)     192.168.10.1.67 > 192.168.1.1.67: BOOTP/DHCP, Request from 00:24:9b:29:81:f3, length 300, hops 1, xid 0xf43325a2, secs 3072, Flags [Broadcast]
          Gateway-IP 192.168.10.1
          Client-Ethernet-Address 00:24:9b:29:81:f3
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: Discover
            Client-ID Option 61, length 7: ether 00:24:9b:29:81:f3
            Hostname Option 12, length 10: "MTGS-SBOOK"
            Vendor-Class Option 60, length 8: "MSFT 5.0"
            Parameter-Request Option 55, length 13:
              Subnet-Mask, Default-Gateway, Domain-Name-Server, Domain-Name
              Router-Discovery, Static-Route, Vendor-Option, Netbios-Name-Server               Netbios-Node, Netbios-Scope, Classless-Static-Route, Classless-Static-Route-Microsoft
              Option 252

    The main router responds:

[Packets seen at Main Router]
12:32:22.215239 Out 80:2a:a8:4f:c4:02 ethertype IPv4 (0x0800), length 346: (tos 0xc0, ttl 64, id 33748, offset 0, flags [none], proto UDP (17), length 330)     192.168.1.1.67 > 192.168.10.1.67: BOOTP/DHCP, Reply, length 302, hops 1, xid 0xf43325a2, secs 3072, Flags [Broadcast]
          Your-IP 192.168.10.133
          Server-IP 192.168.1.1
          Gateway-IP 192.168.10.1
          Client-Ethernet-Address 00:24:9b:29:81:f3
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: Offer
            Server-ID Option 54, length 4: 192.168.1.1
            Lease-Time Option 51, length 4: 86400
            RN Option 58, length 4: 43200
            RB Option 59, length 4: 75600
            Subnet-Mask Option 1, length 4: 255.255.255.0
            BR Option 28, length 4: 192.168.10.255
            Domain-Name Option 15, length 8: "localnet"
            Domain-Name-Server Option 6, length 4: 192.168.1.1
            Default-Gateway Option 3, length 4: 192.168.10.1

[Packet seen at Remote Router]
12:32:22.062199   P 80:2a:a8:4f:c4:02 ethertype IPv4 (0x0800), length 346: (tos 0xc0, ttl 64, id 33748, offset 0, flags [none], proto UDP (17), length 330) 192.168.1.1.67 > 255.255.255.255.68: BOOTP/DHCP, Reply, length 302, hops 1, xid 0xf43325a2, secs 3072, Flags [Broadcast]
          Your-IP 192.168.10.133
          Server-IP 192.168.1.1 [|bootp]

Unfortunately, the DNSMasq Process at the Remote Router never picks up the response to send it to the client. I've tried using the <interface> option of the --dhcp-relay option in DNSMasq, but it doesn't seem to make a difference. ipforwarding is on, and iptables is set to accept everything in both directions.  I can't tell whether DNSMasq at the Remote Router is receiving the response and ignoring it, or if it never makes it to the DNSMasq process.

Is there anyway to crank up the logging on DNSMasq even higher?  Am I right in thinking that DNSMasq should in fact receive this message, process it, and forward the response to the client?  Any tips on how to trouble shoot this?

-- Michael Garrison Stuber

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