[Dnsmasq-discuss] DHCP Relay Problem

[Simon Kelley]

On 12/02/18 21:35, Michael Garrison Stuber wrote:
> 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?
> 


The packet from the DHCP is sent to the relay at 192.168.10.1, which is
correct, and the last packet in your series, "seen at the Remote Router"
is sent to the broadcast address, 255.255.255.255, which is strong
evidence that the relay has, in fact picked up the reply and forwarded
it to the remote client. It's sending it to the broadcast address
because the broadcast flag is on in the DHCP reply. Which interface of
the remote router are you seeing this on. It's possible that the
relay-reply path is picking the wrong interface to send it out on.

As always, we need to know what version of dnsmasq you are running. We
don't just increment those version numbers for fun!


Cheers,

Simon.