[Dnsmasq-discuss] Doing something wrong with dhcp-option=option:router

Ryan Chapman ryan at rchapman.org
Sun Aug 26 20:48:13 BST 2012


Hi Bernard,

> 2) can someone indicate simple way to see what either FreeBSD or Windows machines receive?
You can use tcpdump on the machine running dnsmasq to see what the DHCP response looks like

tcpdump -s 0 -vv -ni <interface> port bootpc or port bootps


As an example, I just tested:

dnsmasq is running on 10.16.1.5 (netmask 255.255.255.0), handing out addresses in the range 10.16.1.190 to 10.16.1.200, the router is at 10.16.1.1.  --port 0 is given to turn off dns

[root at pxe01 etc]# dnsmasq --port 0 --dhcp-range=10.16.1.190,10.16.1.200 --dhcp-option=option:router,10.16.1.1
[root at pxe01 etc]# tcpdump -s 0 -vv -ni eth0 port bootpc or port bootps
tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
15:34:35.390104 IP (tos 0x0, ttl  20, id 0, offset 0, flags [none], proto: UDP (17), length: 576) 0.0.0.0.bootpc > 255.255.255.255.bootps: [udp sum ok] BOOTP/DHCP, Request from 00:50:56:9c:75:ce, length: 548, xid:0x579c75ce, secs:4, flags: [Broadcast] (0x8000)
          Client Ethernet Address: 00:50:56:9c:75:ce
          Vendor-rfc1048:
            DHCP:DISCOVER
            PR:SM+TZ+DG+IEN+NS+RL+HN+BS+DN+SS+RP+EP+VO+SID+VC+BF+T128+T129+T130+T131+T132+T133+T134+T135
            MSZ:1260
            GUID:0.66.28.81.31.120.27.85.238.229.243.224.197.110.93.30.108
            ARCH:0
            NDI:1.2.1
            VC:"PXEClient:Arch:00000:UNDI:002001"
15:34:37.505455 IP (tos 0x0, ttl  20, id 1, offset 0, flags [none], proto: UDP (17), length: 576) 0.0.0.0.bootpc > 255.255.255.255.bootps: [udp sum ok] BOOTP/DHCP, Request from 00:50:56:9c:75:ce, length: 548, xid:0x589c75ce, secs:6, flags: [Broadcast] (0x8000)
          Client Ethernet Address: 00:50:56:9c:75:ce
          Vendor-rfc1048:
            DHCP:DISCOVER
            PR:SM+TZ+DG+IEN+NS+RL+HN+BS+DN+SS+RP+EP+VO+SID+VC+BF+T128+T129+T130+T131+T132+T133+T134+T135
            MSZ:1260
            GUID:0.66.28.81.31.120.27.85.238.229.243.224.197.110.93.30.108
            ARCH:0
            NDI:1.2.1
            VC:"PXEClient:Arch:00000:UNDI:002001"
15:34:38.139657 IP (tos 0x0, ttl  64, id 1952, offset 0, flags [none], proto: UDP (17), length: 328) 10.16.1.5.bootps > 255.255.255.255.bootpc: [bad udp cksum 47eb!] BOOTP/DHCP, Reply, length: 300, xid:0x579c75ce, secs:4, flags: [Broadcast] (0x8000)
          Your IP: 10.16.1.193
          Server IP: 10.16.1.5
          Client Ethernet Address: 00:50:56:9c:75:ce
          Vendor-rfc1048:
            DHCP:OFFER
            SID:10.16.1.5
            LT:3600
            RN:1800
            RB:3150
            SM:255.255.255.0
            BR:10.16.1.255
            BR:10.16.1.255
            NS:10.16.1.5
            DG:10.16.1.1


The DG (default gateway) is what is of interest here.





On Aug 26, 2012, at 11:36 AM, Bernard Higonnet <bthigonnet at gmail.com> wrote:

> Hello,
> 
> I want a machine getting an IP through DHCP to have a specific gateway and not go through the machine running dnsmasq.
> 
> In my dnsmasq.conf I have
> 
> dhcp-option=option:router,192.168.4.1
> 
> 
> running dnsmasq --test says syntax is OK.
> 
> but it would appear (sic) that this isn't working. Both a FreeBSD machine and a Windows7 machine are using the address of the machine running dnsmasq.
> 
> Questions:
> 
> 1) is there some variant of --test which shows what will be sent in reply to a DHCP request?
> 2) can someone indicate simple way to see what either FreeBSD or Windows machines receive?
> 
> TIA
> Bernard Higonnet
> 
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