Query on default gateway with DHCP vs Static address
glenn.satchell at uniq.com.au
glenn.satchell at uniq.com.au
Fri Mar 19 07:58:32 UTC 2021
HI SImon,
Ok that's clearer now - it's the ISC client you use, but an entirely
different DHCP server.
Probably you need to do some packet captures using,say, tcpdump to look
at the actual packets being sent from the dhcp server and the specific
dhcp options being sent.
Also, have a look in the dhclient.conf (location varies with different
Linux distros, eg Ubuntu is /etc/dhcp/dhclient.conf) and seewhat options
itis requesting. eg on Ubuntu in that file I have
option rfc3442-classless-static-routes code 121 = array of unsigned
integer 8;
send host-name = gethostname();
request subnet-mask, broadcast-address, time-offset, routers,
domain-name, domain-name-servers, domain-search, host-name,
dhcp6.name-servers, dhcp6.domain-search, dhcp6.fqdn,
dhcp6.sntp-servers,
netbios-name-servers, netbios-scope, interface-mtu,
rfc3442-classless-static-routes, ntp-servers;
"routers" asks the dhcp server to send the default gateway info.
regards,
Glenn
On 2021-03-19 16:10, SIMON BABY wrote:
> Hi Glenn,
> My linux interface is a DHCP client which is connected to a night hawk
> (cradlepoint) which acts as the DHCP server. So my Linux interface is
> getting a DHCP IP address as gievn below
>
> eth1.15 Link encap:Ethernet HWaddr 00:08:A2:00:00:02
>
> inet addr:192.168.1.76 Bcast:192.168.1.255
> Mask:255.255.255.0
>
> UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
>
> RX packets:457 errors:0 dropped:0 overruns:0 frame:0
>
> TX packets:1782 errors:0 dropped:0 overruns:0 carrier:0
>
> collisions:0 txqueuelen:0
>
> RX bytes:44717 (43.6 KiB) TX bytes:1704498 (1.6 MiB)
>
> The kernel routing table is below. I did not see any default GW in the
> routing table.
>
> Kernel IP routing table
>
> Destination Gateway Genmask Flags MSS Window
> irtt Iface
>
> 10.64.64.67 0.0.0.0 255.255.255.255 UH 0 0
> 0 ppp3
>
> 10.64.64.66 0.0.0.0 255.255.255.255 UH 0 0
> 0 ppp2
>
> 10.64.64.65 0.0.0.0 255.255.255.255 UH 0 0
> 0 ppp1
>
> 255.255.255.255 0.0.0.0 255.255.255.255 UH 0 0
> 0 eth1.13
>
> 192.168.1.0 0.0.0.0 255.255.255.0 U 0 0
> 0 eth1.15
>
> 128.0.0.0 0.0.0.0 255.255.255.0 U 0 0
> 0 eth1.16
>
> 128.1.0.0 0.0.0.0 255.255.0.0 U 0 0
> 0 eth0
>
> 10.20.0.0 0.0.0.0 255.255.0.0 U 0 0
> 0 eth1.13
>
> Regards
>
> Simon
>
> On Thu, Mar 18, 2021 at 8:48 PM <glenn.satchell at uniq.com.au> wrote:
>
>> Hi Simon,
>>
>> On your dhcp server do you specify a gateway? For isc dhcpd it will
>> be
>> "option routers" in the subnet definition settings. Verify that the
>> gateway you specify is in the same subnet as the IP address being
>> assigned to the interface.
>>
>> Can you post the output from "ip route" and "ip address" so we can
>> see
>> what is there?
>>
>> If you are configuring the IP address manually then it depends on
>> your
>> particular version of linux on how that is done. Usually you will
>> specify an IP address, netmask and gateway- but the files used vary
>> from
>> distribution to distribution. eg Ubuntu uses /etc/netplan/*yaml or
>> /etc/network/interfaces depending on the version. CentOS uses
>> /etc/sysconfig/network-scripts/ifcfg-eth0. Or it may use Network
>> Manager
>> and the associated GUI to manage the configuration.
>>
>> regards,
>> Glenn
>>
>> On 2021-03-19 14:34, SIMON BABY wrote:
>>> Hi Glenn,
>>> It did not show the default route of 0.0.0.0/0 [1] [1] with route
>> -n. One
>>> thing I noticed is, I can ping to another Address only with -I
>>> interface_name. For example I can send and receive packets with
>> ping
>>> -I eth0 8.8.8.8.
>>> If I use ping 8.8.8.8 it is not working. Am I missing anything
>> here?
>>>
>>> Regards
>>> Simon
>>>
>>> On Thu, Mar 18, 2021 at 8:15 PM <glenn.satchell at uniq.com.au>
>> wrote:
>>>
>>>> Hi Simon,
>>>>
>>>> The default route will show up with a destination of 0.0.0.0/0
>> [1] [1]
>>>> if you
>>>> use route -n.
>>>>
>>>> If you use "ip route" it will show up with a destination of
>>>> "default" in
>>>> the first column.
>>>>
>>>> regards,
>>>> Glenn
>>>>
>>>> On 2021-03-19 13:54, SIMON BABY wrote:
>>>>> Hi Peter,
>>>>> Thank you so much for helping.
>>>>>
>>>>> Yes, I am using ISC dhcpd. My query is with ISC DHCPd I can ping
>>>> to
>>>>> any network, but I did not see the ISC gateway in the kernel
>> route
>>>>> table. Will it be stored somewhere else so that when packet goes
>>>> out,
>>>>> the hardware (NIC) gets this information?
>>>>>
>>>>> Regards
>>>>> Simon
>>>>>
>>>>> On Thu, Mar 18, 2021 at 5:07 PM Peter Yardley
>>>>> <peter.martin.yardley at gmail.com> wrote:
>>>>>
>>>>>> Hi,
>>>>>>
>>>>>> This is prolly a problem with your configuration. Are you using
>>>> ISC
>>>>>> DHCPd? If so could provide a relevant snippet of your
>>>> configuration,
>>>>>> anonymised if necessary.
>>>>>>
>>>>>> I used to configure subnets like this …
>>>>>>
>>>>>> subnet 10.20.50.0 netmask 255.255.254.0 {
>>>>>> option subnet-mask 255.255.254.0;
>>>>>> option broadcast-address 10.20.51.255;
>>>>>> option routers 10.20.50.1;
>>>>>>
>>>>>> pool {
>>>>>> # A device for Blah
>>>>>> host HMTXYZ { hardware ethernet 00:40:9d:54:4c:c7;
>>>>>> fixed-address 10.20.50.32; }
>>>>>> …
>>>>>>
>>>>>> range 10.20.51.0 138.25.51.254;
>>>>>> }
>>>>>>
>>>>>> Of course there are many other correct possiblities
>>>>>>
>>>>>>> On 19 Mar 2021, at 10:03 am, SIMON BABY <simonkbaby at gmail.com>
>>>>>> wrote:
>>>>>>>
>>>>>>> Hi,
>>>>>>> I have a query about the DHCP provided IP address vs Static IP
>>>>>> address configured manually. When i get the IP address from
>>>> DHCP, I
>>>>>> can ping to 8.8.8.8 (google public server) where as
>>>>>>> with a statically configured IP address, I could not. When I
>>>>>> looked into the route table (route -n ), I did not see any
>>>> default
>>>>>> gateway with DHCP or Statically configured IP. (I am testing
>>>> with
>>>>>> Linux).
>>>>>>>
>>>>>>> 1. Can I know when the packet goes out , how did the hardware
>>>>>> (NIC) find the default gateway in the case of the DHCP provided
>>>> IP
>>>>>> address since I did not see any default gw IP in the routing
>>>> table?
>>>>>>> 2. With statically configured IP, if I explicitly configure a
>>>>>> default GW, I can see packets are going out and receiving. I
>> can
>>>> see
>>>>>> the default GW in the routing table. Do I need a default GW to
>>>> send
>>>>>> packets from a statically configured IP address ?
>>>>>>>
>>>>>>> Thank you for your time
>>>>>>>
>>>>>>> Regards
>>>>>>> Simon
>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> ISC funds the development of this software with paid support
>>>>>> subscriptions. Contact us at https://www.isc.org/contact/ for
>>>> more
>>>>>> information.
>>>>>>>
>>>>>>> dhcp-users mailing list
>>>>>>> dhcp-users at lists.isc.org
>>>>>>> https://lists.isc.org/mailman/listinfo/dhcp-users
>>>>>>
>>>>>> Peter Yardley
>>>>>> peter.martin.yardley at gmail.com
>>>>>>
>>>>>> _______________________________________________
>>>>>> ISC funds the development of this software with paid support
>>>>>> subscriptions. Contact us at https://www.isc.org/contact/ for
>>>> more
>>>>>> information.
>>>>>>
>>>>>> dhcp-users mailing list
>>>>>> dhcp-users at lists.isc.org
>>>>>> https://lists.isc.org/mailman/listinfo/dhcp-users
>>>>> _______________________________________________
>>>>> ISC funds the development of this software with paid support
>>>>> subscriptions. Contact us at https://www.isc.org/contact/ for
>> more
>>>>> information.
>>>>>
>>>>> dhcp-users mailing list
>>>>> dhcp-users at lists.isc.org
>>>>> https://lists.isc.org/mailman/listinfo/dhcp-users
>>>> _______________________________________________
>>>> ISC funds the development of this software with paid support
>>>> subscriptions. Contact us at https://www.isc.org/contact/ for
>> more
>>>> information.
>>>>
>>>> dhcp-users mailing list
>>>> dhcp-users at lists.isc.org
>>>> https://lists.isc.org/mailman/listinfo/dhcp-users
>>>
>>>
>>> Links:
>>> ------
>>> [1] http://0.0.0.0/0
>>> _______________________________________________
>>> ISC funds the development of this software with paid support
>>> subscriptions. Contact us at https://www.isc.org/contact/ for more
>>> information.
>>>
>>> dhcp-users mailing list
>>> dhcp-users at lists.isc.org
>>> https://lists.isc.org/mailman/listinfo/dhcp-users
>> _______________________________________________
>> ISC funds the development of this software with paid support
>> subscriptions. Contact us at https://www.isc.org/contact/ for more
>> information.
>>
>> dhcp-users mailing list
>> dhcp-users at lists.isc.org
>> https://lists.isc.org/mailman/listinfo/dhcp-users
>
>
> Links:
> ------
> [1] http://0.0.0.0/0
> _______________________________________________
> ISC funds the development of this software with paid support
> subscriptions. Contact us at https://www.isc.org/contact/ for more
> information.
>
> dhcp-users mailing list
> dhcp-users at lists.isc.org
> https://lists.isc.org/mailman/listinfo/dhcp-users
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