Query on default gateway with DHCP vs Static address

SIMON BABY simonkbaby at gmail.com
Fri Mar 19 05:10:09 UTC 2021


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] 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]
> >> 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
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>
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