How can we change gateway in netstat -rn output

Question

root@prateek-desktop:~# netstat -rn
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
192.168.0.0     0.0.0.0         255.255.255.0   U         0 0          0 eth1

I have a network where my wireless router has static IP address 192.168.0.80 with subnet mask 255.255.255.0 and default gateway as 192.168.0.81. My pc has IP address 192.168.0.81 (same as gateway of my router). I have few other devices like smart phones n laptops connected to wireless router via DHCP connection. Now When I try to ping from my PC to my router static IP address (192.168.0.80) it does not respond anything. no message is being displayed but for other addresses it says destination host is unreachable.

What can be the reason and how can I trouble shoot ? Please help i am stuck in my master thesis at this point.

Answer 1

Since we're struggling here, I think you would benefit from becoming familiar with some more direct hacks.

The ip route command can edit routes temporarily. ip route shows existing routes, it is a more recent equivalent of netstat -rn. ip route add default via $ROUTER_IP will add a default route which uses $ROUTER_IP. (I.e. replace it with the IP you apparently know the router to have, whatever that is). To remove it again, use ip route del default via $ROUTER_IP. Note that default is the same as writing 0.0.0.0/0.

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Note you need to disconnect and reconnect to the network before the "gateway" in Network Settings window - or any other setting there - will take effect.

I tried disconnecting and reconnecting. The netstat -rn is same as above.

Thanks. I don't understand how this could happen - i.e. it suggests to me either a very annoying bug, or you're doing something slightly different, like editing a connection that's actually tied to eth0 instead of eth1. This would be possible to show by taking exhaustive screenshots. (Just for the record, you have given some information that was not quite right or contradictory already).

I can suggest looking at nmcli connection and nmcli connection show id NAME. NAME is taken from the NAME column of the first command. The output from the last command will be quite long.

I would also make sure you have ticked "Make available to other users", and then I would be able to look at a text configuration file written by NetworkManager. This is not as long as nmcli connection show, because it does not include unnecessary default values. On Ubuntu, the config files could be in /etc/network/interfaces, /etc/network/interfaces.d, or /etc/NetworkManager/system-connections.

When I try to ping from my PC to my router static IP address (192.168.0.80) it does not respond anything. no message is being displayed but for other addresses it says destination host is unreachable.

That's neither good nor bad. Some devices block ICMP ping. The fact that it does not say "destination host is unreachable", nor any other message, actually means that this IP address is reachable! As I mentioned in my previous answer, you can confirm it like this:

$ ip -4 neigh
172.16.8.1 dev wlp2s0 lladdr 74:44:01:86:42:d6 REACHABLE

there will also be entries saying INCOMPLETE or something, from any non-existent IP address which you have tried to ping.

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I have assigned my PC gateway as 192.168.0.80

Yes, something did not work. In fact, you are showing no "default gateway" at all. You only have the automatic route for your local network, 192.168.0.*. (That's what 192.168.0.0 with mask 255.255.255.0 means). You will want this route for the local network to stay, and add a second route for your "default gateway".

I'm sorry to tell you that the "Apply" button you clicked in the NetworkManager graphical interface is misleading. (Or whatever it says on the button you used to close the NetworkManager settings window). Unlike most other "Apply" buttons in GNOME, this one does not take effect immediately. It only saves the setting for future use.