routing using default gw or 0.0.0.0

slowerogue · 09-29-2015, 03:00 AM

Hi guys
I am having some confusion using the route in linux.

Code:

[root@AAA network-scripts]# netstat -rn
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
192.168.248.1xx 0.0.0.0         255.255.255.255 UH        0 0          0 eth2
172.19.0.0      0.0.0.0         255.255.128.0   U         0 0          0 eth2
0.0.0.0         172.19.0.1      0.0.0.0         UG        0 0          0 eth2



[root@BBBB network-scripts]# netstat -rn
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
192.168.248.1xx 10.10.200.1     255.255.255.255 UGH       0 0          0 bond0
10.10.0.0       0.0.0.0         255.255.0.0     U         0 0          0 bond0
0.0.0.0         10.10.200.1     0.0.0.0         UG        0 0          0 bond0

A uses 0.0.0.0 to route, able to ping.
A uses its own gateway to route, not able to ping

B uses 0.0.0.0 to route, not able to ping
B uses its own gateway to route, able to ping.

its quite confusing to me sometimes 0.0.0.0 to successful to ping, but sometimes it's the default gateway. because in my understanding the default route will route to the default gateway, and in my example above, they shouldnt be any differences, right?

Please kindly help...THXXXXXXXXXX <3

wildwizard · 09-29-2015, 06:04 AM

You seem very confused.

Firstly you have given an incomplete picture of your network topology, you have 3 different network blocks in use?
Second you seem to have the default gateway confused in your routing table, the default gateway is the one with 0.0.0.0 for the destination.

ie in your B example you have a default gateway of 10.10.200.1 and you also have a host only route to that same gateway which is redundant unless your doing something more complicated that you have failed to mention.

So perhaps you could explain your :-
1. Network layout
2. Your complete routing table
3. What exactly you're pinging

FYI Don't bother hiding parts of private IP address ranges you'll only confuse yourself and wont protect yourself as they are not routable across the internet anyway

slowerogue · 09-29-2015, 08:09 AM

Hi , thanks for the reply.
The reason im hiding the ip because i dont want the people who is working on this project know im posting this(providing the two gateway, and the eth they might recognize it ><).. the same reason why i filter out the routing table because there are other routing information they might recognize(differernt eth)

Sorry i cant provide any network topology simply because i do not know them at all. Actually i am a system admin, not a network guy. The only thing i know is i need add route to ping to the ip 192.168.248.1xx/32, using the gateway provided by the network guy.
These are just 2 examples out of hundred over server i am working on.

So the network guy told me to use their own default gateway to route to the 192.168.248.1xx ip.

Correct me if im wrong.

Code:

A default gw - 172.19.0.1  
B default gw - 10.10.200.1

So, when i add route

Code:

A #route add -host 192.168.248.xxx/32 dev eth2
B #route add -host 192.168.248.xxx/32 gw 10.10.200.1 dev

My question is, why MUST A use 0.0.0.0 and B MUST use 10.10.200.1?