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08-12-2013, 05:02 PM
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#1
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Member
Registered: Nov 2012
Location: SF Bay Area
Posts: 47
Rep: 
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can't ping to different interface
Hello. I have a host1 with 3 ethernet interface eth0, eth1, eth2, and a host2 with only eth0. All of 4 interfaces are in a same subnet 192.168.1.0/24, eth0 is 192.168.1.5/24, eth1 is 192.168.1.8/24, eth2 is 192.168.1.12/24(host1). eth0 on host2 is 192.168.1.2/24. When I connect eth0(host2) and eth1(host1) together they can ping each other, however when I connect eth2(host) and eth0(host1) or eth2(host) together they will NOT ping each other. Any idea what's goning on? I guesss eth1 on host1 occupy some ports but I m not sure..
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08-12-2013, 08:33 PM
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#2
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Moderator
Registered: Mar 2008
Posts: 22,284
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Ping to localhost isn't a very good test. Putting 4 ip's on the same subnet will foul some of the other rules of tcp/ip. Use new subnets for each but I'd consider other tools also.
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08-13-2013, 10:53 PM
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#3
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Member
Registered: Jul 2012
Location: München
Distribution: Debian, CentOS/RHEL
Posts: 587
Rep:
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Quote:
Originally Posted by georgewhr
however when I connect eth2(host) and eth0(host1) or eth2(host) together they will NOT ping each other.
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Just to clarify, which port are you connecting to which?
Quote:
Originally Posted by jefro
Putting 4 ip's on the same subnet will foul some of the other rules of tcp/ip. Use new subnets for each but I'd consider other tools also.
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I don't think this statement is accurate enough; subnets consist of IP's which belong to a single logical group. There's no reason why he can't put all IP's under the same subnet, provided that this logical topology is consistent with his physical layout.
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08-14-2013, 04:54 PM
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#4
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Moderator
Registered: Mar 2008
Posts: 22,284
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I may have missed that up. I was thinking he was wanting a different deal such as ping across a host.
If one had a switch or even a hub where all 4 nics connected to the same time and they had firewall open for icmp and they forced ping to go from a nic ip and they didn't have any gateway or hosts file issue or using fqdn or name deal it should ping. But, we are kind of back to why are you using ping.
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08-17-2013, 02:08 AM
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#5
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Member
Registered: Apr 2013
Location: Arlington, WA
Distribution: Slackware
Posts: 96
Rep:
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Check your routing table (`route -n`).
The last interface configured on the 3-interface host will probably be the interface that packets will be sent out of for the entire subnet. If that interface isn't plugged in, then you won't receive your ICMP "Echo reply" packet.
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08-24-2013, 12:51 PM
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#6
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Member
Registered: Jan 2003
Location: Cambridgeshire, UK
Distribution: Mint (Desktop), Debian (Server)
Posts: 891
Rep: 
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If you put the same subnet on 4 interfaces you will end up with 4 possible routes out fo the host for that subnet.
I beleive that linux picks the first route it finds in the table as the next hop for the subnet, so while you may receive packets on all four interfaces it is mnost likely that all the replies will exit out of a single one.
If all your interfaces are sitting in the same vlan on the same switch you probably won't have a problem, however if you have physically split the subnet without splitting is logically, i.e. subnettting it, then you will run into problems as all your traffic for the subnet will all exit via one interface.
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