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Eugene van der Merwe
Asked: 2013-03-07 00:18:58 +0800 CST

Different Logical Subnet on same Physical network cause Linux Connectivity Issues

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We have two Linux CentOS hosts, A and B.

A = 19x.2xx.6.4 (/24)
B = 19x.2xx.7.4 (/24)

Both these hosts are on the same physical switch. The reason why they are on different logical networks is because in the future they will be on separate physical networks.

A can ping B, and from A you can SSH to B.

You can also TRACEROUTE from A to B, in this case it goes through router C (19x.1x.8.1).

You cannot NMAP from A to B and various other important network utilities fail when connecting from A to B. The general effect is the same as a time-out.

The router in has aliases on the Cisco as such:

interface FastEthernet0/0
 ip address 19x.2xx.6.1 255.255.255.0 secondary
 ip address 19x.2xx.7.1 255.255.255.0 secondary
 ip address 19x.1x.8.1 255.255.255.192

I am at a loss how to fix this problem. Please assist.

linux

1 Answers

  1. Check your firewall rules to see what is being allowed out and what is being allowed in on each of the boxes A and B. Also check to see if there are any intermediate firewalls.

    By default, CentOS has a fairly restrictive set of firewall rules and pretty much it only allows ssh and icmp so this would appear to be what you're seeing.

    You can see what your iptables firewall rules are with

    iptables -Lv

    or just the input chain with 

    iptables -L INPUT -v
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