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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 secondary
 ip address 19x.2xx.7.1 secondary
 ip address 19x.1x.8.1

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

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You can ping, SSH and traceroute from A to B. That is very clearly not a connectivity problem. The problem is that there's a firewall on B that is preventing other types of connections from A. – joeqwerty Mar 6 '13 at 11:50
@joeqwerty: I'm kind of curious as to why you commented with a comment that's exactly the same as my answer ? – Iain Mar 6 '13 at 12:52
I only skimmed your answer and didn't read it in depth. It looked like we were on the same track, hence why I only made a comment and not an answer. – joeqwerty Mar 6 '13 at 12:57

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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