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I've encountered some network behaviour that confuses me while trying to get Java RMI working.

I use netcat to connect to a local machine:

[my_machine]$ nc -w 1 192.168.0.100 60000 && echo success
success

I try to do the same to my server:

[my_machine]$ nc -w 1 my-servers-ip 60000 && echo success

This doesn't work, unless I explicitly listen on the server socket:

[amazon_ec2]$ nc -l 60000 

[my_machine]$ nc -w 1 my-servers-ip 60000 && echo success
success

For the version that fails, the SYN packet receives a RST, ACK in response.

I'm not too knowledgable about this stuff, at this point I only have wild theories such as the one in the question. Any ideas?

Potentially useful details:

Local Machine (192.168.0.100) - Macbook

Remote Machine (Amazon EC2) - Amazon Linux AMI 2012.03 
    Security Group Settings:
        22 (SSH)        0.0.0.0/0
        1099            0.0.0.0/0
        49152-65535     0.0.0.0/0
    "iptables -L" shows no rules set 
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1 Answer 1

up vote 1 down vote accepted

No, TCP connections work the same way for local or remote (routed) servers. The three-way handshake is the same.

You can check

  • some service is actially listening on port 60000 (netstat)
  • the service is not configured to reject non-local clients in some way
  • no firewall is blocking traffic
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And this is why we go to bed and try again in the morning. Thanks. –  Dean Oct 5 '12 at 15:58

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