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At home is a Mac Lion Server running a mail server. I have a VPS thats used as relay for incoming mail, to prevent problems when my home server might go down. Because my ISP sometimes blocks port 25, I'd like to use a different port for my incoming mail (from my VPS to my home server).

I set everything up on port 26 and changes the port forwarding settings in my router (AirPort Extreme), but somehow my VPS cannot connect.

My home server: Mac OS X Lion Server 10.7.1
My VPS: CentOS 5.0 with DirectAdmin 1.39.2

What's working:

  • Telnet-ing from a client in my home network to the home server via it's internal IP on port 25 and 26;
  • Telnet-ing from a client in my home network to the home server via it's external host name on port 25 and 26;
  • Portscan on port 25 and 26 (shows that the server is responding);
  • Telnet-ing from someone else's VPS to the home server via it's external host name on port 25 and 26;
  • Telnet-ing from my own VPS to the home server via it's external host name on port 25 only.

What isn't working:

  • Telnet-ing from my own VPS to the home server via it's external host name on port 26.

What could it be?

  • Listening port on the home server: nope, I can connect to port 26 from a local client or another VPS;
  • Firewall on the home server: no, not turned on and the same as above;
  • Port forwarding on the router: no, I can connect from another VPS and the portscan shows a response;
  • Connection between the home server and the VPS: no, other connections (like on port 25 or a simple ping) do work.
  • Firewall on the VPS: I'm not sure, but even when iptables is turned off I can't telnet to port 26, so I guess it's not the firewall.

Examples:

On a local client:

$ telnet 10.0.1.11 26
Trying 10.0.1.11...
Connected to home.removed.info.
[SMTP connection]

$ telnet 82.74.112.32 26
Trying 82.74.112.32...
Connected to removed.dynamic.ziggo.nl.
[SMTP connection]

On my VPS:

# telnet home.removed.info 25
Trying 82.74.112.32...
Connected to home.removed.info.
[SMTP connection]

# telnet home.removed.info 26
Trying 82.74.112.32...
telnet: connect to address 82.74.112.32: No route to host

On the other VPS:

$ telnet home.removed.info 26
Trying 82.74.112.32...
Connected to home.removed.info.
[SMTP connection]

iptables config on my own VPS (that can't connect on port 26):

# iptables -L -n
Chain INPUT (policy ACCEPT)
target     prot opt source               destination         

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination
share|improve this question
    
You've obscured important information needlessly. (This is supposed to be a publicly reachable mail server, you imply. Its IP address is thus hardly a secret.) This information is important because there's one obvious question that one asks in such circumstances, for ruling out one set of possibilities, that no doubt someone will ask here in due course. –  JdeBP Aug 31 '11 at 21:25
    
@JdeBP Ok, the IP's/hostnames are back. –  Jonathan Aug 31 '11 at 21:39
    
I changed the set-up and made port 26 (external) forward to port 25 (internal). The problem remains the same, but this shows that it has nothing to do with the home server or router. So it has to be my VPS, but I have no idea where it could be. –  Jonathan Aug 31 '11 at 21:48
    
The IP addresses rule out the possibility that the domain name maps to more than one IP address and one is reachable the other not. They also allow people to check things out for themselves. I've just done a traceroute to 82.74.112.32. The ICMP responses from that address are a bit odd. That's where the problem lies. –  JdeBP Aug 31 '11 at 22:08

1 Answer 1

Firewall rules granting access often restrict to whom they are granted - are both VPS' on the same IP block? Examine the fw rule(s) involved for IP requirements (on each of the Lion box, the Airport extreme, and the VPS boxes).

share|improve this answer
    
The VPS'es are totally different. Different providers, different subnets and different OS'es. The firewall on the home server is currently turned off and the mail server is configured to accept connections from all subnets. The AirPort Extreme doesn't have a firewall. It has just NAT and I configured the port forwarding correctly. So It must be something on my own VPS, but I don't know where I have to look … –  Jonathan Aug 31 '11 at 21:33
    
I asked about the firewall rules on the VPS box that isn't working... have you checked those? Outbound filtering is common in some environments. I believe CentOS uses iptables by default -> use 'iptables -L -n' to view them. –  Ram Sep 1 '11 at 18:53
    
As far as I understand, the firewall is not in use. See my first post for the output of the command you gave me. –  Jonathan Sep 1 '11 at 23:18
    
Check out your log files on the VPS box when you try this (e.g. tail -f /var/log/kernel etc) and see if you see evidence of the attempt. –  Ram Sep 2 '11 at 0:40

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