[CentOS] TCP/IP Port Relay

Thu Apr 24 15:47:39 UTC 2008
Frank M. Ramaekers <FRamaekers at ailife.com>

There seems to be a problem with it

Normal (TN3270 traffic originating from the relay host):

10:44:29.667165 IP doylebrunson.ailife.com.46318 > S 707994026:707994026(0) win 5840 <mss 1460,sackOK,timestamp 1913051656 0,nop,wscale 7>
10:44:29.685075 IP > doylebrunson.ailife.com.46318: S 1665830401:1665830401(0) ack 707994027 win 33396 <mss 1452,nop,wscale 0,nop,nop,timestamp 6736695 1913051656>
10:44:29.685137 IP doylebrunson.ailife.com.46318 > . ack 1 win 46 <nop,nop,timestamp 1913051674 6736695>
10:44:29.707089 IP > doylebrunson.ailife.com.46318: P 1:4(3) ack 1 win 33396 <nop,nop,timestamp 6736695 1913051656>
10:44:29.707147 IP doylebrunson.ailife.com.46318 > . ack 4 win 46 <nop,nop,timestamp 1913051696 6736695>


10:44:04.362686 IP it-304.ailife.com.bts-appserver > S 3483673038:3483673038(0) win 65535 <mss 1460,nop,nop,sackOK>
10:44:07.294923 IP it-304.ailife.com.bts-appserver > S 3483673038:3483673038(0) win 65535 <mss 1460,nop,nop,sackOK>

...and the client times out (The TN3270 server is running on port 3270/verismart).

Any ideas?

Frank M. Ramaekers Jr.
Systems Programmer                   MCP, MCP+I, MCSE & RHCE
American Income Life Insurance Co.   Phone: (254)761-6649
1200 Wooded Acres Dr.                Fax:   (254)741-5777
Waco, Texas  76710

-----Original Message-----
From: centos-bounces at centos.org [mailto:centos-bounces at centos.org] On Behalf Of Vahur Jõesalu
Sent: Thursday, April 24, 2008 9:27 AM
To: CentOS mailing list
Subject: Re: [CentOS] TCP/IP Port Relay

hmm, if I understood you correctly, then this should work just fine (on 
linux firewall):

/sbin/iptables -t nat -I PREROUTING -p tcp --dport 23 -j DNAT \
--to telnetserverip:port-number
/sbin/iptables -I FORWARD -i external_interface -o internal_interface \ 
-p tcp -d telnetserverip --dport portnumberontelnetserver -j ACCEPT

after a reboot or firewall service restart it's gone again.


Frank M. Ramaekers wrote:
> What is the best way to temporarily setup a port relay.  I'm trying to
> diagnose a problem with Cisco's latest VPN client and need to determine
> if the problem is port number related.
> e.g.  VPN client---------23-(Linux box to relay port)-------xx-(Telnet
> server)
> (Keep in mind this is temporary and for debugging purposes)
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