No Connection to Mail Server

85 pts.
Tags:
Firewalls
Mail protocols
Packet Loss
Subnets
VLAN
I have a subnet (192.168.12.0) that for some reason the workstations cannot access the mail server (located on the dmz) to retrieve or send mail. The workstations can get internet and any other services but not mail. I have checked the firewall but no changes have been made on the access list. I have 50 subnets or VLANs, and all are working fine expect for this one. I have used extend ping, but get no results. I have used traceroute, packets drop at VLAN interface(all vlan interface are on a core switch). I used wireshark to analyze further and I get resets from mail server. What else can I used to get a understanding of why the mail packets are dropping all of a sudden
ASKED: December 14, 2009  3:44 AM
UPDATED: September 11, 2012  4:33 PM

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I would fire up wireshark on both the mail server as well as one of the workstations and try and send data between the two machines. Then you can look at the data from both sides and see where the problems are.
Can you try telnet to the mail port and post the results?

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  • Labnuke99
    I agree with MrDenny. It seems odd that the mail server would be sending RST packets to clients just on this subnet. I suspect that the packets are actually coming from the DMZ firewall. You can determine this by looking at the MAC addresses in the packets. The IP address will not change but the MAC addresses will in the packets. Capture some packets at the mail server and filter for traffic to/from the problem VLAN. Capture also at a client on the VLAN. Are you certain that no changes were made to firewall filtering rules?
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  • Jdleon
    Yes. No changes were made, but I still check to see if anything was different. This problem started this past Friday, before that everything was working fine.
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  • Manliz2000
    What's different to this subnet compared to your other subnets?? Something's either gone wrong or has been changed since it has been working previously!! You can try setting a telnet session from your firewall which faces the DMZ and work your way back to the subnet, doing the same thing on each firewall. What Firewall are you using? There are different ways that you can trace iterfaces etc to see where the trafficis being dropped. Post up a diagram if you can do (change the IPs obviously) and we can go through it step-by-step!
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