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Problem with Mail Server Receiving Mail from Two WANs

Discussion in 'Networking Issues' started by johncole, Sep 8, 2007.

  1. johncole

    johncole LI Guru Member

    We have a DSL and a Comcast connections for redundancy. We were hoping to have automatic switchover to maintain continuous email connectivity in case a backhoe operator runs amuck. However, for the last year we have not been able to have two MX addresses with two active WANs for receiving mail; we have to switchover manually. Linksys customer support was unable to provide us a solution when we asked over a year ago.

    The problem occurs on the RV016, connected to two WANs, when using port forwarding to a server. When a SMTP input to port 25 comes in on either WAN, port forwarding sends the packet to the server correctly. However, the mail server doesn't know from which WAN the packet came in on. It sends the response back to the gateway, the router. The router must then send the response back through the correct WAN. If the response goes back over the correct WAN, then a connection is established, and all is hunky-dory. However, when the response is sent back through the other WAN, the source of the original input receives a response back with an originating IP address other than the one to which it sent the input, its NAT rejects it, and the connection times out and is dropped.

    For port forwarding to be useful for a mail server with multiple WANs active, the RV016 needs to have session persistence or some kind of WAN binding to return the response to a port-forwarded input through the same WAN from which it was received.

    Any suggestions or ways to solve the problem with two active WANs?

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