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SMTP Proxy dual wan

akonst
Dear all

I am trying to configure a rather strange setup. My customer has two WAN uplinks and a single Internal interface. On this internal interface we have the Internal Network (bound to the internal Interface) lets say it is 192.168.0.x and a seconf Internal Network 192.168.1.x. In the inside we have two seperate Exchange servers one on each network each servicing a large number of domains (more that twenty). Internal routing is handled by an L3 switch so the default gateway og 192.168.1.x network is the L3 switch which routes traffic to 192.16.0.1 which is the Astaro Internal interface IP.

The two Internal Networks must each use a different Extrnal WAN interface. I have created general multipath rules that send all traffic from Internal 1 network to the first External Interface and all traffic from Internal 2 Network. Relevant masquerading NAT rules are created.

Everything works well except the SMTP proxy traffic. SMTP is used in transparent mode with Profiles.

All mail leaving the mail server from 192.168.0.x leaves with the correct External WAN adress (an additinal address on the first External Interface) using source NAT rules. All mail leaving the Mail server of internal lan 192.168.1.0 also leaves with the same External WAN address. I would like this mail server to send out mails using the External IP of the second External Interface.

Temporalily i have added the internal ip of the second mail server (192.168.1.5) on the SMTP proxy Transparent skip list but this of course does not filter outgoing emails. In this way however mails leave with the correct External Interface.

Any advice?

Regards

Antonis Constantinou.


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  • 0
    Antonis, your work-around is the only way to get SMTP traffic out another interface at present.  You might want to vote for: SMTP: Multiple Hostnames/Interfaces Support.  If you need the outbound mail to go through the proxy, your only choice at present is to change the public DNS records of those domains on the second mail server to use the same interface/IP as the first mail server.

    On this internal interface we have the Internal Network (bound to the internal Interface)

    I trust you're talking about the "Internal (Network)" object automatically created by WebAdmin when the Internal interface was defined.  For all manually-created Network definitions, always leave 'Interface: >' instead of binding to a specific interface.

    Cheers - Bob
     
    Sophos UTM Community Moderator
    Sophos Certified Architect - UTM
    Sophos Certified Engineer - XG
    Gold Solution Partner since 2005
    MediaSoft, Inc. USA
  • 0 in reply to BAlfson
    Dear Bob

    Thanks for you answer. Yes i mean the Internal Network Object created automatically. What seems strange to me is that the web proxy does not face this issue. Traffic is filter through the proxy and exits the correct interface that i define in the multi path rules. Anyway i have voted for the future[:)]

    Thanks again

    Regards

    Antonis C
  • 0
    This capability was added for HTTP, but has not yet been added for SMTP.  You can imagine that this requires extra steps to mark a packet, move it differently and then strip the extra marks before sending it.

    Cheers - Bob
     
    Sophos UTM Community Moderator
    Sophos Certified Architect - UTM
    Sophos Certified Engineer - XG
    Gold Solution Partner since 2005
    MediaSoft, Inc. USA
  • 0 in reply to BAlfson

    Re questioning about this Bob.
    Any chance for dual hostnames?

  • 0 in reply to Ol Deda

    Not sure what your question is, Olsi.

    Cheers - Bob

     
    Sophos UTM Community Moderator
    Sophos Certified Architect - UTM
    Sophos Certified Engineer - XG
    Gold Solution Partner since 2005
    MediaSoft, Inc. USA
  • 0 in reply to BAlfson

    Any chance to add additional hostname for SMTP, in the backup wan for example

  • 0 in reply to Ol Deda

    Have to ask the guru Bob here.

    My understanding is you can get the SMTP to leave via different IP via multipath rules but you can only receive on 1 IP ie the SMTP Proxy. Now that might create an issue with RDNS lookups by anti-spam servers etc

    I'll check tomorrow. We have a load balanced UTM cluster 2x SG330 at one location and another 2x SG330 cluster. Each 50 miles away connected via a 1gb PtP.

    Having said that, we are utilizing SMTP proxy at both sites which then relay to exchange load balancers so there are 2x smtp proxy in play here which might explain why RDNS lookups work etc.

    So as below:

    OUTGOING: Exchange Transport Servers (NLB) > SITE A UTM SMTP Proxy > Multipath > 50% to internet & 50% to SITE B UTM SMTP Proxy > Internet

    INCOMING: Primary MX > SITE A UTM Proxy > (NLB) Exchange Transport servers
                       Backup MX > SITE B UTM Proxy > (NLB) Exchange Transport servers

  • 0 in reply to Ol Deda

    Add an additional hostname for what purpose?

    Cheers - Bob

     
    Sophos UTM Community Moderator
    Sophos Certified Architect - UTM
    Sophos Certified Engineer - XG
    Gold Solution Partner since 2005
    MediaSoft, Inc. USA
  • 0 in reply to BAlfson

    As Louis-M mentioned, it may compromise RDNS

    I use SMTP only for one exchange server, and it is in trasnparent mode (lets say im more secure this way)

    I can receive emails from the second WAN (with MX record setup in DNS zone)

    I am afraid to let emails going from wan2 because has different RDNS

  • 0 in reply to Ol Deda

    If there isn't an RDNS entry for an IP address, your mail will most likely be bounced. We actually had this with the setup I've just mentioned above as the A & PTR record hadn't been set up. Just enabling the 2 uplink interfaces pushed 50% of the mail out through the 2nd interface and it bounced.

    Fortunately, we were watching for it and expecting it also. The solution was to create the multipath rule and chuck 100% smtp via the 1st interface until we got the A, PTR & MX records in place.

    As far as I'm aware, most anti-spam solutions will do a reverse lookup first to make sure the sending IP has a valid A & PTR record with it. The mail will get bounced right there if the receiving server can't connect or find a PTR record. They will then move on to the more exotic stuff like SPF, DKIM & DMARC to increase or decrease the score.

    We're quite pleased with our setup and are still tweaking it as we're in the process of completely switching the sites ie SITE B (failover site) will become the primary site in the event of an outage at SITE A (primary site) becoming offline for a reason.

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