Using PBF To Split Services Between ISP's

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Using PBF To Split Services Between ISP's

I have a need to split the traffic going to and coming from my Exchange server based on service. Currently I have both SMTP and 443 traffic coming into and going out of the same ISP (we'll call it A). ISP A is also the default for all incoming and outgoing traffic. I want to split this to have SMTP traffic coming and going through ISP B and leave the 443 traffic on ISP A.

I think I can do this with PBF rules but I'm not totally sure how to go about it. If so here are some questions that I have.

Do I need to create 1 PBF or 2? Incoming and outgoing or just outgoing?

Do I need to create NAT rules for the PBF traffic? I already have NAT for the stuff on ISP A.

Do I need to create regular policies in addition to the PBF. eg. allowing incoming port 25 etc.

Or, am I barking up the wrong tree. I looked at the document for branch office with two ISP's and there are similarities here, but I only want to do this for the one service and not use it for failover at this time.

Thanks in advance,

Kenton


Accepted Solutions
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L6 Presenter

Hi...Yes, you can use PBF to do what you described.  My comments are inline:

Do I need to create 1 PBF or 2? Incoming and outgoing or just outgoing?

- You can have 1 PBF rule but the rule would be 'any any service=tcp/25'.  I recommend using 2 PBF rules for inbound & outbound to match the IP address of your mail server(s).

Do I need to create NAT rules for the PBF traffic? I already have NAT for the stuff on ISP A.

- Yes, you need NAT rule and most likely, you need to use ISP-B's assigned IP address.

Do I need to create regular policies in addition to the PBF. eg. allowing incoming port 25 etc.

- If ISP-A and ISP-B are in the same security zone, then you can leverage your existing security rules.

Thanks.

View solution in original post


All Replies
Highlighted
L6 Presenter

Hi...Yes, you can use PBF to do what you described.  My comments are inline:

Do I need to create 1 PBF or 2? Incoming and outgoing or just outgoing?

- You can have 1 PBF rule but the rule would be 'any any service=tcp/25'.  I recommend using 2 PBF rules for inbound & outbound to match the IP address of your mail server(s).

Do I need to create NAT rules for the PBF traffic? I already have NAT for the stuff on ISP A.

- Yes, you need NAT rule and most likely, you need to use ISP-B's assigned IP address.

Do I need to create regular policies in addition to the PBF. eg. allowing incoming port 25 etc.

- If ISP-A and ISP-B are in the same security zone, then you can leverage your existing security rules.

Thanks.

View solution in original post

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Thanks for the quick reply. I'll post back if I have any additional questions once I get going on it.

Kenton

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So would my two PBF rules look like this?

Direction  Source Zone   Source Server   Destination   Service       Egress I/F

Outgoing    Trusted          Mail Server          Any             SMTP        ISP B

Incoming    Untrusted          Any               ISP B Pub     SMTP         Internal

Thanks,

Kenton

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L6 Presenter

Yes, the outgoing PBF rule looks good.  Make sure that your service=SMTP is where SMTP=tcp/25.

I just realized that we can't control the incoming.  Senders will be sending to your mail server 'smtp.company.com' and this domain will resolve to the IP address on your ISP-A.  So all incoming traffic will come in the current path.

Thanks.

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Thanks, yes the service called SMTP is port 25. As for incoming, I would change the DNS so that smtp.company.com would point to ISP B, I think this would be necessary in any case as the receiving servers might do a reverse lookup and be confused. If incoming DNS was pointing to ISP B would my second rule work?

Thanks,

Kenton

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L6 Presenter

If you change the DNS for smtp.company.com to ISP-B, then 443 traffic destine to smtp.company.com will also come thru ISP-B.  I recall you wanted 443 to stay in ISP-Aand only tcp/25 to use ISP-B.

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That's OK, I intend to create a new DNS name for the SMTP traffic and keep the old DNS name for the 443 traffic. That is not a problem. The critical part is that the SMTP traffic enter and exit the firewall on the same IP from ISP B.

Kenton

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L6 Presenter

The outbound SMTP will work with your PBF rule.  I am not sure if the inbound will work because the replies from your server may take the default route which is ISP-A.  Also, consider how you plan to failover.  When ISP-B is down and SMTP traffic is destined for IP on ISP-B, how can you get this traffic to come to ISP-A, and vice versa?

This would not be an issue if you own the public IP address(es) because  you would use the same public IPs for both ISP-A and ISP-B.

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The main reason I'm doing this is for a temporary workaround to a blacklist that our ISP has managed to get itself on (the entire class C has been blacklisted). So most critical for me is outgoing mail. However, I'm concerned that I will run into problems if mail is coming into a different DNS name and IP than mail is going out.

Is that not something I need to be concerned with?

Kenton

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