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[ScreenOS] How the firewall verifies the source IP address against the SBL server to check if it is spam

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Article ID: KB24876 KB Last Updated: 14 Jun 2012Version: 1.0
Summary:
This article provides information how a firewall verifies the source IP address, against the SBL server, to check if it is spam.
Symptoms:
Cause:

Solution:
Example:

  • The DNS configured on the firewall is 2.2.2.2.

  • The IP address of the SMTP (incoming), which has to be checked for spam, is a.b.c.d.

The SBL process is as follows:

  1. The firewall will check the local white and black list (IP, domain).

  2. If the IP address is not listed, the firewall will send a query to 2.2.2.2 to request for the record of d.c.b.a. <hash>. msgsecurity.juniper.net (the communication is always between the firewall and the configured DNS; in this case, it is 2.2.2.2).

  3. d.c.b.a. <hash>. msgsecurity.juniper.net will be considered as the sub-domain/host of Juniper.net by 2.2.2.2.

  4. If 2.2.2.2 has cached information (a record), it will reply; else 2.2.2.2 will query the authoritative DNS of juniper.net (recursive query).

  5. Necessary intelligence is built in the Juniper authoritative DNS to understand, whether the query is for a record of existing sub-domain (for example, kb.juniper.net) or to check the spam IP in the database.

    1. If the query is actually for the sub-domain, DNS will check the zone file and provide the IP address of the domain/sub-domain (for example, address record of kb.juniper.net = 66.129.230.48).

    2. The DNS will pick only the d.c.b.a IP and forward it to a 3rd party anti-spam database to check whether the IP address is part of the anti-spam list. The <hash> value will be used for authentication.


  6. If the result of point 5(b) is true, authoritative DNS will reply to 2.2.2.2 and then to the firewall with the 127.0.0.x IP as a record of the d.c.b.a domain. <hash>. msgsecurity.juniper.net and the firewall will act accordingly (127.0.0.1 - spam).

  7. 7. If the result of point 5(b) is false, the DNS will replay cannot find d.c.b.a. <hash>. msgsecurity.juniper.net, which will be considered as NXDOMAIN by the firewall (NXDOMAIN -spam status is unknown, or spam license has expired).
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