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possible localhost dns spoof attack

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  • Jamie
    Hi Earlier today I noticed a spammer using my Postfix server as a relay to send out spam. This was puzzling because i had all requisite anti relay host
    Message 1 of 32 , Feb 26 2:32 AM
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      Hi 

      Earlier today I noticed a spammer using my Postfix server as a relay to send out spam. This was puzzling because i had all requisite anti relay host settings applied. Further, it was particularly alarming that Postfix seemed to be receiving the spam messages from localhost as indicated: 

      connect from localhost.localdomain[127.0.0.1] 

      After further analysis, I discovered that the traffic was not in fact being sent from 127.0.0.1. The packets were coming from: 

      113.167.239.162 

      Funnily enough, this IP's DNS resolves to the name "localhost". 

      Christian and I are suspicious of this. Could it be that this DNS name forms the basis of a simple DNS spoof attack that somehow confuses Postfix into thinking that the traffic comes from localhost and therefore, allows the relay to proceed? 

      We would appreciate your thoughts. 

      Jamie
    • Benny Pedersen
      ... check that you have not external nameservers that can resolve localhost into 127.0.0.1, but show logs on what postfix really did, even if sender ip is
      Message 32 of 32 , Feb 28 6:16 AM
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        Jamie skrev den 2013-02-26 11:32:

        > We would appreciate your thoughts.

        check that you have not external nameservers that can resolve localhost
        into 127.0.0.1, but show logs on what postfix really did, even if sender
        ip is localhost it should not allow relaying, unless you have
        permit_mynetwork to early, thats why i only allow smpt auth relaying,
        even from localhost/rfc1918 ips

        to minimise the risk you should only trust localhost nameservers

        and possible it would make sense to reject clients that have mx set as
        localhost or 127.0.0.1

        well i admit it just speculating here, so logs please, with postconf -n
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