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Re: Running namecache service on postfix server?

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  • Viktor Dukhovni
    ... Very easy. If the server is *not* local, you should not trust the AD-bit in its responses without authenticating the nameserver via something like TSIG. I
    Message 1 of 25 , Feb 26 10:21 PM
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      On Tue, Feb 26, 2013 at 08:57:51PM -0500, btb@... wrote:

      > > When Postfix support for DANE (RFC 6698) is introduced, there will
      > > be a requirement to operate a local nameserver that is DNSSEC aware
      > > on any machine that wants to take advantage of peer certificate details
      > > published via DNSSEC to scalably deliver verified TLS email to many
      > > sites without the overhead of local per-site configuration.
      >
      > Why must the nameserver be local?

      Very easy. If the server is *not* local, you should not trust the
      AD-bit in its responses without authenticating the nameserver via
      something like TSIG.

      I am not going to bloat Postfix with TSIG support, this would be
      really silly, when a local cache can take care of that. A fortiori
      I am not going to bloat Postfix with its own RRSIG-validing DNSSEC
      support. Therefore, Postfix support for DANE will be sensibly
      predicated on a *local* DNSSEC verifying cache.

      Unless we add code to check that the resolv.conf in fact only
      contains local servers (I am disinclined to do that also), you will
      be able to "break the warranty" and trust the AD-bit from non-local
      nameservers by telling Postfix to enable DANE even with a resolv.conf
      that points to remote servers. If you do that, you only have yourself
      to blame when lack of TSIG, ... makes it possible to MITM your
      server's ostensibly "secure" email deliveries.

      All, I can say (and will say in the documentation) is that you've
      been warned. Since the fields of "_res" other than "_res.options"
      are not generally documented, there is no reasonable way to perform
      a run-time check that the configured nameservers consist of just
      127.0.0.1 and/or ::1. So the plan is to document the warning clearly
      in all the relevant documents, and leave the rest to the administrator's
      ability to restrain himself from folly.

      Perhaps "postfix check" could generate a warning if DANE is enabled
      and non-local nameservers are found in /etc/resolv.conf (or and/or
      its chroot-jail version).

      --
      Viktor.
    • Robert Moskowitz
      ... Ah, thought there was a MITM lurking around the corner if DNS server not local. Thank you for the details. ... I had to add listen on the server s IP
      Message 2 of 25 , Feb 27 3:21 AM
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        On 02/27/2013 01:21 AM, Viktor Dukhovni wrote:
        > On Tue, Feb 26, 2013 at 08:57:51PM -0500, btb@... wrote:
        >
        >>> When Postfix support for DANE (RFC 6698) is introduced, there will
        >>> be a requirement to operate a local nameserver that is DNSSEC aware
        >>> on any machine that wants to take advantage of peer certificate details
        >>> published via DNSSEC to scalably deliver verified TLS email to many
        >>> sites without the overhead of local per-site configuration.
        >> Why must the nameserver be local?
        > Very easy. If the server is *not* local, you should not trust the
        > AD-bit in its responses without authenticating the nameserver via
        > something like TSIG.
        >
        > I am not going to bloat Postfix with TSIG support, this would be
        > really silly, when a local cache can take care of that. A fortiori
        > I am not going to bloat Postfix with its own RRSIG-validing DNSSEC
        > support. Therefore, Postfix support for DANE will be sensibly
        > predicated on a *local* DNSSEC verifying cache.
        >
        > Unless we add code to check that the resolv.conf in fact only
        > contains local servers (I am disinclined to do that also), you will
        > be able to "break the warranty" and trust the AD-bit from non-local
        > nameservers by telling Postfix to enable DANE even with a resolv.conf
        > that points to remote servers. If you do that, you only have yourself
        > to blame when lack of TSIG, ... makes it possible to MITM your
        > server's ostensibly "secure" email deliveries.

        Ah, thought there was a MITM lurking around the corner if DNS server not
        local. Thank you for the details.

        > All, I can say (and will say in the documentation) is that you've
        > been warned. Since the fields of "_res" other than "_res.options"
        > are not generally documented, there is no reasonable way to perform
        > a run-time check that the configured nameservers consist of just
        > 127.0.0.1 and/or ::1.

        I had to add listen on the server's IP addresses. Some services require
        that.

        > So the plan is to document the warning clearly
        > in all the relevant documents, and leave the rest to the administrator's
        > ability to restrain himself from folly.
        >
        > Perhaps "postfix check" could generate a warning if DANE is enabled
        > and non-local nameservers are found in /etc/resolv.conf (or and/or
        > its chroot-jail version).

        My main DNS server is no longer chrooted, as selinux is claimed to be
        better protection. And people better at OS security than I have vetted it.

        But with this, I realize that I have had to turn off selinux on my mail
        server. Or rather I have not found selinux assistance for all the
        services needed on a mail server that 'does it all'. Therefore got to
        add chroot for bind.
      • Wietse Venema
        ... I think it would be entirely reasonable to share a DNS cache among multiple systems within the same trusted perimeter. One DNS server per host in a farm of
        Message 3 of 25 , Feb 27 3:58 AM
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          Viktor Dukhovni:
          > Perhaps "postfix check" could generate a warning if DANE is enabled
          > and non-local nameservers are found in /etc/resolv.conf (or and/or
          > its chroot-jail version).

          I think it would be entirely reasonable to share a DNS cache among
          multiple systems within the same trusted perimeter. One DNS server
          per host in a farm of mail servers may not be practical.

          Wietse
        • DTNX Postmaster
          ... A local cache on each, forwarding to two or three resolvers that are nearby? Local for DNSSEC verification, nearby cache for performance reasons? Am I
          Message 4 of 25 , Feb 27 6:25 AM
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            On Feb 27, 2013, at 12:58, Wietse Venema <wietse@...> wrote:

            > Viktor Dukhovni:
            >> Perhaps "postfix check" could generate a warning if DANE is enabled
            >> and non-local nameservers are found in /etc/resolv.conf (or and/or
            >> its chroot-jail version).
            >
            > I think it would be entirely reasonable to share a DNS cache among
            > multiple systems within the same trusted perimeter. One DNS server
            > per host in a farm of mail servers may not be practical.

            A local cache on each, forwarding to two or three resolvers that are
            nearby? Local for DNSSEC verification, nearby cache for performance
            reasons? Am I missing something that would make that impractical?

            Cya,
            Jona
          • Robert Moskowitz
            ... In such a case I would run IPsec between them with a policy for only DNS traffic through the tunnel. ESP encapsulation is rather cheap and assures you the
            Message 5 of 25 , Feb 27 6:42 AM
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              On 02/27/2013 06:58 AM, Wietse Venema wrote:
              > Viktor Dukhovni:
              >> Perhaps "postfix check" could generate a warning if DANE is enabled
              >> and non-local nameservers are found in /etc/resolv.conf (or and/or
              >> its chroot-jail version).
              > I think it would be entirely reasonable to share a DNS cache among
              > multiple systems within the same trusted perimeter. One DNS server
              > per host in a farm of mail servers may not be practical.

              In such a case I would run IPsec between them with a policy for only DNS
              traffic through the tunnel. ESP encapsulation is rather cheap and
              assures you the traffic is going where you want it.

              Or if you have very good VLAN control, you could run 802.1AE, but the
              app space cannot tell (typically) if MACsec is working.
            • Robert Moskowitz
              ... Lots of cat skinners out here.
              Message 6 of 25 , Feb 27 6:43 AM
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                On 02/27/2013 09:25 AM, DTNX Postmaster wrote:
                > On Feb 27, 2013, at 12:58, Wietse Venema <wietse@...> wrote:
                >
                >> Viktor Dukhovni:
                >>> Perhaps "postfix check" could generate a warning if DANE is enabled
                >>> and non-local nameservers are found in /etc/resolv.conf (or and/or
                >>> its chroot-jail version).
                >> I think it would be entirely reasonable to share a DNS cache among
                >> multiple systems within the same trusted perimeter. One DNS server
                >> per host in a farm of mail servers may not be practical.
                > A local cache on each, forwarding to two or three resolvers that are
                > nearby? Local for DNSSEC verification, nearby cache for performance
                > reasons? Am I missing something that would make that impractical?

                Lots of cat skinners out here.
              • Viktor Dukhovni
                ... No, and that s pretty much what my original post suggests: ... As you say, one would typically add a couple of additional upstream caches: forward-addr:
                Message 7 of 25 , Feb 27 7:09 AM
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                  On Wed, Feb 27, 2013 at 03:25:41PM +0100, DTNX Postmaster wrote:

                  > > I think it would be entirely reasonable to share a DNS cache among
                  > > multiple systems within the same trusted perimeter. One DNS server
                  > > per host in a farm of mail servers may not be practical.
                  >
                  > A local cache on each, forwarding to two or three resolvers that are
                  > nearby? Local for DNSSEC verification, nearby cache for performance
                  > reasons? Am I missing something that would make that impractical?

                  No, and that's pretty much what my original post suggests:

                  On Tue, Feb 26, 2013 at 04:51:22PM +0000, Viktor Dukhovni wrote:

                  > On Tue, Feb 26, 2013 at 09:58:54AM -0500, Robert Moskowitz wrote:
                  >
                  > Setting up DNSSEC on a local unbound cache that forwards all queries
                  > to an upstream server boils down to:
                  >
                  > /etc/unbound/unbound.conf
                  > server:
                  > ...
                  > trust-anchor: ". IN DS 19036 8 2 49AAC11D7B6F6446702E54A1607371607A1A41855200FD2CE1CDDE32F24E8FB5"
                  >
                  > # Forward all requests to upstream server at 192.0.2.1
                  > forward-zone:
                  > name: "."
                  > forward-addr: "192.0.2.1"

                  As you say, one would typically add a couple of additional upstream caches:

                  forward-addr: "192.0.2.2"
                  forward-addr: "192.0.2.3"

                  --
                  Viktor.
                • Wietse Venema
                  ... I think it would be helpful to give examples of how secure DNS caches can be shared, instead of outright banning this. On non-trivial deployments, DNS
                  Message 8 of 25 , Feb 27 7:20 AM
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                    DTNX Postmaster:
                    > On Feb 27, 2013, at 12:58, Wietse Venema <wietse@...> wrote:
                    >
                    > > Viktor Dukhovni:
                    > >> Perhaps "postfix check" could generate a warning if DANE is enabled
                    > >> and non-local nameservers are found in /etc/resolv.conf (or and/or
                    > >> its chroot-jail version).
                    > >
                    > > I think it would be entirely reasonable to share a DNS cache among
                    > > multiple systems within the same trusted perimeter. One DNS server
                    > > per host in a farm of mail servers may not be practical.
                    >
                    > A local cache on each, forwarding to two or three resolvers that are
                    > nearby? Local for DNSSEC verification, nearby cache for performance
                    > reasons? Am I missing something that would make that impractical?

                    I think it would be helpful to give examples of how "secure DNS"
                    caches can be shared, instead of outright banning this. On non-trivial
                    deployments, DNS and MAIL are managed by different people.

                    Wietse
                  • Viktor Dukhovni
                    ... This was the intent of my original example, I guess I did not draw sufficient attention to the: forward-zone: name: . forward-addr: 192.0.2.1 stanza at
                    Message 9 of 25 , Feb 27 7:43 AM
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                      On Wed, Feb 27, 2013 at 10:20:50AM -0500, Wietse Venema wrote:

                      > > > I think it would be entirely reasonable to share a DNS cache among
                      > > > multiple systems within the same trusted perimeter. One DNS server
                      > > > per host in a farm of mail servers may not be practical.
                      > >
                      > > A local cache on each, forwarding to two or three resolvers that are
                      > > nearby? Local for DNSSEC verification, nearby cache for performance
                      > > reasons? Am I missing something that would make that impractical?
                      >
                      > I think it would be helpful to give examples of how "secure DNS"
                      > caches can be shared, instead of outright banning this. On non-trivial
                      > deployments, DNS and MAIL are managed by different people.

                      This was the intent of my original example, I guess I did not draw
                      sufficient attention to the:

                      forward-zone:
                      name: "."
                      forward-addr: 192.0.2.1

                      stanza at the bottom of the unbound.conf example. We'll need to
                      provide a similar configuration example for BIND, and explain the
                      rationale for both, so other local nameservers could also be
                      supported by an MTA administrator who understands the requirements.

                      --
                      Viktor.
                    • Robert Moskowitz
                      ... True, but we are talking about a namecaching server here, not your standard fare for DNS support people. Or rather they are old hands at deploying caching
                      Message 10 of 25 , Feb 27 7:53 AM
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                        On 02/27/2013 10:20 AM, Wietse Venema wrote:
                        > DTNX Postmaster:
                        >> On Feb 27, 2013, at 12:58, Wietse Venema <wietse@...> wrote:
                        >>
                        >>> Viktor Dukhovni:
                        >>>> Perhaps "postfix check" could generate a warning if DANE is enabled
                        >>>> and non-local nameservers are found in /etc/resolv.conf (or and/or
                        >>>> its chroot-jail version).
                        >>> I think it would be entirely reasonable to share a DNS cache among
                        >>> multiple systems within the same trusted perimeter. One DNS server
                        >>> per host in a farm of mail servers may not be practical.
                        >> A local cache on each, forwarding to two or three resolvers that are
                        >> nearby? Local for DNSSEC verification, nearby cache for performance
                        >> reasons? Am I missing something that would make that impractical?
                        > I think it would be helpful to give examples of how "secure DNS"
                        > caches can be shared, instead of outright banning this. On non-trivial
                        > deployments, DNS and MAIL are managed by different people.

                        True, but we are talking about a namecaching server here, not your
                        standard fare for DNS support people. Or rather they are old hands at
                        deploying caching servers where appropriate and could well supply
                        standard templates for them.

                        RHEL/Centos bind installs as a caching server, requiring very little in
                        edits, though as I pointed out in an earlier message I need to add
                        chroot since I have selinux off on the mail server (I don't think it was
                        postfix, but rather dovecot that forced this). Also I think if I change
                        my DNS address in ifcfg-eth0 to 127.0.0.1 and ::1 I can stop bind
                        listening on the local addresses so even less added to named.conf.

                        But to share a single DNS among a number of mail servers, say in a mail
                        farm that probably has lots of other types of servers running with
                        questionable content, I would want secure tunnels from the mail server
                        to the DNS server and that no longer is a non-trivial exercise. Now you
                        can always use my HIP protocol instead of IKEv2 for keying ESP, but
                        people doing this may want distro provided tunneling.

                        How much resources does a local caching server demand? I would think it
                        is mostly memory for the cache. You may have to throw a couple more Gb
                        at loaded server.
                      • Viktor Dukhovni
                        ... Nothing of the sort, just enable validation of outside domains and exempt local domains if unsigned. TSIG configuration is must more complex and is both
                        Message 11 of 25 , Feb 27 8:10 AM
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                          On Wed, Feb 27, 2013 at 10:53:58AM -0500, Robert Moskowitz wrote:

                          > But to share a single DNS among a number of mail servers, say in a
                          > mail farm that probably has lots of other types of servers running
                          > with questionable content, I would want secure tunnels from the mail
                          > server to the DNS server and that no longer is a non-trivial
                          > exercise.

                          Nothing of the sort, just enable validation of outside domains and
                          exempt local domains if unsigned. TSIG configuration is must more
                          complex and is both beyond our reasonable ability to document with
                          specificity (too many variants between GSSAPI, and other security
                          mechanisms) and the ability of most administrators to configure.

                          The same goes for IPSEC, ...

                          > How much resources does a local caching server demand? I would think
                          > it is mostly memory for the cache. You may have to throw a couple
                          > more Gb at loaded server.

                          GB is the wrong order of magnitude. A megabyte of RAM should be
                          more than enough for local cache on most mail servers. Just need
                          room in the cache for the MX, A, TLSA and RRSIG of the 10 highest
                          volume destination domains and the A and PTR records of the 10
                          highest volume clients.

                          The purpose of the local cache (before DANE support) is to reduce
                          latency for the highest volume requests and to give the MTA
                          administrator the flexibility to craft custom local MX RRsets in
                          suitable local zones:

                          example.net.localhost. IN MX 0 internal-mx1.example.net.
                          example.net.localhost. IN MX 0 internal-mx2.example.net.

                          example.com.localhost. IN MX 0 gw1.localhost.
                          example.com.localhost. IN MX 0 gw2.localhost.

                          gw1.localhost. IN A 192.0.2.1
                          gw2.localhost. IN A 192.0.2.2

                          Then one can add transport table entries:

                          example.net smtp:example.net.localhost
                          example.com smtp:example.com.localhost

                          these won't break DNSSEC zone validation since "localhost" would
                          be a local unsigned zone. With DANE + DNSSEC the local cache also
                          makes it possible to trust the AD-bit without jumping through hoops
                          with TSIG or implementing DNSSEC validation in Postfix.

                          I think we've beaten this thread to death, I'm done for now.

                          --
                          Viktor.
                        • Robert Moskowitz
                          ... And I thank you for all you have said.
                          Message 12 of 25 , Feb 27 8:21 AM
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                            On 02/27/2013 11:10 AM, Viktor Dukhovni wrote:
                            > I think we've beaten this thread to death, I'm done for now.

                            And I thank you for all you have said.
                          • Robert Moskowitz
                            ... On Centos 6.3 (bind 9.8.2 with security patches) I did: yum install bind bind-chroot In /etc/sysconfig/network-scripts/ifcfg-eth0 set: DNS1=127.0.0.1
                            Message 13 of 25 , Feb 27 8:42 AM
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                              On 02/27/2013 10:43 AM, Viktor Dukhovni wrote:
                              > On Wed, Feb 27, 2013 at 10:20:50AM -0500, Wietse Venema wrote:
                              >
                              >>>> I think it would be entirely reasonable to share a DNS cache among
                              >>>> multiple systems within the same trusted perimeter. One DNS server
                              >>>> per host in a farm of mail servers may not be practical.
                              >>> A local cache on each, forwarding to two or three resolvers that are
                              >>> nearby? Local for DNSSEC verification, nearby cache for performance
                              >>> reasons? Am I missing something that would make that impractical?
                              >> I think it would be helpful to give examples of how "secure DNS"
                              >> caches can be shared, instead of outright banning this. On non-trivial
                              >> deployments, DNS and MAIL are managed by different people.
                              > This was the intent of my original example, I guess I did not draw
                              > sufficient attention to the:
                              >
                              > forward-zone:
                              > name: "."
                              > forward-addr: 192.0.2.1
                              >
                              > stanza at the bottom of the unbound.conf example. We'll need to
                              > provide a similar configuration example for BIND, and explain the
                              > rationale for both, so other local nameservers could also be
                              > supported by an MTA administrator who understands the requirements.

                              On Centos 6.3 (bind 9.8.2 with security patches) I did:

                              yum install bind bind-chroot

                              In /etc/sysconfig/network-scripts/ifcfg-eth0 set:

                              DNS1=127.0.0.1
                              DNS2=::1

                              ifdown eth0; ifup eth0

                              Add to /var/named/chroot/etc/named.conf options section:

                              forward only;
                              forwarders {
                              'IPv4 addr of forwarded server';
                              'IPv6 addr of forwarded server';
                              'etc.';
                              };


                              service bind start
                              chkconfig bind on

                              You CAN use 'forward first' and then if your forward server is
                              unreachable, your caching server will go out on the net to the '.'
                              servers and walk down from there. Look at 'first' as opportunistic local
                              forwarding and 'only' as forced local forwarding.
                            • Reindl Harald
                              ... hopefully to your own TRSUTABLE forwarders and not to ISP resolvers which all of their mangeling and the problems if you use spamhaus.org and such
                              Message 14 of 25 , Feb 27 8:47 AM
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                                Am 27.02.2013 17:42, schrieb Robert Moskowitz:
                                > On Centos 6.3 (bind 9.8.2 with security patches) I did:
                                >
                                > yum install bind bind-chroot
                                >
                                > In /etc/sysconfig/network-scripts/ifcfg-eth0 set:
                                >
                                > DNS1=127.0.0.1
                                > DNS2=::1
                                >
                                > ifdown eth0; ifup eth0
                                >
                                > Add to /var/named/chroot/etc/named.conf options section:
                                >
                                > forward only;
                                > forwarders {
                                > 'IPv4 addr of forwarded server';
                                > 'IPv6 addr of forwarded server';
                                > 'etc.';
                                > };

                                hopefully to your own TRSUTABLE forwarders and not
                                to ISP resolvers which all of their mangeling and
                                the problems if you use spamhaus.org and such blacklists
                                that you get blocked
                              • Robert Moskowitz
                                ... Yes, you ONLY forward to servers where there is agreement that you MAY use them as forwarders. This is typically your own main DNS servers. Otherwise, it
                                Message 15 of 25 , Feb 27 9:05 AM
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                                  On 02/27/2013 11:47 AM, Reindl Harald wrote:
                                  >
                                  > Am 27.02.2013 17:42, schrieb Robert Moskowitz:
                                  >> On Centos 6.3 (bind 9.8.2 with security patches) I did:
                                  >>
                                  >> yum install bind bind-chroot
                                  >>
                                  >> In /etc/sysconfig/network-scripts/ifcfg-eth0 set:
                                  >>
                                  >> DNS1=127.0.0.1
                                  >> DNS2=::1
                                  >>
                                  >> ifdown eth0; ifup eth0
                                  >>
                                  >> Add to /var/named/chroot/etc/named.conf options section:
                                  >>
                                  >> forward only;
                                  >> forwarders {
                                  >> 'IPv4 addr of forwarded server';
                                  >> 'IPv6 addr of forwarded server';
                                  >> 'etc.';
                                  >> };
                                  > hopefully to your own TRSUTABLE forwarders and not
                                  > to ISP resolvers which all of their mangeling and
                                  > the problems if you use spamhaus.org and such blacklists
                                  > that you get blocked

                                  Yes, you ONLY forward to servers where there is agreement that you MAY
                                  use them as forwarders. This is typically your own main DNS servers.
                                  Otherwise, it runs 'out-of-the-box' as a caching server using the
                                  regular '.' hints to find things.

                                  Another tidbit is you should firewall access to port 53. Your caching
                                  server is only for you. It is listening only on localhost, but why open
                                  up a port not needed.
                                • Viktor Dukhovni
                                  ... Perhaps Postfix could benefit from a DNS_README.html, with examples tuning a local cache for MX overrides, RBLDNSD integration using an internal RBL zone,
                                  Message 16 of 25 , Feb 27 9:06 AM
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                                    On Wed, Feb 27, 2013 at 05:47:28PM +0100, Reindl Harald wrote:

                                    > ... more DNS related suggestions ...

                                    Perhaps Postfix could benefit from a DNS_README.html, with examples
                                    tuning a local cache for MX overrides, RBLDNSD integration using
                                    an internal RBL zone, DNSSEC support, and any other DNS-related
                                    best-practices for an MTA.

                                    Anyone care to volunteer an initial draft? Use one of the existing
                                    documents in the "proto/" directory of the Postfix source distribution
                                    as a starting point.

                                    --
                                    Viktor.
                                  • DTNX Postmaster
                                    ... Review the examples given again, please. Why would you need to firewall a local nameserver that ONLY listens on the localhost interface? Cya, Jona
                                    Message 17 of 25 , Feb 27 9:26 AM
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                                      On Feb 27, 2013, at 18:05, Robert Moskowitz <rgm@...> wrote:

                                      > Another tidbit is you should firewall access to port 53. Your caching server is only for you. It is listening only on localhost, but why open up a port not needed.

                                      Review the examples given again, please. Why would you need to firewall
                                      a local nameserver that ONLY listens on the localhost interface?

                                      Cya,
                                      Jona
                                    • Robert Moskowitz
                                      ... I would hope you are running local firewall, and only opening what is needed. Just pointing out that there is no need to open port 53 as it is only used
                                      Message 18 of 25 , Feb 27 9:38 AM
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                                        On 02/27/2013 12:26 PM, DTNX Postmaster wrote:
                                        > On Feb 27, 2013, at 18:05, Robert Moskowitz <rgm@...> wrote:
                                        >
                                        >> Another tidbit is you should firewall access to port 53. Your caching server is only for you. It is listening only on localhost, but why open up a port not needed.
                                        > Review the examples given again, please. Why would you need to firewall
                                        > a local nameserver that ONLY listens on the localhost interface?

                                        I would hope you are running local firewall, and only opening what is
                                        needed. Just pointing out that there is no need to open port 53 as it
                                        is only used local.

                                        Also about chroot. Only needed if you disable selinux.
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