Re: Running namecache service on postfix server?
On 02/27/2013 01:21 AM, Viktor Dukhovni wrote: On Tue, Feb 26, 2013 at 08:57:51PM -0500, b...@bitrate.net 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.
Re: Running namecache service on postfix server?
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
Re: Running namecache service on postfix server?
On Feb 27, 2013, at 12:58, Wietse Venema wie...@porcupine.org 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
Re: Running namecache service on postfix server?
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.
Re: Running namecache service on postfix server?
On 02/27/2013 09:25 AM, DTNX Postmaster wrote: On Feb 27, 2013, at 12:58, Wietse Venema wie...@porcupine.org 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.
Re: Running namecache service on postfix server?
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 +, 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.
Re: Running namecache service on postfix server?
DTNX Postmaster: On Feb 27, 2013, at 12:58, Wietse Venema wie...@porcupine.org 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
Re: Running namecache service on postfix server?
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.
Re: Running namecache service on postfix server?
On 02/27/2013 10:20 AM, Wietse Venema wrote: DTNX Postmaster: On Feb 27, 2013, at 12:58, Wietse Venema wie...@porcupine.org 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.
Re: Running namecache service on postfix server?
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.
Re: Running namecache service on postfix server?
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.
Re: Running namecache service on postfix server?
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.
Re: Running namecache service on postfix server?
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
signature.asc
Description: OpenPGP digital signature
Re: Running namecache service on postfix server?
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.
Re: Running namecache service on postfix server?
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.
Re: Running namecache service on postfix server?
On Feb 27, 2013, at 18:05, Robert Moskowitz r...@htt-consult.com 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
Re: Running namecache service on postfix server?
On 02/27/2013 12:26 PM, DTNX Postmaster wrote: On Feb 27, 2013, at 18:05, Robert Moskowitz r...@htt-consult.com 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.
Re: Running namecache service on postfix server?
Am 26.02.2013 15:58, schrieb Robert Moskowitz: I have recently updated my DNS server and am observing the traffic from my mail server to constantly query for names. Some of these names are frequent requests, for example: zen.spamhaus.org. So I was thinking that I could benefit from running a namecaching setup on my mail server platform. This would cut down on traffic and time on my mail server. Is this a practice that is common? Are there any downsizes to doing this? virtually anybody with a large mail/connection traffic has a chaching nameserver on his mailservers running, it is common practice signature.asc Description: OpenPGP digital signature
Re: Running namecache service on postfix server?
On 02/26/2013 10:10 AM, Reindl Harald wrote: Am 26.02.2013 15:58, schrieb Robert Moskowitz: I have recently updated my DNS server and am observing the traffic from my mail server to constantly query for names. Some of these names are frequent requests, for example: zen.spamhaus.org. So I was thinking that I could benefit from running a namecaching setup on my mail server platform. This would cut down on traffic and time on my mail server. Is this a practice that is common? Are there any downsizes to doing this? virtually anybody with a large mail/connection traffic has a chaching nameserver on his mailservers running, it is common practice thanks. One more piece learned.
Re: Running namecache service on postfix server?
On Tue, Feb 26, 2013 at 09:58:54AM -0500, Robert Moskowitz wrote: I have recently updated my DNS server and am observing the traffic from my mail server to constantly query for names. Some of these names are frequent requests, for example: zen.spamhaus.org. So I was thinking that I could benefit from running a namecaching setup on my mail server platform. This would cut down on traffic and time on my mail server. Is this a practice that is common? Are there any downsizes to doing this? 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. Consider not only deploying a local cache, but also making sure that it is DNSSEC aware. I recommend unbound from nlnetlabs.nl. Of course you don't have to use DANE and TLS, but you still benefit from a local cache regardless. Setting up DNSSEC on a local unbound cache that forwards all queries to an upstream server boils down to: /etc/unbound/unbound.conf server: ... other server settings ... # # Local (non-public) cache listens only on the loopback interface. # interface: 127.0.0.1 interface: ::1 access-control: 127.0.0.1 allow access-control: ::1 allow # # Enable internal non-DNSSEC RFC 1918 nets. # local-zone: 10.in-addr.arpa. nodefault domain-insecure: 10.in-addr.arpa. # local-zone: 16.172.in-addr.arpa. nodefault domain-insecure: 16.172.in-addr.arpa. # local-zone: 17.172.in-addr.arpa. nodefault domain-insecure: 17.172.in-addr.arpa. ... # local-zone: 30.172.in-addr.arpa. nodefault domain-insecure: 30.172.in-addr.arpa. # local-zone: 31.172.in-addr.arpa. nodefault domain-insecure: 31.172.in-addr.arpa. # local-zone: 168.192.in-addr.arpa. nodefault domain-insecure: 168.192.in-addr.arpa. # # Internal domains may map to private addresses, # and may not be DNSSEC signed. # private-domain: example.com. domain-insecure: example.com. # # root zone key fingerprint, get your copy from a trusted source. # AND update it from time to time if and when the root zone key is # updated. # trust-anchor: . IN DS 19036 8 2 49AAC11D7B6F6446702E54A1607371607A1A41855200FD2CE1CDDE32F24E8FB5 # Forward all requests to upstream server at 192.0.2.1 # This server must not forward queries for internal # names (forward or reverse) to the public internet. # forward-zone: name: . forward-addr: 192.0.2.1 -- Viktor.
Re: Running namecache service on postfix server?
On Feb 26, 2013, at 17:51, Viktor Dukhovni postfix-us...@dukhovni.org wrote: On Tue, Feb 26, 2013 at 09:58:54AM -0500, Robert Moskowitz wrote: I have recently updated my DNS server and am observing the traffic from my mail server to constantly query for names. Some of these names are frequent requests, for example: zen.spamhaus.org. So I was thinking that I could benefit from running a namecaching setup on my mail server platform. This would cut down on traffic and time on my mail server. Is this a practice that is common? Are there any downsizes to doing this? 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. Consider not only deploying a local cache, but also making sure that it is DNSSEC aware. I recommend unbound from nlnetlabs.nl. Of course you don't have to use DANE and TLS, but you still benefit from a local cache regardless. Another advantage of a local cache is that you can intercept or redirect DNS queries. We have a dedicated zone for our own DNS blacklist, for example, that redirects to rbldnsd on a different port. Postfix (and postscreen) query this local blacklist via DNS, just like they would with Spamhaus blacklists, the BRBL etc. We currently use BIND, but would recommend Unbound as well, that's what we'll be moving towards for DNSSEC support. HTH, Jona
Re: Running namecache service on postfix server?
On Feb 26, 2013, at 11.51, Viktor Dukhovni postfix-us...@dukhovni.org wrote: On Tue, Feb 26, 2013 at 09:58:54AM -0500, Robert Moskowitz wrote: I have recently updated my DNS server and am observing the traffic from my mail server to constantly query for names. Some of these names are frequent requests, for example: zen.spamhaus.org. So I was thinking that I could benefit from running a namecaching setup on my mail server platform. This would cut down on traffic and time on my mail server. Is this a practice that is common? Are there any downsizes to doing this? 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? i gather the point is to be able to trust the dns responses, which of course goes without saying - but there are methods for accomplishing this in scenarios with a non local nameserver, aren't there? i think rfc 6698 speaks to this briefly? -ben
Re: Running namecache service on postfix server?
On 02/26/2013 08:57 PM, b...@bitrate.net wrote: On Feb 26, 2013, at 11.51, Viktor Dukhovni postfix-us...@dukhovni.org wrote: On Tue, Feb 26, 2013 at 09:58:54AM -0500, Robert Moskowitz wrote: I have recently updated my DNS server and am observing the traffic from my mail server to constantly query for names. Some of these names are frequent requests, for example: zen.spamhaus.org. So I was thinking that I could benefit from running a namecaching setup on my mail server platform. This would cut down on traffic and time on my mail server. Is this a practice that is common? Are there any downsizes to doing this? 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? i gather the point is to be able to trust the dns responses, which of course goes without saying - but there are methods for accomplishing this in scenarios with a non local nameserver, aren't there? i think rfc 6698 speaks to this briefly? I don't think there is a MUST there in the IETF tradition. More of a SHOULD; I think it is a matter of performance, and perhaps security (I would have to net it out; definitely less 'room' for a MITM). I suspect people with experience in this area (mine is elsewhere in the IETF and IEEE 802) can well list the advantages of 'co-location'.
Re: Running namecache service on postfix server?
On Tue, Feb 26, 2013 at 08:57:51PM -0500, b...@bitrate.net 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.
