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тАО02-27-2003 09:54 AM
тАО02-27-2003 09:54 AM
Sendmail DNS issues
I know this has been brought up before, but I can't seem to find the exact solution so I???ll give it a post also....
We have 2 servers running hpux 11 and sendmail 8.12. One is inbound traffic and one is outbound traffic. The outbound server is getting the error Deferred: Name server: xxxx.xxxx.com.: host name lookup failure message at a high rate (maybe 1 in 20 emails to legitimate sites). It is failing on such sites as Yahoo, Hp, and Excite for example. If I do nslookups from the server outbound itself, it finds the correct ip.
On some occasions, it will fail 2 of 3 times before it finally finds the ip and sends the email out or it might even fail completely. We are using out isp's resolver and there have been no changes made to our firewall. It just seems to be getting slowly worse as we went from 15-20 a year ago to 400-600 failures now.
I understand that network congestion plays a part in these messages, but I don't understand why sendmail fails to lookup records but I can from the same server?
Is there anything that I can do about these dns issues? Are there any timeouts that you would suggest we change? Anything at all?
Thanks in advance!!
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тАО02-27-2003 10:05 AM
тАО02-27-2003 10:05 AM
Re: Sendmail DNS issues
Consider running a caching only name server.
Folks used to split name servers and sendmail servers. With the power and memory capabilities of today's servers, splitting doesn't make much sense.
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тАО02-27-2003 10:44 AM
тАО02-27-2003 10:44 AM
Re: Sendmail DNS issues
Berlene
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тАО02-27-2003 10:51 AM
тАО02-27-2003 10:51 AM
Re: Sendmail DNS issues
We have tried to talk people into allowing dns on the sendmail servers, but the security teams firmly says no but we are running out of options before that is our last resort. What are the security impacts of hosting dns on sendmail servers?
As far as the resolver.timeouts in the sendmail.cf, are there any suggested times other then the defaults? What would you recommend?
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тАО02-27-2003 11:09 AM
тАО02-27-2003 11:09 AM
Re: Sendmail DNS issues
2) You've got to run a resolver somewhere - the closer to the mail server, the better.
Any options available in resolver(3) are available as an option to ResolverOptions in sendmail.cf (omit the RES_ prefix from resolver(3)).
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тАО02-27-2003 01:04 PM
тАО02-27-2003 01:04 PM
Re: Sendmail DNS issues
Timeout.resolver.retrans=30s
and recycle sendmail
Berlene
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тАО02-27-2003 01:17 PM
тАО02-27-2003 01:17 PM
Re: Sendmail DNS issues
forward only;
forwarders {
10.1.1.1;
10.1.2.1;
};
Then in your resolv.conf, use:
domain mydomain.com
nameserver 127.0.0.1
Your firewall should prevent anyone on the internet from getting to that server, so you should be OK security-wise - particularly since your incoming mail server is another machine. As far as anyone is concerned, your server would look (on teh network) no different than it does today.
-Vince
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тАО03-04-2003 12:05 PM
тАО03-04-2003 12:05 PM
Re: Sendmail DNS issues
If the failure is absolute on all messages, this could explain it.
AOL is configured to reject mail from servers who's hostname or sendmail name can not resolved by DNS.
Yahoo accepts the mail but doesn't deliver it.
You can manually modify the Dj parameter in your sendmail file.
/etc/mail/sendmail.cf
Change
#Dj$w.Foo.COM
to
Djvaliddomain.com
Save the file
/sbin/init.d/sendmail stop
/sbin/init.d/sendmail start
If validdomain.com will resolve on the public Internet they will accept your mail.
If however you use a mail relay server with the DS directive in sendmail.cf the above instructions do not apply.
SEP
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