Configuring sendmail to use an external STMP server (I don't understand some magic)

crisostomo_enrico · 10-16-2008, 11:38 AM

Hi.

I have a server (server0.devel.mydomain.com) and I wanted its sendmail to send mail to our ISP MTA (mail.mydomain.com). Looking in internet and in some other LQ threads I understand that I can do this by using the smart host (DS) configuration parameter. I expected indeed a failure before modifying sendmail.cf.

With my surprise, using mailx to send a test message to myuser@gmail.com, sendmail sent the message successfully and I don't understand the magic because the DS configuration parameter was still empty on sendmail.cf.

First thing: the domainname command shows devel.mydomain.com and I don't understand exactly where it's set. There's no /etc/defaultdomain, correctly because I assume it's just for NIS(+). /etc/hosts is:

Code:

#/etc/hosts
[...]
192.168.1.50 server0 server0.devel.mydomain.com loghost
[...]

#/etc/nsswitch.conf
[...]
hosts    files dns
ipnodes  files dns
[...]

and then there's DNS running on server0 with an entry for server0 in domain devel.mydomain.com. So, which one of this is setting domain to devel.mydomain.com?

Last thing: knowing that the machine says that it's domain is devel.mydomain.com, how and why did sendmail (with an empty DS) decided to relay to mail.mydomain.com:25?

EDIT: I noticed that IP address of mail.mydomain.com and mydomain.com are the same.

Thanks in advance for pointers to some explanation.
Enrico

stuart_cherrington · 10-17-2008, 06:06 AM

On MX Records:

It comes down to MX (Mail eXchanger) records in your DNS.

Can't remember the exact syntax, but if you run nslookup/dig you can look for MX servers in your local domain and this is what Sendmail did.

SmartHosts are basically hosts are virtual IP/hostnames for several MX servers. I.e. SmartHost mail0.domain.com mayhave mail1.domain.com and mail2.mydom.com as its MX servers, this way if mail0 goes down, mail1 keeps on serving.

I Think (Don't quote me) that the Domain name settings are retrieved from DNS as well now. the /etc/domainname file is usually blank on Sol8 and above hosts, mainly becuase many companies now run one host in multiple domains and this file just confuses the poor thing!

Stuart.

crisostomo_enrico · 10-19-2008, 01:24 PM

Thanks for your information, stuart_cherrington.
Domain name seems to come from DNS configuration, indeed.

Bye,
Enrico.