Sendmail on a LAN

[JD]

  On 08/17/2010 08:47 AM, Gordon Messmer wrote:
> On 08/16/2010 10:46 AM, JD wrote:
>> Clearly, a full setup of DNS server for your domain
>> must be set up, per this wiki, along with mx records ...etc.
>>
>> Does this prevent one from settiing up and using sendmail
>> on a LAN to send and receive email to/from the outside world?
> Not by itself, but I don't like the advice in that tutorial.  It
> suggests a configuration with a catch-all address.  Long-term, you'll
> find that you have to turn this off or else your catch-all will receive
> an enormous amount of spam sent by spammers who used a dictionary attack
> and found that every address they test is valid on your system.  Without
> a catch-all, there's no purpose in using the virtual user feature at
> all, so the howto is somewhat more complicated than it needs to be.
>
> Beyond that, it does not address several practical concerns with setting
> up a mail server.  First, you'll need a static address and a proper PTR
> for it.  You won't be able to set up a PTR without a static address, and
> if the reverse lookup for your IP address isn't valid, many systems will
> refuse your mail.  The reverse lookup (PTR) for your address must be a
> hostname that resolves to your IP.
>
> aa.bb.cc.dd ->  PTR myhost.example.com
> myhost.example.com ->  A aa.bb.cc.dd
>
> You should also look up your IP address on a blacklist watcher:
> http://www.dnswatch.info/dns/rbl-lookup
>
> If you're listed in one of the dynamic IP blacklists, you want to set up
> a smart host through which you'll relay mail.  Many sites will otherwise
> reject your messages.  Likewise, if your ISP prevents you from making
> outbound connections to port 25, you'll need to use a smart host,
> because you couldn't send mail any other way.
>
>> I understand that some things need to  be set up so that sendmail
>> sends headers that use a routable IP address as the source of
>> the message. Is it possible to make sendmail use my router's
>> public IP address in the message headers? How?
> No, that's not quite right.  Sendmail should be configured to use a
> valid hostname for HELO, but the IP address will be recorded in a header
> which is set by the receiver of the message.  You can't do anything to
> change that.

Well, that's very interesting.
I am on the verge of giving up.

Re:  a.b.c.d ==> valid.host.name
and valid.host.name ==> a.b.c.d

does not seem to apply to the google smtp server I use for Thunderbird.
To wit:

$ nslookup
 > smtp.gmail.com
Server:        156.154.70.1
Address:    156.154.70.1#53

Non-authoritative answer:
smtp.gmail.com    canonical name = gmail-smtp-msa.l.google.com.
Name:    gmail-smtp-msa.l.google.com
Address: 74.125.127.109


 > 74.125.127.109
Server:        156.154.70.1
Address:    156.154.70.1#53

Non-authoritative answer:
109.127.125.74.in-addr.arpa    name = pz-in-f109.1e100.net.

Authoritative answers can be found from:


 > pz-in-f109.1e100.net
Server:        156.154.70.1
Address:    156.154.70.1#53

Non-authoritative answer:
Name:    pz-in-f109.1e100.net
Address: 74.125.127.109


So, Thunderbird client does not seem to mind that
reverse lookup does not match the name smtp.gmail.com