Dale Moore's Home Page
About Dale Moore
If you have to contact me, please feel free to send email to
I work for Facilities in the School
of Computer Science at Carnegie Mellon
University.
I have been at Carnegie Mellon University since 1976. I've worked on lots
of different systems programming stuff.
The
mission of the School of Computer Science Computing Facilities
- We provide leading-edge computing support to meet the needs of our
customers in the School of Computer Science.
- Our services are comprehensive and flexible.
- We take pride in contributing to the success of Carnegie Mellon and
continually strive to improve our services and value.
Some of my best projects
Other tidbits of information.
Domain Name System MX records are not IPV4 addresses
Or
Why usairwaysmobile.com gets it wrong
The Domain Name System DNS is used to lookup the IP addresses of
domains like CNN.COM or CS.CMU.EDU . This IP address is used at
the packet layer to route network traffic. In addition to
address information, DNS can be used to provide other information
about a domain, including Sender Policy Framework SPF and
Mail Exchange MX information.
The MX record is a way of saying, if you want to send email to this
domain via Simple Mail Transfer Protocol SMTP you should
contact these hosts listed as MX's for this domain.
For example, if we were to send email to Harry.Bovik@CS.CMU.EDU ,
the mail system would look to see if the domain CS.CMU.EDU has
any MX records. The MX records for CS.CMU.EDU are
- MX-LB-01.SRV.CS.CMU.EDU with a priority of 10
- MX-LB-02.SRV.CS.CMU.EDU with a priority of 10
- MX-LB-03.SRV.CS.CMU.EDU with a priority of 10
Because CS.CMU.EDU domain has MX records, we must use the information
in the MX to contact that domain. If the domain has no
MX records, then it is expected that we will use the IP addresses of that domain instead.
When we receive email, it is reasonable to put some basic
sanity checks on the domain portion of the envelope mail from address.
We implement these sanity checks so that any email non-delivery reports can
be sent back. We refuse to accept any email where we have no available
path to send information back to the envelope sender.
- The envelope email from domain must resolve (or be found in DNS), and the domain must
have either MX records or an IP address.
If the domain doesnt resolve, we dont have to accept the email.
- At least one of the domains listed in the MX records must resolve
with an IP address. We would like for all of them to resolve with IP addresses,
but at least one is the minimum acceptable standard.
- If the domain of the envelope from address has mx records, the ip addresses of the domains listed in the MX records must not
be on a list of reserved non-routable ip addresses. For example, if the MX domain resolves to
127.0.0.1, and that domain is not our domain, we know that we will not be able to reach that domain.
- If the domain of the envelope from address has no mx records, the ip address of that domain must
be routable. We want to avoid accepting email from user@localhost .
A common mistake is to use IPV4 style addresses in MX records instead of Fully Qualified Domain Names FQDN .
An example of this mistake is usairwaysmobile.com . They may have changed their behaviour since I wrote this page,
so, please feel free to check for yourself.
The company USAirways is an airline. They recently started a program where some of their customers can
get updates on their mobile devices about flights, delays, and other information. Some of the updates
that are sent out as sent via email with a from address of something@usairwaysmobile.com .
The domain usairwaysmobile.com has the following MX information.
- 98.129.101.42 with a priority of 10
- 87.246.86.74 with a priority of 10
The problem is that 98.129.101.42 is not a domain name, it is an IPV4 address. Nor is 87.246.86.74 .
The specification for DNS and SMTP are quire clear. The MX domains must be FQDN's.
Because of this misconfiguration of usairwaysmobile.com, the email that they attempt to send is
rejected.
There are some sites that are forgiving of this problem. Many email sites are not forgiving.
Even if CS.CMU.EDU hosts were reconfigured to accept email from domains with this sort of problem,
there are many others that would not. Sites that we forward email to would be quite reasonable
to reject email with an envelope from domain with this problem.
This is not a problem with our local domain,
it is a problem with domains like usairwaysmobile.com that dont have proper MX information.
We could be exceptionally forgiving of others misconfigurations, and bad behaviour,
but that approach has helped fuel the spam problems of the last few decades.
A more correct approach in this case would be if usairwaysmobile.com had the following MX information
- test1.mobiqua.net with a priority 10
- prodmail1.mobiqua.net with a priority 10
I have tried several different approaches in contacting someone responsible for this
example domain. I have tried many different email addresses multiple times at usairways. I've tried
making phone calls. But all of my efforts appear to have fallen on deaf ears (or
unmonitored mailboxes). I sincerely hope that they will soon find enlightenment on
this small issue.
Written by Dale Moore (Last updated )
