As a case study, we consider the changes made to a locally-maintained version of MMDF by one of the authors to make it IPv6-capable. MMDF is a mail transfer agent (MTA), similar in purpose to Sendmail or Postfix. It interacts with the network in three ways.
To accept incoming mail by SMTP.
To send outgoing mail by SMTP.
To look up DNS records for the routing of mail.
These components of MMDF are relatively modularized—there is an SMTP channel daemon that runs from inetd for accepting mail, an SMTP channel program that sends mail by SMTP and a table system for looking up hosts in the DNS.
A server handling an incoming SMTP connection, must get the address of the
remote machine so that it can be included in the Received: headers
of the email. This address is obtained by calling
getpeername, but the code must now expect
a response containing an IPv4 or IPv6 address. We achieved this by
rather than a
sockaddr_in. MMDF's old code had a limit
of 250 bytes for a hostname; we replaced this with the new constant
NI_MAXHOST , which is the length of the longest hostname you will
need to deal with.
The original MMDF code then called
gethostbyaddr to try to convert the
address into a hostname. We replaced this call with a
getnameinfo call. For the SMTP header, we
need to know if the address can be resolved to a hostname or not, so
we can first call
NI_NAMEREQD flag, to determine the hostname. To prevent spoofing ...