IP_SRCROUTE
Add IP source-routing to $_ Tune with confENVDEF
Mail is normally transported over networks with TCP/IP. At the IP layer, packets are usually constructed to be point-to-point—from one host to another. IP packets can also be constructed to contain source-routing information—from one host, through a second, then to a final host.
Although such source routing (when used) is generally
legitimate, it can also be used to generate
fraudulent mail. V8.7 and above
sendmail attempt to extract
source-routing information from the initial
connection’s IP information. If any is found,
sendmail adds that
information to the $_ defined macro ($_ on page 801) for use
in the Received:
header (Received: on page
1162). The $_
defined macro is usually used like this:
Received: from $s ($_) ...
where $_ will
contain information such as the following when IP
source-routing information is found:
IP source-routing information ↓ user@host.domain [!@hostC@hostB:hostA] ↑ RFC1413 identd information
IP source-routing information is presented inside
square brackets. If routing is strict, the
information is prefixed with an exclamation mark.
The format of the information is made to resemble
that of source-route addressing (see also the
DontPruneRoutes
option, DontPruneRoutes on page
1024). In this example, the IP packets will go first
to hostC, then to
hostB, and
finally to hostA.
The inclusion of code to support this reporting is determined by the IP_SRCROUTE definition in your Build m4 file:
APPENDDEF(`confENVDEF', ...
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