Name
IP_SRCROUTE
Synopsis
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 attempts to extract source-routing
information from the initial connection’s IP
information. If any is found, sendmail adds that
information to the $_ defined-macro ($_) for use in the Received:
header (Received:). 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). 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', `-DIP_SRCROUTE=1') ←turn on support APPENDDEF(`confENVDEF', ...
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