postinst is in inetutils-telnet 2:1.9.4-1build1.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 | #!/bin/sh
set -e
update-alternatives \
--install /usr/bin/telnet telnet /usr/bin/inetutils-telnet 100 \
--slave /usr/share/man/man1/telnet.1.gz telnet.1.gz \
/usr/share/man/man1/inetutils-telnet.1.gz
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