Interface names must fit in a buffer of size IFNAMSIZ (16) including a trailing \0, so they must be 15 or less characters.

A) 
Passt has an off by one error in the checking for the -I option meaning that it will only accept interface names of 14 or less characters.

  To reproduce:

1. ./pasta -I foo456789abcdef

  Expected results: pasta works normally
  Actual results: "Invalid interface name: foo456789abcdef"

B)

In conf_ports() we have the same error, plus some additional ones, meaning we only accept interface names of 12 or less characters.

  To reproduce:

1. ./pasta -I foo456789abcd --config-net
    To create an environment with a 13 character name
2. ./pasta -t %foo456789abcd/8000

    Expected results: inner pasta works normally
    Actual results: "Invalid port specifier %foo456789abcd/8000"