Mappings are applied iteratively (page 332)More...
Reported on: 2005-07-26
In chapter 6. Debian system administration on page 332:
When the second paragraph in the subsection "Interface mappings" in section 6.7.1 states that only the first matching mapping is applied, it is only partially correct. Mappings are applied iteratively until no further mappings match.
It is true that a configuration identifier is mapped by the first matching mapping stanza; mapping stanzas further down in the file are not applied.
However, the result of a mapping is another configuration identifier. After applying a mapping, ifupdown repeats the process and tries to find the first mapping whose glob matches the new configuration identifier. This process continues until no more matching mapping stanzas are available, at which point the iface stanzas are considered.
This allows for interface configuration hierarchies and conditional mappings. For instance, in combination with guessnet (see p. 335ff), you can run certain location checks only if others were successful; you may only want to check for the presence of a certain router if a specific wireless access point is available.