New submission from Kat Hoessel <[email protected]>:
IPv4 and IPv6 interface objects of the `ipaddress` library show unexpected
behaviour when arithmetic operators like + (increment) and - (decrement) are
run on them.
`ipaddress` interfaces can be incremented/decremented, similar to IP addresses.
Currently, there are two problems with how these operations are implemented for
interfaces within this library.
This applies to both IPv4 and IPv6, and may yield incorrect and inconsistent
results.
1. Increment/decrement on an interface overrides subnet prefix length
The resulting interface will always have a prefix length of /32 for IPv4, /128
for IPv6, regardless of the original interface prefix. (See iPython code
examples below.)
This behaviour is incomprehensible and unexpected.
2. Increment/decrement on an interface does not respect subnet arithmetic
For example,
ipaddress.ip_interface('10.0.0.1/24') + 255
has no valid result under subnet arithmetic.
`ipaddress` however, incorrectly returns
IPv4Interface('10.0.1.0/32')
It is likely that the behaviour seen in 2) logically follows after 1).
Both are assumedly related to the fact that IPv*Interface is a subclass of
IPv*Address with inherited properties.
To achieve consistent behaviour for interfaces under +/-, I would suggest to
either
a) make sure that +/- operators on interfaces preserve the prefix and respect
subnet arithmetic, or
b) forbid +/- operators on interfaces.
My preferred solution would be a). I would be interested in providing a patch,
given that the observed behaviour is confirmed as a bug.
----------------------------------------------------------------
### IPv4
In [1]: import ipaddress
In [2]: iff = ipaddress.ip_interface('10.0.0.1/24')
In [3]: iff
Out[3]: IPv4Interface('10.0.0.1/24')
In [4]: iff1 = iff + 1
In [5]: iff1
Out[5]: IPv4Interface('10.0.0.2/32')
In [6]: iff.network.prefixlen
Out[6]: 24
In [7]: iff1.network.prefixlen
Out[7]: 32
### overflow
In [8]: iff + 255 # this should not work
Out[8]: IPv4Interface('10.0.1.0/32')
### IPv6
In [9]: iff = ipaddress.ip_interface('1:1:1:2::1/64')
In [10]: iff
Out[10]: IPv6Interface('1:1:1:2::1/64')
In [11]: iff + 1
Out[11]: IPv6Interface('1:1:1:2::2/128')
### version information
In [12]: import sys
In [13]: sys.version
Out[13]: '3.8.3 (default, Aug 31 2020, 16:03:14) \n[GCC 8.3.1 20191121 (Red Hat
8.3.1-5)]'
----------
components: Library (Lib)
messages: 392417
nosy: katsel
priority: normal
severity: normal
status: open
title: ipaddress: increment/decrement on IPv*Interface disregards bitmask prefix
versions: Python 3.8
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Python tracker <[email protected]>
<https://bugs.python.org/issue43986>
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