Add ipaddress property to get name of reverse DNS PTR record

[leonn]

BPO	20480
Nosy	@ncoghlan, @pitrou, @ericvsmith
Files	ipaddress_reverse_names.patch: patch implementing the feature ipaddress_reverse_names_v2.patch: patch adding the reverse_pointer property ipaddress_reverse_names_v3.patch: patch with updated documentation ipaddress_reverse_names_v3_alt.patch: alternative patch without trailing dots ipaddress_reverse_names_v4.patch: no trailing dots, with consistent variable naming

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/ericvsmith'
closed_at = <Date 2014-04-14.16:59:19.574>
created_at = <Date 2014-02-02.01:22:28.083>
labels = ['type-feature', 'library']
title = 'Add ipaddress property to get name of reverse DNS PTR record'
updated_at = <Date 2014-04-14.16:59:19.574>
user = 'https://bugs.python.org/leonn'

bugs.python.org fields:

activity = <Date 2014-04-14.16:59:19.574>
actor = 'eric.smith'
assignee = 'eric.smith'
closed = True
closed_date = <Date 2014-04-14.16:59:19.574>
closer = 'eric.smith'
components = ['Library (Lib)']
creation = <Date 2014-02-02.01:22:28.083>
creator = 'leonn'
dependencies = []
files = ['33851', '33852', '33865', '33870', '33940']
hgrepos = []
issue_num = 20480
keywords = ['patch']
message_count = 23.0
messages = ['209933', '209935', '209936', '209937', '209938', '209939', '209941', '209960', '209962', '209963', '209977', '209980', '209981', '209982', '209986', '209987', '209995', '209996', '210000', '210049', '210378', '210379', '216112']
nosy_count = 6.0
nosy_names = ['ncoghlan', 'pitrou', 'eric.smith', 'pmoody', 'python-dev', 'leonn']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue20480'
versions = ['Python 3.5']

[leonn]

I was missing a method to compute the reverse DNS name for an IP address, and I felt this is something that would belong in the ipaddress module; so here’s a patch for the ipaddress module adding a reverse_name property to IPv?Address.

This is an example:

>>> ipaddress.ip_address("127.0.0.1").reverse_name
'1.0.0.127.in-addr.arpa.'
>>> ipaddress.ip_address("2001:db8::1").reverse_name
'1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa.'

Would this be an acceptable feature for inclusion in the ipaddress module?

[ericvsmith]

I think the functionality is reasonable for this module.

When I originally read the bug title, I was concerned that it was actually doing a reverse DNS lookup, which would not be appropriate. But now I realize it's just computing the name that would be used for the reverse lookup.

I'm not sure "reverse_name" is the best name for this, but I don't have any better suggestions, and I could live with this name.

Changing version to 3.5, since that's the first version this could be added to.

[ncoghlan]

Heh, my initial reaction based on the issue title was the same as Eric's, but yes, I agree the pure text manipulation proposed in the patch is actually a good fit.

Rather than "reverse_name" (which I feel is ambiguous about whether or not it does the DNS lookup to resolve back to the canonical name for the address), I would suggest the attribute name "reverse_pointer" or "reverse_record" (with a slight preference for the former, as indicated in the updated issue title).

[pitrou]

Thanks for the patch, Leon. Is the trailing dot actually desired?

>>> ipaddress.ip_address("127.0.0.1").reverse_name
'1.0.0.127.in-addr.arpa.'

Also, to accept your contribution, we will need you to fill a contributor's agreement. See http://www.python.org/psf/contrib/

[leonn]

Thanks for the feedback, I agree "reverse_pointer" is a better, less ambiguous name for the property. I’ve amended the patch to reflect this suggestion.

Regarding the trailing dot, I felt it more appropriate to have it that to leave it out, but I don’t have a strong opinion on this. It has to be there in the DNS query, but most tools will automatically add it to the query if it’s not specified. The "host" tool from bind-utils is undecided as well when printing the output, it includes the trailing dot when querying IPv4 addresses, but not for IPv6 addresses.

Including the trailing dot has the advantage that the output could be directly fed into other code that strictly requires it, like the dnspython module.

I’ll sign and submit the contributor’s agreement as soon as I’m within reach of a printer (likely within the next 24 hours).

[ncoghlan]

Gah, we still haven't fixed the contributor license docs on the main
CLA page (hopefully we'll finally have that sorted later this month).
In the meantime, if you go directly to
http://www.python.org/psf/contrib/contrib-form/ it should give you the
option to sign electronically:
http://www.python.org/psf/contrib/contrib-form/

[leonn]

Oh nice, then fewer trees have to die. I’ve now signed the contributor’s agreement.

[ericvsmith]

I'd prefer to leave the trailing dots. It's the technically correct thing to do.

And as they say, technically correct is the best kind of correct!

[ericvsmith]

The patch looks good to me, save for a versionadded tag in the docs. I've mentioned it on Rietveld.

Not to get in to bikeshedding on the name, but I'd prefer something with PTR in the name. I'm an old-timer, and think of these as "PTR lookups" (because they are!). If I were looking for it, that's what I'd search for.

And if not in the property name, then at least the documentation should mention it.

[ericvsmith]

Or, now that I think about it some more, maybe leave the name as reverse_pointer and just mention PTR records in the documentation.

[pitrou]

I'd prefer to leave the trailing dots. It's the technically correct thing to do.

It depends what we're talking about. Hostnames (in the general sense)
don't carry a trailing dot except in DNS queries. So I'd argue the
trailing dot is more part of the DNS protocol than of the hostname
itself.

As for what the "host" command does, it doesn't add any trailing dots
here:

$ host 127.0.0.1
1.0.0.127.in-addr.arpa domain name pointer XXX.
$ host ::1
1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.ip6.arpa domain name pointer XXX.

(this is bind9-host from Ubuntu)

The Wikipedia page doesn't use trailing dots:
http://en.wikipedia.org/wiki/Reverse_DNS_lookup

Neither do the examples from the RIPE itself:
http://www.ripe.net/data-tools/dns/reverse-dns/how-to-set-up-reverse-delegation