[lwip-devel] [bug #58473] Trailing dot in hostname results in extraneous label (and corrupt packet)

[Mate Varga]

URL:
  <https://savannah.nongnu.org/bugs/?58473>

                 Summary: Trailing dot in hostname results in extraneous label
(and corrupt packet)
                 Project: lwIP - A Lightweight TCP/IP stack
            Submitted by: matevarga
            Submitted on: Sun 31 May 2020 09:31:13 PM UTC
                Category: DNS
                Severity: 3 - Normal
              Item Group: Faulty Behaviour
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: None
            lwIP version: git head

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Details:

Expected behavior:
------------------

When a hostname ending with a dot are looked up using lwip_gethostbyname(),
then the DNS query's QNAME section will contain the right number of labels. 

For example, an A query for foo.barbaz. should result in an UDP packet
fragment like 

3foo6barbaz0101

(3 char long label: foo, 6 char long label: barbaz, type: 1, class 1)

Actual behavior
---------------

LWIP considers the trailing dot as a separate label with zero lenght.
Therefore the above example will result in the following packet fragment:

3foo6barbaz00101

(3 char long label: foo, 6 char long label: barbaz, 0 char long label: , type:
1, class 1)

DNS servers like dnsmasq won't be able to parse this, and it'll likely be
interpreted as a * query with class 256. Site-local lookups will break.

I have attached the patch. I intend to submit a unit test as well.



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File Attachments:


-------------------------------------------------------
Date: Sun 31 May 2020 09:31:13 PM UTC  Name:
0001-Does-not-add-an-empty-label-when-encountering-traili.patch  Size: 1KiB  
By: matevarga

<http://savannah.nongnu.org/bugs/download.php?file_id=49191>

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