[systemd-devel] [PATCH] network: allow domain names up to 255 characters

[Nick Owens]

sorry, "supercalifragilisticexpialidocioussupercalifragilisticexpialidocious"
was a bad example. it would not be valid since it is longer than a
label.

On Sun, Apr 12, 2015 at 9:37 AM, Lennart Poettering
<lennart at poettering.net> wrote:
> On Sun, 12.04.15 09:21, Nick Owens (mischief at offblast.org) wrote:
>
>> On Sun, Apr 12, 2015 at 6:35 AM, Lennart Poettering
>> <lennart at poettering.net> wrote:
>> > On Fri, 10.04.15 13:03, Nick Owens (mischief at offblast.org) wrote:
>> >
>> >> From: mischief <mischief at offblast.org>
>> >>
>> >> The maximum domain name size is larger than the maximum host name size.
>> >> The smaller limit causes valid domains provided by DHCP or .network
>> >> files to be silently ignored.
>> >
>> > Hmm?
>> >
>> > Can you give an example?
>>
>> if you set the Domains key in the [Network] section of a
>> systemd.network file to a domain longer than a label, then it will be
>> ignored. the same is true if your DHCP server sends a domain in option
>> 15 (domain name) that is longer than a label. it will be ignored too.
>> both of these code paths call 'hostname_is_valid', which will fail if
>> passed something larger than a label, which a domain name can be.
>
> "longer than a label"? What do you mean by that? The function should
> perfectly consider multi-label names valid? Are you saying you cannot
> set the domain name "foo.bar."?

a multi-label name is valid. however, hostname_is_valid will reject
any domain name (multi-label or not) longer than HOST_NAME_MAX, the
size of a label.

>
> Hmm, so the DHCP spec explicitly declares that options 15 and 12 are
> about the DNS hostname, where RFC 1035 is normative. Our function
> hostname_is_valid() currently does not validate host names according
> to RFC 1035, but is in some way stricter (by enforcing Linux' own semantics
> on the length, and by limiting the charset drastically) and in other
> ways less strict (by not enforce label length.)
>
> I am pretty sure we should leave hostname_is_valid() the way it is, to
> be used when setting local hostnames and things like that. However,
> the DHCP code should really validate according to RFC 1035 instead,
> since that's what the spec says...
>
> Implementation-wise this probably means we should move
> src/resolve/resolved-dns-domain.[ch] into src/shared/dns-domain.[ch]
> and then add a call there that works similar to dns_name_normalize()
> but doesn't actually normalize, but simply validates.

the intention of the domainname_is_valid function in this patch is to
validate domain names, which can be up to 255 octets. there appears to
be no standardized macro for this size, so i added one.

>
> Hope that makes sense?
>
> Lennart
>
> --
> Lennart Poettering, Red Hat