Does getaddrinfo() need to be rebuilt for SCTP support?

Does getaddrinfo() need to be rebuilt for SCTP support?

[Jonathan T. Leighton]

I've run into some behavior of getaddrinfo() that I think may be 
incorrect. When getaddrinfo() is called with a numeric service argument 
(as opposed to a name), it returns a list of addrinfo structures for 
each protocol that it knows about, without referencing the /etc/services 
database. After installing lksctp I expected this list to include 
IPPROTO_SCTP, but it doesn't. It looks to me like getaddrinfo() is 
actually only returning addrinfo structures for all the protocols that 
is knew about when it was built. Is this the correct behavior? Do I need 
to rebuild glibc? Thanks for any help.

- Jon

Does getaddrinfo() need to be rebuilt for SCTP support?

[Jonathan T. Leighton]

I've run into some behavior of getaddrinfo() that I think may be 
incorrect. When getaddrinfo() is called with a numeric service argument 
(as opposed to a name), it returns a list of addrinfo structures for 
each protocol that it knows about, without referencing the /etc/services 
database. After installing lksctp I expected this list to include 
IPPROTO_SCTP, but it doesn't. It looks to me like getaddrinfo() is 
actually only returning addrinfo structures for all the protocols that 
is knew about when it was built. Is this the correct behavior? Does 
glibc need to be rebuilt after installing lksctp? Thanks for any help.

- Jon

RE: Does getaddrinfo() need to be rebuilt for SCTP support?

[David Laight]

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Re: Does getaddrinfo() need to be rebuilt for SCTP support?

[Jonathan T. Leighton]

Hi David -

Thanks for the response.

On 11/7/16 5:11 AM, David Laight wrote:
> From: Jonathan T. Leighton
>> Sent: 05 November 2016 15:15
>> I've run into some behavior of getaddrinfo() that I think may be
>> incorrect. When getaddrinfo() is called with a numeric service argument
>> (as opposed to a name), it returns a list of addrinfo structures for
>> each protocol that it knows about, without referencing the /etc/services
>> database. After installing lksctp I expected this list to include
>> IPPROTO_SCTP, but it doesn't. It looks to me like getaddrinfo() is
>> actually only returning addrinfo structures for all the protocols that
>> is knew about when it was built. Is this the correct behavior? Does
>> glibc need to be rebuilt after installing lksctp? Thanks for any help.
> What you don't want is programs being given SCTP addresses when they
> don't really want them.

I'm not sure I agree with this. getaddrinfo() can return addrinfo 
structures with IPPROTO_UDP and IPPROTO_IP (in addition to IPPROTO_TCP) 
- assuming that every returned addrinfo structure uses IPPROTO_TCP is a 
problem. Code needs to be able to select among the returned addrinfo 
structures appropriately, and including IPPROTO_SCTP shouldn't break 
such code.

> I've seen problems in the past (not Linux) where programs have tried
> to use addresses on all available protocols when they really just meant
> TCP or UDP.

IMO this is just bad coding.

> I don't think any of the address lookup functions are service specific,
> AA and AAAA records probably aren't even protocol specific (below IP).
>
> So programs that want to use SCTP can lookup TCP and then substitute SCTP
> when they create the socket.

Sure, but this is a hack. On a system with SCTP installed, SCTP 
shouldn't be treated like a second class citizen.

Looking at the getaddrinfo() source shows that it's written to return 
addrinfo structures with IPPROTO_SCTP, but only if it is built on a 
system with SCTP installed. That suggests to me that installing lksctp 
should include replacing glibc with a version that was built with SCTP 
enabled.

- Jon

> 	David
>

Re: Does getaddrinfo() need to be rebuilt for SCTP support?

[Jonathan T. Leighton]

On 11/07/2016 11:04 AM, Jonathan T. Leighton wrote:
> Hi David -
>
> Thanks for the response.
>
> On 11/7/16 5:11 AM, David Laight wrote:
>> From: Jonathan T. Leighton
>>> Sent: 05 November 2016 15:15
>>> I've run into some behavior of getaddrinfo() that I think may be
>>> incorrect. When getaddrinfo() is called with a numeric service argument
>>> (as opposed to a name), it returns a list of addrinfo structures for
>>> each protocol that it knows about, without referencing the 
>>> /etc/services
>>> database. After installing lksctp I expected this list to include
>>> IPPROTO_SCTP, but it doesn't. It looks to me like getaddrinfo() is
>>> actually only returning addrinfo structures for all the protocols that
>>> is knew about when it was built. Is this the correct behavior? Does
>>> glibc need to be rebuilt after installing lksctp? Thanks for any help.
>> What you don't want is programs being given SCTP addresses when they
>> don't really want them.
>
> I'm not sure I agree with this. getaddrinfo() can return addrinfo 
> structures with IPPROTO_UDP and IPPROTO_IP (in addition to 
> IPPROTO_TCP) - assuming that every returned addrinfo structure uses 
> IPPROTO_TCP is a problem. Code needs to be able to select among the 
> returned addrinfo structures appropriately, and including IPPROTO_SCTP 
> shouldn't break such code.
>
>> I've seen problems in the past (not Linux) where programs have tried
>> to use addresses on all available protocols when they really just meant
>> TCP or UDP.
>
> IMO this is just bad coding.

I think I need to revise my opinion somewhat here. I've been advised 
that there is significant code which assumes that protocol 0 and type 
SOCK_STREAM means TCP. The workaround for getaddrinfo() is to include 
IPPROTO_SCTP in the hints when using SCTP, and that works fine for me.

Thanks again for your input David.

- Jon

>> I don't think any of the address lookup functions are service specific,
>> AA and AAAA records probably aren't even protocol specific (below IP).
>>
>> So programs that want to use SCTP can lookup TCP and then substitute 
>> SCTP
>> when they create the socket.
>
> Sure, but this is a hack. On a system with SCTP installed, SCTP 
> shouldn't be treated like a second class citizen.
>
> Looking at the getaddrinfo() source shows that it's written to return 
> addrinfo structures with IPPROTO_SCTP, but only if it is built on a 
> system with SCTP installed. That suggests to me that installing lksctp 
> should include replacing glibc with a version that was built with SCTP 
> enabled.
>
> - Jon
>
>>     David
>>
>