Re: gethostbyname and getaddrinfo - multiple DNS entries for a hostname

[Andrew Johnson <anj@aps.anl.gov>]

Hi Eric,

On 2013-03-09 Eric Norum wrote:
> It sounds like getaddrinfo is a better routine to use.  Will this cause
>  problems for folks on old POSIX platforms?  I suppose that those so
>  affected could use the existing gethostbyname code by copying it from the
>  posix directory to, say, solaris? Or should this be a 3.15 change and
>  we'll wave goodbye to those folks?

Our solaris-10 systems have getaddrinfo() so I think we could safely make such 
a change in the osi/posix area — it was apparently in POSIX.1-2001 according 
to the Linux man-pages.  If someone wants to send me a patch or propose 
merging a branch containing it on Launchpad I'd be happy to take it.

The only question is whether this should be a 3.15 change; I don't think it 
has to be, since it's not really adding functionality and is fixing a problem 
in the 3.14 code.  However from what Matt says it will change the behavior 
from the existing code, so I will listen to any arguments people have against 
putting it in 3.14.

- Andrew

> On Mar 9, 2013, at 2:41 AM, <matthew.pearson@diamond.ac.uk> wrote:
> > Hi,
> >
> > In the POSIX code in libCom for networking (posix/osdSock.c) there is the
> > use of gethostbyname to map a hostname to IP address. This is what the
> > Asyn function drvAsynIPPort uses to map hostname to IP address (that we
> > use in IOC startup scripts).
> >
> > We have a problem at Diamond for a few systems that move around different
> > beamlines. These typically comprise a few ethernet attached devices and
> > an Epics IOC running on Linux. The IP addresses of the device that an IOC
> > can talk to changes depending on which beamline it's running on, but the
> > hostname can stay the same. So in our DNS server we have multiple IPv4
> > address for a hostname, and we would like to be able to use the hostname
> > in the IOC startup script.
> >
> > The problem is our DNS server can't/doesn't prioritise local subnets, so
> > our hostname to IP address lookup can return any value from the list (and
> > only one value is returned). At least, that's what gethostbyname does in
> > hostToIPAddr in posix/osdSock.c. However, I've found that if I use
> > getaddrinfo instead I always get the correct IP address for the local
> > subnet first (in the list of returned IP address). So getaddrinfo seems
> > to be matching the localhost to the list of possible IP addresses.
> >
> > getaddrinfo seems to be the newer POSIX way of doing DNS lookups, and has
> > superseded gethostbyname. From the man page: The gethostbyname*() and
> > gethostbyaddr*() functions are obsolete. Applications should use
> > getaddrinfo(3) and getnameinfo(3) instead.
> >
> >
> > Has anyone else had this problem with multiple DNS entries? Any one see a
> > problem with changing hostToIPAddr to use getaddrinfo instead of
> > gethostbyname?
> >
> > In the meantime we may start doing our own hostname to IP mapping, using
> > getaddrinfo, and only ever pass an IP address into drvAsynIPPort.
> >
> > Cheers,
> > Matt

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