A really compresensive FAQ is not yet available for OpenSLP so please
send
your questions to the OpenSLP mailing lists:
openslp-devel@lists.sourceforge.net
openslp-users@lists.sourceforge.net
Q: Where is the configure script to build OpenSLP?
A: Did you read section 3 of the README? You need
to run autogen.sh to
generate the configure script.
Q: How do I build OpenSLP on Windows?
A: The MSVC project files used by the developers who ported
OpenSLP to win32
available in the source directories. If you
do not use MSVC and you are a
Windows developer, then you will be used to trying
to get MSVC makes to
work with your tools
Q: Will OpenSLP work on my operating system
A: Yes, the OpenSLP code has proven to be very portable.
It currently works
many operating systems including: Linux, BSD, Solaris,
Tru64, HPUX, UnixWare,
OSR5, and Win32
Q: I am having trouble discovering attributes using FindAttr()
and "slptool
findattrs". The functions seem to execute properly,
and the services URL's
can be discovered, but no attributes are returned.
I am registering
services in slp.reg files. I don't think it is my
syntax in the slp.reg
file, because the example registrations in that file
do not return
attributes either. Can anyone help?
A: If you just want to use slptool to see if things are
working, you need to
do the following:
Contents of the slp.reg:
------------------------
service:myservice1.x://myhost.caldera.com,en,65535
owner=Matt Peterson
email=mpeterson@caldera.com
service:myservice1.x://yourhost.yourdomain.com,en,65535
owner=Kim Jackson
email=bjackson@yourhost.yourdomain.com
IMPORTANT: Restart slpd and check the /var/log/slpd.log
to ensure that
there were no errors during parsing of the .reg file
Use slptool to find attributes
------------------------------
$ slptool findsrvs service:myservice1.x
service:myservice1.x://myhost.caldera.com.com,65535
service:myservice1.x://yourhost.yourdomain.com,65535
$ slptool findattrs service:myservice1.x://myhost.mydomain.com
(owner=Matt Peterson),(email=mpeterson@caldera.com)
$ slptool findattrs service:myservice1.x://yourhost.caldera.com
(owner=Kim Jackson),(email=bjackson@yourhost.yourdomain.com)
Note that you need to supply the service-url as returned by findsrvs
Q: I have a multi-homed machine and OpenSLP is not working.
A: Please read the updated installation guide
http://www.openslp.org/doc/html/UsersGuide/Installation.html.
There are special instructions for users of multi-homed
machines.
Q: In our development lab, the multicast SLP requests work
just fine.
However, in our SVT lab, the multicasts requests never
work. We always
have to edit the slp.conf file and turn on broadcast.
Have any others seen
this? Do you recall what the solution was?
We have spent a great deal of
time trying to figure this one out without success.
A: Yes, others have seen this behavior -- I know I have.
I should put this in
the FAQ because I get a lot of questions. The
following is a list of the
most common problems along with trouble shooting and
resolution info:
No multicast route
-------------------
A very common problem with some OS installations (especially
Linux) is
that there is no multicast or default route set up.
On systems with BSD
derived TCP/IP stacks (nearly all OSes), broadcast
and multicast traffic
are delivered using the unicast routing table.
If the unicast routing
table does not have either a default route or an explicit
multicast route,
then the kernel does not know where to send the SLP
datagram and returns
an error 101 - network unreachable error which translates
into a SLPError
-23 SLP_NETWORK_ERROR. A quick test is to try to ping
SLP multicast:
$ ping 239.255.255.253
If ping returns an error that the network was unreachable,
you will need
to set up a default route or a multicast route.
Note you may not get any responses to the ping.
This may not indicate
a problem. The only thing to be concerned about
is if there is an error
actually sending the ping.
Please read the installation instructions for more
information on how
to install a multicast route:
http://www.openslp.org/doc/html/UsersGuide/Installation.html
The "smart switch /stupid router" problem
------------------------------------------
The smart switch / stupid router (or no router) problem
is something that
happens on switched ethernet networks only.
If you do not have a
switched ethernet network, then you do not have this
problem. If you do
have a switched ethernet network, then you may have
this problem if you
are using newer switching hardware. The reason
is that ethernet
switching hardware is smart enough to monitor IGMP
traffic and only
forward multicast ethernet frames to those ports that
are connected to a
host that has maintained the appropriate IGMP conversations
with the
router.
At a very high level, IGMP works like this. First,
the host joins the
multicast group by sending the router an IGMP message.
The router
responds periodically with request to the host to
see if the host is
still interested in multicast traffic. Since
IGMP conversations are
handled transparently by the kernel level IP stack
implementations, most
developers and users do not even realize anything
is happening.
However, "smart" ethernet switches do realize something
is happening!
If they do not see the IGMP messages being sent from
the router to a host
that is plugged into a given port of the switch, then
they will will not
forward multicast ethernet frames to that port.
This is good and bad.
It is good because it makes multicast extremely efficient
in terms of
physical network usage. However, it also makes
it so multicast will not
work at all if a router does not exist (or does not
support IGMP) to
maintain it's end of the IGMP conversation.
Trouble Shooting:
Monitor IGMP traffic! Make sure that periodic
IGMP traffic is happening
on your network. IGMP traffic can be monitored
on Linux (and many other
OSes) with the following command:
# tcpdump igmp
Issue this command before starting slpd. You
will notice that several
IGMP "report" messages are sent. The important
thing to look for a IGMP
"query" message from the router. If you do not
see the IGMP query
message from the router then you will soon find that
you will no longer
see any "report" messages either.
Another good test is to try to ping the multicast address
and see where
it is visable.
$ ping 239.255.255.253
Finally, the best advice is to read the normally untouched
section of
your ethernet switch manual that describes how the
switch handles
multicast. Stupid/inexpensive switches treat
multicast frames exactly
like broadcast frames which means that they are forwarded
to every port
of the switch. Smart/Expensive switches often
allow this behavior to be
configured. If you are on a network without
a router, then it is
possible that you might need to "dumb down" your switch.
Broken NIC driver
------------------
Some NICs do not support multicast operation, so the
driver does the
work by placing the NIC into permiscuous mode
(accept everything) then
the driver filters out what is not needed. The
problem with this is
that sometimes on a very busy ethernet, the NIC buffers
may not be able
to keep up with all the traffic and some frames will
be dropped. This
is normally not a problem since SLP is designed to
work on unreliable
physical networks, but if enough frames are dropped,
OpenSLP may not
be able to find DAs or other services. This
would result in erratic
behavior.
Q: I'm pretty new to SLP. I just installed the Open-SLP package.
It comes
with the SLP daemon(slpd). Is it the Directory agent?
A: OpenSLP consists of two binary pieces of software.
One is libslp.so and the
other is slpd.
libslp.so is the SLP API implementation library that
is linked to applications
that will be using SLP to register and locate services.
See
http://www.openslp.org/doc/html/ProgrammersGuide/index.html
for more information
on how this library can be used to write more intelligent
applications.
slpd is the SA/DA daemon. In otherwords, slpd
has two modes of operation. slpd
can be an SA or it can be an SA and a DA.
In SA only mode, slpd keeps track services that were
registered by applications
that link to libslp.so and call SLPReg(). As
an SA slpd responds appropriately
to multicast requests for location of services that
it has registrations for.
In SA/DA mode slpd additionally acts as an DA (directory
agent). The existence
of a DA is optional, but desireable on large networks
to reduce multicast
traffic. To enable DA functionality in slpd,
you will need to set the
.net.slp.isDA=true setting in the /etc/slp.conf file.
In deploying OpenSLP you should be aware of the following guidelines:
1 - Existence of a DA (directory agent) is optional
but beneficial on larger
networks to reduce the amount of multicast SLP traffic.
On smaller networks
where multicast traffic is not a problem it is simpler
to operate without a DA.
2 - As a general rule there should only be one DA per
IP network. Too many DAs
on the same network will cause a reduction in performance.
3 - With OpenSLP it is not required that ALL machines
run slpd however it is
highly recommended. This is because the OpenSLP
implementation uses slpd to
cache important information that would otherwise need
to be discovered via
multicast or broadcast. Therefore, for optimal
performance, slpd should run on
all machines as an SA with only a very few (usually
1) machine running slpd as a
DA.
4 - In deployments where it is not possible to run
slpd on all machines it is at
least REQUIRED that slpd run on ALL machines that
run software that calls
SLPReg() or SLPDeReg().