Re: bringing /etc/services up to date

On Jul 7, 2004, at 2:13 PM, Matthew D. Fuller wrote:
> On Wed, Jul 07, 2004 at 10:25:58AM -0700 I heard the voice of
> Brooks Davis, and lo! it spake thus:
[ ...with regard to Colin's proposal to make /etc/services much 
larger... ]
>> Can you check how much this change slows down inetd startup with a
>> few services enabled?  The traditional argument against this is that
>> reading the whole IANA service file takes too long.  If the
>> difference isn't measurable, the the argument is bogus, but I'm not
>> sure that's the case.

Oh, the difference is easy to measure: a trivial program which calls 
getservbyport() for the first 1000 ports takes 2.17 seconds to run with 
the 73K /etc/services file.  Using a 106K services file from nmap-3.51 
takes 2.95 seconds.

Using a 587K /etc/services file from 
http://www.iana.org/assignments/port-numbers gives a time of 16.85 
seconds, so the lookup time seems to be closely linear to the file 
size, about 2.8 ms per lookup per 100K worth of /etc/services file, at 
least on the machine I was using to test.  :-)

>> The alternative solution would be to add
>> optional database backing.  That shouldn't be too hard to do, and
>> there are several examples to work from.
>
> In theory, any program reading the data should be using
> getservby{name,port}() and friends, since it avoids code duplication
> and handles NIS and such already.  So, hashing it into a DB and fixing
> those functions to match would probably work.

Matthew is exactly correct, programs should be using getservby*().

Lots of systems implement some external caching system (nscd, lookupd, 
etc) which those library calls access rather than iterating through the 
/etc/services file and related sources directly within each process 
which calls getservby*().  I'm not sure whether the sources for 
Solaris' nscd are handy, but I believe the sources for lookupd and 
friends are at:

http://developer.apple.com/darwin/projects/opendirectory/

-- 
-Chuck

PS: Sources for the trivial program mentioned above:

#include <netdb.h>

int main(int argc, char *argv[]) {
   int port;
   struct servent *se;

   for (port = 1; port <= 1000; port++) {
     se = getservbyport(port, "tcp");
   }
}