Re: 5.2.1 Trap Handler causes snmptrapd to exit from broken pipe

Michael Higgins wrote:

> Robert Story wrote:
>
>> On Fri, 04 Mar 2005 10:46:02 -0800 Michael wrote:
>> MH> Using a simple setup as described in MH>
>> http://www.net-snmp.org/tutorial/tut.../snmptrap.html,
>> when MH> a trap is received, the snmptrapd process fails from a
>> broken pipe. I MH> then use the same setup with 5.0.9 and everything
>> functions as designed.
>> MH> MH> I did a little digging and it looks like the problem is in
>> MH> agent/mibgroup/utilities/execute.c in run_exec_command(). I
>> don't know MH> a whole lot about how this is architected, but it
>> appears that two MH> threads are responsible for making this happen.
>> One gets the trap MH> handler process running and the other writes to
>> its stdin and waits for MH> any output.
>> MH> MH> What I see happening is the thread that is initiating the
>> trap handler MH> does some manipulation of file descriptors, closing
>> the standard ones MH> and redirecting them to pipes. When it does a
>> close(2), this function MH> never returns. (Is this stderr?)
>>
>> Yes, and it is odd that it doesn't return..
>>
>> MH> In the meantime, the other thread MH> marches ahead and attempts
>> to write to the pipe. This is where the MH> broken pipe happens and
>> the process terminates.
>>
>> Apparently snmpd handles this by ignoring SIGPIPE. Add this code in
>> snmptrapd.c, near the other signal handlers:
>>
>> #ifdef SIGPIPE
>> signal(SIGPIPE, SIG_IGN); /* 'Inline' failure of wayward readers */
>> #endif
>> MH> I didn't see it bugged either. Should it be?
>>
>> Yes, anything that causes the daemon to crash is very bad.
>>
>>
>>
> Thanks for the reply and suggestions. I can ignore SIGPIPE, but I'll
> also have to skip the close(2) call so that the trap handler can be
> started. I wonder what the effect of that might be?
>
> Mike
>
>

I added the code to ignore the SIGPIPE signal and found that I also had
to remove the close(2) to get to the execv() call that runs the trap
handler. In the snmptrapd.conf file for 5.0.9 I had:

traphandle IF-MIB::linkDown "/home/mhiggins/src/snmp/traps.sh" linkDown
traphandle IF-MIB::linkUp "/home/mhiggins/src/snmp/traps.sh" linkUp

This caused execv to fail with errno 2, no such file or directory. Thre
must be an error in tokenize_exec_command() because it was return the
command with the leading " still in place. When I changed the
snmptrapd.conf to:

traphandle IF-MIB::linkDown /home/mhiggins/src/snmp/traps.sh linkDown
traphandle IF-MIB::linkUp /home/mhiggins/src/snmp/traps.sh linkUp

The trap handler completed successfully. I'll look into that to see I
can find a fix. Now the only thing left at the moment is a formatting
issue. I have some OID values that are getting a linefeed erroneously
added at some point. I'm suspecting realloc_format_trap() in
apps/snmptrapd_log.c. I'll look at that too.

Thanks again for the suggestion. Any idea if removing close(2) might
have some sort of side effect? When I figure out the other two items
I'll submit a bug with suggested fixes.

-- Mike





-------------------------------------------------------
SF email is sponsored by - The IT Product Guide
Read honest & candid reviews on hundreds of IT Products from real users.
Discover which products truly live up to the hype. Start reading now.
http://ads.osdn.com/?ad_id=6595&alloc_id=14396&op=click
_______________________________________________
Net-snmp-users mailing list
Net-snmp-users@lists.sourceforge.net
Please see the following page to unsubscribe or change other options:
https://lists.sourceforge.net/lists/...net-snmp-users