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IDS Forum

Re: How to Monitor Open Connection Tiime???

Posted By: Fernando Nunes
Date: Wednesday, 1 December 2010, at 8:07 p.m.

In Response To: Re: How to Monitor Open Connection Tiime??? (Cesar Inacio Martins)

On Wed, Dec 1, 2010 at 6:01 PM, Cesar Inacio Martins <
cesar_inacio_martins@yahoo.com.br> wrote:

> Hi Fernando!
> Thanks for your message!
> Sorry take to long to answer, I don't know why the last 2 days of
> messages incoming from IIUG has arrived just now.
>
> So, about the gethostbyaddr() , I'm not sure about that because isn't
> what we see on the strace .
>

I wrote a simple program in Fedora, to test your situation.... Several
interesting things came up. They're not very helpful, but the prove Informix
innocence :)

On strace you will not see the reference to gethostbyaddr. Only in a
debugger or if you force a stack trace in the precise moment...

> The request of DNS reverse lookup, appear to be executed "manually" by
> Informix or the strace just traced the gethostbyaddr() too.
> Check the output marked with "##" by me.
> where: 172.18.0.57 and 172.18.0.119 are the OLD DNS when the server was
> started. When we run this strace, the resolv.conf already have new
> values (for at least 2 days):
>
> 11:24:28 munmap(0x2abeb6d75000, 4096) = 0
> 11:24:28 socket(PF_INET, SOCK_DGRAM, IPPROTO_IP) = 3
> ##11:24:28 connect(3, {sa_family=AF_INET, sin_port=htons(53),
> sin_addr=inet_addr("172.18.0.57")}, 28) = 0
> 11:24:28 fcntl(3, F_GETFL) = 0x2 (flags O_RDWR)
> 11:24:28 fcntl(3, F_SETFL, O_RDWR|O_NONBLOCK) = 0
> 11:24:28 poll([{fd=3, events=POLLOUT}], 1, 0) = 1 ([{fd=3,
> revents=POLLOUT}])
> ##11:24:28 sendto(3,
> "\242;\1\0\0\1\0\0\0\0\0\0\003185\00248\00222\003172\7in-ad"..., 44,
> MSG_NOSIGNAL, NULL, 0) = 44
> ##11:24:28 poll([{fd=3, events=POLLIN}], 1, 5000) = 0 (Timeout)
> 11:24:33 socket(PF_INET, SOCK_DGRAM, IPPROTO_IP) = 4
> 11:24:33 connect(4, {sa_family=AF_INET, sin_port=htons(53),
> sin_addr=inet_addr("172.18.0.119")}, 28) = 0
> 11:24:33 fcntl(4, F_GETFL) = 0x2 (flags O_RDWR)
> .... they continue trying to third DNS server...
> (check the timestamp,.. 5 seconds of timeout delay)
>
> Before you ask, the Linux Admin already try change the timeout (option
> into resolv.conf) and don't have effect....
>
>
2nd interesting observation... I put the program in loop... reading an IP
from the console and trying to reverse DNS it.
Between loops I changed my resolve.conf.... It didn't matter for the running
process. I belive gethostbyaddr creates some static structures. First time
it runs it reads the resolv.conf, but it doesn't happen again... Probably
due to performance reasons. But it's inconvenient in your situation....:

open("/etc/resolv.conf", O_RDONLY) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=55, ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) =
0xb776e000
read(3, "# Generated by NetworkManager\nna"..., 4096) = 55
read(3, "", 4096) = 0
close(3)

This is why it keeps trying the old DNS...

Sorry , yes is a 11.50 FC7 GE over Linux Red Hat 5.5
> This option to create a group is nice, but I not sure if is viable for
> this environment because have a lot of applications spread on the
> company (.net/windows, java/web, C/Linux) what we will need care about
> to change the SQLHOSTS...
>
> The nsswitch.conf is ok, it isn't modified from they default values...
> and the problem is with connections what become from out of our network
> (what don't exists into /etc/hosts) and create consequences to local
> connections (what exists in hosts) when the instance get trouble to try
> resolve their hostnames (not local clients)
>

Are there many client IPs from outside the network? If the number is low you
could put them in /etc/hosts...

>
> And the problem what we detected is over MSC, because is it what freeze...
> The most weird thing is, when the MSC "freeze" (stuck running in active
> threads: onstat -g act) they stack dump (onstat -g stk #threadid) don't
> change anything, still showing in yield process...and just change the
> status from sleep to running....
>
>
Now... none of these are good news. I really can't see a good solution
besides restart... You're a victim of how the TCP/IP name resolution works
Apparently nscd will not work also, because gethostbyaddr only tries it the
first time...

Some other thoughts:

1- You could launch more msc VPs. The new ones should pick up the correct
DNS. And before you ask, no, you cannot remove MSC VPs...
But with more MSC VPs your chances of getting stuck should be lower
2- (this is very weird...) You could use IPTABLEs to hijack connections to
the OLD DNS and redirect them to the new one... Not sure if this is
possible, but you would have to create a rule for the old IP, port 53 and
protocol UDP.
3- You could "hack" the network to make the old IP available. This could be
done by putting a machine on the network with that IP, or eventually by
hacking the ARP table
4- you could create another TCP interface on the machine (how?) with the IP
of the old DNS server. I tried putting my own IP on the /etc/resolv.conf
file and the reverse DNS returns to "quick" (although I don't have anything
listening on port 53)...
The problem you're facing comes from the fact that the IP cannot be reached.
If you make it go to an existing IP it will not resolv, but it will be
quick.

Naturally 2, 3 and 4 are "crazy" suggestions... But I don't see any "sane"
alternative to an instance stop.

Regards.

--
Fernando Nunes
Portugal

http://informix-technology.blogspot.com
My email works... but I don't check it frequently...

--000e0cd1e1646c4c6404966309a2

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