Re: TCP?

Karl Pearson <karlp@xxxxxxxxxxxxxxxx> · Thu, 16 Oct 2008 13:23:21 -0600 (MDT)

On Thu, 16 Oct 2008, Rick Stevens wrote:

Karl Pearson wrote:

On Thu, October 16, 2008 12:05 pm, Rick Stevens wrote:

Karl Pearson wrote:

On Thu, October 16, 2008 11:31 am, Rick Stevens wrote:

I should have mentioned that you'll need to "service sshd restart" after
making the change to sshd_config.  You'll need to restart sendmail as
well if you change its config.

And if you're asking "why doesn't it fail when I specify 'localhost'?"
remember that localhost is in your /etc/hosts file.  When you "ssh
localhost" or "telnet localhost 25", the host AND CLIENT IPs are
127.0.0.1 which corresponds to "localhost" in /etc/hosts.

I try to give full explanations when I post and I missed it that time.
Sorry!

I had already tried DNS settings. I edited nsswitch.conf and changed it 

to

files first, then dns, with this line:
hosts:      files dns [NOTFOUND=return] files

and made sure the IPs are in /etc/hosts, which they already were. One 

email

went out with alpine from another PC, but nothing more and it took just
about
long enough to time out. Watching it in ps ax showed

startup with 172.20.20.100
then
cmd read: 172.20.20.100

but all the other ones show the startup line only, then when it times out 

on

the client, it changes from the IP to the entry in the hosts file. It's 

like

something is preventing things from moving. I'm wondering if I have a bad
cable or switch.

I am going to try sshd_config right now and see if that works. Remember,
this

is the box that got duplicate libs which I had to manually delete. There 

are

a
few duplicates again. It may not be related....

Okay, changing UseDNS no didn't help at all. Still taking a couple 

minutes

to
connect via ssh.

Ok, then check your default routes, "netstat -rn" and verify that the
one with "UG" is indeed your default gateway.

It checks out okay.

Okey doke.  Just checking.  With updates and such (and that goddamn
POS called "NetworkManager"), anything's possible.

I disable NetworkMangler on my servers at install time.

I'm not sure how, but it works fine. Let me 'splain...

In checking for things in /etc/mail/sendmail.mc and /etc/mail/submit.mc I
found that one can just type

make (or make -C /etc/mail)

in /etc/mail and submit.mc will be transferred to submit.cf as if m4 had 

been

run. Same with sendmail.mc

Actually, sendmail.cf is the output of m4 after processing sendmail.mc
(and others).

Yes, nice, huh? Why doesn't sendmail.cf actually act like a .conf file one 

can just modify? Like I haven't manually modified them for years anyway :)

I've also seen that using make restart works on some distros, and so tried 

it,

and that works, too.

Most of them to a "cd /etc/mail;make" before they actually start
sendmail (either in a "restart" or a "start" scenario).

Yes, but new to me.

In any case, that's not the apparent solution, which IS DNS related, but 

not

in the way you or I thought. I changed the line in sendmail.mc:

FEATURE(`accept_unresolvable_domains')dnl

to

dnl FEATURE(`accept_unresolvable_domains')dnl

and things started working again as they had several days ago. Now then,

please understand that nothing had been changed on the server at all. 

Nothing.

That's why I still think there may be a hardware issue somewhere. The 

feature

above has been enabled since the system was installed 7 weeks ago.

Yes, anything that's to be put into the sendmail.cf must begin with
"dnl".  Things without "dnl" are directives to m4 itself.

dnl is the starting line of a not-to-be-used feature, so adding dnl turned 

it off. Or am I missing something?

BTW, m4 is a right pain in the arse...why sendmail.org decided to standardize 

on it is beyond my comprehension.

Yes it is. Which is why I was happy to see that 'make' does the same thing 

now.

As far as DNS issues are concerned...well, there's much to check.
Verify that /etc/resolv.conf has the actual DNS servers in it and verify
you can ping them from the host in question.  In some cases, your router
does DNS for you via a proxy.

I had checked that stuff before emailing the list. I should have mentioned 

it. The solution isn't a real solution as some email still times out, but 

the majority is able to be delivered to port 25

Next, verify that you have TCP and UDP port 53 open so DNS queries can
be handled, both in the iptables config and in your external firewall
(amazing how many of them block DNS).  You can use "related,established"
in your iptables rules if you are concerned about security.

DNS is open. I do my own DNS and my ISP 'secondary's my changes on a 

schedule based on serial. iptables is set to related,established already. 

That's been in place for a few years now across server upgrades.

But, like email sending, ssh is now slow again... But not as slow as 

before. I'm still leaning toward hardware being the issue, though I 

haven't been able to track it down yet.

One thing I just tried for grins and giggles is to switch nsswitch.conf 

from:

hosts: dns files
to
hosts: files dns

and things seem much faster again. But, that was at dns files forever 

(okay, seven weeks on this server).

Karl

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