[wplug] Redhat 9 Slow starting - sendmail / sm-client

[Poyner, Brandon]

Just to clarify, the loopback address is also important for routing.  If
your machine's eth0 interface has the IP address 192.168.0.10 and you
ssh from that machine to itself at 192.168.0.10, it will actually use
the lo0 interface.  It really wouldn't make much sense to send the
packets to the ethernet device anyway.  You can see this by examining
the kernel IP routing cache with 'netstat -nrC' in Linux.

Kernel IP routing cache
Source          Destination     Gateway         Flags   MSS Window  irtt
Iface
192.168.0.10    192.168.0.1     192.168.0.1            1500 0          0
eth0
192.168.0.10    192.168.0.10    192.168.0.10    l     16436 0          0
lo
127.0.0.1       127.0.0.1       127.0.0.1       l     16436 0          0
lo

Loopback addresses can be bound to by services that you wish only to be
available locally, since there should be no external route or device
associated to them. 

There are other types of loopbacks that are not associated with
networking, such as loopback block devices.

Brandon Poyner
Network Engineer II
CCAC - College Office
412-237-3086


-----Original Message-----
From: Vanco, Don [mailto:don.vanco at agilysys.com] 
Sent: Wednesday, December 10, 2003 8:57 AM
To: 'wplug at wplug.org'
Subject: RE: [wplug] Redhat 9 Slow starting - sendmail / sm-client


Eric wrote:
>> From: wplug-admin at wplug.org [mailto:wplug-admin at wplug.org] On Behalf
>> Of Vanco, Don Sent: Tuesday, December 09, 2003 1:22 AM
>> Subject: RE: [wplug] Redhat 9 Slow starting - sendmail / sm-client
>> 
>> ...
>> It has been my experience in the past that simply rearranging
>> the first line
>> in /etc/hosts to read:
>> 127.0.0.1        localhost     localhost.localdomain
>> ....resolves the slow startup (it is not, in fact, "hung") issue with
>> sendmail, yet no one has ever been able to explain to me why
>> this might
>> work.  This has been true since RH6.2
>> 
> 
> Sweet!  I changed it up like this:
> $ cat /etc/hosts
> 127.0.0.1     localhost  localhost.localdomain  butanone
> 192.168.1.100 acetone $

	Note the other posters' comment re: using the hostname in the
loopback line (127.0.0.1 is referred to as the "loopback" device and is
in
place to allow binding of clients, protocols, applications, etc without
using a "real" network connection - it's essential for other network
things
to function in most cases)  If you're using a static IP (or an IP as
reserved in your DHCP server) for this machine you'd be best to create a
line entry for it in /etc/hosts.

> it booted up with no delay.  could you point me to an authoritative
> guide to dns that resides online?  or would i be better off picking
> up a copy of the o'reilly dns/bind tome?
	I have that book and it's good, but don't read it after a warm
glass
of milk or a turkey sammich as you may never come to.  Someone else
might
have a suggestion for another printed volume that's more "fun" - but you
might be best served by looking at resources on line (read: the linux
documentation project).

Don
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