[Novalug] observations of CentOS6 route and ifconfig

Walt Smith waltechmail@yahoo.com
Fri Dec 16 12:14:48 EST 2016




some observations of CentOS6 route and ifconfig
based on past performance:

In years past, I would do a ifconfig and usually get 
at least one, often 2 interfaces showing.  One was lo.
Now, for years, this was every the time.  IOW, 
* something * was always there.


With CentOS6, The route table will be totally empty
while interfaces eth0 and ppp0 were down.

This was a surprise the first time.   Afer some while,
the pattern I noticed was that if I brought up eth0,
then lo came up -or- if I brought up ppp, then lo
also came up.  Taking down the interfaces would make
lo go away.

Some posts below area bit of cut/paste, sorry.
Any obvious errors, I'll answer best I can.


thx,
Walt..........

------------------------------



[root@core-bltm-69-72-61-124 CentOS-6.8]# route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface

[root@core-bltm-69-72-61-124 CentOS-6.8]# ## dialup using kppp at gui
[root@core-bltm-69-72-61-124 CentOS-6.8]# ifconfig eth0

[root@core-bltm-69-72-61-124 CentOS-6.8]# route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
209.163.113.81  0.0.0.0         255.255.255.255 UH    0      0        0 ppp0
192.168.1.0     0.0.0.0         255.255.255.0   U     0      0        0 eth0
0.0.0.0         0.0.0.0         0.0.0.0         U     0      0        0 ppp0


[root@core-bltm-69-72-61-124 CentOS-6.8]# ifconfig ppp0 down
[root@core-bltm-69-72-61-124 CentOS-6.8]# route del default 
[root@core-bltm-69-72-61-124 CentOS-6.8]# ifconfig eth0 down

###   no interfaces brought up
[root@core-bltm-69-72-61-124 CentOS-6.8]# ifconfig
[root@core-bltm-69-72-61-124 CentOS-6.8]#



[root@core-bltm-69-72-61-124 CentOS-6.8]# ifconfig
### Today, I took down 3 interfaces but was still left with lo.
###  This is the first time I've seen this.

[root@core-bltm-69-72-61-124 CentOS-6.8]# ifconfig
lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:49555 errors:0 dropped:0 overruns:0 frame:0
          TX packets:49555 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:4178887 (3.9 MiB)  TX bytes:4178887 (3.9 MiB)


[root@core-bltm-69-72-61-124 CentOS-6.8]# ifconfig ppp0 up

[root@core-bltm-69-72-61-124 CentOS-6.8]# route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
209.163.113.81  *               255.255.255.255 UH    0      0        0 ppp0

[root@core-bltm-69-72-61-124 CentOS-6.8]# route add default ppp0

[root@core-bltm-69-72-61-124 CentOS-6.8]# route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
mgmt-apx01.bltm *               255.255.255.255 UH    0      0        0 ppp0
default         *               0.0.0.0         U     0      0        0 ppp0

[root@core-bltm-69-72-61-124 CentOS-6.8]# ifconfig eth0 up

[root@core-bltm-69-72-61-124 CentOS-6.8]# route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
mgmt-apx01.bltm *               255.255.255.255 UH    0      0        0 ppp0
192.168.1.0     *               255.255.255.0   U     0      0        0 eth0
default         *               0.0.0.0         U     0      0        0 ppp0

[root@core-bltm-69-72-61-124 CentOS-6.8]# ping 204.255.212.1 ## to real internet thru ppp0 
PING 204.255.212.1 (204.255.212.1) 56(84) bytes of data.
^C
--- 204.255.212.1 ping statistics ---
4 packets transmitted, 0 received, 100% packet loss, time 3300ms


[root@core-bltm-69-72-61-124 CentOS-6.8]# 

### it's usually at this point that a "route" command would wait a VERY 
long time before displaying route information. (At the momoent, this is
not the case, but if frequently happens.  Very frequently.  So I need
to know what route/ifconfig util statements make it happen.  But more,
why ?   At the moment, a broken route command would display not differently
than what I've shown.   Hopefully, next time I can recreate better.
But ow it's documents and maybe someone else will have seen or know about it.

Analogy... OK, it's not..... but still...
kinda reminds of the undocuments bugs in my 200  4  Niss     an..
If I sit and rev the engine up or down slightly,  there are spots
in the engine RPM gauge I can make not move:  software.
IF I modulate the gas and brake slightly ( push on both pedals at the
same time or with slight timing differences, I can make the transmission
drop out entirely, and then it tries to re-engage with a rather powerful
"bang"... which could damage it.   Indeed I think thats one casue of
why I needed a replacement at one time.  Again, transmission defect.
Maybe software.  But bug not posted anywhere I can find.


Walt..........


----
 The government is lawless, not the press (people).
 ( [Supreme Court] Justice Douglas re: The Pentagon Papers )


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