Loopback interface missing

Martin.B · 09-24-2019, 07:44 PM

Ping to 127.0.0.1 times out, ifconfig doesn't have lo. How can I get it back?

System setup: Ubuntu 14.04. Two network cards, one goes to the house network and from there to the internet, one goes to local hardware.

Code:

sudo ifup lo
ifup: interface lo already configured

Code:

ifconfig lo
lo: error fetching interface information: Device not found

Doesn't help. Things I tried: ifup lo, change /etc/network/interfaces (see commented out lines there, below), mess with 70-persistent-net.rules as suggested here, reboot of course, connect another network card via USB to look for changes, no success. The problem looks similar to this old thread but there ifup lo recovered the loopback.

Things worked months ago, the problem might have been caused by some update.

One more observation: Normally trying to ping 127.0.0.1 just times out, but if I disconnect the cable going to the house network I get "network is unreachable". It seems to look for 127.0.0.1 in the local network where it doesn't find it of course.

ifconfig:

Code:

eth4      Link encap:Ethernet  HWaddr 00:24:e8:45:8f:10  
          inet6 addr: fe80::224:e8ff:fe45:8f10/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:180 errors:0 dropped:0 overruns:0 frame:0
          TX packets:16 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:35088 (35.0 KB)  TX bytes:3183 (3.1 KB)
          Interrupt:21 Memory:f7fe0000-f8000000 

eth5      Link encap:Ethernet  HWaddr d8:eb:97:71:be:e5  
          inet addr:192.168.153.124  Bcast:192.168.153.255  Mask:255.255.255.0
          inet6 addr: fe80::daeb:97ff:fe71:bee5/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:149995 errors:0 dropped:0 overruns:0 frame:0
          TX packets:37251 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:48465215 (48.4 MB)  TX bytes:5040261 (5.0 MB)

ifconfig -a finds some additional device I don't understand:

Code:

ttyUSB0   Link encap:Local Loopback  
          LOOPBACK  MTU:65536  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

/etc/network/interfaces:

Code:

auto lo
 iface lo inet loopback
# iface lo inet static
# address 127.0.0.1
# netmask 255.0.0.0

jefro · 09-26-2019, 04:07 PM

Hello and welcome to LQ.

This makes me think it is OK.
"sudo ifup lo
ifup: interface lo already configured"

Try ifconfig -a instead maybe.

Might be worth using the ip command now. https://www.cyberciti.biz/faq/linux-...-usage-syntax/

Ser Olmy · 09-26-2019, 04:12 PM

Quote:

Originally Posted by Martin.B

Ping to 127.0.0.1 times out, ifconfig doesn't have lo. How can I get it back?

This statement really threw me off, because the loopback interface appears automagically when IP support is loaded (or compiled into the kernel). One can certainly mess with the IP address assigned to the loopback i/f, but how could it be gone and IP support still be there?

Quote:

Originally Posted by Martin.B

System setup: Ubuntu 14.04. Two network cards, one goes to the house network and from there to the internet, one goes to local hardware.

Code:

sudo ifup lo
ifup: interface lo already configured

Ah, so the interface is present. Phew!

Quote:

Originally Posted by Martin.B

Code:

ifconfig lo
lo: error fetching interface information: Device not found

...but it isn't called "lo" anymore...?

Quote:

Originally Posted by Martin.B

ifconfig -a finds some additional device I don't understand:

Code:

ttyUSB0   Link encap:Local Loopback  
          LOOPBACK  MTU:65536  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

You know, I think you just found your loopback interface.

How on earth it came to be renamed to "ttyUSB0", I have no idea, but I'm willing to bet good money it has something to do with your udev configuration.

My guess is that at some point, udev decided it would be a splendid idea to ifrename your loopback interface and turn it into "ttyUSB0".

Do you by any chance have any USB peripherals connected?

Martin.B · 09-26-2019, 04:47 PM

This was stuck in moderation for a bit, I found a solution in the meantime. It looks like lo was renamed, but I don't know why. Thanks for the answers.

Code:

ip link set TTYUSB0 up

solved the problem.

Quote:

Originally Posted by Ser Olmy

Do you by any chance have any USB peripherals connected?

Some lab devices.

Ser Olmy · 09-26-2019, 05:30 PM

Quote:

Originally Posted by Martin.B

Some lab devices.

They wouldn't happen to be usb-to-serial interfaces, by any chance? For control or data acquisition?

Compare the udev ruleset with the output from lsusb. I wouldn't be at all surprised if a manufacturer of niche equipment decided to play fast and loose with USB IDs and ended up creating a conflict. I've seen enough hardware reporting "0000" as IDs and serial numbers in my time.

Martin.B · 09-27-2019, 03:46 AM

There is one USB to serial interface, but it doesn't use 0000. There is a rule calling the serial interface TTYUSB0, but I don't understand why it plays loopback at the same time.
Anyway, my problem has been solved.