Cannot enable Promiscuous Mode for network device

felixspechtiosb-inaf · January 24, 2020, 3:00pm

Hello, i would like to enable the promiscuous mode for the ‘eth0’ network device. I tried this with the following commands: 1) ip link set eth0 promisc on 2) ifconfig eth0 promisc Verificiation via ip a shows it is not set. After reboot of the system a log message via dmesg shows device eth0 entered promiscuous mode, but it is still not activated for the device. Is there any way to activate the mode? Version is 2019.9 Best regards, fspecht

Martin_PLCnext_Techn · January 28, 2020, 4:15pm

Hi fspecht, I am not sure how to check for certain that the interface is in promiscuous mode … but perhaps it is? Here is what I tried:

[](mailto:admin@axcf2152:~$)[admin@axcf2152:~$](mailto:admin@axcf2152:~$) su  
> Password:   
> [](mailto:root@axcf2152:/opt/plcnext/#)[root@axcf2152:/opt/plcnext/#](mailto:root@axcf2152:/opt/plcnext/#) ip a  
> 1: lo:  mtu 65536 qdisc noqueue qlen 1000  
>  link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00  
>  inet 127.0.0.1/8 scope host lo  
>  valid_lft forever preferred_lft forever  
>  inet6 ::1/128 scope host   
>  valid_lft forever preferred_lft forever  
> 2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq qlen 1000  
>  link/ether 00:a0:45:9c:a1:13 brd ff:ff:ff:ff:ff:ff  
>  inet 192.168.178.221/24 brd 192.168.178.255 scope global eth0  
>  valid_lft forever preferred_lft forever  
>  inet6 fe80::2a0:45ff:fe9c:a113/64 scope link   
>  valid_lft forever preferred_lft forever  
> 3: [](mailto:sit0@NONE)[sit0@NONE>:  mtu 1480 qdisc noop qlen 1000  
>  link/sit 0.0.0.0 brd 0.0.0.0  
> 4: docker0:  mtu 1500 qdisc noqueue   
>  link/ether 02:42:ec:9d:25:76 brd ff:ff:ff:ff:ff:ff  
>  inet 172.17.0.1/16 brd 172.17.255.255 scope global docker0  
>  valid_lft forever preferred_lft forever  
> ](mailto:sit0@NONE</a)[](mailto:root@axcf2152:/opt/plcnext/#)[root@axcf2152:/opt/plcnext/#](mailto:root@axcf2152:/opt/plcnext/#)   
> [](mailto:root@axcf2152:/opt/plcnext/#)[root@axcf2152:/opt/plcnext/#](mailto:root@axcf2152:/opt/plcnext/#) ifconfig eth0  
> eth0 Link encap:Ethernet HWaddr 00:A0:45:9C:A1:13   
>  inet addr:192.168.178.221 Bcast:192.168.178.255 Mask:255.255.255.0  
>  inet6 addr: fe80::2a0:45ff:fe9c:a113/64 Scope:Link  
>  UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1  
>  RX packets:12102 errors:0 dropped:38 overruns:0 frame:0  
>  TX packets:2344 errors:0 dropped:0 overruns:0 carrier:0  
>  collisions:0 txqueuelen:1000   
>  RX bytes:1225350 (1.1 MiB) TX bytes:329862 (322.1 KiB)  
>  Interrupt:29 Base address:0xe000

[](mailto:root@axcf2152:/opt/plcnext/#)[root@axcf2152:/opt/plcnext/#](mailto:root@axcf2152:/opt/plcnext/#) ifconfig eth0 promisc  
> [](mailto:root@axcf2152:/opt/plcnext/#)[root@axcf2152:/opt/plcnext/#](mailto:root@axcf2152:/opt/plcnext/#) ip a  
> 1: lo:  mtu 65536 qdisc noqueue qlen 1000  
>  link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00  
>  inet 127.0.0.1/8 scope host lo  
>  valid_lft forever preferred_lft forever  
>  inet6 ::1/128 scope host   
>  valid_lft forever preferred_lft forever  
> 2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP100> mtu 1500 qdisc mq qlen 1000  
>  link/ether 00:a0:45:9c:a1:13 brd ff:ff:ff:ff:ff:ff  
>  inet 192.168.178.221/24 brd 192.168.178.255 scope global eth0  
>  valid_lft forever preferred_lft forever  
>  inet6 fe80::2a0:45ff:fe9c:a113/64 scope link   
>  valid_lft forever preferred_lft forever  
> 3: [](mailto:sit0@NONE)[sit0@NONE>:  mtu 1480 qdisc noop qlen 1000  
>  link/sit 0.0.0.0 brd 0.0.0.0  
> 4: docker0:  mtu 1500 qdisc noqueue   
>  link/ether 02:42:ec:9d:25:76 brd ff:ff:ff:ff:ff:ff  
>  inet 172.17.0.1/16 brd 172.17.255.255 scope global docker0  
>  valid_lft forever preferred_lft forever  
> ](mailto:sit0@NONE</a)[](mailto:root@axcf2152:/opt/plcnext/#)[root@axcf2152:/opt/plcnext/#](mailto:root@axcf2152:/opt/plcnext/#) ifconfig eth0  
> eth0 Link encap:Ethernet HWaddr 00:A0:45:9C:A1:13   
>  inet addr:192.168.178.221 Bcast:192.168.178.255 Mask:255.255.255.0  
>  inet6 addr: fe80::2a0:45ff:fe9c:a113/64 Scope:Link  
>  UP BROADCAST RUNNING PROMISC MULTICAST MTU:1500 Metric:1  
>  RX packets:12358 errors:0 dropped:38 overruns:0 frame:0  
>  TX packets:2399 errors:0 dropped:0 overruns:0 carrier:0  
>  collisions:0 txqueuelen:1000   
>  RX bytes:1251098 (1.1 MiB) TX bytes:339883 (331.9 KiB)  
>  Interrupt:29 Base address:0xe000

You can see that the ifconfig command indicates that the interface is in promiscuous mode, and the ip a command - although not indicating this explicitly, like you would expect - does change its status from LOWER_UP to LOWER_UP100. I have been unable to find out what “LOWER_UP100” means, but it seems to have something to do with the change to promiscuous mode. Hope this helps. ~ Martin.