Putting a network interface up/down from command line
In Xubuntu 14.04, I tried to use both ip and ifconfig to handle a network interface, but they gave the same result.
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sudo ifconfig wlan0 down sudo ip link set wlan0 down |
both correcly put down the interface and the connectivity does not work; but then
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sudo ifconfig wlan0 up sudo ip link set wlan up |
did not restore the connectivity!
This is the output of ip link show after putting the interface down:
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wlan0: <BROADCAST,MULTICAST> mtu 1500 qdisc mq state DOWN group default qlen 1000 link/ether <my_MAC_address> brd ff:ff:ff:ff:ff:ff inet 192.168.1.29/24 brd 192.168.1.255 scope global wlan0 valid_lft forever preferred_lft forever |
and this is the output after putting the interface up:
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wlan0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc mq state DOWN group default qlen 1000 link/ether <my_MAC_address> brd ff:ff:ff:ff:ff:ff inet 192.168.1.29/24 brd 192.168.1.255 scope global wlan0 valid_lft forever preferred_lft forever |
So it has no carrier and I can’t access the web, but it has an IP!
1) Why? Shouldn’t the up command restore the previous situation? I had to turn off and on the physical switch of the wireless board to browse again the web. I also tried with dhclient -r wlan0 and dhclient wlan0, but the result was that neither the physical switch was useful and I had to restart the whole system.
2) Even after putting the interface down, the GUI connectivity icon was active and a connection to the wireless Access-Point was normally shown (even if no webpages were actually available). Why?
1)This answer is only for a wired connection. Use ifup and ifdown, not ifconfig, unless you want to manually specify the parameters. ifup will look at your network configuration in /etc/network/interfaces (or wherever your distro puts it).
2)If you’re using wpa for your wifi, you cannot connect to it with just ifup; a helper application must be configured and running (usually wpa_supplicant). Usually your distro will provide you with network scripts to perform these tasks automatically.