wireshark enabled "promisc" mode but ifconfig displays not

edit

There's promiscuous mode and there's promiscuous mode.

Originally, the only way to enable promiscuous mode on Linux was to turn on the IFF_PROMISC flag on the interface; that flag showed up in the output of command such as ifconfig.

In the 2.2 kernel (i.e., a long time ago), a second mechanism was added; that mechanism does not set the IFF_PROMISC flag, so the interface being in promiscuous mode does not show up in the output of ifconfig, and it does not require promiscuous mode to be turned off manually - closing the last descriptor on which promiscuous mode was requested suffices.

libpcap uses the second mechanism if it's available; tcpdump and Wireshark both use libpcap to do packet capturing, so they'll use the second mechanism on any Linux system with a 2.2 or later kernel. Therefore, neither tcpdump nor Wireshark will, when capturing in promiscuous mode, cause ifconfig to show "PROMISC".

However:

• If you're capturing on an Ethernet that's on a switched network, promiscuous mode isn't sufficient to capture other machine's traffic, because that traffic probably isn't going to be sent to your switch port; see the Wireshark Wiki's page on Ethernet capture setup for more information;
• If you're capturing on Wi-Fi, promiscuous mode might not do anything at all - you'd need to capture in monitor mode, and set up Wireshark to be able to decrypt traffic if it's a "protected" network using WEP, WPA, WPA2, or WPA3. See the Wireshark Wiki's page on Wi-Fi capture setup for information on monitor mode and the Wireshark Wiki's "how to decrypt 802.11" page for information on that topic.