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I have a Netlink socket, subscribed to destroy events, by using bind with NF_NETLINK_CONNTRACK_DESTROY OR'd into the groups parameter.

When looking through the nfnetlink_compat.h, though, I see that there're more flags than just NEW, UPDATE, and DESTROY - there's an EXP_XYZ variant for each flag as well:

#define NF_NETLINK_CONNTRACK_NEW            0x00000001
#define NF_NETLINK_CONNTRACK_UPDATE         0x00000002
#define NF_NETLINK_CONNTRACK_DESTROY        0x00000004
#define NF_NETLINK_CONNTRACK_EXP_NEW        0x00000008
#define NF_NETLINK_CONNTRACK_EXP_UPDATE     0x00000010
#define NF_NETLINK_CONNTRACK_EXP_DESTROY    0x00000020

I can't find the EXP_XYZ flags documented anywhere I've looked. Where can I find more information on these flags?

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These flags exist to handle notifications about conntrack expectations, which are conntrack RELATED states that are stored in a table and handled by the kernel; you can be notified if an expectation is being added, deleted or modified. This table can be manipulated by user programs like conntrack-tools toolset but are used by kernel modules like the FTP and SIP helpers.

That said, I've never had to use this feature directly: it's usually handled automatically.

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