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Network monitoring tools ss, lsof -i, netstat -ltupw and similar have limitations like namespace and thus they don't show the whole machine connections. (for namespace created with ip we can see other connections with ip -all netns exec ss).

The kernel state the connections here /proc/net/tcp[udp][raw] and namespace connections can be seen here /proc/$pid/net/tcp[udp][raw] (detail)

What are the other possible connections that are not seen by ss/lsof/netstat and how to monitor them?

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    I guess that a start of an answer would be 1. connection outside the kernel like Intel me 2. namespaces 3. other kernel modules that would use a particular interface...
    – intika
    Jun 3, 2019 at 13:03
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    Any connection created via raw sockets? the sockets will appear in the info obtained with sock_diag (as used by ss, etc), but not the "connections" an app may build on top of them. Your question is too open-ended, so I doubt that any adequate answer can be made. The only true way is to capture all the packets on the wire and look at them. Jun 3, 2019 at 13:22
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    @intika You likely know more than me but maybe this is helpful: 1) NFQUEUE can reveal connections as they happen, but it might be tricky to set up and not miss anything (OpenSnitch has a number of closed issues covering that part, although I'm not sure if there's anything about Layer2 there) 2) I'm guessing that kernel extensions are able to make unnoticed connections if they talk directly to NIC (not directly applicable on Linux, but I'm observing something like this on a Mac, where VirtualBox is able to bypass Little Snitch when in bridged mode). Jun 3, 2019 at 16:28
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    Very fast connections - those are not going to be visible. If you're doing something like watch -n1 ss you won't see connections established and closed within less than 1 second. Jun 3, 2019 at 18:12
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    @intika "Raw ip packets and raw ethernet packets both use the kernel socket and thus are visible to the listed tools" Neither ss, nor lsof, nor netstat knows anything abouts packets, raw or cooked. They only display info about sockets, gathered from ioctls, /proc files and (more recently) from the *diag netlink interface. Knowing that process X has a raw socket open won't tell you much about what it's using it for, unlike a tcp socket, which has a peer, a port, etc. Jun 3, 2019 at 18:42

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