如何找到TCP套接字的inode？

Question

How do I tie values in the 'inode' column of /proc/net/tcp to files in /proc/<pid>/fd/ ?

I was under the impression that the inode column in the TCP had a decimal representation of the socket's inode, but that doesn't seem to be the case.

For example, if I run telnet localhost 80 , I see the following (telnet is pid 9021).

/proc/net/tcp contains

sl  local_address rem_address   st tx_queue rx_queue tr tm->when retrnsmt   uid  timeout inode
23: 0100007F:CE2A 0100007F:0050 01 00000000:00000000 00:00000000 00000000  1000        0 361556 1 00000000 20 0 0 10 -1

which makes me think that the inode of the socket connected to 127.0.0.1:80 is 361556. But if I run ls --inode -alh /proc/9021/fd , I see

349886 lrwx------ 1 me me 64 Dec 26 10:51 3 -> socket:[361556]

The inode is 349886, which is different from the value in the inode column of the tcp table: 361556. But the link target seems to have the right name. Similarly, stat /proc/9021/3 shows:

File: ‘/proc/9021/fd/3’ -> ‘socket:[361556]’
Size: 64            Blocks: 0          IO Block: 1024   symbolic link
Device: 3h/3d   Inode: 349886      Links: 1

What's the number in the inode column of tcp table? Why doesn't it line up with the inode as reported by ls or stat ?

(I'm running Ubuntu 14.10, if that matters)

Answer 1

The inode shown by ls and stat is for the symlink that points to the inode associated with the socket. Running ls -iLalh shows the right inode. Ditto for stat -L .

Herpa derp derp. I only figured this out when I was composing my question. ;_;

Answer 2

Inode id represent a file id per fs mount (proc, sys, ntfs, ext...), so as you probably understand you deal with two different fs here: procfs and some pseudo socket fs.

The files under the /proc/pid/fd/ directories are soft links which have inode representation in the procfs fs. These links are "pointing" to different "fs" - socket fs.

What stat -L and ls -iLalh do, is to give you the inode of the file the links points to. You can do this also explicitly with readlink /proc/#pid/fd/#fdnum