Getting a socket's 'struct sock' information

gistfile1.txt

You can get even more detailed information about a socket from kernel's internal socket struct.
For example, we will try to get nginx listening (on port 81) socket's backlog length

NOTE: You may need to install kernel debug info if not already installed
# yum-config-manager --enable "amzn-main-debuginfo" --enable "amzn-updates-debuginfo"
# yum -y install kernel-debuginfo kernel-devel

Or get socket info for listening socket on port 81
# ss -len | grep :81
tcp    LISTEN     0      511       *:81                    *:*                   ino:29842919 sk:55 <->

Get its sk buff address
# grep 29842919 /proc/net/tcp
   8: 00000000:0051 00000000:0000 0A 00000000:00000000 00:00000000 00000000     0        0 29842919 1 ffff8800e9d1da00 100 0 0 10 0

Now get the details, for example, the backlog length
# gdb /usr/lib/debug/lib/modules/`uname -r`/vmlinux /proc/kcore
................
................
Reading symbols from /usr/lib/debug/lib/modules/4.4.51-40.69.amzn1.x86_64/vmlinux...done.
[New process 1]
Core was generated by `root=LABEL=/ console=tty1 console=ttyS0 selinux=0 LANG=en_US.UTF-8 KEYTABLE=us'.
................
(gdb) set print pretty on
(gdb) p *(struct sock *)0xffff8800e9d1da00
................
................
  sk_ack_backlog = 0,
  sk_max_ack_backlog = 511,
................
................

It seems kafka (and some other processes) are listening on ipv6 addresses (on ports 9092,9093 and 9094) (as you had noted), as shown by

lafolle@yoss:~/Downloads/confluent-3.3.0$ ss -lnt
State       Recv-Q Send-Q                                                        Local Address:Port                                                                       Peer Address:Port
LISTEN      0      128                                                                       *:25672                                                                                 *:*
LISTEN      0      80                                                                127.0.0.1:3306                                                                                  *:*
LISTEN      0      128                                                                       *:5355                                                                                  *:*
LISTEN      0      1024                                                              127.0.0.1:11211                                                                                 *:*
LISTEN      0      128                                                                       *:4369                                                                                  *:*
LISTEN      0      128                                                                       *:22                                                                                    *:*
LISTEN      0      5                                                                 127.0.0.1:631                                                                                   *:*
LISTEN      0      1024                                                                      *:15672                                                                                 *:*
LISTEN      0      100                                                               127.0.0.1:25                                                                                    *:*
LISTEN      0      50                                                                       :::9092                                                                                 :::*
LISTEN      0      50                                                                       :::9093                                                                                 :::*
LISTEN      0      50                                                                       :::2181                                                                                 :::*
LISTEN      0      50                                                                       :::9094                                                                                 :::*
LISTEN      0      50                                                                       :::26471                                                                                :::*
LISTEN      0      50                                                                       :::17639                                                                                :::*
LISTEN      0      128                                                                      :::5672                                                                                 :::*
LISTEN      0      128                                                                      :::5355                                                                                 :::*
LISTEN      0      511                                                                      :::80                                                                                   :::*
LISTEN      0      128                                                                      :::4369                                                                                 :::*
LISTEN      0      50                                                                       :::11027                                                                                :::*
LISTEN      0      128                                                                      :::22                                                                                   :::*
LISTEN      0      5                                                                       ::1:631                                                                                  :::*
LISTEN      0      100                                                                     ::1:25                                                                                   :::*
LISTEN      0      50                                                                       :::12447                                                                                :::*
LISTEN      0      128                                                                      :::12865                                                                                :::*

inode info for sockets on ipv6 are found in /proc/net/tcp6 as opposed to /proc/net/tcp.

lafolle@yoss:~/Downloads/confluent-3.3.0$ cat /proc/net/tcp6
  sl  local_address                         remote_address                        st tx_queue rx_queue tr tm->when retrnsmt   uid  timeout inode
   0: 00000000000000000000000000000000:2384 00000000000000000000000000000000:0000 0A 00000000:00000000 00:00000000 00000000  1000        0 133817 1 0000000000000000 100 0 0 10 0
   1: 00000000000000000000000000000000:2385 00000000000000000000000000000000:0000 0A 00000000:00000000 00:00000000 00000000  1000        0 134614 1 0000000000000000 100 0 0 10 0
   2: 00000000000000000000000000000000:0885 00000000000000000000000000000000:0000 0A 00000000:00000000 00:00000000 00000000  1000        0 134159 1 0000000000000000 100 0 0 10 0
   3: 00000000000000000000000000000000:2386 00000000000000000000000000000000:0000 0A 00000000:00000000 00:00000000 00000000  1000        0 133980 1 0000000000000000 100 0 0 10 0
   4: 00000000000000000000000000000000:6767 00000000000000000000000000000000:0000 0A 00000000:00000000 00:00000000 00000000  1000        0 133974 1 0000000000000000 100 0 0 10 0
   5: 00000000000000000000000000000000:44E7 00000000000000000000000000000000:0000 0A 00000000:00000000 00:00000000 00000000  1000        0 134489 1 0000000000000000 100 0 0 10 0
   6: 00000000000000000000000000000000:1628 00000000000000000000000000000000:0000 0A 00000000:00000000 00:00000000 00000000   132        0 27118 1 0000000000000000 100 0 0 10 0
   7: 00000000000000000000000000000000:14EB 00000000000000000000000000000000:0000 0A 00000000:00000000 00:00000000 00000000   102        0 26182 1 0000000000000000 100 0 0 10 0
   8: 00000000000000000000000000000000:0050 00000000000000000000000000000000:0000 0A 00000000:00000000 00:00000000 00000000     0        0 24507 1 0000000000000000 100 0 0 10 0
   9: 00000000000000000000000000000000:1111 00000000000000000000000000000000:0000 0A 00000000:00000000 00:00000000 00000000   132        0 29266 1 0000000000000000 100 0 0 10 0
  10: 00000000000000000000000000000000:2B13 00000000000000000000000000000000:0000 0A 00000000:00000000 00:00000000 00000000  1000        0 134154 1 0000000000000000 100 0 0 10 0
  11: 00000000000000000000000000000000:0016 00000000000000000000000000000000:0000 0A 00000000:00000000 00:00000000 00000000     0        0 48605 1 0000000000000000 100 0 0 10 0
  12: 00000000000000000000000001000000:0277 00000000000000000000000000000000:0000 0A 00000000:00000000 00:00000000 00000000     0        0 68754 1 0000000000000000 100 0 0 10 0
  13: 00000000000000000000000001000000:0019 00000000000000000000000000000000:0000 0A 00000000:00000000 00:00000000 00000000     0        0 27378 1 0000000000000000 100 0 0 10 0
  14: 00000000000000000000000000000000:309F 00000000000000000000000000000000:0000 0A 00000000:00000000 00:00000000 00000000  1000        0 133843 1 0000000000000000 100 0 0 10 0
  15: 00000000000000000000000000000000:3241 00000000000000000000000000000000:0000 0A 00000000:00000000 00:00000000 00000000     0        0 33243 1 0000000000000000 100 0 0 10 0
  16: 0000000000000000FFFF00000100007F:7158 0000000000000000FFFF00000101007F:2386 08 00000000:00000001 02:00022FB2 00000000  1000        0 133982 2 0000000000000000 20 4 28 10 -1
  17: 0000000000000000FFFF00000100007F:0885 0000000000000000FFFF00000100007F:7790 01 00000000:00000000 00:00000000 00000000  1000        0 135623 1 0000000000000000 20 4 31 10 -1
  18: 0000000000000000FFFF00000100007F:0885 0000000000000000FFFF00000100007F:7788 01 00000000:00000000 00:00000000 00000000  1000        0 130845 1 0000000000000000 20 4 31 10 -1
  19: 0000000000000000FFFF00000100007F:7788 0000000000000000FFFF00000100007F:0885 01 00000000:00000000 00:00000000 00000000  1000        0 132599 1 0000000000000000 20 4 30 10 -1
  20: 0000000000000000FFFF00000100007F:7790 0000000000000000FFFF00000100007F:0885 01 00000000:00000000 00:00000000 00000000  1000        0 130016 1 0000000000000000 20 4 30 10 -1
  21: 0000000000000000FFFF00000100007F:81D6 0000000000000000FFFF00000101007F:2385 08 00000000:00000001 02:00022C8A 00000000  1000        0 132624 2 0000000000000000 20 4 28 10 -1
  22: 0000000000000000FFFF00000100007F:573A 0000000000000000FFFF00000101007F:2384 08 00000000:00000001 02:00022B2E 00000000  1000        0 132603 2 0000000000000000 20 4 28 10 -1
  23: 0000000000000000FFFF00000100007F:0885 0000000000000000FFFF00000100007F:778C 01 00000000:00000000 00:00000000 00000000  1000        0 129993 1 0000000000000000 20 4 31 10 -1
  24: 0000000000000000FFFF00000100007F:778C 0000000000000000FFFF00000100007F:0885 01 00000000:00000000 00:00000000 00000000  1000        0 136261 1 0000000000000000 20 4 30 10 -1

Here the inode of socket in which we're interested in is present, but corresponding address (?) of socket is 0 (and hence socket can't be accessed using gdb.)

So, the problem, I think, boils down to "why socket address (?) is 0 for all active sockets in /proc/net/ipv6?".
But interestingly socket address is 0 also for all sockets in /proc/net/tcp.