netstat showing multiple lines for each listening socket

[Thomas Hungenberg]

Hello,

we have been running some BIND nameservers on Debian-based systems for many years.

Until (including) Debian 10 with BIND 9.11.5, netstat always showed only one line
per listening socket, e.g.

tcp        0      0 10.x.x.x:53             0.0.0.0:*               LISTEN      1234/named
tcp        0      0 127.0.0.1:53            0.0.0.0:*               LISTEN      1234/named
udp        0      0 10.x.x.x:53             0.0.0.0:*                           1234/named
udp        0      0 127.0.0.1:53            0.0.0.0:*                           1234/named


We noticed that with Debian 11 and 12 (BIND 9.16.48 / 9.18.24), netstat instead
shows multiple (on some systems four, on others up to 20) completely identical lines
for each listening socket, like this:

tcp        0      0 10.x.x.x:53             0.0.0.0:*               LISTEN      1234/named
tcp        0      0 10.x.x.x:53             0.0.0.0:*               LISTEN      1234/named
tcp        0      0 10.x.x.x:53             0.0.0.0:*               LISTEN      1234/named
tcp        0      0 10.x.x.x:53             0.0.0.0:*               LISTEN      1234/named
tcp        0      0 127.0.0.1:53            0.0.0.0:*               LISTEN      1234/named
tcp        0      0 127.0.0.1:53            0.0.0.0:*               LISTEN      1234/named
tcp        0      0 127.0.0.1:53            0.0.0.0:*               LISTEN      1234/named
tcp        0      0 127.0.0.1:53            0.0.0.0:*               LISTEN      1234/named
udp        0      0 10.x.x.x:53             0.0.0.0:*                           1234/named
udp        0      0 10.x.x.x:53             0.0.0.0:*                           1234/named
udp        0      0 10.x.x.x:53             0.0.0.0:*                           1234/named
udp        0      0 10.x.x.x:53             0.0.0.0:*                           1234/named
udp        0      0 127.0.0.1:53            0.0.0.0:*                           1234/named
udp        0      0 127.0.0.1:53            0.0.0.0:*                           1234/named
udp        0      0 127.0.0.1:53            0.0.0.0:*                           1234/named
udp        0      0 127.0.0.1:53            0.0.0.0:*                           1234/named


We wonder what is causing this and if this is intended behaviour?


    - Thomas