Does :::80 in netstat output means only ipv6 or ipv6+ipv4

ipv4ipv6networking

If I am listening on :::80, is it listening on all ipv6 or all ipv6+ipv4?

This is my netstat -tln:

tcp        0      0 :::8080                     :::*

Best Answer

A listening socket that is bound to ::, i.e. any address IPv6 address (INADDR6_ANY), may or may not also listen to connections using IPv4. This depends from several things:

  • Some operating systems are what is known as dual stack, and on those operating systems this depends from whether the IPV6_V6ONLY socket option is set on the listening socket (by the program that created the socket). Linux-based operating systems and FreeBSD are examples of such operating systems.

    The default behavior if the option is not explicitly set by a program is OS dependent. On Linux-based operating systems, for example, you can change this default by writing 0 or 1 to /proc/sys/net/ipv6/bindv6only.

  • Some other operating systems are not dual stack, and on those operating systems one cannot ever listen to both IPv6 and IPv4 with a single socket. OpenBSD is one such operating system.

On some operating systems, the output of netstat will tell you whether the socket is dual-stack. FreeBSD's netstat reports dual-stack sockets as tcp46 and udp46 in the first column of the output, for examples.

Thanks for your answer @Johan Myreen I want to improve this answer with examples.

I am testing the ipv6_only behavior with both values.

1.

cat /proc/sys/net/ipv6/bindv6only
0
nc -6 -l 80
#server started
# netstat
tcp6       0      0 :::80                   :::*                    LISTEN     
# nc client
nc localhost 80
test
# server response 
nc -6 -l 80
test
# from ipv6 now
 nc ::1 80
test ipv6
# server response
nc -6 -l 80
test ipv6

2.

cat /proc/sys/net/ipv6/bindv6only
1
# server started
nc -6 -l 80
# connect to ipv4
nc localhost 80
nc: connect to localhost port 80 (tcp) failed: Connection refused
# connect to ipv6 
nc ::1 80 
test ipv6 
# server respose
nc -6 -l 80
test ipv6

from above results we can see that value of /proc/sys/net/ipv6/bindv6only deciding the behaviour of ipv6 only or ipv6+ipv4

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