Linux – How to disable IPv4-mapped IPv6

ipv6linux

On my Linux box, I have various daemons which can bind to all IPv6-enabled interfaces on ::. When they do so, Linux sends IPv4 requests to that daemon mapped as, for instance, ::ffff:198.51.100.37.

I would instead like IPv4 connections to be refused and only IPv6 connections accepted when a daemon binds to ::. To receive IPv4 connections, I want the daemon to have to explicitly bind to 0.0.0.0 (as well as ::).

In other words, I want to run a service exclusively on IPv6, and not on IPv4.

Is there a way to accomplish this?

Best Answer

This is controlled by the net.ipv6.bindv6only sysctl. Add the following to /etc/sysctl.conf and run sudo sysctl -p to effect the change.

net.ipv6.bindv6only=1

Applications can also explicitly only bind to the IPv6 address instead of changing this globally, for example, nginx has the ipv6only option to the listen directive. This corresponds to the IPV6_V6ONLY option to setsockopt().

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Linux Bash – How to Sort du -h Output by Size

As of GNU coreutils 7.5 released in August 2009, sort allows a -h parameter, which allows numeric suffixes of the kind produced by du -h:

du -hs * | sort -h

If you are using a sort that does not support -h, you can install GNU Coreutils. E.g. on an older Mac OS X:

brew install coreutils
du -hs * | gsort -h

From sort manual:

-h, --human-numeric-sort compare human readable numbers (e.g., 2K 1G)

Linux:/sbin/dhclient to bind to a specific interface

DHCP by definition works by the client using the broadcast address to find all responding DHCP servers. It then chooses one of them (based on an algorithm, and AFAIK this is mostly to do with response times) and asks for an address. This address is then bound to the interface that is being configured.

Again, by definition, broadcasts are sent on all network interfaces. So, unless you are willing to write your own DHCP client (the sources are freely available), there is not much you can do about that.

However, you could use a different port. Read the man page for dhclient for details. This would then also require a DHCP server that listens on that port. The standard ISC DHCP server supports this with the -p option. Again, read the man page for dhcpd.

Best Answer

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Linux:/sbin/dhclient to bind to a specific interface

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