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Date:   Thu, 25 Apr 2019 22:34:24 +0200
From:   Willy Tarreau <w@1wt.eu>
To:     Marco Davids <mdavids@forfun.net>
Cc:     linux-kernel@vger.kernel.org
Subject: Re: How to turn off IPv4 without disabling IPv6
Message-ID: <20190425203424.GA14855@1wt.eu>
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On Thu, Apr 25, 2019 at 06:42:52PM +0200, Marco Davids wrote:
> Op Thu, Apr 25, 2019 at 13:22, Nico Schottelius wrote:
> > if I cannot turn off IPv4, I cannot test what needs to be fixed.
> 
> You know what? I actually agree with Nico on this.
> 
> It's 2019 and the adoption of IPv6 is actually gaining momentum (at last).
> 
> This is absolutely the time to seriously start thinking about unbundling
> IP-stacks the kernel, so that IPv4 can be truly disabled at compile time.
> 
> That will allow for further testing and fixes, just as Nico suggests.

While I can understand the value in doing this, I think that there's
much more value in being able to disable it at run time, precisely
because if you have to reboot to a different kernel for each and every
minor application issue you meet, it will take ages before you converge
to something usable.

Probably that for such tests instead you should use a sysctl to allow/deny
IPv4 socket creation. It should be more than enough for program validation.
Something like the following code (not even compile-tested) could possibly
be sufficient.

Just my two cents,
Willy

-------------

diff --git a/include/net/netns/ipv4.h b/include/net/netns/ipv4.h
index 104a666..aa9ac80 100644
--- a/include/net/netns/ipv4.h
+++ b/include/net/netns/ipv4.h
@@ -83,6 +83,8 @@ struct netns_ipv4 {
 	struct xt_table		*nat_table;
 #endif
 
+	int sysctl_disable;
+
 	int sysctl_icmp_echo_ignore_all;
 	int sysctl_icmp_echo_ignore_broadcasts;
 	int sysctl_icmp_ignore_bogus_error_responses;
diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
index eab3ebd..0784c41 100644
--- a/net/ipv4/af_inet.c
+++ b/net/ipv4/af_inet.c
@@ -255,6 +255,9 @@ static int inet_create(struct net *net, struct socket *sock, int protocol,
 	int try_loading_module = 0;
 	int err;
 
+	if (net->ipv4.sysctl_disable)
+		return -EAFNOSUPPORT;
+
 	if (protocol < 0 || protocol >= IPPROTO_MAX)
 		return -EINVAL;
 
diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index eeb4041..73a7ead 100644
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c
@@ -555,6 +555,13 @@ static struct ctl_table ipv4_table[] = {
 
 static struct ctl_table ipv4_net_table[] = {
 	{
+		.procname	= "disable",
+		.data		= &init_net.ipv4.sysctl_disable,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec
+	},
+	{
 		.procname	= "icmp_echo_ignore_all",
 		.data		= &init_net.ipv4.sysctl_icmp_echo_ignore_all,
 		.maxlen		= sizeof(int),