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SubjectRe: How to turn off IPv4 without disabling IPv6
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),
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