[Dnsmasq-discuss] [RESEND PATCH] Only bind IPv6 wildcard when it is enabled

[Geert Stappers]

On Thu, Jan 21, 2021 at 02:20:58PM +0100, Pali Rohár wrote:
> Hello!
> 
> On Thursday 21 January 2021 14:00:10 Matthias May wrote:
> > On Linux when IPv6 is disabled, one would not expect Dnsmasq to bind
> > the IPv6 wildcard.
> 
> And it is a problem? I would expect that if IPv6 is disabled then
> applications (including dnsmasq) cannot bind to the IPv6 address for
> listening... Or can applications bind to IPv6 address also when IPv6 is
> disabled? I just have not caught what is the problem (from description).

Our mailinglist archive has some extra information.  Check 2020-07-06.

How to integrate that into the patch description
is indeed a good question.

 
> > This patch adds a condition to the wildcard bind function, which checks
> > on Linux if IPv6 is disabled.
> > 
> > Signed-off-by: Matthias May <matthias.may at westermo.com>
> > Signed-off-by: Zefir Kurtisi <zefir.kurtisi at westermo.com>
> > ---
> >  src/dnsmasq.h |  1 +
> >  src/network.c | 24 +++++++++++++-----------
> >  src/util.c    | 22 ++++++++++++++++++++++
> >  3 files changed, 36 insertions(+), 11 deletions(-)
> > 
> > diff --git a/src/dnsmasq.h b/src/dnsmasq.h
> > index 4220798..2cfb1cb 100644
> > --- a/src/dnsmasq.h
> > +++ b/src/dnsmasq.h
> > @@ -1292,6 +1292,7 @@ int read_write(int fd, unsigned char *packet, int size, int rw);
> >  void close_fds(long max_fd, int spare1, int spare2, int spare3);
> >  int wildcard_match(const char* wildcard, const char* match);
> >  int wildcard_matchn(const char* wildcard, const char* match, int num);
> > +int is_ipv6_disabled(void);
> >  #ifdef HAVE_LINUX_NETWORK
> >  int kernel_version(void);
> >  #endif
> > diff --git a/src/network.c b/src/network.c
> > index c7d002b..0d35fb7 100644
> > --- a/src/network.c
> > +++ b/src/network.c
> > @@ -990,19 +990,21 @@ void create_wildcard_listeners(void)
> > 
> >    l = create_listeners(&addr, !!option_bool(OPT_TFTP), 1);
> > 
> > -  memset(&addr, 0, sizeof(addr));
> > +  if (!is_ipv6_disabled()) {
> > +    memset(&addr, 0, sizeof(addr));
> >  #ifdef HAVE_SOCKADDR_SA_LEN
> > -  addr.in6.sin6_len = sizeof(addr.in6);
> > +    addr.in6.sin6_len = sizeof(addr.in6);
> >  #endif
> > -  addr.in6.sin6_family = AF_INET6;
> > -  addr.in6.sin6_addr = in6addr_any;
> > -  addr.in6.sin6_port = htons(daemon->port);
> > -
> > -  l6 = create_listeners(&addr, !!option_bool(OPT_TFTP), 1);
> > -  if (l)
> > -    l->next = l6;
> > -  else
> > -    l = l6;
> > +    addr.in6.sin6_family = AF_INET6;
> > +    addr.in6.sin6_addr = in6addr_any;
> > +    addr.in6.sin6_port = htons(daemon->port);
> > +
> > +    l6 = create_listeners(&addr, !!option_bool(OPT_TFTP), 1);
> > +    if (l)
> > +      l->next = l6;
> > +    else
> > +      l = l6;
> > +  }
> > 
> >    daemon->listeners = l;
> >  }
> > diff --git a/src/util.c b/src/util.c
> > index 5f13027..5cd461f 100644
> > --- a/src/util.c
> > +++ b/src/util.c
> > @@ -787,6 +787,28 @@ int wildcard_matchn(const char* wildcard, const char* match, int num)
> >    return (!num) || (*wildcard == *match);
> >  }
> > 
> > +#ifndef HAVE_LINUX_NETWORK
> > +/* implement for other platforms */
> > +int is_ipv6_disabled(void)
> > +{
> > +	return 0;
> > +}
> > +#else /* HAVE_LINUX_NETWORK */
> > +int is_ipv6_disabled(void)
> > +{
> > +	FILE *f;
> > +	char *fname = "/proc/sys/net/ipv6/conf/all/disable_ipv6";
> > +	char buf[4];
> > +	int ipv6_disabled = 0;
> > +	if ((f = fopen(fname, "r"))) {
> > +		if (fgets(buf, 4, f))
> > +			ipv6_disabled = atoi(buf) == 1;
> > +		fclose(f);
> > +	}
> > +	return ipv6_disabled;
> 
> This check is incorrect. If IPv6 support is disabled at kernel compile
> time then fopen() fails and this function returns 0, meaning IPv6 is
> enabled.
 
I'm looking forward to see what other say about.

And I say:  Thanks for reviewing.



> > +}
> > +#endif /* HAVE_LINUX_NETWORK */
> > +
> >  #ifdef HAVE_LINUX_NETWORK
> >  int kernel_version(void)
> >  {
> > -- 
> > 2.27.0
> > 


Regards
Geert Stappers
-- 
Silence is hard to parse