Re: [PATCH 3/3] udp: Check for answers to forwarded DNS queries before handling local redirects

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Nov 03, 2022 at 12:04:43AM +0100, Stefano Brivio wrote:
> Now that we allow loopback DNS addresses to be used as targets for
> forwarding, we need to check if DNS answers come from those targets,
> before deciding to eventually remap traffic for local redirects.
> 
> Otherwise, the source address won't match the one configured as
> forwarder, which means that the guest or the container will refuse
> those responses.
> 
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
> ---
>  udp.c | 17 ++++++++---------
>  1 file changed, 8 insertions(+), 9 deletions(-)
> 
> diff --git a/udp.c b/udp.c
> index 4b201d3..7c77e09 100644
> --- a/udp.c
> +++ b/udp.c
> @@ -680,8 +680,10 @@ static void udp_sock_fill_data_v4(const struct ctx *c, int n,
>  	src = ntohl(b->s_in.sin_addr.s_addr);
>  	src_port = ntohs(b->s_in.sin_port);
>  
> -	if (src >> IN_CLASSA_NSHIFT == IN_LOOPBACKNET ||
> -	    src == INADDR_ANY || src == ntohl(c->ip4.addr_seen)) {
> +	if (c->ip4.dns_fwd && src == htonl(c->ip4.dns[0]) && src_port == 53) {

I guess this is not a newly introduced bug, but for the case of
multiple host nameservers, don't you need to check against everything
in the ip4.dns[] array, not just entry 0?

> +		b->iph.saddr = c->ip4.dns_fwd;
> +	} else if (IPV4_IS_LOOPBACK(src) || src == INADDR_ANY ||
> +		   src == ntohl(c->ip4.addr_seen)) {
>  		b->iph.saddr = c->ip4.gw;
>  		udp_tap_map[V4][src_port].ts = now->tv_sec;
>  		udp_tap_map[V4][src_port].flags |= PORT_LOCAL;
> @@ -692,9 +694,6 @@ static void udp_sock_fill_data_v4(const struct ctx *c, int n,
>  			udp_tap_map[V4][src_port].flags |= PORT_LOOPBACK;
>  
>  		bitmap_set(udp_act[V4][UDP_ACT_TAP], src_port);
> -	} else if (c->ip4.dns_fwd &&
> -		   src == htonl(c->ip4.dns[0]) && src_port == 53) {
> -		b->iph.saddr = c->ip4.dns_fwd;
>  	} else {
>  		b->iph.saddr = b->s_in.sin_addr.s_addr;
>  	}
> @@ -770,6 +769,10 @@ static void udp_sock_fill_data_v6(const struct ctx *c, int n,
>  	if (IN6_IS_ADDR_LINKLOCAL(src)) {
>  		b->ip6h.daddr = c->ip6.addr_ll_seen;
>  		b->ip6h.saddr = b->s_in6.sin6_addr;
> +	} else if (!IN6_IS_ADDR_UNSPECIFIED(&c->ip6.dns_fwd) &&
> +		   IN6_ARE_ADDR_EQUAL(src, &c->ip6.dns[0]) && src_port == 53) {
> +		b->ip6h.daddr = c->ip6.addr_seen;
> +		b->ip6h.saddr = c->ip6.dns_fwd;
>  	} else if (IN6_IS_ADDR_LOOPBACK(src)			||
>  		   IN6_ARE_ADDR_EQUAL(src, &c->ip6.addr_seen)	||
>  		   IN6_ARE_ADDR_EQUAL(src, &c->ip6.addr)) {
> @@ -794,10 +797,6 @@ static void udp_sock_fill_data_v6(const struct ctx *c, int n,
>  			udp_tap_map[V6][src_port].flags &= ~PORT_GUA;
>  
>  		bitmap_set(udp_act[V6][UDP_ACT_TAP], src_port);
> -	} else if (!IN6_IS_ADDR_UNSPECIFIED(&c->ip6.dns_fwd) &&
> -		   IN6_ARE_ADDR_EQUAL(src, &c->ip6.dns[0]) && src_port == 53) {
> -		b->ip6h.daddr = c->ip6.addr_seen;
> -		b->ip6h.saddr = c->ip6.dns_fwd;
>  	} else {
>  		b->ip6h.daddr = c->ip6.addr_seen;
>  		b->ip6h.saddr = b->s_in6.sin6_addr;

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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