[PATCH] net: dns: Generate and verify transaction ID

[Jules Maselbas]

Hi,

I would like some feedback on how to select a dns_req_id.
Although ths is likely not very critical to barebox, I think using both
dns_timer_start plus random32 is a bit overkill. Maybe simply using
random is sufficient.

On Thu, May 05, 2022 at 12:08:05PM +0200, Jules Maselbas wrote:
> The transaction ID wasn't verified on received DNS responses, plus the
> ID needs to be difficult to predict in order to avoid MitM (man in the
> middle) being able to easily forge responses.
> 
> Signed-off-by: Jules Maselbas <jmaselbas at kalray.eu>
> ---
>  net/dns.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/net/dns.c b/net/dns.c
> index 78588b96f..9ad316e33 100644
> --- a/net/dns.c
> +++ b/net/dns.c
> @@ -58,6 +58,7 @@ struct header {
>  
>  static struct net_connection *dns_con;
>  static uint64_t dns_timer_start;
> +static uin32_t dns_req_id;
>  static int dns_state;
>  static IPaddr_t dns_ip;
>  
> @@ -70,9 +71,12 @@ static int dns_send(const char *name)
>  	unsigned char *p, *s, *fullname, *dotptr;
>  	const unsigned char *domain;
>  
> +	/* generate a random transaction id */
> +	dns_req_id = dns_timer_start + random32();
I am wondering if using only one of dns_timer_start or randome32 is
sufficient on its own. For the record musl uses clock_gettime without
random at all.

>  	/* Prepare DNS packet header */
>  	header           = (struct header *)packet;
> -	header->tid      = 1;
> +	header->tid      = htons(dns_req_id);
>  	header->flags    = htons(0x100);	/* standard query */
>  	header->nqueries = htons(1);		/* Just one query */
>  	header->nanswers = 0;
> @@ -127,6 +131,10 @@ static void dns_recv(struct header *header, unsigned len)
>  
>  	pr_debug("%s\n", __func__);
>  
> +	/* Only accept responses with the expected request id */
> +	if (ntohs(header->id) != dns_req_id)
> +		return;
> +
>  	/* We sent 1 query. We want to see more that 1 answer. */
>  	if (ntohs(header->nqueries) != 1)
>  		return;
> -- 
> 2.17.1
>