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@xxxxxxxxx>
> ---
> 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
>
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