On Thu, May 12, 2022 at 04:37:26PM +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.
>
> The ID is generated from the time of the request, probably not strongly
> unpredictable, this what musl does and it is considered to be enough.
>
> Signed-off-by: Jules Maselbas <jmaselbas@xxxxxxxxx>
> ---
> v2: fix the dns_req_id type to uint16_t, added pr_debug when incorrect id
> is received, drop uses of the random32.
>
> net/dns.c | 12 +++++++++++-
> 1 file changed, 11 insertions(+), 1 deletion(-)
Applied, thanks
Sascha
>
> diff --git a/net/dns.c b/net/dns.c
> index 78588b96f..8b5e8d59e 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 uint16_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 "difficult" to predict transaction id */
> + dns_req_id = dns_timer_start + (dns_timer_start >> 16);
> +
> /* 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 +131,12 @@ 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->tid) != dns_req_id) {
> + pr_debug("DNS response with incorrect id\n");
> + 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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