Re: [PATCH v4 7/9] tcp: make tcp_rst_no_conn() respond with correct MAC address

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

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On Tue, Aug 19, 2025 at 11:10:03PM -0400, Jon Maloy wrote:
> tcp_rst_no_conn() needs to identify and specify which source MAC
> address to use when sending an RST to the guest. This is because
> it doesn't have access to any flow structure where this address
> could be fetched.

I'm not sure we actually need to preserve MAC in this case.  This
happens when the guest sends a TCP packet that doesn't appear to
belong to any existing flow.  That means that the guest is somehow
already out of sync with what's going on in the outside world.  So,
it's hard to imagine any scenario where the guest needs the correct
MAC of this peer that it already has a wrong idea of the state of.

> Signed-off-by: Jon Maloy <jmaloy@redhat.com>
> ---
>  tcp.c | 17 +++++++++++++++--
>  1 file changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/tcp.c b/tcp.c
> index bdcd477..28f9ef5 100644
> --- a/tcp.c
> +++ b/tcp.c
> @@ -309,6 +309,7 @@
>  #include "tcp_internal.h"
>  #include "tcp_buf.h"
>  #include "tcp_vu.h"
> +#include "netlink.h"
>  
>  #ifndef __USE_MISC
>  /* From Linux UAPI, missing in netinet/tcp.h provided by musl */
> @@ -1888,17 +1889,29 @@ static void tcp_rst_no_conn(const struct ctx *c, int af,
>  			    const struct tcphdr *th, size_t l4len)
>  {
>  	struct iov_tail payload = IOV_TAIL(NULL, 0, 0);
> +	unsigned char src_mac[ETH_ALEN];
> +	union inany_addr tgt;
>  	struct tcphdr *rsth;
>  	char buf[USHRT_MAX];
>  	uint32_t psum = 0;
>  	size_t rst_l2len;
> +	int ifi;
>  
>  	/* Don't respond to RSTs without a connection */
>  	if (th->rst)
>  		return;
>  
> +	/* Respond with true MAC address if remote host is on
> +	 * the template interface's network segment
> +	 */
> +	ifi = af == AF_INET ? c->ifi4 : c->ifi6;
> +	memcpy(src_mac, c->our_tap_mac, ETH_ALEN);
> +	inany_from_af(&tgt, af, daddr);
> +	if (!fwd_inany_nat(c, &tgt))
> +		nl_neigh_mac_get(nl_sock, &tgt, ifi, src_mac);
> +
>  	if (af == AF_INET) {
> -		struct iphdr *ip4h = tap_push_l2h(c, buf, NULL, ETH_P_IP);
> +		struct iphdr *ip4h = tap_push_l2h(c, buf, src_mac, ETH_P_IP);
>  		const struct in_addr *rst_src = daddr;
>  		const struct in_addr *rst_dst = saddr;
>  
> @@ -1908,7 +1921,7 @@ static void tcp_rst_no_conn(const struct ctx *c, int af,
>  					      *rst_src, *rst_dst);
>  
>  	} else {
> -		struct ipv6hdr *ip6h = tap_push_l2h(c, buf, NULL, ETH_P_IPV6);
> +		struct ipv6hdr *ip6h = tap_push_l2h(c, buf, src_mac, ETH_P_IPV6);
>  		const struct in6_addr *rst_src = daddr;
>  		const struct in6_addr *rst_dst = saddr;
>  

-- 
David Gibson (he or they)	| 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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