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From: Stefano Brivio <sbrivio@redhat.com>
To: David Gibson <david@gibson.dropbear.id.au>
Cc: passt-dev@passt.top
Subject: Re: [PATCH 1/7] udp: Don't attempt to translate a 0.0.0.0 source address
Date: Thu, 22 Feb 2024 18:46:02 +0100	[thread overview]
Message-ID: <20240222184602.554e3f5e@elisabeth> (raw)
In-Reply-To: <20240221232115.1376333-2-david@gibson.dropbear.id.au>

On Thu, 22 Feb 2024 10:21:09 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> If an incoming packet has a source address of 0.0.0.0 we translate that to
> the gateway address.  This doesn't really make sense, because we have no
> way to do a reverse translation for reply packets.

Well, we would translate that back to a loopback address, which is fine
if we take 0.0.0.0 as "This host on this network". Actually, after my
previous note based on RFC 6890, I went and had a look at RFC 1122,
section 3.2.1.3, which also states that 0.0.0.0:

  MUST NOT be sent, except as a source address as part of an
  initialization procedure by which the host learns its own IP address.

...so I guess dropping it here is fine.

By the way, I added this originally as part of commit 6488c3e8489d
("tcp, udp: Replace loopback source address by gateway address") on the
basis that 0.0.0.0 could be used in lieu of a loopback address, but
sure, we shouldn't even get it from the kernel to start with.

> Certain UDP protocols do use an unspecified source address in some
> circumstances (e.g. DHCP).  These generally either require no reply, a
> multicast reply, or provide a suitable reply address by other means.
> 
> In none of those cases does translating it in passt/pasta make sense.  The
> best we can really do here is just leave it as is.
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  udp.c | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/udp.c b/udp.c
> index a3961bfd..d2f8027c 100644
> --- a/udp.c
> +++ b/udp.c
> @@ -599,7 +599,6 @@ static size_t udp_update_hdr4(const struct ctx *c, int n, in_port_t dstport,
>  	    src_port == 53) {
>  		b->iph.saddr = c->ip4.dns_match.s_addr;
>  	} else if (IN4_IS_ADDR_LOOPBACK(&b->s_in.sin_addr) ||
> -		   IN4_IS_ADDR_UNSPECIFIED(&b->s_in.sin_addr)||
>  		   IN4_ARE_ADDR_EQUAL(&b->s_in.sin_addr, &c->ip4.addr_seen)) {
>  		b->iph.saddr = c->ip4.gw.s_addr;
>  		udp_tap_map[V4][src_port].ts = now->tv_sec;

-- 
Stefano


  reply	other threads:[~2024-02-22 17:47 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2024-02-21 23:21 [PATCH 0/7] Assorted small fixes for UDP David Gibson
2024-02-21 23:21 ` [PATCH 1/7] udp: Don't attempt to translate a 0.0.0.0 source address David Gibson
2024-02-22 17:46   ` Stefano Brivio [this message]
2024-02-23  4:03     ` David Gibson
2024-02-21 23:21 ` [PATCH 2/7] udp: Set pif in epoll reference for ephemeral host sockets David Gibson
2024-02-21 23:21 ` [PATCH 3/7] udp: Unconditionally clear act flag in udp_timer_one() David Gibson
2024-02-27 11:56   ` Stefano Brivio
2024-02-28  1:54     ` David Gibson
2024-02-21 23:21 ` [PATCH 4/7] udp: Separate bound sockets from flags David Gibson
2024-02-27 11:56   ` Stefano Brivio
2024-02-28  2:30     ` David Gibson
2024-02-28  3:53       ` David Gibson
2024-02-21 23:21 ` [PATCH 5/7] udp: Small streamline to udp_update_hdr4() David Gibson
2024-02-21 23:21 ` [PATCH 6/7] udp: Fix incorrect usage of IPv6 state in IPv4 path David Gibson
2024-02-21 23:21 ` [PATCH 7/7] udp: Remove unnecessary test for unspecified addr_out David Gibson

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