Re: [PATCH net] icmp: don't send out ICMP messages with a source address of 0.0.0.0

[David Ahern <dsahern@gmail.com>]

On 6/15/21 5:07 AM, Toke Høiland-Jørgensen wrote:
> When constructing ICMP response messages, the kernel will try to pick a
> suitable source address for the outgoing packet. However, if no IPv4
> addresses are configured on the system at all, this will fail and we end up
> producing an ICMP message with a source address of 0.0.0.0. This can happen
> on a box routing IPv4 traffic via v6 nexthops, for instance.
> 
> Since 0.0.0.0 is not generally routable on the internet, there's a good
> chance that such ICMP messages will never make it back to the sender of the
> original packet that the ICMP message was sent in response to. This, in
> turn, can create connectivity and PMTUd problems for senders. Fortunately,
> RFC7600 reserves a dummy address to be used as a source for ICMP
> messages (192.0.0.8/32), so let's teach the kernel to substitute that
> address as a last resort if the regular source address selection procedure
> fails.
> 
> Below is a quick example reproducing this issue with network namespaces:
> 
> ip netns add ns0
> ip l add type veth peer netns ns0
> ip l set dev veth0 up
> ip a add 10.0.0.1/24 dev veth0
> ip a add fc00:dead:cafe:42::1/64 dev veth0
> ip r add 10.1.0.0/24 via inet6 fc00:dead:cafe:42::2
> ip -n ns0 l set dev veth0 up
> ip -n ns0 a add fc00:dead:cafe:42::2/64 dev veth0
> ip -n ns0 r add 10.0.0.0/24 via inet6 fc00:dead:cafe:42::1
> ip netns exec ns0 sysctl -w net.ipv4.icmp_ratelimit=0
> ip netns exec ns0 sysctl -w net.ipv4.ip_forward=1
> tcpdump -tpni veth0 -c 2 icmp &
> ping -w 1 10.1.0.1 > /dev/null
> tcpdump: verbose output suppressed, use -v[v]... for full protocol decode
> listening on veth0, link-type EN10MB (Ethernet), snapshot length 262144 bytes
> IP 10.0.0.1 > 10.1.0.1: ICMP echo request, id 29, seq 1, length 64
> IP 0.0.0.0 > 10.0.0.1: ICMP net 10.1.0.1 unreachable, length 92
> 2 packets captured
> 2 packets received by filter
> 0 packets dropped by kernel
> 
> With this patch the above capture changes to:
> IP 10.0.0.1 > 10.1.0.1: ICMP echo request, id 31127, seq 1, length 64
> IP 192.0.0.8 > 10.0.0.1: ICMP net 10.1.0.1 unreachable, length 92

We should capture this use case in a test script. There is already an
icmp_redirect.sh; how about starting an icmp.sh script?

> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")

This should be the one that allows IPv6 nexthops with IPv4 routes.

Fixes: d15662682db2 ("ipv4: Allow ipv6 gateway with ipv4 routes")

> Reported-by: Juliusz Chroboczek <jch@irif.fr>
> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
> ---
>  include/uapi/linux/in.h | 3 +++
>  net/ipv4/icmp.c         | 7 +++++++
>  2 files changed, 10 insertions(+)
> 
> diff --git a/include/uapi/linux/in.h b/include/uapi/linux/in.h
> index 7d6687618d80..d1b327036ae4 100644
> --- a/include/uapi/linux/in.h
> +++ b/include/uapi/linux/in.h
> @@ -289,6 +289,9 @@ struct sockaddr_in {
>  /* Address indicating an error return. */
>  #define	INADDR_NONE		((unsigned long int) 0xffffffff)
>  
> +/* Dummy address for src of ICMP replies if no real address is set (RFC7600). */
> +#define	INADDR_DUMMY		((unsigned long int) 0xc0000008)
> +
>  /* Network number for local host loopback. */
>  #define	IN_LOOPBACKNET		127
>  
> diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c
> index 7b6931a4d775..752e392083e6 100644
> --- a/net/ipv4/icmp.c
> +++ b/net/ipv4/icmp.c
> @@ -759,6 +759,13 @@ void __icmp_send(struct sk_buff *skb_in, int type, int code, __be32 info,
>  		icmp_param.data_len = room;
>  	icmp_param.head_len = sizeof(struct icmphdr);
>  
> +	/* if we don't have a source address at this point, fall back to the
> +	 * dummy address instead of sending out a packet with a source address
> +	 * of 0.0.0.0
> +	 */
> +	if (!fl4.saddr)
> +		fl4.saddr = htonl(INADDR_DUMMY);
> +
>  	icmp_push_reply(&icmp_param, &fl4, &ipc, &rt);
>  ende:
>  	ip_rt_put(rt);
> 

Reviewed-by: David Ahern <dsahern@kernel.org>