netdev - Re: [PATCH iproute2] iproute get: force rtm_dst

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Message-ID: <5a165b08-9394-6c64-efe7-2f141b498b76@gmail.com>
Date:   Sun, 24 Jan 2021 10:26:52 -0700
From:   David Ahern <dsahern@...il.com>
To:     Luca Boccassi <bluca@...ian.org>, netdev@...r.kernel.org
Cc:     stephen@...workplumber.org
Subject: Re: [PATCH iproute2] iproute get: force rtm_dst_len to 32/128

On 1/24/21 8:53 AM, Luca Boccassi wrote:
> Since NETLINK_GET_STRICT_CHK was enabled, the kernel rejects commands
> that pass a prefix length, eg:
> 
>  ip route get `1.0.0.0/1
>   Error: ipv4: Invalid values in header for route get request.
>  ip route get 0.0.0.0/0
>   Error: ipv4: rtm_src_len and rtm_dst_len must be 32 for IPv4

Those are not the best responses from the kernel for the mask setting. I
should have been clearer about src and dst masks.

> 
> Since there's no point in setting a rtm_dst_len that we know is going
> to be rejected, just force it to the right value if it's passed on
> the command line.
> 
> Bug-Debian: https://bugs.debian.org/944730
> Reported-By: Clément 'wxcafé' Hertling <wxcafe@...afe.net>
> Signed-off-by: Luca Boccassi <bluca@...ian.org>
> ---
> As mentioned by David on:
> 
> https://www.spinics.net/lists/netdev/msg624125.html
> 
>  ip/iproute.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/ip/iproute.c b/ip/iproute.c
> index ebb5f160..3646d531 100644
> --- a/ip/iproute.c
> +++ b/ip/iproute.c
> @@ -2069,7 +2069,12 @@ static int iproute_get(int argc, char **argv)
>  			if (addr.bytelen)
>  				addattr_l(&req.n, sizeof(req),
>  					  RTA_DST, &addr.data, addr.bytelen);
> -			req.r.rtm_dst_len = addr.bitlen;
> +			if (req.r.rtm_family == AF_INET)
> +				req.r.rtm_dst_len = 32;
> +			else if (req.r.rtm_family == AF_INET6)
> +				req.r.rtm_dst_len = 128;
> +			else
> +				req.r.rtm_dst_len = addr.bitlen;
>  			address_found = true;
>  		}
>  		argc--; argv++;
> 

Since the kernel used to blindly ignore the mask, having iproute2 fix it
up seems acceptable.

I think it would be good to educate the user about invalid settings as
well - get them to fix scripts and mind set.