From mboxrd@z Thu Jan  1 00:00:00 1970
From: Stephen Hemminger <stephen@networkplumber.org>
Subject: Re: [PATCHv2 net-next iproute2] gre: add support for ERSPAN tunnel
Date: Thu, 10 Aug 2017 15:12:13 -0700
Message-ID: <20170810151213.095a4505@xeon-e3>
References: <1502396067-21274-1-git-send-email-u9012063@gmail.com>
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Cc: netdev@vger.kernel.org, Meenakshi Vohra <mvohra@vmware.com>,
        Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
To: William Tu <u9012063@gmail.com>
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On Thu, 10 Aug 2017 13:14:27 -0700
William Tu <u9012063@gmail.com> wrote:

> +
> +		if (greinfo[IFLA_GRE_ERSPAN_INDEX])
> +			erspan_idx = rta_getattr_u32(greinfo[IFLA_GRE_ERSPAN_INDEX]);
>  	}

Are you missing a ntohl() here?

It is encoded as htonl() when sending to kernel.

 		addattr32(n, 1024, IFLA_GRE_FWMARK, fwmark);
+		addattr32(n, 1024, IFLA_GRE_ERSPAN_INDEX, htonl(erspan_idx));

Is erspan_idx == 0 a reserved value?
You should only send erspan_idx to kernel if it is given on command line.