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    Subject[PATCH 47/98] netfilter: xt_TCPMSS: Fix violation of RFC879 in absence of MSS option
    Date
    3.5.7.17 -stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Phil Oester <kernel@linuxace.com>

    commit 409b545ac10d9548929557a75ad86540f59a2c83 upstream.

    The clamp-mss-to-pmtu option of the xt_TCPMSS target can cause issues
    connecting to websites if there was no MSS option present in the
    original SYN packet from the client. In these cases, it may add a
    MSS higher than the default specified in RFC879. Fix this by never
    setting a value > 536 if no MSS option was specified by the client.

    This closes netfilter's bugzilla #662.

    Signed-off-by: Phil Oester <kernel@linuxace.com>
    Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
    Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
    ---
    net/netfilter/xt_TCPMSS.c | 6 ++++++
    1 file changed, 6 insertions(+)

    diff --git a/net/netfilter/xt_TCPMSS.c b/net/netfilter/xt_TCPMSS.c
    index 71a266d..4960b66 100644
    --- a/net/netfilter/xt_TCPMSS.c
    +++ b/net/netfilter/xt_TCPMSS.c
    @@ -124,6 +124,12 @@ tcpmss_mangle_packet(struct sk_buff *skb,

    skb_put(skb, TCPOLEN_MSS);

    + /* RFC 879 states that the default MSS is 536 without specific
    + * knowledge that the destination host is prepared to accept larger.
    + * Since no MSS was provided, we MUST NOT set a value > 536.
    + */
    + newmss = min(newmss, (u16)536);
    +
    opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
    memmove(opt + TCPOLEN_MSS, opt, tcplen - sizeof(struct tcphdr));

    --
    1.8.1.2


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