• James Chapman's avatar
    l2tp: remove configurable payload offset · 5c097f55
    James Chapman authored
    [ Upstream commit 900631ee ]
    
    If L2TP_ATTR_OFFSET is set to a non-zero value in L2TPv3 tunnels, it
    results in L2TPv3 packets being transmitted which might not be
    compliant with the L2TPv3 RFC. This patch has l2tp ignore the offset
    setting and send all packets with no offset.
    
    In more detail:
    
    L2TPv2 supports a variable offset from the L2TPv2 header to the
    payload. The offset value is indicated by an optional field in the
    L2TP header.  Our L2TP implementation already detects the presence of
    the optional offset and skips that many bytes when handling data
    received packets. All transmitted packets are always transmitted with
    no offset.
    
    L2TPv3 has no optional offset field in the L2TPv3 packet
    header. Instead, L2TPv3 defines optional fields in a "Layer-2 Specific
    Sublayer". At the time when the original L2TP code was written, there
    was talk at IETF of offset being implemented in a new Layer-2 Specific
    Sublayer. A L2TP_ATTR_OFFSET netlink attribute was added so that this
    offset could be configured and the intention was to allow it to be
    also used to set the tx offset for L2TPv2. However, no L2TPv3 offset
    was ever specified and the L2TP_ATTR_OFFSET parameter was forgotten
    about.
    
    Setting L2TP_ATTR_OFFSET results in L2TPv3 packets being transmitted
    with the specified number of bytes padding between L2TPv3 header and
    payload. This is not compliant with L2TPv3 RFC3931. This change
    removes the configurable offset altogether while retaining
    L2TP_ATTR_OFFSET for backwards compatibility. Any L2TP_ATTR_OFFSET
    value is ignored.
    Signed-off-by: 's avatarJames Chapman <jchapman@katalix.com>
    Signed-off-by: 's avatarDavid S. Miller <davem@davemloft.net>
    Signed-off-by: 's avatarSasha Levin <sashal@kernel.org>
    5c097f55