• Florian Westphal's avatar
    ip_fragment: don't forward defragmented DF packet · d6b915e2
    Florian Westphal authored
    We currently always send fragments without DF bit set.
    
    Thus, given following setup:
    
    mtu1500 - mtu1500:1400 - mtu1400:1280 - mtu1280
       A           R1              R2         B
    
    Where R1 and R2 run linux with netfilter defragmentation/conntrack
    enabled, then if Host A sent a fragmented packet _with_ DF set to B, R1
    will respond with icmp too big error if one of these fragments exceeded
    1400 bytes.
    
    However, if R1 receives fragment sizes 1200 and 100, it would
    forward the reassembled packet without refragmenting, i.e.
    R2 will send an icmp error in response to a packet that was never sent,
    citing mtu that the original sender never exceeded.
    
    The other minor issue is that a refragmentation on R1 will conceal the
    MTU of R2-B since refragmentation does not set DF bit on the fragments.
    
    This modifies ip_fragment so that we track largest fragment size seen
    both for DF and non-DF packets, and set frag_max_size to the largest
    value.
    
    If the DF fragment size is larger or equal to the non-df one, we will
    consider the packet a path mtu probe:
    We set DF bit on the reassembled skb and also tag it with a new IPCB flag
    to force refragmentation even if skb fits outdev mtu.
    
    We will also set DF bit on each fragment in this case.
    
    Joint work with Hannes Frederic Sowa.
    Reported-by: default avatarJesse Gross <jesse@nicira.com>
    Signed-off-by: default avatarFlorian Westphal <fw@strlen.de>
    Acked-by: default avatarHannes Frederic Sowa <hannes@stressinduktion.org>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    d6b915e2
ip.h 16.2 KB