While playing around with the IPv6 extension headers i found a strange
behaviour when it comes to the IPv6 fragmentation header. First i used the
thc-ipv6 toolkit with that i could trigger the issue below in the first place.
I used the ipv6header match to check for those extension headers and
noticed that sometimes the match for "check if frag header set" didn't
match although i saw it in tcpdump/tshark and all the other checks for
ext headers worked.
I added a debug output after the "nexthdr = ipv6_hdr(skb)->nexthdr;" in
"net/ipv6/netfilter/ip6t_ipv6header.c" and saw that with special packets
the skb just returns the last nexthdr. So i'm quite confused why.
I generate packets with scapy and the easiest packet to reproduce it is
a simple ICMPv6 Echo Request packet with a fragmentation header.
##[ IPv6 ]###
version= 6L
tc= 0L
fl= 0L
plen= 16
nh= Fragment Header
hlim= 64
src= FOO
dst= BAR
###[ IPv6 Extension Header - Fragmentation header ]###
nh= ICMPv6
res1= 0L
offset= 0L
res2= 0L
m= 0L
id= 0
###[ ICMPv6 Echo Request ]###
type= Echo Request
code= 0
cksum= 0x848e
id= 0x0
seq= 0x0
data= ''
Which looks like this in tcpdump:
IP6 (hlim 64, next-header Fragment (44) payload length: 16) FOO > BAR frag
(0x00000000:0|8) ICMP6, echo request, seq 0
But if i printk the value of nexthdr it's just 58 for ICMPv6. The same packet
generate with dst,hopbyhop,routing header results in nexthdr value being
60,0,43 and then the 58.
So i can narrow it down to the frag header missing in the skb info at this point.
Does anyone have an idea why this happens and where i might need to look for
this issue?
Thanks.
--
Andreas Herz
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