Re: minimum ICMPv6 message size vs. RPL's DIS
From: Hannes Frederic Sowa <hidden>
Date: 2013-07-25 06:17:33
On Wed, Jul 24, 2013 at 08:28:52PM -0300, Werner Almesberger wrote:
The kernel assumes that any valid ICMPv6 message is at least eight bytes long, for example in net/ipv6/raw.c:icmpv6_filter The DIS message of RPL (RFC 6550 section 6.2, from the 6LoWPAN world), has a minimum length of six bytes, and is thus blocked by icmpv6_filter. Contiki OS sends this sort of messages and is thus ignored by an user-space RPL application on Linux (in my case simpleRPL). Nodes will eventually join the network regardless, but this can take hours instead of seconds. RFC 4443 seems to allow even a zero-sized body, making the minimum allowable size four bytes. A similar restriction exists in net/ipv6/icmp.c:icmpv6_rcv but causes no harm there since all potential recipients at that point expect a larger payload. Now, while this is easy enough to fix on the Linux side (see below), I wonder if it is really a Linux bug or if I (and the authors of RPL in Contiki) may have missed some minimum size requirement for ICMPv6 messages stated elsewhere. Opinions ?
Browsing the RFCs, I do think you are correct. Did you check what IPv4 does? (what a strange errata to RFC 4443...)
quoted hunk ↗ jump to hunk
diff --git a/net/ipv6/raw.c b/net/ipv6/raw.c index c45f7a5..539534c 100644 --- a/net/ipv6/raw.c +++ b/net/ipv6/raw.c@@ -112,7 +112,7 @@ static int icmpv6_filter(const struct sock *sk, const struct sk_buff *skb) const struct icmp6hdr *hdr; hdr = skb_header_pointer(skb, skb_transport_offset(skb), - sizeof(_hdr), &_hdr); + 4, &_hdr); if (hdr) { const __u32 *data = &raw6_sk(sk)->filter.data[0]; unsigned int type = hdr->icmp6_type;
Hmm, maybe we should update the icmp header to something like
struct icmp6hdr {
struct icmp6hdr_head {
__u8 icmp6_type;
...;
} icmpv6_head;
#define icmp6_type icmpv6_head.icmp6_type
and just change the type in icmpv6_filter to icmp6hdr_head.
Hmm, there is a bug in this function, _hdr must not be a pointer.
Greetings,
Hannes