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SubjectDefragging/refragging IPv6 packets
Hello,

My name is Brad Chapman and I am currently attempting to port the
iptables ip_conntrack component of Netfilter to the IPv6 protocol. My
port is almost feature-complete, but I cannot figure out how to properly
defragment/refragment IPv6 packets. Also, I have written an IPv6 header
parsing function to grab headers from the IPv6 header chain. I would
appreciate some feedback on that as well. Below are some code examples.


DEFRAGGING PACKETS:

/* Returns defragged sk_buff, or NULL */
/* FIXME: is this the "right" way to do it? -- BC */
struct sk_buff *ip6_ct_gather_frags(struct sk_buff *skb)
{
#ifdef CONFIG_NETFILTER_DEBUG
unsigned int old_debug = skb->nf_debug;
#endif
struct frag_hdr *fhdr = (struct frag_hdr *)
ipv6_get_header(skb->nh.ipv6h,
NEXTHDR_FRAGMENT,
NULL);
local_bh_disable();
if (!ipv6_reassembly(skb, (u8 *)fhdr))
return NULL;
local_bh_enable();

skb->nfcache |= NFC_ALTERED;
#ifdef CONFIG_NETFILTER_DEBUG
skb->nf_debug = old_debug;
#endif
return skb;
}

REFRAGGING PACKETS:

/* This gets the fragmentable part - I think -- BC */
static int ip6_refrag_getfrag(const void *data, struct in6_addr *addr,
char *buff, unsigned int offset, unsigned int len)
{
return 0;
}

static unsigned int ip6_refrag(unsigned int hooknum,
struct sk_buff **pskb,
const struct net_device *in,
const struct net_device *out,
int (*okfn)(struct sk_buff *))
{
struct frag_hdr *fhdr = (struct frag_hdr *)
ipv6_get_header((*pskb)->nh.ipv6h,
NEXTHDR_FRAGMENT,
NULL);
struct flowi *flow;
int ret;
__u8 protonum;

/* Verify the protocol */
if (!ipv6_get_header((*pskb)->nh.ipv6h, NEXTHDR_PROTOANY, &protonum))
return NF_DROP;

/* We've seen it coming out the other side: confirm */
ip6_confirm(hooknum, pskb, in, out, okfn);

/* Local packets are never produced too large for their
interface. We degfragment them at LOCAL_OUT, however,
so we have to refragment them here. */
if ((fhdr) && !(fhdr->frag_off & htons(IP6_NON_FRAG_OFFSET))) {

/* Build the flowlabel */
flow.proto = (int)protonum;
ipv6_addr_copy(flow.fl6_src, (*pskb)->nh.ipv6h->saddr);
ipv6_addr_copy(flow.fl6_dst, (*pskb)->nh.ipv6h->daddr);
flow.fl6_flowlabel = 0;

if (protonum == NEXTHDR_TCP) {
flow.uli_u.ports.sport = (*pskb)->nh.tcph->sport;
flow.uli_u.ports.dport = (*pskb)->nh.tcph->dport;
}
else if (protonum == NEXTHDR_UDP) {
flow.uli_u.ports.sport = (*pskb)->nh.udph->sport;
flow.uli_u.ports.dport = (*pskb)->nh.udph->dport;
}
else if (protonum == NEXTHDR_ICMP) {
struct icmp6hdr *icmph;
icmph = (struct icmp6hdr *)ipv6_get_header((*pskb)->nh.ipv6h,
NEXTHDR_ICMP,
NULL);
if (!icmph)
return NF_DROP;

flow.uli_u.icmpt.type = icmph->icmp6_type;
flow.uli_u.icmpt.code = icmph->icmp6_code;
}
else
return NF_DROP;

ret = ip6_build_xmit((*pskb)->sk,
ip6_refrag_getfrag,
(*pskb)->data,
flow,
(*pskb)->nh.ipv6h->payload_len,
NULL, 0, 0);

if (ret < 0)
return NF_DROP;
return NF_STOLEN;
}
return NF_ACCEPT;
}

PARSING IPV6 HEADERS

/* Find a particular header in the IPv6 header chains */
struct ipv6_opt_hdr *
ipv6_get_header(const struct ipv6hdr *ipv6h,
__u8 header,
__u8 *result)
{
struct ipv6_opt_hdr *newhdr = NULL;
__u8 *hdrptr = (__u8 *)&ipv6h->nexthdr;
int hdrlen, length;

IP_NF_ASSERT(header != NEXTHDR_NONE);
IP_NF_ASSERT(*hdrptr != NEXTHDR_NONE);

/* Start bouncing */
length = sizeof(struct ipv6hdr);
hdrlen = 0;
while (*hdrptr != NEXTHDR_NONE)
{
if (header == NEXTHDR_PROTOANY &&
(*hdrptr == NEXTHDR_TCP ||
*hdrptr == NEXTHDR_UDP ||
*hdrptr == NEXTHDR_ICMP) &&
result != NULL)
{
*result = *hdrptr;
newhdr = (struct ipv6_opt_hdr *)hdrptr;
break;
} else if (*hdrptr == header) {
newhdr = (struct ipv6_opt_hdr *)hdrptr;
break;
} else if (hdrlen > length) {
break;
}
hdrlen = sizeof((__u8 *)hdrptr);
hdrptr = (__u8 *)(hdrptr + hdrlen);
length -= hdrlen;
}
return newhdr;
}



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