Hello, Thanks for your time reviewing! On Wed, 2021-08-25 at 19:05 +0200, Pablo Neira Ayuso wrote: > Hi, > > On Mon, Aug 09, 2021 at 04:10:36PM +1200, Cole Dishington wrote: > > Adds support for masquerading into a smaller subset of ports - > > defined by the PSID values from RFC-7597 Section 5.1. This is part of > > the support for MAP-E and Lightweight 4over6, which allows multiple > > devices to share an IPv4 address by splitting the L4 port / id into > > ranges. > > > > Co-developed-by: Anthony Lineham <anthony.lineham@xxxxxxxxxxxxxxxxxxx> > > Signed-off-by: Anthony Lineham <anthony.lineham@xxxxxxxxxxxxxxxxxxx> > > Co-developed-by: Scott Parlane <scott.parlane@xxxxxxxxxxxxxxxxxxx> > > Signed-off-by: Scott Parlane <scott.parlane@xxxxxxxxxxxxxxxxxxx> > > Signed-off-by: Blair Steven <blair.steven@xxxxxxxxxxxxxxxxxxx> > > Signed-off-by: Cole Dishington <Cole.Dishington@xxxxxxxxxxxxxxxxxxx> > > Reviewed-by: Florian Westphal <fw@xxxxxxxxx> > [...] > > Looking at the userspace logic: > > https://scanmail.trustwave.com/?c=20988&d=6vim4fcVLjPkIbLUDqz3Tj2W4gXWNCkYa5llWggBjA&u=https%3a%2f%2fpatchwork%2eozlabs%2eorg%2fproject%2fnetfilter-devel%2fpatch%2f20210716002219%2e30193-1-Cole%2eDishington%40alliedtelesis%2eco%2enz%2f > > Chunk extracted from void parse_psid(...) > > > offset = (1 << (16 - offset_len)); > > Assuming offset_len = 6, then you skip 0-1023 ports, OK. > > > psid = psid << (16 - offset_len - psid_len); > > This psid calculation is correct? Maybe: > > psid = psid << (16 - offset_len); PSID port numbers have the form [offset|PSID|j] and 16 = offset_length + PSID_length + j_length. The PSID calculation above is bit shifting the passed psid up j_length. The userspace tool accepts the unshifted psid to be consistent with how RFC7597 specified it (see RFC7597 Appendix A. Examples). > > instead? > > psid=0 => 0 << (16 - 6) = 1024 > psid=1 => 1 << (16 - 6) = 2048 > > This is implicitly assuming that 64 PSIDs are available, each of them > taking 1024 ports, ie. psid_len is 6 bits. But why are you subtracting > the psid_len above? > > > /* Handle the special case of no offset bits (a=0), so offset loops */ > > min = psid; > > OK, this line above is the minimal port in the range > > > if (offset) > > min += offset; > > ... which is incremented by the offset (to skip the 0-1023 ports). > > > r->min_proto.all = htons(min); > > r->max_proto.all = htons(min + ((1 << (16 - offset_len - psid_len)) - 1)); > > Here, you subtract psid_len again, not sure why. Each PSID port range is made up of many smaller contiguous port sub-ranges (except for the special case of offset_len = 0) e.g. for PSID=0x34,psid_length=8,psid_offset=6 the ranges are 1232-1235, 2256-2259, ..., 63696-63699, 64720-64723 (Taken from rfc7597 Appendix A. Examples). The above calculation is selecting the first sub-range. Max is computed by finding j_length and filling it with 1's. > > > r->base_proto.all = htons(offset); > > base is set to offset, ie. 1024. > > > r->flags |= NF_NAT_RANGE_PSID; > > r->flags |= NF_NAT_RANGE_PROTO_SPECIFIED; > > Now looking at the kernel side. > > > diff --git a/net/netfilter/nf_nat_masquerade.c b/net/netfilter/nf_nat_masquerade.c > > index 8e8a65d46345..19a4754cda76 100644 > > --- a/net/netfilter/nf_nat_masquerade.c > > +++ b/net/netfilter/nf_nat_masquerade.c > > @@ -55,8 +55,31 @@ nf_nat_masquerade_ipv4(struct sk_buff *skb, unsigned int hooknum, > > newrange.flags = range->flags | NF_NAT_RANGE_MAP_IPS; > > newrange.min_addr.ip = newsrc; > > newrange.max_addr.ip = newsrc; > > - newrange.min_proto = range->min_proto; > > - newrange.max_proto = range->max_proto; > > + > > + if (range->flags & NF_NAT_RANGE_PSID) { > > + u16 base = ntohs(range->base_proto.all); > > + u16 min = ntohs(range->min_proto.all); > > + u16 off = 0; > > + > > + /* xtables should stop base > 2^15 by enforcement of > > + * 0 <= offset_len < 16 argument, with offset_len=0 > > + * as a special case inwhich base=0. > > I don't understand this comment. This is a sanity check. The userspace tool restricts offset_len to the specified range and since base = 2^(16 - offset_len) (or base = 0 for the special case of offset_len = 16) the below condition should never be true. However, if base greater than 1<<15 was allowed, a divide by zero error would occur on the block below. > > > + */ > > + if (WARN_ON_ONCE(base > (1 << 15))) > > + return NF_DROP; > > + > > + /* If offset=0, port range is in one contiguous block */ > > + if (base) > > + off = prandom_u32_max(((1 << 16) / base) - 1); > > Assuming the example above, base is set to 1024. Then, off is a random > value between UINT16_MAX (you expressed this as 1 << 16) and the base > which is 1024 minus 1. > > So this is picking a random off (actually the PSID?) between 0 and 63. > What about clashes? I mean, two different machines behind the NAT > might get the same off. > > > + newrange.min_proto.all = htons(min + base * off); > > min could be 1024, 2048, 3072... you add base which is 1024 * off. > > Is this duplicated? Both calculated in user and kernel space? Each PSID value defines many contiguous port sub-ranges. The randomly chosen off selects the ith sub-range for a given PSID e.g. off=1 would select 2256-2259 for rfc7597 Appendix A. Examples. The userspace tool calculates the min and max of the first sub-range for a given psid, whereas the above randomly selects one of the sub-ranges for a given psid. j_length determines how large each sub-range will be, so for small j_length values there still is the risk the chosen sub-range will be exhausted. > > > + newrange.max_proto.all = htons(ntohs(newrange.min_proto.all) + ntohs(range->max_proto.all) - min); > > I'm stopping here, I'm getting lost. > > My understanding about this RFC is that you would like to split the > 16-bit ports in ranges to uniquely identify the host behind the NAT. > > Why don't you just you just select the port range from userspace > utilizing the existing infrastructure? I mean, why do you need this > kernel patch? If utilizing existing infrastruture to install PSID port ranges a lot of rules would be required as each PSID port range is made up of many smaller sub-ranges. e.g. (from rfc7597 Appendix A. Examples) for psid_length=8,offset_length=6 each PSID would need 63 NF_NAT_RANGE_PROTO_SPECIFIED rules, hence a total of 16128 rules if all the PSIDs were allocated. > > Florian already suggested: > > > Is it really needed to place all of this in the nat core? > > > > The only thing that has to be done in the NAT core, afaics, is to > > suppress port reallocation attmepts when NF_NAT_RANGE_PSID is set. > > > > Is there a reason why nf_nat_masquerade_ipv4/6 can't be changed instead > > to do what you want? > > > > AFAICS its enough to set NF_NAT_RANGE_PROTO_SPECIFIED and init the > > upper/lower boundaries, i.e. change input given to nf_nat_setup_info(). > > extracted from: > > https://scanmail.trustwave.com/?c=20988&d=6vim4fcVLjPkIbLUDqz3Tj2W4gXWNCkYa5s0Bg8JjA&u=https%3a%2f%2fpatchwork%2eozlabs%2eorg%2fproject%2fnetfilter-devel%2fpatch%2f20210422023506%2e4651-1-Cole%2eDishington%40alliedtelesis%2eco%2enz%2f
