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ICMPv6: Recognise ND option 38 (PREF64) #1109

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Mar 19, 2025
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ICMPv6: Recognise ND option 38 (PREF64) #1109

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40 changes: 40 additions & 0 deletions print-icmp6.c
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Original file line number Diff line number Diff line change
Expand Up @@ -283,6 +283,7 @@ struct nd_opt_hdr { /* Neighbor discovery option header */
#define ND_OPT_ROUTE_INFO 24 /* RFC4191 */
#define ND_OPT_RDNSS 25
#define ND_OPT_DNSSL 31
#define ND_OPT_PREF64_INFORMATION 38 /* RFC8781 */

struct nd_opt_prefix_info { /* prefix information */
nd_uint8_t nd_opt_pi_type;
Expand Down Expand Up @@ -354,6 +355,13 @@ struct nd_opt_route_info { /* route info */
/* prefix follows */
};

struct nd_opt_pref64 { /* PREF64 option */
nd_uint8_t nd_opt_pref64_type;
nd_uint8_t nd_opt_pref64_len;
nd_uint16_t nd_opt_pref64_slplc; /* 13bit lft + 3bit PLC */
nd_uint32_t nd_opt_pref64_words[3]; /* highest 96 bits of prefix */
};

/*
* icmp6 namelookup
*/
Expand Down Expand Up @@ -495,6 +503,7 @@ struct rr_result { /* router renumbering result message */

static const char *get_rtpref(u_int);
static const char *get_lifetime(uint32_t);
static const char *get_pref64_len_repr(uint16_t);
static void print_lladdr(netdissect_options *ndo, const u_char *, size_t);
static int icmp6_opt_print(netdissect_options *ndo, const u_char *, int);
static void mld6_print(netdissect_options *ndo, const u_char *);
Expand Down Expand Up @@ -734,6 +743,7 @@ static const struct tok icmp6_opt_values[] = {
{ ND_OPT_HOMEAGENT_INFO, "homeagent information"},
{ ND_OPT_NONCE, "nonce"},
{ ND_OPT_ROUTE_INFO, "route info"},
{ ND_OPT_PREF64_INFORMATION, "pref64"},
{ 0, NULL }
};

Expand Down Expand Up @@ -774,6 +784,20 @@ get_lifetime(uint32_t v)
}
}

static const char *
get_pref64_len_repr(uint16_t v)
{
static const char *prefixlen_str[] = {
"96", "64", "56", "48", "40", "32"
};

v = v & 0x0007;
if (v < 6)
return prefixlen_str[v];
else
return "??";
}

static void
print_lladdr(netdissect_options *ndo, const uint8_t *p, size_t l)
{
Expand Down Expand Up @@ -1416,10 +1440,12 @@ icmp6_opt_print(netdissect_options *ndo, const u_char *bp, int resid)
const struct nd_opt_advinterval *opa;
const struct nd_opt_homeagent_info *oph;
const struct nd_opt_route_info *opri;
const struct nd_opt_pref64 *op64;
const u_char *cp, *ep, *domp;
nd_ipv6 in6;
size_t l;
u_int i;
uint16_t w;

cp = bp;
/* 'ep' points to the end of available data. */
Expand Down Expand Up @@ -1533,6 +1559,20 @@ icmp6_opt_print(netdissect_options *ndo, const u_char *bp, int resid)
ND_PRINT(", lifetime=%s",
get_lifetime(GET_BE_U_4(opri->nd_opt_rti_lifetime)));
break;
case ND_OPT_PREF64_INFORMATION:
op64 = (const struct nd_opt_pref64 *)op;
if (opt_len != 2)
ND_PRINT("%s", "bad option length! ");
w = GET_BE_U_2(op64->nd_opt_pref64_slplc);
memset(&in6, 0, sizeof(in6));
GET_CPY_BYTES(&in6, op64->nd_opt_pref64_words,
sizeof(op64->nd_opt_pref64_words));
ND_PRINT("%s/%s (plc %u), lifetime %us",
ip6addr_string(ndo, (const u_char *)&in6),
get_pref64_len_repr(w),
w & 0x0007,
Comment on lines +1572 to +1573
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It feels a little odd to have the w & 0x0007 both inside get_pref64_len_repr() and in the ND_PRINT args. It might be more clear to use a temp variable, maybe named plc, and then pass that to both get_pref64_len_repr() and in the ND_PRINT arg list. Obviously this is pretty minor.

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The arg of get_pref64_len_repr() is eventually used to index memory, so IMHO the mask should not be removed from get_pref64_len_repr() anyway; it should be local to the array member access. Otherwise, in the case where the compiler does not inline the call, we would not be far from introducing a gadget to read garbage memory; it would take a single human error. In my view, the resulting code would be harder to maintain.

A temporary variable declaration five screens away, only useful for opt 38, does not address the point above.

One way to reduce repetition would be to abstract "%s/%u (plc %u)" into its own function instead of get_pref64_len_repr():

static void
print_pref64(netdissect_options *ndo, const u_char *in6, size_t w)
{
	static const char *lengths[] = {
		"96", "64", "56", "48", "40", "32"
	};
	const char *plen;
	uint16_t plc = w & 0x0007;

	if (plc < 6)
		plen = lengths[plc];
	else
		plen = "??";
	ND_PRINT("%s/%s (plc %u)",
	         ip6addr_string(ndo, in6),
	         plen, plc);
}

..., and then use it in icmp6_opt_print(). Or, maybe, even print the lifetime in here.
The mask is local to the memory access, so it is hard to make a mistake. We do not have to write w & 0x0007 twice.
That might or might not be easier to read.

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Sometimes the cure is worse than the disease. While the two masks jumped out at me as unusual, I don't think it's a blocker. Thanks for humoring me.

w & 0xfff8);
break;
default:
if (ndo->ndo_vflag <= 1) {
print_unknown_data(ndo,cp+2,"\n\t ", (opt_len << 3) - 2); /* skip option header */
Expand Down
2 changes: 2 additions & 0 deletions tests/TESTLIST
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Original file line number Diff line number Diff line change
Expand Up @@ -249,6 +249,8 @@ icmpv6_nodeinfo_replyipv6 icmpv6_nodeinfo_replyipv6.pcap icmpv6_nodeinfo_replyip
icmpv6-ns-nonce-v0 icmpv6-ns-nonce.pcap icmpv6-ns-nonce-v0.out
icmpv6-ns-nonce-v1 icmpv6-ns-nonce.pcap icmpv6-ns-nonce-v1.out -v
icmpv6-ns-nonce-v2 icmpv6-ns-nonce.pcap icmpv6-ns-nonce-v2.out -vv
icmpv6-ra-pref64-v1 icmpv6-ra-pref64.pcap icmpv6-ra-pref64-v1.out -v
icmpv6-ra-pref64-v2 icmpv6-ra-pref64.pcap icmpv6-ra-pref64-v2.out -vv

# SPB tests
spb spb.pcap spb.out
Expand Down
20 changes: 20 additions & 0 deletions tests/icmpv6-ra-pref64-v1.out
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Original file line number Diff line number Diff line change
@@ -0,0 +1,20 @@
1 2023-12-04 20:18:21.401201 IP6 (flowlabel 0x9fc72, hlim 255, next-header ICMPv6 (58), payload length 72) fe80::e015:81ff:feb4:b945 > ff02::1: [icmp6 sum ok] ICMP6, router advertisement, length 72
hop limit 80, Flags [other stateful], pref medium, router lifetime 500s, reachable time 0ms, retrans timer 0ms
source link-address option (1), length 8 (1): e2:15:81:b4:b9:45
prefix info option (3), length 32 (4): 2001:db8:cc:dd::/64, Flags [onlink], valid time 3600s, pref. time 1800s
pref64 option (38), length 16 (2): 2001:db8:1:64:ff9b::/96 (plc 0), lifetime 0s
2 2023-12-04 20:18:24.401773 IP6 (flowlabel 0x9fc72, hlim 255, next-header ICMPv6 (58), payload length 72) fe80::e015:81ff:feb4:b945 > ff02::1: [icmp6 sum ok] ICMP6, router advertisement, length 72
hop limit 80, Flags [other stateful], pref medium, router lifetime 500s, reachable time 0ms, retrans timer 0ms
source link-address option (1), length 8 (1): e2:15:81:b4:b9:45
prefix info option (3), length 32 (4): 2001:db8:cc:dd::/64, Flags [onlink], valid time 3600s, pref. time 1800s
pref64 option (38), length 16 (2): 2001:db8:0:64:ff9b::/?? (plc 6), lifetime 1800s
3 2023-12-04 20:18:27.402345 IP6 (flowlabel 0x9fc72, hlim 255, next-header ICMPv6 (58), payload length 72) fe80::e015:81ff:feb4:b945 > ff02::1: [icmp6 sum ok] ICMP6, router advertisement, length 72
hop limit 80, Flags [other stateful], pref medium, router lifetime 500s, reachable time 0ms, retrans timer 0ms
source link-address option (1), length 8 (1): e2:15:81:b4:b9:45
prefix info option (3), length 32 (4): 2a00:f480:cc:dd::/64, Flags [onlink], valid time 3600s, pref. time 1800s
pref64 option (38), length 16 (2): 2001:db8:0:64:ff9b::/96 (plc 0), lifetime 1800s
4 2023-12-04 20:18:30.402917 IP6 (flowlabel 0x9fc72, hlim 255, next-header ICMPv6 (58), payload length 72) fe80::e015:81ff:feb4:b945 > ff02::1: [icmp6 sum ok] ICMP6, router advertisement, length 72
hop limit 80, Flags [other stateful], pref medium, router lifetime 500s, reachable time 0ms, retrans timer 0ms
source link-address option (1), length 8 (1): e2:15:81:b4:b9:45
prefix info option (3), length 32 (4): 2001:db8:cc:dd::/64, Flags [onlink], valid time 3600s, pref. time 1800s
pref64 option (38), length 16 (2): 2001:db8:0:64:ff9b::/96 (plc 0), lifetime 65528s
36 changes: 36 additions & 0 deletions tests/icmpv6-ra-pref64-v2.out
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Original file line number Diff line number Diff line change
@@ -0,0 +1,36 @@
1 2023-12-04 20:18:21.401201 IP6 (flowlabel 0x9fc72, hlim 255, next-header ICMPv6 (58), payload length 72) fe80::e015:81ff:feb4:b945 > ff02::1: [icmp6 sum ok] ICMP6, router advertisement, length 72
hop limit 80, Flags [other stateful], pref medium, router lifetime 500s, reachable time 0ms, retrans timer 0ms
source link-address option (1), length 8 (1): e2:15:81:b4:b9:45
0x0000: e215 81b4 b945
prefix info option (3), length 32 (4): 2001:db8:cc:dd::/64, Flags [onlink], valid time 3600s, pref. time 1800s
0x0000: 4080 0000 0e10 0000 0708 0000 0000 2001
0x0010: 0db8 00cc 00dd 0000 0000 0000 0000
pref64 option (38), length 16 (2): 2001:db8:1:64:ff9b::/96 (plc 0), lifetime 0s
0x0000: 0000 2001 0db8 0001 0064 ff9b 0000
2 2023-12-04 20:18:24.401773 IP6 (flowlabel 0x9fc72, hlim 255, next-header ICMPv6 (58), payload length 72) fe80::e015:81ff:feb4:b945 > ff02::1: [icmp6 sum ok] ICMP6, router advertisement, length 72
hop limit 80, Flags [other stateful], pref medium, router lifetime 500s, reachable time 0ms, retrans timer 0ms
source link-address option (1), length 8 (1): e2:15:81:b4:b9:45
0x0000: e215 81b4 b945
prefix info option (3), length 32 (4): 2001:db8:cc:dd::/64, Flags [onlink], valid time 3600s, pref. time 1800s
0x0000: 4080 0000 0e10 0000 0708 0000 0000 2001
0x0010: 0db8 00cc 00dd 0000 0000 0000 0000
pref64 option (38), length 16 (2): 2001:db8:0:64:ff9b::/?? (plc 6), lifetime 1800s
0x0000: 070e 2001 0db8 0000 0064 ff9b 0000
3 2023-12-04 20:18:27.402345 IP6 (flowlabel 0x9fc72, hlim 255, next-header ICMPv6 (58), payload length 72) fe80::e015:81ff:feb4:b945 > ff02::1: [icmp6 sum ok] ICMP6, router advertisement, length 72
hop limit 80, Flags [other stateful], pref medium, router lifetime 500s, reachable time 0ms, retrans timer 0ms
source link-address option (1), length 8 (1): e2:15:81:b4:b9:45
0x0000: e215 81b4 b945
prefix info option (3), length 32 (4): 2a00:f480:cc:dd::/64, Flags [onlink], valid time 3600s, pref. time 1800s
0x0000: 4080 0000 0e10 0000 0708 0000 0000 2a00
0x0010: f480 00cc 00dd 0000 0000 0000 0000
pref64 option (38), length 16 (2): 2001:db8:0:64:ff9b::/96 (plc 0), lifetime 1800s
0x0000: 0708 2001 0db8 0000 0064 ff9b 0000
4 2023-12-04 20:18:30.402917 IP6 (flowlabel 0x9fc72, hlim 255, next-header ICMPv6 (58), payload length 72) fe80::e015:81ff:feb4:b945 > ff02::1: [icmp6 sum ok] ICMP6, router advertisement, length 72
hop limit 80, Flags [other stateful], pref medium, router lifetime 500s, reachable time 0ms, retrans timer 0ms
source link-address option (1), length 8 (1): e2:15:81:b4:b9:45
0x0000: e215 81b4 b945
prefix info option (3), length 32 (4): 2001:db8:cc:dd::/64, Flags [onlink], valid time 3600s, pref. time 1800s
0x0000: 4080 0000 0e10 0000 0708 0000 0000 2001
0x0010: 0db8 00cc 00dd 0000 0000 0000 0000
pref64 option (38), length 16 (2): 2001:db8:0:64:ff9b::/96 (plc 0), lifetime 65528s
0x0000: fff8 2001 0db8 0000 0064 ff9b 0000
Binary file added tests/icmpv6-ra-pref64.pcap
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