Re: [PATCH v5 12/13] ndp: Support advertising multiple prefixes in Router Advertisement

On Sun, Feb 22, 2026 at 12:44:44PM -0500, Jon Maloy wrote:

We extend NDP to advertise all suitable IPv6 prefixes in Router Advertisements, per RFC 4861. Observed and link-local addresses, plus addresses with a prefix length != 64, are excluded.

Signed-off-by: Jon Maloy --- ndp.c | 127 +++++++++++++++++++++++++++++++++++++++------------------- 1 file changed, 85 insertions(+), 42 deletions(-)

diff --git a/ndp.c b/ndp.c index ed8c6ae..cf64f3f 100644 --- a/ndp.c +++ b/ndp.c @@ -32,6 +32,8 @@ #include "passt.h" #include "tap.h" #include "log.h" +#include "fwd.h" +#include "conf.h"

#define RT_LIFETIME 65535

@@ -82,7 +84,7 @@ struct ndp_na { } __attribute__((packed));

/** - * struct opt_prefix_info - Prefix Information option + * struct opt_prefix_info - Prefix Information option header * @header: Option header * @prefix_len: The number of leading bits in the Prefix that are valid * @prefix_flags: Flags associated with the prefix @@ -99,6 +101,16 @@ struct opt_prefix_info { uint32_t reserved; } __attribute__((packed));

+/** + * struct ndp_prefix - Complete Prefix Information option with prefix + * @info: Prefix Information option header + * @prefix: IPv6 prefix + */ +struct ndp_prefix { + struct opt_prefix_info info; + struct in6_addr prefix; +} __attribute__((__packed__)); + /** * struct opt_mtu - Maximum transmission unit (MTU) option * @header: Option header @@ -140,27 +152,23 @@ struct opt_dnssl { } __attribute__((packed));

/** - * struct ndp_ra - NDP Router Advertisement (RA) message + * struct ndp_ra_hdr - NDP Router Advertisement fixed header * @ih: ICMPv6 header * @reachable: Reachability time, after confirmation (ms) * @retrans: Time between retransmitted NS messages (ms) - * @prefix_info: Prefix Information option - * @prefix: IPv6 prefix - * @mtu: MTU option - * @source_ll: Target link-layer address - * @var: Variable fields */ -struct ndp_ra { +struct ndp_ra_hdr { struct icmp6hdr ih; uint32_t reachable; uint32_t retrans; - struct opt_prefix_info prefix_info; - struct in6_addr prefix; - struct opt_l2_addr source_ll; +} __attribute__((__packed__));

- unsigned char var[sizeof(struct opt_mtu) + sizeof(struct opt_rdnss) + - sizeof(struct opt_dnssl)]; -} __attribute__((packed, aligned(__alignof__(struct in6_addr)))); +/* Maximum RA message size: header + prefixes + source_ll + mtu + rdnss + dnssl */ +#define NDP_RA_MAX_SIZE (sizeof(struct ndp_ra_hdr) + \ + INANY_MAX_ADDRS * sizeof(struct ndp_prefix) + \ + sizeof(struct opt_l2_addr) + \ + sizeof(struct opt_mtu) + sizeof(struct opt_rdnss) + \ + sizeof(struct opt_dnssl))

/** * struct ndp_ns - NDP Neighbor Solicitation (NS) message @@ -231,6 +239,46 @@ void ndp_unsolicited_na(const struct ctx *c, const struct in6_addr *addr) ndp_na(c, &in6addr_ll_all_nodes, addr); }

+/** + * ndp_prefix_fill() - Fill prefix options for all suitable addresses + * @c: Execution context + * @buf: Buffer to write prefix options into + * + * Fills buffer with Prefix Information options for all non-linklocal, + * non-observed addresses with prefix_len == 64 (required for SLAAC). + * + * Return: number of bytes written + */ +static size_t ndp_prefix_fill(const struct ctx *c, unsigned char *buf) +{ + int skip = CONF_ADDR_OBSERVED | CONF_ADDR_LINKLOCAL; + const struct inany_addr_entry *e; + struct ndp_prefix *p; + size_t offset = 0; + + for_each_addr(e, c, AF_INET6) { + if (e->flags & skip)

Again, this will skip OBSERVED addresses that are also USER|HOST.

+ continue; + /* SLAAC requires /64 prefix */ + if (e->prefix_len != 64) + continue; + + p = (struct ndp_prefix *)(buf + offset); + p->info.header.type = OPT_PREFIX_INFO; + p->info.header.len = 4; /* 4 * 8 = 32 bytes */ + p->info.prefix_len = 64; + p->info.prefix_flags = 0xc0; /* L, A flags */ + p->info.valid_lifetime = ~0U; + p->info.pref_lifetime = ~0U; + p->info.reserved = 0; + p->prefix = e->addr.a6; + + offset += sizeof(struct ndp_prefix); + } + + return offset; +} + /** * ndp_ra() - Send an NDP Router Advertisement (RA) message * @c: Execution context @@ -238,7 +286,15 @@ void ndp_unsolicited_na(const struct ctx *c, const struct in6_addr *addr) */ static void ndp_ra(const struct ctx *c, const struct in6_addr *dst) { - struct ndp_ra ra = { + unsigned char buf[NDP_RA_MAX_SIZE] + __attribute__((__aligned__(__alignof__(struct in6_addr)))); + struct ndp_ra_hdr *hdr = (struct ndp_ra_hdr *)buf; + struct opt_l2_addr *source_ll; + unsigned char *ptr; + size_t prefix_len; + + /* Build RA header */ + *hdr = (struct ndp_ra_hdr){ .ih = { .icmp6_type = RA, .icmp6_code = 0, @@ -247,32 +303,22 @@ static void ndp_ra(const struct ctx *c, const struct in6_addr *dst) .icmp6_rt_lifetime = htons_constant(RT_LIFETIME), .icmp6_addrconf_managed = 1, }, - .prefix_info = { - .header = { - .type = OPT_PREFIX_INFO, - .len = 4, - }, - .prefix_len = 64, - .prefix_flags = 0xc0, /* prefix flags: L, A */ - .valid_lifetime = ~0U, - .pref_lifetime = ~0U, - }, - .prefix = IN6ADDR_ANY_INIT, - .source_ll = { - .header = { - .type = OPT_SRC_L2_ADDR, - .len = 1, - }, - }, }; - struct inany_addr_entry *e = first_v6(c); - unsigned char *ptr = NULL; - - ASSERT(e);

- ra.prefix = e->addr.a6; + /* Fill prefix options */ + prefix_len = ndp_prefix_fill(c, buf + sizeof(struct ndp_ra_hdr)); + if (prefix_len == 0) { + /* No suitable prefixes to advertise */ + return; + }

- ptr = &ra.var[0]; + /* Add source link-layer address option */ + ptr = buf + sizeof(struct ndp_ra_hdr) + prefix_len;

Somewhat idiomatic trick you can use here is ptr = (char *)(hdr + 1) + prefix_len;

+ source_ll = (struct opt_l2_addr *)ptr; + source_ll->header.type = OPT_SRC_L2_ADDR; + source_ll->header.len = 1; + memcpy(source_ll->mac, c->our_tap_mac, ETH_ALEN); + ptr += sizeof(struct opt_l2_addr);

if (c->mtu) { struct opt_mtu *mtu = (struct opt_mtu *)ptr; @@ -346,10 +392,7 @@ static void ndp_ra(const struct ctx *c, const struct in6_addr *dst) } }

- memcpy(&ra.source_ll.mac, c->our_tap_mac, ETH_ALEN); - - /* NOLINTNEXTLINE(clang-analyzer-security.PointerSub) */ - ndp_send(c, dst, &ra, ptr - (unsigned char *)&ra); + ndp_send(c, dst, buf, ptr - buf); }

/** -- 2.52.0

-- David Gibson (he or they) | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you, not the other way | around. http://www.ozlabs.org/~dgibson