Re: [PATCH v2 4/8] udp: forward external source mac address through tap interface

On Thu, 12 Jun 2025 00:21:48 -0400 Jon Maloy wrote:

We forward the incoming mac address through the tap interface when receiving incoming packets from network local hosts. Packets from the local host are excepted from this rule, and are still forwarded with the default passt/pasta mac address as source.

This is a part of the solution to bug #120

Signed-off-by: Jon Maloy --- udp.c | 29 +++++++++++++---------------- 1 file changed, 13 insertions(+), 16 deletions(-)

diff --git a/udp.c b/udp.c index 65a52e0..ae8fbaf 100644 --- a/udp.c +++ b/udp.c @@ -133,11 +133,8 @@ static int udp_splice_init[IP_VERSIONS][NUM_PORTS]; /* UDP header and data for inbound messages */ static struct udp_payload_t udp_payload[UDP_MAX_FRAMES];

-/* Ethernet header for IPv4 frames */ -static struct ethhdr udp4_eth_hdr; - -/* Ethernet header for IPv6 frames */ -static struct ethhdr udp6_eth_hdr; +/* Ethernet headers for IPv4 and IPv6 frames */ +static struct ethhdr udp_eth_hdr[UDP_MAX_FRAMES];

An alternative could be to keep two separated sets of headers. It avoids setting eh->h_proto every time. I'm not sure if it's worth it, perhaps a quick throughput test would be a good idea, just to be sure it's fine (I don't expect any substantial regression).

/** * struct udp_meta_t - Pre-cooked headers for UDP packets @@ -214,8 +211,10 @@ void udp_portmap_clear(void) */ void udp_update_l2_buf(const unsigned char *eth_d, const unsigned char *eth_s) { - eth_update_mac(&udp4_eth_hdr, eth_d, eth_s); - eth_update_mac(&udp6_eth_hdr, eth_d, eth_s); + int i; + + for (i = 0; i < UDP_MAX_FRAMES; i++) + eth_update_mac(&udp_eth_hdr[i], eth_d, eth_s); }

/** @@ -238,6 +237,7 @@ static void udp_iov_init_one(const struct ctx *c, size_t i)

*siov = IOV_OF_LVALUE(payload->data);

+ tiov[UDP_IOV_ETH] = IOV_OF_LVALUE(udp_eth_hdr[i]); tiov[UDP_IOV_TAP] = tap_hdr_iov(c, &meta->taph); tiov[UDP_IOV_PAYLOAD].iov_base = payload;

@@ -253,9 +253,6 @@ static void udp_iov_init(const struct ctx *c) { size_t i;

- udp4_eth_hdr.h_proto = htons_constant(ETH_P_IP); - udp6_eth_hdr.h_proto = htons_constant(ETH_P_IPV6); - for (i = 0; i < UDP_MAX_FRAMES; i++) udp_iov_init_one(c, i); } @@ -362,21 +359,21 @@ static void udp_tap_prepare(const struct mmsghdr *mmh, struct iovec (*tap_iov)[UDP_NUM_IOVS] = &udp_l2_iov[idx]; struct udp_payload_t *bp = &udp_payload[idx]; struct udp_meta_t *bm = &udp_meta[idx]; + struct ethhdr *eh = (*tap_iov)[UDP_IOV_ETH].iov_base;

Nit: this could be moved after the declaration of tap_iov, so that we keep initialisers from longest to shortest.

size_t l4len;

+ eth_update_mac(eh, 0, toside->mac); if (!inany_v4(&toside->eaddr) || !inany_v4(&toside->oaddr)) { l4len = udp_update_hdr6(&bm->ip6h, bp, toside, mmh[idx].msg_len, no_udp_csum); - tap_hdr_update(&bm->taph, l4len + sizeof(bm->ip6h) + - sizeof(udp6_eth_hdr)); - (*tap_iov)[UDP_IOV_ETH] = IOV_OF_LVALUE(udp6_eth_hdr); + tap_hdr_update(&bm->taph, l4len + sizeof(bm->ip6h) + ETH_HLEN); + eh->h_proto = htons_constant(ETH_P_IPV6); (*tap_iov)[UDP_IOV_IP] = IOV_OF_LVALUE(bm->ip6h); } else { l4len = udp_update_hdr4(&bm->ip4h, bp, toside, mmh[idx].msg_len, no_udp_csum); - tap_hdr_update(&bm->taph, l4len + sizeof(bm->ip4h) + - sizeof(udp4_eth_hdr)); - (*tap_iov)[UDP_IOV_ETH] = IOV_OF_LVALUE(udp4_eth_hdr); + tap_hdr_update(&bm->taph, l4len + sizeof(bm->ip4h) + ETH_HLEN); + eh->h_proto = htons_constant(ETH_P_IP); (*tap_iov)[UDP_IOV_IP] = IOV_OF_LVALUE(bm->ip4h); } (*tap_iov)[UDP_IOV_PAYLOAD].iov_len = l4len;

-- Stefano