Re: [PATCH v2 5/8] tcp: forward external source mac address through tap interface

On Thu, 12 Jun 2025 00:21:49 -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 --- tcp.c | 5 ++++- tcp_buf.c | 27 +++++++++++++-------------- tcp_internal.h | 2 +- tcp_vu.c | 5 ++--- 4 files changed, 20 insertions(+), 19 deletions(-)

diff --git a/tcp.c b/tcp.c index f43c1e2..777f345 100644 --- a/tcp.c +++ b/tcp.c @@ -920,7 +920,7 @@ static void tcp_fill_header(struct tcphdr *th, * @no_tcp_csum: Do not set TCP checksum */ void tcp_fill_headers(const struct tcp_tap_conn *conn, - struct tap_hdr *taph, + struct tap_hdr *taph, struct ethhdr *eh, struct iphdr *ip4h, struct ipv6hdr *ip6h, struct tcphdr *th, struct iov_tail *payload, const uint16_t *ip4_check, uint32_t seq, bool no_tcp_csum) @@ -952,6 +952,7 @@ void tcp_fill_headers(const struct tcp_tap_conn *conn, psum = proto_ipv4_header_psum(l4len, IPPROTO_TCP, *src4, *dst4); } + eh->h_proto = htons_constant(ETH_P_IP); }

if (ip6h) { @@ -972,7 +973,9 @@ void tcp_fill_headers(const struct tcp_tap_conn *conn, &ip6h->saddr, &ip6h->daddr); } + eh->h_proto = htons_constant(ETH_P_IPV6); } + eth_update_mac(eh, 0, tapside->mac);

tcp_fill_header(th, conn, seq);

diff --git a/tcp_buf.c b/tcp_buf.c index d1fca67..2dbeca2 100644 --- a/tcp_buf.c +++ b/tcp_buf.c @@ -40,8 +40,7 @@ /* Static buffers */

/* Ethernet header for IPv4 and IPv6 frames */ -static struct ethhdr tcp4_eth_src; -static struct ethhdr tcp6_eth_src; +static struct ethhdr tcp_eth_hdr[TCP_FRAMES_MEM];

Same comment as for the UDP implementation.

static struct tap_hdr tcp_payload_tap_hdr[TCP_FRAMES_MEM];

@@ -71,8 +70,10 @@ static struct iovec tcp_l2_iov[TCP_FRAMES_MEM][TCP_NUM_IOVS]; */ void tcp_update_l2_buf(const unsigned char *eth_d, const unsigned char *eth_s) { - eth_update_mac(&tcp4_eth_src, eth_d, eth_s); - eth_update_mac(&tcp6_eth_src, eth_d, eth_s); + int i; + + for (i = 0; i < TCP_FRAMES_MEM; i ++)

While 'i ++' is surely valid, it's somewhat weird and we never add that extra whitespace elsewhere.

+ eth_update_mac(&tcp_eth_hdr[i], eth_d, eth_s); }

/** @@ -85,8 +86,8 @@ void tcp_sock_iov_init(const struct ctx *c) struct iphdr iph = L2_BUF_IP4_INIT(IPPROTO_TCP); int i;

- tcp6_eth_src.h_proto = htons_constant(ETH_P_IPV6); - tcp4_eth_src.h_proto = htons_constant(ETH_P_IP); + for (i = 0; i < TCP_FRAMES_MEM; i ++)

Same here.

+ eth_update_mac(&tcp_eth_hdr[i], NULL, c->our_tap_mac);

for (i = 0; i < ARRAY_SIZE(tcp_payload); i++) { tcp6_payload_ip[i] = ip6; @@ -164,6 +165,7 @@ static void tcp_l2_buf_fill_headers(const struct tcp_tap_conn *conn, struct tap_hdr *taph = iov[TCP_IOV_TAP].iov_base; const struct flowside *tapside = TAPFLOW(conn); const struct in_addr *a4 = inany_v4(&tapside->oaddr); + struct ethhdr *eh = iov[TCP_IOV_ETH].iov_base; struct ipv6hdr *ip6h = NULL; struct iphdr *ip4h = NULL;

@@ -172,7 +174,7 @@ static void tcp_l2_buf_fill_headers(const struct tcp_tap_conn *conn, else ip6h = iov[TCP_IOV_IP].iov_base;

- tcp_fill_headers(conn, taph, ip4h, ip6h, th, &tail, + tcp_fill_headers(conn, taph, eh, ip4h, ip6h, th, &tail, check, seq, no_tcp_csum); }

@@ -194,14 +196,12 @@ int tcp_buf_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags) int ret;

iov = tcp_l2_iov[tcp_payload_used]; - if (CONN_V4(conn)) { + if (CONN_V4(conn)) iov[TCP_IOV_IP] = IOV_OF_LVALUE(tcp4_payload_ip[tcp_payload_used]); - iov[TCP_IOV_ETH].iov_base = &tcp4_eth_src; - } else { + else iov[TCP_IOV_IP] = IOV_OF_LVALUE(tcp6_payload_ip[tcp_payload_used]); - iov[TCP_IOV_ETH].iov_base = &tcp6_eth_src; - }

+ iov[TCP_IOV_ETH] = IOV_OF_LVALUE(tcp_eth_hdr[tcp_payload_used]); payload = iov[TCP_IOV_PAYLOAD].iov_base; seq = conn->seq_to_tap; ret = tcp_prepare_flags(c, conn, flags, &payload->th, @@ -259,11 +259,10 @@ static void tcp_data_to_tap(const struct ctx *c, struct tcp_tap_conn *conn, check = &iph->check; } iov[TCP_IOV_IP] = IOV_OF_LVALUE(tcp4_payload_ip[tcp_payload_used]); - iov[TCP_IOV_ETH].iov_base = &tcp4_eth_src; } else if (CONN_V6(conn)) { iov[TCP_IOV_IP] = IOV_OF_LVALUE(tcp6_payload_ip[tcp_payload_used]); - iov[TCP_IOV_ETH].iov_base = &tcp6_eth_src; } + iov[TCP_IOV_ETH].iov_base = &tcp_eth_hdr[tcp_payload_used]; payload = iov[TCP_IOV_PAYLOAD].iov_base; payload->th.th_off = sizeof(struct tcphdr) / 4; payload->th.th_x2 = 0; diff --git a/tcp_internal.h b/tcp_internal.h index 36c6533..6c2d1ef 100644 --- a/tcp_internal.h +++ b/tcp_internal.h @@ -167,7 +167,7 @@ void tcp_rst_do(const struct ctx *c, struct tcp_tap_conn *conn); struct tcp_info_linux;

void tcp_fill_headers(const struct tcp_tap_conn *conn, - struct tap_hdr *taph, + struct tap_hdr *taph, struct ethhdr *eh, struct iphdr *ip4h, struct ipv6hdr *ip6h, struct tcphdr *th, struct iov_tail *payload, const uint16_t *ip4_check, uint32_t seq, bool no_tcp_csum); diff --git a/tcp_vu.c b/tcp_vu.c index f3914c7..da1fb37 100644 --- a/tcp_vu.c +++ b/tcp_vu.c @@ -135,7 +135,7 @@ int tcp_vu_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags) flags_elem[0].in_sg[0].iov_len = hdrlen + optlen; payload = IOV_TAIL(flags_elem[0].in_sg, 1, hdrlen);

- tcp_fill_headers(conn, NULL, ip4h, ip6h, th, &payload, + tcp_fill_headers(conn, NULL, eh, ip4h, ip6h, th, &payload, NULL, seq, !*c->pcap);

if (*c->pcap) { @@ -315,7 +315,6 @@ static void tcp_vu_prepare(const struct ctx *c, struct tcp_tap_conn *conn, eh = vu_eth(base);

memcpy(eh->h_dest, c->guest_mac, sizeof(eh->h_dest)); - memcpy(eh->h_source, c->our_tap_mac, sizeof(eh->h_source));

/* initialize header */

@@ -339,7 +338,7 @@ static void tcp_vu_prepare(const struct ctx *c, struct tcp_tap_conn *conn, th->ack = 1; th->psh = push;

- tcp_fill_headers(conn, NULL, ip4h, ip6h, th, &payload, + tcp_fill_headers(conn, NULL, eh, ip4h, ip6h, th, &payload, *check, conn->seq_to_tap, no_tcp_csum); if (ip4h) *check = &ip4h->check;

-- Stefano