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diff --git a/share/man/man4/epair.4 b/share/man/man4/epair.4
--- a/share/man/man4/epair.4
+++ b/share/man/man4/epair.4
@@ -25,7 +25,7 @@
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
-.Dd August 12, 2025
+.Dd September 4, 2025
.Dt EPAIR 4
.Os
.Sh NAME
@@ -108,6 +108,29 @@
can have a
.Xr vlan 4
configured on top of it.
+.Pp
+The
+.Nm
+has RXCSUM and RXCSUM6 enabled because it may receive a packet where the
+checksum has already been validated by a physical interface.
+The
+.Nm
+supports TXCSUM and TXCSUM6 for TCP and UDP, but only by forwarding the order
+to compute the checksum.
+Thus, when using an
+.Nm
+interface, a TCP or UDP sender can offload checksum computation
+to a physical interface.
+Note that, in case the packet does not leave the host, the checksum is
+unnecessary and will be ignored if offloaded.
+Such packets contain an incorrect checksum, since it is not computed yet.
+TXCSUM and TXCSUM6 are synchronized between the
+.Nm
+interface pair (i.e., enabling/disabling the capability on one end
+enables/disables it on the other end).
+In case one end is in a bridge and the bridge disabled TXCSUM or TXCSUM6,
+this avoids a sender to send packets with checksum offloading into the
+bridge by using the other end.
.Sh SEE ALSO
.Xr ioctl 2 ,
.Xr altq 4 ,
diff --git a/sys/net/if_epair.c b/sys/net/if_epair.c
--- a/sys/net/if_epair.c
+++ b/sys/net/if_epair.c
@@ -69,6 +69,7 @@
#include <net/if_media.h>
#include <net/if_private.h>
#include <net/if_types.h>
+#include <net/if_vlan_var.h>
#include <net/netisr.h>
#ifdef RSS
#include <net/rss_config.h>
@@ -434,6 +435,21 @@
imr->ifm_active = IFM_ETHER | IFM_10G_T | IFM_FDX;
}
+/*
+ * Update ifp->if_hwassist according to the current value of ifp->if_capenable.
+ */
+static void
+epair_caps_changed(struct ifnet *ifp)
+{
+ uint64_t hwassist = 0;
+
+ if (ifp->if_capenable & IFCAP_TXCSUM)
+ hwassist |= CSUM_IP_TCP | CSUM_IP_UDP;
+ if (ifp->if_capenable & IFCAP_TXCSUM_IPV6)
+ hwassist |= CSUM_IP6_TCP | CSUM_IP6_UDP;
+ ifp->if_hwassist = hwassist;
+}
+
static int
epair_ioctl(struct ifnet *ifp, u_long cmd, caddr_t data)
{
@@ -461,6 +477,44 @@
error = 0;
break;
+ case SIOCGIFCAP:
+ ifr->ifr_reqcap = ifp->if_capabilities;
+ ifr->ifr_curcap = ifp->if_capenable;
+ error = 0;
+ break;
+ case SIOCSIFCAP:
+ /*
+ * Enable/disable capabilities as requested, besides
+ * IFCAP_RXCSUM(_IPV6), which always remain enabled.
+ * Incoming packets may have the mbuf flag CSUM_DATA_VALID set.
+ * Without IFCAP_RXCSUM(_IPV6), this flag would have to be
+ * removed, which does not seem helpful.
+ */
+ ifp->if_capenable = ifr->ifr_reqcap | IFCAP_RXCSUM |
+ IFCAP_RXCSUM_IPV6;
+ epair_caps_changed(ifp);
+ /*
+ * If IFCAP_TXCSUM(_IPV6) has been changed, change it on the
+ * other epair interface as well.
+ * A bridge disables IFCAP_TXCSUM(_IPV6) when adding one epair
+ * interface if another interface in the bridge has it disabled.
+ * In that case this capability needs to be disabled on the
+ * other epair interface to avoid sending packets in the bridge
+ * that rely on this capability.
+ */
+ sc = ifp->if_softc;
+ if ((ifp->if_capenable ^ sc->oifp->if_capenable) &
+ (IFCAP_TXCSUM | IFCAP_TXCSUM_IPV6)) {
+ sc->oifp->if_capenable &=
+ ~(IFCAP_TXCSUM | IFCAP_TXCSUM_IPV6);
+ sc->oifp->if_capenable |= ifp->if_capenable &
+ (IFCAP_TXCSUM | IFCAP_TXCSUM_IPV6);
+ epair_caps_changed(sc->oifp);
+ }
+ VLAN_CAPABILITIES(ifp);
+ error = 0;
+ break;
+
default:
/* Let the common ethernet handler process this. */
error = ether_ioctl(ifp, cmd, data);
@@ -572,8 +626,11 @@
ifp->if_dname = epairname;
ifp->if_dunit = unit;
ifp->if_flags = IFF_BROADCAST | IFF_SIMPLEX | IFF_MULTICAST;
- ifp->if_capabilities = IFCAP_VLAN_MTU;
- ifp->if_capenable = IFCAP_VLAN_MTU;
+ ifp->if_capabilities = IFCAP_VLAN_MTU | IFCAP_TXCSUM |
+ IFCAP_TXCSUM_IPV6 | IFCAP_RXCSUM | IFCAP_RXCSUM_IPV6;
+ ifp->if_capenable = IFCAP_VLAN_MTU | IFCAP_TXCSUM |
+ IFCAP_TXCSUM_IPV6 | IFCAP_RXCSUM | IFCAP_RXCSUM_IPV6;
+ epair_caps_changed(ifp);
ifp->if_transmit = epair_transmit;
ifp->if_qflush = epair_qflush;
ifp->if_start = epair_start;

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