Date: Mon, 15 Dec 2025 10:45:16 +0000 From: bugzilla-noreply@freebsd.org To: net@FreeBSD.org Subject: [Bug 291420] [epair] Bad UDP packet checksum with epair(4) and txcsum enabled Message-ID: <bug-291420-7501-p1PFcClXxv@https.bugs.freebsd.org/bugzilla/> In-Reply-To: <bug-291420-7501@https.bugs.freebsd.org/bugzilla/> References: <bug-291420-7501@https.bugs.freebsd.org/bugzilla/>
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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=291420 --- Comment #27 from Timo Voelker <timo.voelker@fh-muenster.de> --- (In reply to leper from comment #22) Thanks for sharing your setup. If I got it right, it looks like this: DNS-Jail Host VM-Jail Ubuntu-VM e0b_dns---e0a_dns e0a_vmjail---e0b_vmjail tap0---vtnet0 \ / \ / bridge0 bridge1 That's a good example where bridge capability synchronization fails. In an unpatched FreeBSD tap0 has no txcsum and it cannot be enabled. When adding tap0 to bridge1, the bridge disables txcsum on all other member interfaces (and will disable txcsum on interfaces newly added to that bridge). Thus, it disables txcsum on e0b_vmjail, which, due to the capability sync between epair interfaces, will disable txcsum on e0a_vmjail. The core problem here is that the bridge does not notice changes made after adding an interface. In this example, disabling txcsum on e0a_vmail does not result in disabling txcsum on e0a_dns, which would disable txcsum on e0b_dns. With txcsum enabled on e0b_dns, the DNS-Server uses checksum offloading for outgoing packets and writes only the header checksum in the checksum field. For a packet to the Ubuntu-VM, bridge1 forwards the packet that still needs a valid checksum out over tap0 which has txcsum disabled. I guess we need to add txcsum to the bridge, which then computes the checksum in software if the outgoing interface does not support txcsum. -- You are receiving this mail because: You are on the CC list for the bug.help
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