[Bug]: No connectivity on Virtualbox 7.2.1 r170274 (Fedora 42 Host, XP Pro x86 Guest) ok in 7.1.12 r169651

[FranceBB]

Version

trunk/main

Host OS Type

Linux

Host OS name + version

Fedora 42 Workstation

Host Architecture

x86

Guest OS Type

Windows

Guest Architecture

x86

Guest OS name + version

Windows XP Professional x86

Component

Network

What happened?

This issue affects the current master branch, namely 7.2.1 r170274.
When Windows XP Professional x86 is started with virt-io and NAT it acquires the IP Address and the correct DNS, however no website is actually browsable.
In 7.2.0 (the latest stable), prior to a patch committed by Jack Doherty the guest couldn't acquire any DNS, however after the patch (7.2.1 r170274) the guest can #136
Although websites are now pingable by the Windows XP guest and the names are resolved (for instance, pinging google.co.uk now results in the IP address being displayed and the ping being successful), it's still not possible to actually browse websites as the requests end up in a timeout.

One notable thing is that IPv6 is enabled on XP, however the host only has an IPv4 address and therefore the guest is expected to do everything in IPv4 anyway.

This is only affecting Windows XP Professional x86 as the patch for 7.2.1 solved the problem in both Windows 8.1 Professional x64 and Windows 10 Enterprise x64.

How can we reproduce this?

Run a Windows XP Professional x86 VM on a Fedora 42 Workstation x64 host with virt-io NAT on Virtualbox 7.2.1 r170274 with both IPv4 and IPv6 enabled. The guest will be able to ping but not browse any website.

Perform the same operation on 7.1.12 r169651 and there won't be any issues.

In attachment you're gonna find both logs: the one from Virtualbox 7.2.1 r170274 displaying the issue and the one from Virtualbox 7.1.12 r169651 not displaying the issue.

WindowsEmbedded-2025-08-24-21-39-10_Vbox_7_2_1_r170274.log
WindowsEmbedded-2025-08-24-22-24-14_Vbox_7_1_12_r169651.log

Did you upload all of your necessary log files, screenshots, etc.?

• Yes, I've uploaded all pertinent files to this issue.

[FranceBB]

For reference, this is still there in the new 7.2.6.
At this point, given that pinging and resolving the domains works (i.e ICMP/DNS succeeds) but visiting website doesn't (i.e TCP traffic HTTP/HTTPS) as it times out across all network adapters (Virtio, Intel PRO/1000 MT Desktop, PCNet Fast III), there's definitely something that changed in the NAT engine in how it handles TCP segmentation or handshakes that causes some issues on XP.

Since telnet google.com 80 works (connecting to a blank screen), the 3-way TCP handshake is succeeding. The failure only occurs when the guest tries to send a data payload (like an HTTP GET or TLS Client Hello), which likely triggers the segmentation logic that is currently broken. In other words, what I think is happening is that small packets pass, but any packet requiring segmentation by the NAT engine is dropped.

[FranceBB]

Reverting back to the 7.1.x branch works in fact even the latest 7.1.16 works.

In other words, 7.1.16 r172425 is ok.

7.1.0 ok
7.1.2 ok
7.1.4 ok
7.1.6 ok
7.1.8 ok
7.1.10 ok
7.1.12 ok
7.1.14 ok
7.1.16 ok

7.2.1 not ok (issue introduced in r170274)
7.2.2 not ok
7.2.4 not ok
7.2.6 not ok