[PATCH net-next] selftests: net: do not detect PPPoX loopback

[PATCH net-next] selftests: net: do not detect PPPoX loopback

[Qingfang Deng]

By default, pppd attempts to detect loopbacks on the underlying
interface using a pseudo-randomly generated magic number and checks if
the same value is received. The seed for the PRNG is a hash of hostname
XOR current time XOR pid, which is likely to collide on NIPA, causing
false positives. Disable magic number generation.

Reported-by: Matthieu Baerts <matttbe@kernel.org>
Fixes: 7af2a94f4dcf ("selftests: net: add tests for PPPoL2TP")
Signed-off-by: Qingfang Deng <qingfang.deng@linux.dev>
---
 tools/testing/selftests/net/ppp/pppoe-server-options | 1 +
 1 file changed, 1 insertion(+)

diff --git a/tools/testing/selftests/net/ppp/pppoe-server-options b/tools/testing/selftests/net/ppp/pppoe-server-options
index 66c8c9d319e9..cd586be7061b 100644
--- a/tools/testing/selftests/net/ppp/pppoe-server-options
+++ b/tools/testing/selftests/net/ppp/pppoe-server-options
@@ -1,2 +1,3 @@
 noauth
 noipdefault
+nomagic
-- 
2.43.0

Re: [PATCH net-next] selftests: net: do not detect PPPoX loopback

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net-next.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Wed,  3 Jun 2026 14:17:44 +0800 you wrote:
> By default, pppd attempts to detect loopbacks on the underlying
> interface using a pseudo-randomly generated magic number and checks if
> the same value is received. The seed for the PRNG is a hash of hostname
> XOR current time XOR pid, which is likely to collide on NIPA, causing
> false positives. Disable magic number generation.
> 
> Reported-by: Matthieu Baerts <matttbe@kernel.org>
> Fixes: 7af2a94f4dcf ("selftests: net: add tests for PPPoL2TP")
> Signed-off-by: Qingfang Deng <qingfang.deng@linux.dev>
> 
> [...]

Here is the summary with links:
  - [net-next] selftests: net: do not detect PPPoX loopback
    https://git.kernel.org/netdev/net-next/c/2eb9a81a7f73

You are awesome, thank you!
-- 
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Re: [PATCH net-next] selftests: net: do not detect PPPoX loopback

[Matthieu Baerts]

Hi Qingfang,

On 03/06/2026 08:17, Qingfang Deng wrote:
> By default, pppd attempts to detect loopbacks on the underlying
> interface using a pseudo-randomly generated magic number and checks if
> the same value is received. The seed for the PRNG is a hash of hostname
> XOR current time XOR pid, which is likely to collide on NIPA, causing
> false positives. Disable magic number generation.

Thank you for the fix!

It looks like the test is no longer flaky [1], so I just unignored it on
NIPA.

[1] https://netdev.bots.linux.dev/contest.html?skip=0&test=pppol2tp-sh

Cheers,
Matt