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At the current moment, the logs for martian packets are as follows:
```
martian source {DST} from {SRC}, on dev {DEV}
martian destination {DST} from {SRC}, dev {DEV}
```
These messages feel rather hard to understand in production, especially
the "martian source" one, mostly because it is grammatically ambitious
to parse which part is now the source address and which part is the
destination address. For example, "{DST}" may there be interpreted as
the actual source address due to following the word "source", thereby
implying the actual source address to be the destination one.
Personally, I discovered this bug while toying around with TUN
interfaces and using them as a tunnel (receiving packets via a TUN
interface and sending them over a TCP stream; receiving packets from a
TCP stream and writing them to a TUN).[^1]
When these IP addresses contained local IPs (i.e. 10.0.0.0/8 in source
and destination), everything worked fine. However, sending them to a
real routable IP address on the internet led to them being treated as a
martian packet, obviously. Using a few sysctl(8) and iptables(8)
settings[^2] fixed it, but while debugging I found the log message
starting with "martian source" rather confusing, as I was unsure on
whether the packet that gets dropped was the packet originating from me
or the response from the endpoint, as "martian source <ROUTABLE IP>"
could also be falsely interpreted as the response packet being martian,
due to the word "source" followed by the routable IP address, implying
the source address of that packet is set to this IP, as explained above.
In the end, I had to look into the source code of the kernel on where
this error message gets generated, which is usually an indicator of
there being room for improvement with regard to this error message.
In terms of improvement, this commit changes the error messages for
martian source and martian destination packets as follows:
```
martian source (src={SRC}, dst={DST}, dev={DEV})
martian destination (src={SRC}, dst={DST}, dev={DEV})
```
These new wordings leave pretty much no room for ambiguity as all
parameters are prefixed with a respective key explaining their semantic
meaning.
See also the following thread on LKML.[^3]
[^1]: <https://backreference.org/2010/03/26/tuntap-interface-tutorial>
[^2]: sysctl net.ipv4.ip_forward=1 && \
iptables -A INPUT -i tun0 -j ACCEPT && \
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
[^3]: <https://lore.kernel.org/all/aSd4Xj8rHrh-krjy@4944566b5c925f79/>
Signed-off-by: Clara Engler <cve@cve.cx>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20260101125114.2608-1-cve@cve.cx
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
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Linux kernel ============ The Linux kernel is the core of any Linux operating system. It manages hardware, system resources, and provides the fundamental services for all other software. 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Start your kernel development journey here: * Getting Started: Documentation/process/development-process.rst * Your First Patch: Documentation/process/submitting-patches.rst * Coding Style: Documentation/process/coding-style.rst * Build System: Documentation/kbuild/index.rst * Development Tools: Documentation/dev-tools/index.rst * Kernel Hacking Guide: Documentation/kernel-hacking/hacking.rst * Core APIs: Documentation/core-api/index.rst Academic Researcher ------------------- Explore the kernel's architecture and internals: * Researcher Guidelines: Documentation/process/researcher-guidelines.rst * Memory Management: Documentation/mm/index.rst * Scheduler: Documentation/scheduler/index.rst * Networking Stack: Documentation/networking/index.rst * Filesystems: Documentation/filesystems/index.rst * RCU (Read-Copy Update): Documentation/RCU/index.rst * Locking Primitives: Documentation/locking/index.rst * Power Management: Documentation/power/index.rst Security Expert --------------- Security documentation and hardening guides: * Security Documentation: Documentation/security/index.rst * LSM Development: Documentation/security/lsm-development.rst * Self Protection: Documentation/security/self-protection.rst * Reporting Vulnerabilities: Documentation/process/security-bugs.rst * CVE Procedures: Documentation/process/cve.rst * Embargoed Hardware Issues: Documentation/process/embargoed-hardware-issues.rst * Security Features: Documentation/userspace-api/seccomp_filter.rst Backport/Maintenance Engineer ----------------------------- Maintain and stabilize kernel versions: * Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst * Backporting Guide: Documentation/process/backporting.rst * Applying Patches: Documentation/process/applying-patches.rst * Subsystem Profile: Documentation/maintainer/maintainer-entry-profile.rst * Git for Maintainers: Documentation/maintainer/configure-git.rst System Administrator -------------------- Configure, tune, and troubleshoot Linux systems: * Admin Guide: Documentation/admin-guide/index.rst * Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst * Sysctl Tuning: Documentation/admin-guide/sysctl/index.rst * Tracing/Debugging: Documentation/trace/index.rst * Performance Security: Documentation/admin-guide/perf-security.rst * Hardware Monitoring: Documentation/hwmon/index.rst Maintainer ---------- Lead kernel subsystems and manage contributions: * Maintainer Handbook: Documentation/maintainer/index.rst * Pull Requests: Documentation/maintainer/pull-requests.rst * Managing Patches: Documentation/maintainer/modifying-patches.rst * Rebasing and Merging: Documentation/maintainer/rebasing-and-merging.rst * Development Process: Documentation/process/maintainer-handbooks.rst * Maintainer Entry Profile: Documentation/maintainer/maintainer-entry-profile.rst * Git Configuration: Documentation/maintainer/configure-git.rst Hardware Vendor --------------- Write drivers and support new hardware: * Driver API Guide: Documentation/driver-api/index.rst * Driver Model: Documentation/driver-api/driver-model/driver.rst * Device Drivers: Documentation/driver-api/infrastructure.rst * Bus Types: Documentation/driver-api/driver-model/bus.rst * Device Tree Bindings: Documentation/devicetree/bindings/ * Power Management: Documentation/driver-api/pm/index.rst * DMA API: Documentation/core-api/dma-api.rst Distribution Maintainer ----------------------- Package and distribute the kernel: * Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst * ABI Documentation: Documentation/ABI/README * Kernel Configuration: Documentation/kbuild/kconfig.rst * Module Signing: Documentation/admin-guide/module-signing.rst * Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst * Tainted Kernels: Documentation/admin-guide/tainted-kernels.rst Communication and Support ========================= * Mailing Lists: https://lore.kernel.org/ * IRC: #kernelnewbies on irc.oftc.net * Bugzilla: https://bugzilla.kernel.org/ * MAINTAINERS file: Lists subsystem maintainers and mailing lists * Email Clients: Documentation/process/email-clients.rst