MPTCPD(8)

💖 MPTCPD — the multipath TCP daemon, bestie edition 💖

okay so this is the manual page for mptcpd (that's section 8 of the System Management Commands manual, iykyk) and I am so excited to tell you about it because this little daemon is genuinely doing the most 💅

---

✨ NAME ✨

mptcpd — multipath TCP daemon 🦋

---

🌸 SYNOPSIS 🌸

mptcpd [-d?V] [-l DEST] [--debug] [--addr-flags=FLAGS]
       [--notify-flags=FLAGS] [--plugin-dir=DIR] [--path-manager=PLUGIN]
       [--load-plugins=PLUGINS] [--help] [--usage] [--log=DEST]
       [--version]

---

👑 DESCRIPTION 👑

So mptcpd is a daemon for Linux based operating systems, and what it does is handle multipath TCP path management stuff — you know, multipath TCP, the thing from IETF RFC 8684 — but it does all of that in user space, not in the kernel. no cap.

The way it talks to the Linux kernel is through a generic netlink connection, and through that connection it:

• tracks per-connection information 👀
• keeps up with the available network interfaces 🌐
• requests new MPTCP subflows 🚀
• handles requests for subflows 🙋
• etc. (yes the original says etc., we love an open-ended list) ✨

---

💅 OPTIONS 💅

mptcpd accepts the following command line options, and bestie, I'm listing every single one:

• -d, --debug — turn on debug log messages. the receipts, basically 🐛
• -?, --help — show the mptcpd help information
• --usage — show the short-and-sweet mptcpd usage information
• -l DEST, --log=DEST — send log output to DEST, where DEST can be stderr, syslog or journal 📝
• --addr-flags=FLAGS — set the flags for the announced address, where FLAGS is a comma separated list holding one or more of the flags subflow, signal, backup, fullmesh, and laminar. these are the ones plugins that deal with the in-kernel path manager may use when advertising addresses. like, for example:

--addr-flags=subflow

• --notify-flags=FLAGS — the address notification flags, where FLAGS is a comma separated list holding one or more of these:
  • existing — notify plugins about the addresses that already exist at the moment mptcpd starts up 🌱
  • skip_link_local — ignore (so, do not notify about) [ipv6] link local address updates 🙈
  • skip_loopback — ignore (so, do not notify about) host (loopback) address updates 🔁
  • check_route — only notify about an address if a default route is actually available from that address and the related device. if the route check fails, it gets re-done after a little timeout, which is there to let e.g. DHCP configure the host properly. heads up though: complex policy routing configuration may confuse or circumvent this check 😭

These flags decide whether mptcpd plugins get notified when the related addresses are updated. for example:

--notify-flags=existing,skip_link_local,skip_loopback,check_route

• --plugin-dir=DIR — set the plugin directory to DIR.

⚠️ DIR should not be world writable, for security reasons. periodt, don't do it 🔒

• --path-manager=PLUGIN — set the default path manager plugin to PLUGIN, which overrides plugin priorities 👑
• --load-plugins=PLUGINS — set the plugins to load on startup, where PLUGINS is a comma separated list holding one or more plugin names 🧚
• -V, --version — show the mptcpd version information ⭐

---

🎀 FILES 🎀

• /etc/mptcpd/mptcpd.conf — the location of the mptcpd system configuration file. that's where the vibes live 💜

---

😭 REPORTING BUGS 😭

Report bugs to mptcp@lists.linux.dev. fr fr, they want to hear about it 🫶

---

🌈 SEE ALSO 🌈

ip-mptcp(8) — go read that one too bestie, it's giving related content ✨

---

and that's the whole page!! it's the Multipath TCP Daemon manual, dated 2022-08-19, from the MPTCPD(8) page, and honestly? iconic. go manage some paths, you main character 🦋💖