Summary

Multi-port USB serial adapters (e.g., MosChip MCS7840, FTDI FT4232H) experience symlink collisions in /dev/serial/by-bus/ because all ports on the same USB device create identical symlink names. This prevents access to ports beyond the last-enumerated port via the console-server.

Current Behavior

When a multi-port USB serial adapter is connected:

• Device: MosChip MCS7840 (4 ports per chip)
• USB Path: 1-1.4.1
• Expected: 4 accessible ports
• Actual: Only 1 symlink created: usb0b1.4.1p1.0
• Result: Only the last port (port 3) is accessible; ports 0-2 are unreachable

The issue occurs in /etc/udev/rules.d/90-vyos-serial.rules where both conditions create the same symlink name:

udev
ENV{ID_PATH}=="?*", ENV{.ID_PORT}=="",   PROGRAM="/bin/sh -c 'echo $env{ID_PATH} | cut -d- -f3- | tr -d - | sed -e 0,/:/s//b/ | sed -e 0,/:/s//p/'", SYMLINK+="serial/by-bus/$result"
ENV{ID_PATH}=="?*", ENV{.ID_PORT}=="?*", PROGRAM="/bin/sh -c 'echo $env{ID_PATH} | cut -d- -f3- | tr -d - | sed -e 0,/:/s//b/ | sed -e 0,/:/s//p/'", SYMLINK+="serial/by-bus/$result"

The .ID_PORT variable is captured but never used in the symlink name.

Expected Behavior

Each port should have a unique symlink in /dev/serial/by-bus/:

• usb0b1.4.1p1.0_port0 → ttyUSB7
• usb0b1.4.1p1.0_port1 → ttyUSB8
• usb0b1.4.1p1.0_port2 → ttyUSB9
• usb0b1.4.1p1.0_port3 → ttyUSB10

Proposed Fix

Modify the second rule in 90-vyos-serial.rules to append the port number when .ID_PORT is set:

Current (line 19):

udev
ENV{ID_PATH}=="?*", ENV{.ID_PORT}=="?*", PROGRAM="/bin/sh -c 'echo $env{ID_PATH} | cut -d- -f3- | tr -d - | sed -e 0,/:/s//b/ | sed -e 0,/:/s//p/'", SYMLINK+="serial/by-bus/$result"

Proposed:

udev
ENV{ID_PATH}=="?*", ENV{.ID_PORT}=="?*", PROGRAM="/bin/sh -c 'echo $env{ID_PATH} | cut -d- -f3- | tr -d - | sed -e 0,/:/s//b/ | sed -e 0,/:/s//p/ | sed \"s/$/_port$env{.ID_PORT}/\"'", SYMLINK+="serial/by-bus/$result"

This change:

1. Appends _port and the port number to the symlink name
2. Only affects multi-port devices (when .ID_PORT is set)
3. Maintains backward compatibility for single-port devices
4. Uses underscore _ (not dash -) to match VyOS console-server naming expectations

Testing Results

Tested successfully on VyOS Stream 2025.11 with:

Hardware:

• MosChip MCS7840 (8-port adapter with 2x 4-port chips)
• FTDI FT4232H (4-port adapter)
• Multiple single-port USB serial adapters

Results:

• All 8 ports from MCS7840 accessible via unique symlinks
• All 4 ports from FT4232H accessible
• Single-port adapters unchanged (backward compatible)
• Console-server can connect to all ports remotely
• Symlinks persist across reboots and USB re-enumeration

Before fix:

$ ls /dev/serial/by-bus/ | grep 1.4
usb0b1.4.1p1.0  # Only last port accessible
usb0b1.4.2p1.0  # Only last port accessible

After fix:

$ ls /dev/serial/by-bus/ | grep 1.4
usb0b1.4.1p1.0_port0
usb0b1.4.1p1.0_port1
usb0b1.4.1p1.0_port2
usb0b1.4.1p1.0_port3
usb0b1.4.2p1.0_port0
usb0b1.4.2p1.0_port1
usb0b1.4.2p1.0_port2
usb0b1.4.2p1.0_port3

Impact

• Scope: Affects users with multi-port USB serial adapters using console-server
• Breaking Change: No - single-port devices behave identically
• Risk: Low - only modifies symlink creation for multi-port devices

Affected Multi-Port Adapters

Common devices that will benefit:

• MosChip MCS7820/MCS7840 (2/4 port)
• FTDI FT2232H, FT4232H (2/4 port)
• Prolific PL2303 multi-port variants
• StarTech/IOGear multi-port USB serial hubs
• Any USB serial adapter with multiple ports on a single USB device