When troubleshooting Debian systems, having access to complete boot messages is crucial. The traditional dmesg output only captures kernel ring buffer messages, missing critical components like udev events, syslog entries, and early userspace initialization.

To capture all boot messages systematically, we need to combine several techniques:

# Method 1: Persistent journalctl (systemd systems)
journalctl --boot --no-pager > /var/log/full_boot.log

# Method 2: Serial console redirection (for early boot)
console=ttyS0,115200n8 earlyprintk=serial,ttyS0,115200

Modify /etc/rsyslog.conf to ensure all facilities are logged during boot:

# Enable imklog for kernel messages
module(load="imklog")

# Log everything to a dedicated boot file
if $syslogtag contains 'kernel' then {
    action(type="omfile" file="/var/log/kernel-boot.log")
}

For systems without systemd, create a custom boot logger:

#!/bin/bash
# /etc/init.d/bootlogger
exec > /var/log/full-boot.log 2>&1
while read line; do
    echo "[$(date '+%b %d %H:%M:%S')] $line"
done

To specifically capture udev messages that often appear in syslog but not dmesg:

# Create udev rule to log all events
echo 'ACTION=="*", RUN+="/bin/logger -t udev \"%k %p %b\""' > \
/etc/udev/rules.d/99-log-events.rules

Add these kernel parameters in GRUB for verbose logging:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash loglevel=7 debug earlyprintk=vga"

Combine all logs into a single timeline:

#!/bin/bash
{
    cat /var/log/boot.log
    journalctl -b --no-pager
    dmesg
    grep -h 'udev' /var/log/syslog*
} | sort -k 1,2 > /var/log/full_boot_analysis.txt

---

When troubleshooting Debian systems, many developers find themselves needing to review all boot messages, not just partial logs. The default logging mechanisms often split messages across multiple files (/var/log/syslog, /var/log/daemon.log, /var/log/kern.log), making comprehensive analysis difficult.

For modern Debian systems using systemd, the most comprehensive method is:

journalctl -b

To filter specific boot:

journalctl -b -1  # Previous boot
journalctl -b 0   # Current boot

For kernel-specific messages:

journalctl -k --boot=0

For older sysvinit systems, configure bootlogd:

sudo apt install bootlogd
echo "BOOTLOGD_ENABLE=yes" | sudo tee -a /etc/default/bootlogd

This will create /var/log/boot containing early boot messages.

To capture the complete kernel ring buffer before it's overwritten:

dmesg -T > /var/log/dmesg-full.log

For persistent storage across reboots, add to /etc/rc.local:

dmesg -T > /var/log/dmesg-$(date +%Y%m%d-%H%M%S).log

Create a consolidated boot log script (/usr/local/bin/fullbootlog):

#!/bin/bash
{
    echo "=== DMESG OUTPUT ==="
    dmesg -T
    
    echo "=== SYSTEMD JOURNAL ==="
    journalctl -b 0 --no-pager
    
    echo "=== SYSLOG FILES ==="
    cat /var/log/syslog /var/log/daemon.log /var/log/kern.log | grep -a "$(date -d @$(stat -c %X /var/log/syslog) +'%b %d %H:%M')"
} | less

Make executable:

chmod +x /usr/local/bin/fullbootlog

For udev warnings like SYSFS{}= will be removed, add udev rules debugging:

udevadm control --log-priority=debug
journalctl -u systemd-udevd --no-pager