After setting up encrypted partitions and installing Arch Linux, I relized that Arch’s initramfs with the default encrypt hook can’t mount an encrypted root filesystem with detached LUKS header. I had to modify it for that to work.

First, I looked at the detached header instructions on Arch Wiki - they made a modified version of the encrypt mkinitcpio hook that expected the detached header in a file inside initramfs. That didn’t work for me, because I have the detached header on a separate partition. (Their solution was meant for carrying a boot partition with initramfs and LUKS header on an USB stick.)

But from there, I could easily adapt the hook to do what I needed to do. It was just a matter of adding a kernel commandline option for specifying the header’s location. Here’s a diff between the original encrypt hook and my adapted encrypt2:

--- /lib/initcpio/hooks/encrypt	2015-06-18 00:58:22.000000000 +0200
+++ /etc/initcpio/hooks/encrypt2	2015-07-11 11:06:37.649509904 +0200
@@ -49,11 +49,18 @@
         echo "Use 'cryptdevice=${root}:root root=/dev/mapper/root' instead."
+    local headerFlag=false
     for cryptopt in ${cryptoptions//,/ }; do
         case ${cryptopt} in
                 cryptargs="${cryptargs} --allow-discards"
+            header.*)
+                cryptargs="${cryptargs} --header ${cryptopt#header.*}"
+                headerFlag=true
+                ;;
                 echo "Encryption option '${cryptopt}' not known, ignoring." >&2
@@ -61,7 +68,7 @@
     if resolved=$(resolve_device "${cryptdev}" ${rootdelay}); then
-        if cryptsetup isLuks ${resolved} >/dev/null 2>&1; then
+        if $headerFlag || cryptsetup isLuks ${resolved} >/dev/null 2>&1; then
             [ ${DEPRECATED_CRYPT} -eq 1 ] && warn_deprecated
             # If keyfile exists, try to use that

There’s also /etc/initcpio/install/encrypt2, but it’s identical to /lib/initcpio/install/encrypt.

To use this hook, enable it in mkinitcpio.conf (and disable the old encrypt hook in case it was enabled), and add header./path/to/header to your cryptsetup options in kernel commandline.

Eg. if - like me - without detached header you’d have: root=/dev/mapper/root rw cryptdevice=/dev/sda6:root:allow-discards and you have the detached LUKS header on /dev/sdb2, change it to: root=/dev/mapper/root rw cryptdevice=/dev/sda6:root:allow-discards,header./dev/sdb2

My setup is identical, except I used /dev/disk/by-partuuid/... instead /dev/sdXY, so that it recognizes partitons by their GUID in GPT, and the configuration doesn’t become invalid when the disks switch places, or something… not that it’s likely to happen on a laptop, but still…