Give same-subnet secondary NICs a distinct DHCP hostname so the bare name tracks the primary

set-dhcp-hostname-on-secondary-interface.sh

#!/usr/bin/env bash

set -Eeuo pipefail

primary-line()    { ip -o route show default | awk '{m=0;for(i=1;i<=NF;i++)if($i=="metric")m=$(i+1);print m"\t"$0}' | sort -n | head -1 | cut -f2-; }
primary-iface()   { primary-line | grep -oP 'dev \K\S+'; }
default-gateway() { primary-line | grep -oP 'via \K\S+'; }
iface-subnet()    { ip -o -4 route show dev "$1" scope link proto kernel | awk '{print $1; exit}'; }
primary-subnet()  { iface-subnet "$(primary-iface)"; }
secondary-ifaces() {
    local p net i
    p="$(primary-iface)"; net="$(iface-subnet "$p")"
    for i in $(ls /sys/class/net); do
        [ "$i" = "$p" ] && continue
        [ "$(iface-subnet "$i")" = "$net" ] && echo "$i"
    done
}
sys-hostname()          { hostnamectl --static; }
desired-hostname()      { echo "${DESIRED_HOSTNAME:-$(sys-hostname)}-$1"; }
conn-for-iface()        { nmcli -t -g GENERAL.CONNECTION device show "$1"; }
current-dhcp-hostname() { nmcli -g ipv4.dhcp-hostname connection show "$1"; }
localdomain()           { resolvectl domain "$1" 2>/dev/null | sed 's/.*): //' | awk '{print $1}'; }

run() {
    if [ "${MODE:-preview}" = apply ]; then "$@"; return; fi
    local a q=''; for a in "$@"; do q+=" ${a@Q}"; done
    echo "  DRY-RUN: would run:$q"
}

rehost-hint() {
    echo "  make it live (safe even over SSH on $1): tmux new-session -d -s dhcp-rehost-$1 'sudo nmcli connection down \"$2\"; sleep 2; sudo nmcli connection up \"$2\"'"
}

dot-warn() {
    local base p ld
    base="${DESIRED_HOSTNAME:-$(sys-hostname)}"
    case "$base" in *.*)
        p="$(primary-iface)"; ld="$(localdomain "$p")"
        echo "WARNING: Your hostname is $base -- depending on the DHCP server / gateway on interface $p, this is likely to resolve as ${base%%.*}.${ld:-<localdomain>}" >&2
    ;; esac
}

set-plan() {
    local secs i conn want have
    secs="$(secondary-ifaces)"
    [ -z "$secs" ] && { echo "no same-subnet secondary interfaces; nothing to do" >&2; [ "${MODE:-}" = apply ] && exit 3; exit 0; }
    dot-warn
    for i in $secs; do
        conn="$(conn-for-iface "$i")"; want="$(desired-hostname "$i")"; have="$(current-dhcp-hostname "$conn")"
        echo "[$i] conn \"$conn\"  current=${have:-<unset>}  desired=$want"
        [ "$have" = "$want" ] && { echo "  already set; nothing to do"; continue; }
        run sudo nmcli connection modify "$conn" ipv4.dhcp-hostname "$want"
        rehost-hint "$i" "$conn"
    done
}

preview() { MODE=preview set-plan; }
apply()   { MODE=apply   set-plan; }

revert() {
    local secs i conn have noop=false
    secs="$(secondary-ifaces)"
    [ -z "$secs" ] && { echo "no same-subnet secondary interfaces; nothing to revert" >&2; exit 3; }
    for i in $secs; do
        conn="$(conn-for-iface "$i")"; have="$(current-dhcp-hostname "$conn")"
        echo "[$i] conn \"$conn\"  current=${have:-<unset>}"
        [ -z "$have" ] && { echo "  already default; nothing to do"; continue; }
        MODE=apply run sudo nmcli connection modify "$conn" ipv4.dhcp-hostname ''
        rehost-hint "$i" "$conn"; noop=true
    done
    $noop && { echo "all secondaries already default; nothing to do" >&2; exit 3 ; } || true
}

usage() {
    local me; me="$(basename "$0")"
    cat >&2 <<EOF
$me - give same-subnet secondary NICs a distinct DHCP hostname so the bare name tracks the primary.

subcommands:
  preview            show what apply would change (DRY-RUN)
  apply              set ipv4.dhcp-hostname on each secondary; prints how to make it live
  revert             clear the override, back to default DHCP hostname
  usage [--verbose]  this message${1:+ (with plumbing exposed for the curious)}
EOF
    [ "${1:-}" = --verbose ] && cat >&2 <<EOF

plumbing:
  primary-iface              interface owning the lowest-metric default route
  default-gateway            gateway IP of that route
  primary-subnet             CIDR of the primary's IPv4
  secondary-ifaces           same-subnet interfaces (newline-delimited)
  desired-hostname <iface>   \${DESIRED_HOSTNAME:-\$(hostnamectl --static)}-<iface>
  conn-for-iface <iface>     NetworkManager connection on a device
  current-dhcp-hostname <conn>
  localdomain <iface>        first search domain (resolvectl)

environment:
  DESIRED_HOSTNAME   override \$(hostnamectl --static)
EOF
}

[ $# -eq 0 ] && { usage; exit 1; }
"$@"