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I tried establishing the connection in all possible orders between the two peers. I even restarted both the server and my local machine to make sure that there was no orphaned or zombie instances of nc hugging the socket that had somehow evaded detection.

I tried establishing the connection in all possible orders between the two peers. I even restarted both the server and my local machine to make sure that there were no orphaned or zombie instances of nc hugging the socket that had somehow evaded detection.

#!/usr/bin/env bash

# Usage: ./lanchat <local_ip>:<local_port> <remote_ip>:<remote_port>

# set -x
set -o errexit -o nounset -o pipefail

IFS=':' read -a socket <<< "$1"
LOCAL_IP=${socket[0]}
LOCAL_PORT=${socket[1]}

IFS=':' read -a socket <<< "$2"
REMOTE_IP=${socket[0]}
REMOTE_PORT=${socket[1]}

RECV_FIFO=".tmp.lanchat"

trap "rm '$RECV_FIFO'; kill 0" EXIT

mkfifo "$RECV_FIFO"

# EDIT: Following @Kamil Maciorowski's suggestion, removing the `-q 0` part below solves the issue.
while true; do nc -n -l -q 0 -s "$LOCAL_IP" -p "$LOCAL_PORT" > "$RECV_FIFO"; done &

TMUX_TOP="while true; do cat '$RECV_FIFO'; done"
TMUX_BOTTOM="while IFS= read -r line; do nc -n -q 0 '$REMOTE_IP' '$REMOTE_PORT' <<< \$line; done"

tmux new "$TMUX_TOP" \; split -v "$TMUX_BOTTOM"

#!/usr/bin/env bash

# Usage: ./lanchat <local_ip>:<local_port> <remote_ip>:<remote_port>

# set -x
set -o errexit -o nounset -o pipefail

IFS=':' read -a socket <<< "$1"
LOCAL_IP=${socket[0]}
LOCAL_PORT=${socket[1]}

IFS=':' read -a socket <<< "$2"
REMOTE_IP=${socket[0]}
REMOTE_PORT=${socket[1]}

RECV_FIFO=".tmp.lanchat"

trap "rm '$RECV_FIFO'; kill 0" EXIT

mkfifo "$RECV_FIFO"

# EDIT: As per @Kamil Maciorowski's suggestion, removing the `-q 0` part below solves the issue.
while true; do nc -n -l -q 0 -s "$LOCAL_IP" -p "$LOCAL_PORT" > "$RECV_FIFO"; done &

TMUX_TOP="while true; do cat '$RECV_FIFO'; done"
TMUX_BOTTOM="while IFS= read -r line; do nc -n -q 0 '$REMOTE_IP' '$REMOTE_PORT' <<< \$line; done"

tmux new "$TMUX_TOP" \; split -v "$TMUX_BOTTOM"

#!/usr/bin/env bash

# Usage: ./lanchat <local_ip>:<local_port> <remote_ip>:<remote_port>

# set -x
set -o errexit -o nounset -o pipefail

IFS=':' read -a socket <<< "$1"
LOCAL_IP=${socket[0]}
LOCAL_PORT=${socket[1]}

IFS=':' read -a socket <<< "$2"
REMOTE_IP=${socket[0]}
REMOTE_PORT=${socket[1]}

RECV_FIFO=".tmp.lanchat"

trap "rm '$RECV_FIFO'; kill 0" EXIT

mkfifo "$RECV_FIFO"

while true; do nc -n -l -q 0 -s "$LOCAL_IP" -p "$LOCAL_PORT" > "$RECV_FIFO"; done &

TMUX_TOP="while true; do cat '$RECV_FIFO'; done"
TMUX_BOTTOM="while IFS= read -r line; do nc -n -q 0 '$REMOTE_IP' '$REMOTE_PORT' <<< \$line; done"

tmux new "$TMUX_TOP" \; split -v "$TMUX_BOTTOM"

#!/usr/bin/env bash

# Usage: ./lanchat <local_ip>:<local_port> <remote_ip>:<remote_port>

# set -x
set -o errexit -o nounset -o pipefail

IFS=':' read -a socket <<< "$1"
LOCAL_IP=${socket[0]}
LOCAL_PORT=${socket[1]}

IFS=':' read -a socket <<< "$2"
REMOTE_IP=${socket[0]}
REMOTE_PORT=${socket[1]}

RECV_FIFO=".tmp.lanchat"

trap "rm '$RECV_FIFO'; kill 0" EXIT

mkfifo "$RECV_FIFO"

# EDIT: Following @Kamil Maciorowski's suggestion, removing the `-q 0` part below solves the issue.
while true; do nc -n -l -q 0 -s "$LOCAL_IP" -p "$LOCAL_PORT" > "$RECV_FIFO"; done &

TMUX_TOP="while true; do cat '$RECV_FIFO'; done"
TMUX_BOTTOM="while IFS= read -r line; do nc -n -q 0 '$REMOTE_IP' '$REMOTE_PORT' <<< \$line; done"

tmux new "$TMUX_TOP" \; split -v "$TMUX_BOTTOM"