netcap: capture and sum bytes send for command

netcap

#!/usr/bin/env bash
set -euo pipefail

if [[ $# -lt 1 ]]; then
    echo "usage: netcap <command> [args...]"
    exit 1
fi

RAND=$(head -c 32 /dev/urandom | tr -dc a-z0-9 | head -c 6)
NS="netcap-$RAND"
VETH_HOST="veth-$RAND"
VETH_NS="eth0"
SUBNET="10.200.200.0/24"
HOST_IP="10.200.200.1"
NS_IP="10.200.200.2"
PCAP="capture-${$}.pcap"

cleanup() {
    ip link del "$VETH_HOST" 2>/dev/null || true
    ip netns del "$NS" 2>/dev/null || true
    rm -rf /etc/netns/$NS 2>/dev/null || true
}
trap cleanup EXIT


# Pre-clean in case previous run crashed
ip link del "$VETH_HOST" 2>/dev/null || true
ip netns del "$NS" 2>/dev/null || true

echo "[+] Creating namespace"
ip netns add "$NS"

mkdir -p /etc/netns/$NS
echo "nameserver 1.1.1.1" > /etc/netns/$NS/resolv.conf
echo "nameserver 8.8.8.8" >> /etc/netns/$NS/resolv.conf

echo "[+] Creating veth pair"
VETH_PEER="vpeer-$RAND"

ip link add "$VETH_HOST" type veth peer name "$VETH_PEER"
ip link set "$VETH_PEER" netns "$NS"

ip netns exec "$NS" ip link set "$VETH_PEER" name "$VETH_NS"

echo "[+] Configuring host side"
ip addr add "$HOST_IP/24" dev "$VETH_HOST"
ip link set "$VETH_HOST" up

echo "[+] Configuring namespace side"
ip netns exec "$NS" ip addr add "$NS_IP/24" dev "$VETH_NS"
ip netns exec "$NS" ip link set "$VETH_NS" up
ip netns exec "$NS" ip link set lo up
ip netns exec "$NS" ip route add default via "$HOST_IP"

echo "[+] Enabling NAT"
iptables -t nat -A POSTROUTING -s "$SUBNET" -j MASQUERADE

echo "[+] Starting tcpdump → $PCAP"
tcpdump -i "$VETH_HOST" -w "$PCAP" '(tcp or udp) and not multicast' &
TCPDUMP_PID=$!

sleep 0.5

echo "[+] Running command in namespace"
ip netns exec "$NS" "$@"

sleep 0.5

kill "$TCPDUMP_PID"
wait "$TCPDUMP_PID" 2>/dev/null || true

echo "[+] Calculating totals..."

TOTAL_BYTES=$(tcpdump -nn -r "$PCAP" 2>/dev/null | \
    awk '
        /length/ {
            for (i=1; i<=NF; i++) {
                if ($i == "length") {
                    sum += $(i+1)
                }
            }
        }
        END { print sum+0 }'
)

human() {
    local bytes=$1
    if (( bytes > 1024*1024 )); then
        printf "%.2f MiB" "$(echo "$bytes / 1024 / 1024" | bc -l)"
    elif (( bytes > 1024 )); then
        printf "%.2f KiB" "$(echo "$bytes / 1024" | bc -l)"
    else
        printf "%d B" "$bytes"
    fi
}

echo
echo "Traffic summary:"
echo "  Packets: $(tcpdump -r "$PCAP" 2>/dev/null | wc -l)"
echo "  Total:   $(human "$TOTAL_BYTES")"
echo
echo "[+] Capture saved to $PCAP"