/usr/sbin/vznetaddbr is in vzctl 4.9.4-5.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42  | #!/bin/sh
#
# Add virtual network interfaces (veth's) in a container to a bridge on CT0
CONFIGFILE=/etc/vz/conf/$VEID.conf
. $CONFIGFILE
NETIFLIST=$(printf %s "$NETIF" |tr ';' '\n')
if [ -z "$NETIFLIST" ]; then
   echo >&2 "According to $CONFIGFILE, CT$VEID has no veth interface configured."
   exit 1
fi
for iface in $NETIFLIST; do
    bridge=
    host_ifname=
    for str in $(printf %s "$iface" |tr ',' '\n'); do
	case "$str" in
	    bridge=*|host_ifname=*)
		eval "${str%%=*}=\${str#*=}" ;;
	esac
    done
    [ "$host_ifname" = "$3" ] ||
	continue
    [ -n "$bridge" ] ||
	bridge=vmbr0
    echo "Adding interface $host_ifname to bridge $bridge on CT0 for CT$VEID"
    ip link set dev "$host_ifname" up
    ip addr add 0.0.0.0/0 dev "$host_ifname"
    echo 1 >"/proc/sys/net/ipv4/conf/$host_ifname/proxy_arp"
    echo 1 >"/proc/sys/net/ipv4/conf/$host_ifname/forwarding"
    brctl addif "$bridge" "$host_ifname"
    break
done
exit 0
 |