File "mtu.sh"

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#!/bin/bash
# Objetivo: forca um MSS fixo para remediar o problema do Path MTU Discovery
IPTABLES=`which iptables`
#
function compute_mss() {
    local MTU
    local IP_HEADER_LENGTH
    local ESP_HEADER_LENGTH
    local AH_HEADER_LENGTH
    local IPSEC_HEADER_LENGTH
    local ENCRYPTION_HEADER_LENGTH
    local SUBTOTAL_DATA_LENGTH
    local DISCARDED_DATA_LENGTH
    local DATA_LENGTH

    MTU=$1
   
    IP_HEADER_LENGTH=40
    ESP_HEADER_LENGTH=8
    AH_HEADER_LENGTH=12
    IPSEC_HEADER_LENGTH=`echo "$ESP_HEADER_LENGTH + $AH_HEADER_LENGTH" | bc`
    ENCRYPTION_HEADER_LENGTH=16 #CBC initial value
    SUBTOTAL_DATA_LENGTH=`echo "$MTU - $IP_HEADER_LENGTH - $IPSEC_HEADER_LENGTH - $ENCRYPTION_HEADER_LENGTH" | bc`
    DISCARDED_DATA_LENGTH=`echo "$SUBTOTAL_DATA_LENGTH % 16" | bc`
    DATA_LENGTH=`echo "$SUBTOTAL_DATA_LENGTH - $DISCARDED_DATA_LENGTH" | bc`
    MSS=$DATA_LENGTH
}
#
MSS=-1
MTU=1255
compute_mss $MTU
#echo $MTU
#
$IPTABLES -I FORWARD 1 -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --set-mss $MSS
$IPTABLES -I OUTPUT 1 -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --set-mss $MSS