H Hiblot

I Ziegler-Devin

R Fournet

Pierre-Alexandre A Glaude
email: pierre-alexandre.glaude@univ-lorraine.fr

Steam reforming of methane in a synthesis gas from biomass gasification

Keywords: Steam reforming, pyrolysis, methane, syngas, kinetic modeling

 Experimental and modelling study of pyrolysis and of steam reforming of methane  Investigation of the kinetic effect of gases issued from biomass gasification  Steam shows little effect on CH4 conversion but allows the oxidation of products  CO2 promotes the reaction while H2 inhibits it  High temperature necessary to the reforming favors the formation of soot precursors

Introduction

Environmental concerns, especially greenhouse gases emissions, have led to an increased interest in the use of renewable energies. Biomass is widely used in combustion but can also be involved in more advanced applications such as the production of liquid combustibles (GTL) or the cogeneration by means of a fuel cell [START_REF] Demirbas | Potential applications of renewable energy sources, biomass combustion problems in boiler power systems and combustion related environmental issues[END_REF].

These applications require the production of a high quality synthesis gas (syngas, mixture of CO and H2). Cellulosic biomass may be a promising feedstock through the gasification processes [START_REF] Klose | On the intrinsic reaction rate of biomass char gasification with carbon dioxide and steam[END_REF][START_REF] Blasi | Modeling chemical and physical processes of wood and biomass pyrolysis[END_REF]. Wood gasification is a heterogeneous process between a carboncontaining solid and an oxidizer such as O2, CO2 or water steam [START_REF] Gómez-Barea | Modeling of biomass gasification in fluidized bed[END_REF][START_REF] Yoon | Non-catalytic autothermal gasification of woody biomass[END_REF]. The main products are CO and H2, but large fractions of hydrocarbons, char and soot, which are undesirable to achieve a high efficiency and avoid fouling, are yielded too. Three phases can be distinguished [START_REF] Evans | Molecular characterization of the pyrolysis of biomass[END_REF]. The solid phase includes soot and char. The liquid phase involves the condensable intermediates named as tars, while carbon oxides, hydrogen, water, methane and other light hydrocarbons remain in the gas phase. The hot gases from the gasification need to be cleaned drastically to reach the severe specifications of fuel cells or catalytic processes [START_REF] Tijmensen | Exploration of the possibilities for production of Fischer Tropsch liquids and power via biomass gasification[END_REF]. Catalytic post treatment are usually used to clean the gases at the exhaust of the gasification [START_REF] Shen | Recent progresses in catalytic tar elimination during biomass gasification or pyrolysis-A review[END_REF], but the severe conditions, especially the high temperature, lead to a short life time for the catalytic bed. An non catalytic alternative can be a high temperature steam gasification and reforming of a mixture of biomass and methane, which permits to optimize hydrogen yields for downstream liquid fuel synthesis and shows a rather good efficiency over autothermal process [START_REF] Palumbo | Co-processing methane in high temperature steam gasification of biomass[END_REF][START_REF] Palumbo | High temperature thermochemical processing of biomass and methane for high conversion and selectivity to H2-enriched syngas[END_REF]. Another alternative process would be a homogeneous high temperature reforming of tars (T > 1273 K) and light hydrocarbons at the exhaust of the gasifier. This would allow a cleaning of the gas and an improvement of the global yield by oxidizing the remaining hydrocarbons into CO and H2 [START_REF] Anis | Tar reduction in biomass producer gas via mechanical, catalytic and thermal methods: A review[END_REF]. The technical evaluation of such an alternative is required to know precisely the kinetics of the hydro reforming of hydrocarbons. These reactions are also involved in other processes proposed for the production of hydrogen, such as solar driven methane [START_REF] Pregger | Prospects of solar thermal hydrogen production processes[END_REF] or carbonaceous material steam reforming [START_REF] Trommer | Hydrogen production by steam-gasification of petroleum coke using concentrated solar power-I. Thermodynamic and kinetic analyses[END_REF][START_REF] Z'graggen | Hydrogen production by steam-gasification of carbonaceous materials using concentrated solar energy -V. Reactor modeling, optimization, and scale-up[END_REF] or auto-thermal reforming using partial oxidation of hydrocarbons [START_REF] Karim | The production of hydrogen through the uncatalyzed partial oxidation of methane in an internal combustion engine[END_REF][START_REF] Kim | Coupled transport and kinetics in the mixing region for hydrocarbon autothermal reforming applications[END_REF][START_REF] Xie | Syngas production from oxidative methane reforming and CO cleaning with water gas shift reaction[END_REF][START_REF] Guiberti | Experimental and numerical analysis of non-catalytic partial oxidation and steam reforming of CH4/O2/N2/H2O mixtures including the impact of radiative heat losses[END_REF]. Up to now, modeling of gas phase reforming or water gas shift reaction in thermal processes often relies on thermochemical equilibrium [START_REF] Stendardo | High quality syngas production via steam-oxygen blown bubbling fluidised bed gasifier[END_REF] or global stoichiometric equations [START_REF] Kim | Numerical analysis of a 20-kWe biogas steam reformer in PEMFC applications[END_REF].

Few studies have been devoted to the reaction of hydrocarbons and tars with water steam. Jönsson [START_REF] Jönsson | Thermal cracking of tars and hydrocarbons by addition of steam and oxygen in the cracking zone[END_REF] studied the conversion of methane in a gas obtained from wood pyrolysis, in a tubular reactor. The conversion was found to be maximum at 1523 K (60% conversion) and to be not very sensitive to the amounts of water and oxygen, while hydrogen showed an inhibiting effect. Tijmensen [START_REF] Tijmensen | The production of Fischer Tropsch liquids and power through biomass gasification[END_REF] studied the reaction of methane with water between 973 and 1373 K between 10 and 35 bar and proposed a global reaction rate. Valin et al. [START_REF] Valin | Upgrading biomass pyrolysis gas by conversion of methane at high temperature: Experiments and modelling[END_REF] followed the conversion of methane in a pilot scale tubular reactor in a gas mixture representative of that obtained after the gasification of wood. 90% of the hydrocarbon was oxidized at 1653 K for a residence time of 2s and they noted also the inhibiting effect of hydrogen [START_REF] Dufour | Mechanisms and Kinetics of Methane Thermal Conversion in a Syngas[END_REF].

Reactions of tars have been also the subject of some studies. At high temperature, tars are mostly composed of aromatic and polyaromatic compounds [START_REF] Evans | Molecular characterization of the pyrolysis of biomass[END_REF]. The major components are benzene, toluene, indene, naphthalene, and phenol [START_REF] Brage | Tar evolution profiles obtained from gasification of biomass and coal[END_REF]. These compounds could then be used as surrogates for the reactions of tars. Jess [START_REF]Mechanisms and kinetics of thermal reactions of aromatic hydrocarbons from pyrolysis of solid fuels[END_REF] studied the steam reforming of benzene, toluene, and naphthalene in a tubular reactor between 973 K and 1673 K under 1,6 bar. Toluene was found to be completely consumed at 1300 K while a temperature of 1700 K was necessary to convert benzene.

Although many detailed kinetic models exist for the thermal decomposition and the combustion of light hydrocarbons, especially for methane [START_REF] Olsvik | Modelling of the decomposition of methane at 1273 K in a plug flow reactor at low conversion[END_REF][START_REF] Ranzi | A Wide Range Modeling Study of Methane Oxidation[END_REF][START_REF] Tan | Oxidation and Ignition of Methane-Propane and Methane-Ethane-Propane Mixtures: Experiments and Modeling[END_REF][START_REF] Marinov | Chemical kinetic modeling of a methane opposed-flow diffusion flame and comparison to experiments[END_REF][START_REF] Petersen | Kinetics modeling of shock-induced ignition in low-dilution CH4/O2 mixtures at high pressures and intermediate temperatures[END_REF][START_REF] Smith | GRI-Mech 3.0[END_REF][START_REF] Hughes | Development and testing of a comprehensive chemical mechanism for the oxidation of methane[END_REF][START_REF] Zhu | Reforming of CH4 by partial oxidation: thermodynamic and kinetic analyses[END_REF][START_REF] De Ferrières | Experimental and numerical investigation of low-pressure laminar premixed synthetic natural gas/O2/N2 and natural gas/H2/O2/N2 flames[END_REF][START_REF] Konnov | The pseudo-catalytic promotion of nitric oxide oxidation by ethane at low temperatures[END_REF][START_REF] Metcalfe | A Hierarchical and Comparative Kinetic Modeling Study of C1 -C2 Hydrocarbon and Oxygenated Fuels[END_REF], these models were not designed to simulate the slow oxidation by water in a reducing atmosphere containing mainly CO and H2. Reactions at very high temperature in a mixture rich in H2O, CO, CO2 and H2 need further investigations. The present work aimed to go further than the process study by Valin et al. [START_REF] Valin | Upgrading biomass pyrolysis gas by conversion of methane at high temperature: Experiments and modelling[END_REF] and to investigate the kinetic behavior and interactions of the different compounds in well-defined conditions. An experimental and modelling study of the thermal cracking of methane has been performed at high temperature in a synthetic gas representative of that produced by the gasification of biomass. The effect of temperature and composition has been investigated and is discussed.

Experiments

The experimental study has been performed in a continuous flow reactor under a near atmospheric pressure (800 Torr, 1.07 bar). The reactor was a tubular mullite pipe (60 cm length and 2.4 cm internal diameter) of internal volume 271 cm 3 involving a S/V ratio of 1.67 cm -1 . The mullite, which is an oxide of silicon and aluminum, allows a heating up to 1900 K. Comparisons were made at lower temperature, between 973 and 1273 K, with a fused silica reactor to check that no wall reaction occurs. Results were found identical in the case of the pyrolysis of ethane up to 1273 K. As pyrolytic carbon deposits were formed in the hottest part of the reactor, the effect of reactor aging was checked in another set of experiments. For a fixed temperature, thermal decomposition of methane, ethylene and acetylene was performed in neat and in coked reactors. It appeared that the formation of carbon deposit had a little promoting influence on the consumption of acetylene, without affecting the conversion of methane and other C2 hydrocarbons. Experiments were then performed in all cases in clean reactors. A horizontal Vecstar oven allowed heating the reactor up to 1870 K. Temperature profiles were measured by a type S thermocouple coated in an alumina pipe. Uncertainty is estimated to ± 20K. Figure 1 presents temperature profiles. A plateau of 16 cm length was reached at the set point temperature but lower temperature regions cannot be neglected in modeling. The reactor can be considered as a plug flow reactor in the studied conditions between 1000 K and 1800 K.

For a residence time of 0.68 s calculated in the 16 cm long hot section of the reactor, the Péclet number lies between 120 and 140 as a function of the temperature, whereas a value above 50 allows to consider the reactor as a plug flow reactor. Messer and Air Liquide supplied gases, with a purity at least above 99.9 %. Water was added through a Bronkhorst liquid mass flow controller coupled with a controlled evaporator mixer from the same supplier. For all the experiments, residence time was fixed to 0.68 s (in the hot area), pressure was 800 Torr (1,07 bar) and argon was used as diluent in order to make negligible the thermal effect and the volumetric expansion of the flow due to the reaction.

On-line analyses were done by gas chromatography. Permanent gases were quantified by a thermal conductivity detector (TCD) and hydrocarbons by a flame ionization detector (FID). Light species, i.e. H2, CO, CO2, methane, ethane, ethylene and acetylene were separated with a Carbosphere packed column. The uncertainties are around 5%; the detection threshold is below 1 ppm with FID, it is 30 and 10 ppm for hydrogen and carbon oxides with TCD, respectively. Heavier compounds up to C4 were separated with a Hayesep D packed column. When C2 were added in the reacting mixture to test the kinetic effect of the coking of the walls, propene, allene (propadiene), propyne, 1-butene, 1,3-butadiene, vinylacetylene and diacetylene could be detected (not presented here).

Modeling

A detailed kinetic mechanism has been developed in order to simulate and analyze the experimental results of steam reforming. It was based on a model for the combustion of C0-C2 species, which includes also a reaction base for the reaction of C3-C5 unsaturated hydrocarbons validated in flame and shock tube conditions [START_REF] Fournet | Experimental and modeling of oxidation of acetylene, propyne, allene and 1,3-butadiene[END_REF][START_REF] Gueniche | A comparative study of the formation of aromatics in rich methane flames doped by unsaturated compounds[END_REF]. The C0-C2 reaction base contains all unimolecular and bimolecular reactions between species involving less than three carbon atoms and was especially validated against experimental results for the combustion of methane and ethane. Pressure dependence is taken into account for reaction of light species when data are available in the literature.

In the mechanism, reactions involving HO2, H2O2 and O2 were negligible and were removed since molecular oxygen was not present. The enthalpy of reaction of the hydroxyl radical, which is the main species responsible for steam reforming, was updated according to Burcat and Ruscic [START_REF] Burcat | Third Millennium Ideal Gas and Condensed Phase Thermochemical Database for Combustion[END_REF]. Several kinetic parameters were updated in order to include recent recommended values for the sensitive reactions. Data from the review by Baulch et al [START_REF] Baulch | Evaluated Kinetic Data for Combustion Modeling: Supplement II[END_REF] were used for the H-abstraction by a H-atom and OH radical on methane, by H-atom on methyl radical producing H2 and CH2 diradical or the decomposition of vinyl radical to ethylene and H-atom. Special care was given to reactions of methyl radical and unsaturated species with H2O and OH radical, which can be involved in the reforming process. Combination of methyl and hydroxyl radical yields numerous products: methanol if the adduct is stabilized [START_REF] Baulch | Evaluated Kinetic Data for Combustion Modeling: Supplement II[END_REF], or decomposition products such as water and CH2 diradical [START_REF] Jasper | Kinetics of the Reaction of Methyl Radical with Hydroxyl Radical and Methanol Decomposition †[END_REF], formaldehyde and H2 [START_REF] Dean | Bimolecular QRRK analysis of methyl radical reactions[END_REF], or methoxy radical and H-atom. Important reactions in the reforming of hydrocarbons are the addition of OH radical on unsaturated hydrocarbons, yielding the first C-O bonds that lead ultimately to the production of CO. These reactions are usually not sensitive in combustion and both rate constants and products are subject to discussion. In the case of the reactions OH+C2H4 and OH+C2H2, two pathways are possible: the H-atom abstraction and the formation of an adduct, which decomposes. In the case of ethylene, the adduct can decompose through three routes producing CH3 and HCHO, CH2CHOH and an H-atom, and CH3CHO and an H-atom, respectively [START_REF] Baulch | Evaluated Kinetic Data for Combustion Modeling: Supplement II[END_REF]. The formation of methyl radical and formaldehyde is dominant and the concurrent routes have been neglected. According to the literature, the rate parameters of the H-abstraction and of the addition were supposed to be similar (k=10 13 exp(-5940 cal mol -1 /RT) cm 3 mol -1 s -1 ), and their sum equal to the value proposed for the global reaction of OH with ethylene [START_REF] Baulch | Evaluated Kinetic Data for Combustion Modeling: Supplement II[END_REF]. These choices led to average values in comparison to studies of the literature. In the case of reactions between OH and acetylene, the data for the H-abstraction was taken from Tsang and Hampson [START_REF] Tsang | Chemical Kinetic Data Base for Combustion Chemistry. Part I. Methane and Related Compounds[END_REF]. The addition was supposed to produce mainly ketene (CH2CO) and an H-atom with a rate constant of 1.110 13 exp(-7174 cal mol -1 /RT) cm 3 mol -1 s -1 [START_REF] Woods | C1/C2 chemistry in fuel-rich post-flame gases: Detailed kinetic modelling[END_REF]. Carbon dioxide has shown a promoting effect on the steam reforming of hydrocarbons. By analogy with reaction of CH3 (and C2H5) with CO2, yielding CO and CH3O (or C2H5O, respectively) [START_REF] Glaude | Chemical kinetic modeling of dimethyl carbonate in an opposed-flow diffusion flame[END_REF], reactions of C2H3 and C2H radical with CO2 were added:

C2H3 + CO2  CH2CHO + CO C2H + CO2  HCCO + CO
The pre-exponential factors were estimated to 10 13 cm 3 mol -1 s -1 , as in the case of the addition of a radical on a double bond [START_REF] Buda | Progress toward a unified detailed kinetic model for the autoignition of alkanes from C4 to C10 between 600 and 1200 K[END_REF]. The activation energies were estimated from an Evans-Polanyi plot made with the reference reactions of methyl and ethyl radicals [START_REF] Glaude | Chemical kinetic modeling of dimethyl carbonate in an opposed-flow diffusion flame[END_REF], which led to activation energies of 17 kcal mol -1 and 0 kcal mol -1 , respectively.

Simulations were performed with the CHEMKIN suite [START_REF] Kee | CHEMKIN-II: A FORTRAN Chemical Kinetics Package for the Analysis of Gas-Phase Chemical Kinetics[END_REF]. The plug flow reactor was modeled as a cascade of small continuously stirred reactors taking into account the experimental temperature profile.

Results and Discussion

Influence of water steam on methane pyrolysis

The degradation of pure methane has been studied as a function of temperature with and without the addition of water (i.e. pyrolysis condition). The initial fractions of methane and water were 0.63 % and 4.8%, respectively, diluted in argon. The profiles of methane and of the main products are presented in figure 2 in the pyrolysis and the case of the addition of water. An interesting result is that the reactivity of methane is not very sensitive to the addition of water. Its consumption starts around 1400 K in these conditions and the conversion is total around 1700 K with and without water. Both experiments and modeling show that water slightly promotes the reaction below 1523 K but inhibits it above this temperature. Conversely, the addition of water changes dramatically the product distribution: while the pyrolysis leads to large amounts of C2, especially acetylene, solid carbon and soot, the steam reforming produces H2, CO and CO2 as major products, but also small amounts of C2. Ethylene reaches a maximum of about 60 ppm (mol) at 1423 K (150 ppm in pyrolysis at the same temperature), and acetylene an amount of about 550 ppm (mol) at 1523 K (to compare with a maximum of 2000 ppm at 1500 K in pyrolysis).

The model reproduces fairly well the profiles and the influence of temperature in pyrolysis and in presence of steam. The fraction of final products, i.e. hydrogen and carbon oxides, are well simulated. In pyrolysis, the model tends to overestimate acetylene at the highest temperatures, above 1573 K. The formation of solid carbon at the reactor wall and of soot becomes important in pyrolysis at these temperatures, whereas the gas phase model does not take into account these heterogeneous reactions. Studies on the carbon vapor deposition have shown that acetylene is one of the main precursor of pyrolytic carbon. A rate constant for the global reaction of deposition C2H2  2 C(S) + H2 was determined in the case of carbon vapor infiltration during propane pyrolysis [START_REF] Ziegler-Devin | Pyrolysis of propane for CVI of pyrocarbon: Part III: Experimental and modeling study of the formation of pyrocarbon[END_REF]. The same reaction has been tested in the present mechanism. The pre-exponential factor has been corrected to take into account the change of the surface/volume ratio between both reactors, and multiplied by a factor 2, within the range of uncertainty. The rate constant equals finally 1.610 5 exp(-40 000 cal mol -1 /RT) s -1 . Hydrogen, ethylene and acetylene profiles obtained in pyrolysis with this modified mechanism are shown in Figure 2 (dotted lines). The heterogeneous reaction allows to reproduce perfectly the experimental profile of hydrogen above 1600 K. The most dramatic effect can be seen on acetylene fraction.

Instead of an increase of the concentration to a plateau, acetylene profile reaches a maximum, as observed experimentally, and decreases at high temperature to produce solid carbon. Even if the temperature of the maximum is slightly shifted toward high temperature compared to the experiments, it is important to note that the heterogeneous reaction, determined in another reactor during propane pyrolysis, permits to reproduce the present data without further adjustment. The effect is rather moderate on ethylene profile, even if a better fit of the experimental data is obtained at the highest temperatures.

No effect was found on other species, methane and hydrogen and the reaction was found completely negligible in the case of steam reforming.

The low sensitivity of water on the global reactivity is explained by the low reactivity of water: the initial reaction channels consuming methane remains similar in both cases, i.e. a H-abstraction by a H-atom (or an OH radical when water is added) producing a methyl radical which is not very reactive and leads to bimolecular reactions. Water does not allow a new promoting route even at the highest temperatures. The slight promoting effect of water below 1523 K is due first to the reaction of H-atoms with water to produce H2 and OH radicals. These latter react faster by H-atom abstraction with methane than H atoms.

To a lesser extent, methyl radical reacts with H2O to produce an H-atom and methanol. This latter molecule, much more reactive than methane, acts somewhat as a degenerate branching agent in unimolecular decompositions, which produce new radicals. When the temperature increases, OH radical reacts more with acetylene to yield an H-atom and ketene. The latter react by addition with an H-atom to form CH3CO that decompose immediately into CO and a methyl radical. The scheme consists then in replacing an OH radical by a methyl radical, much of which yielding back methane by termination with an H-atom. Furthermore, the highest amount of H2 in pyrolysis at high temperature inhibits methane conversion, as discussed below.

Figure 2: Profiles of species during the steam reforming of methane. In the cases of methane, H2, and C2, experimental results in pyrolysis condition without water are also displayed. XCH4=0.63%, XH2O=4.8%, diluent argon. Residence time is 0.68s. Points represent experimental results and lines simulations. Dash lines refer to the pyrolysis simulation, dotted lines to the simulation including a reaction of formation of solid carbon.

Influence of H2 on methane pyrolysis

The influence of hydrogen in reactants (0-11 %) has been investigated during the pyrolysis of neat methane. Figure 3 displays the amount of methane as a function of the fraction of hydrogen added for two temperatures. At 1523 K, a strong inhibiting effect appears even for amounts of hydrogen as low as 1 %. When amounts larger than 10% are added, the conversion of methane is close to be stopped. The fractions of hydrogen and of C2 products at the outlet of the reactor at 1523 K are plotted in Figure 4 as a function of H2 at the inlet of the reactor. A large decrease of the amount of acetylene is observed in both experiments and simulations when hydrogen is added. Conversely, fractions of ethylene and ethane remain almost constant when hydrogen is added. The model reproduces very well the amount of H2 produced in addition to that present as reactant. As maximum mole fraction reached by C2 species are slightly shifted toward higher temperatures (Figure 1), ethylene and acetylene amounts are somewhat overestimated for the lowest additions of hydrogen in figure 4. This is due to the significant uncertainties on reactions of consumption of acetylene with water and OH at high temperatures, but trends are however well predicted. At 1673 K, H2 keeps an inhibiting effect, but less dramatic (Figure 3): while the conversion of pure methane is close to 99%, the addition of 11% of hydrogen makes it drop to around 70%. Whatever the temperature, the model reproduces very well the experimental observations. Reactions of methyl radicals explain the strong inhibiting effect of H2. In thermal decomposition, methane yields methyl radicals by initiation (reaction 1) or by H-atom abstraction by another Hatom (reaction 2). CH3 combines with itself to produce C2 (reaction 3 and 4), or reacts in the reverse reactions with H or H2 to yield methane. The addition of H2 in pyrolysis favors these latter and strongly limit the formation of C2 and subsequent products.

Influence of carbon oxides on methane pyrolysis

A possible kinetic effect of the addition of carbon oxides on the thermal reaction of methane has been also investigated. Carbon monoxide and dioxide are contained in large amounts in gas from biomass or coal gasification. An increasing mole fraction of CO was added to 0.63% of methane, up to 7% at 1523 and up to 10% at 1673 K (Figure 5).

Whatever the temperature and the fraction of additive, no change can be observed in methane conversion, which reaches 75% at 1523K and 99.3% at 1673 K (60% and 98% by simulation, respectively). The amount of reaction products; i.e. hydrogen, ethylene, and acetylene remains also constant at both temperatures whatever the fraction of CO, as illustrated in Figure 5 in the case of hydrogen. Contrariwise, CO2 addition shows a promoting effect on the reactivity, as displayed in Figure 6. At 1523K, neat methane conversion in pyrolysis reaches 78% experimentally.

The conversion increases up to 97% for a 10% mole fraction of carbon dioxide in the reactive mixture. Simultaneously, CO2 is slightly consumed (experimental points are below the fraction at the reactor inlet in figure 6) and CO appears among the reaction products. The amount of hydrogen and C2 decreases, as can be seen in the bottom of Figure 6. The model reproduces well the trends, considering the slight underprediction of neat methane conversion at 1523K. In these blends, CO2 acts as an oxidizer through the reaction CO2 + H  CO + OH. The hydroxyl radicals react hereafter with methane or H2, reducing the inhibiting influence of this latter. They also can add on unsaturated C2 to produce oxygenated compound, such as formaldehyde, which decomposes quickly at this high temperature. Note that the direct reaction between CO2 and methyl radical to produce CO and methoxy radical, included in the model, remains negligible.

Methane reforming in complex mixtures

The reaction of a mixture representative of a gas issued from biomass gasification has also been investigated as a function of the temperature. The blend contained methane (1.26%), CO (5%), H2 (5%), and H2O (5%) diluted in argon. Results are presented in figure 7. It appears that the reactivity of the hydrocarbon is lower than in the case of the steam reforming presented in figure 2. The inhibiting effect of hydrogen shifts the reaction towards higher temperatures. Methane consumption starts around 1423 K (1400K for pure methane) and a temperature higher than 1723 K is necessary to reform completely the hydrocarbon. H2 and carbon oxides are the major products. CO mole fraction exhibits a V shape profile: it is converted to CO2 at intermediate temperature by OH radical but at higher temperature, the reverse reduction of CO2 by H-atom and the reforming of methane to CO make its production predominant. The production of CO2 is only due to the reaction of CO with OH radicals, which rate constant is well-known. The correct simulation of the amount of CO2 indicates that the amount of OH radicals is correctly predicted by the model. Ethylene and acetylene are produced in important amounts at intermediate temperatures. Note that if methane concentration is twice that used in the study of the reforming of pure methane, the amount of acetylene reaches a maximum as high as 3500 ppm against 550 in the first conditions. This explains the significant formation of soot and carbon deposit that was observed experimentally. The model reproduces well both reactivity and product distribution, the simulation of the amounts of C2 compounds is somewhat underestimated. Methane consumption starts at lower temperature than in the case of the CO blend.

An experimental methane conversion around 6% is achieved at 1423K, whereas the COcontaining mixture does not react. At 1573K, the conversion reaches 60% in the CO2 blend and 45% in the CO blend, respectively. The amount of the final products from reforming are also modified. In the CO-containing mixture, methane reforming leads to the formation of hydrogen, CO and C2, which are converted thereafter to CO and H2.

Meanwhile, a small part of CO is oxidized into CO2. When CO is replaced by CO2 in the blend, this compound acts as an oxidizer, as discussed above in the case of pure methane pyrolysis. The water gas shift reaction, induced by the reaction of an H-atom with CO2 yielding CO and an OH radical, leads to the oxidation of H2 into water. In Figure 8, it appears that half of the CO2 content is reduced to CO at high temperature, while the profile of H2 exhibits a V-shape as a function of temperature. At moderate methane conversion, up to 1573 K, hydrogen consumption exceeds the production from the hydrocarbon reforming. When the temperature increases, the hydrogen formation from methane balances the oxidation of H2 by CO2. The promoting effect of CO2 on the reforming reaction is also noticeable on the profiles of ethylene and acetylene. The amounts of unsaturated C2 are almost the same for both mixture up to 1523 K, but differentiate at higher temperatures. In the case of the CO2 blend, peak values are reached at a lower temperature and with a lower maximum amount than in the case of the CO blend (2500 ppm of acetylene instead of 3600 ppm, 144 ppm of ethylene instead of 326 ppm). The higher concentration of hydroxyl radical promoted by the reaction of carbon dioxide permits a larger consumption of methyl radical by termination with OH and furthermore favors ethylene and acetylene consumption, which produces oxygenated species. The model shows that in all the conditions studied, the hydrocarbon reforming does not go mainly through direct reactions of molecules or hydrocarbon radicals with water, but by the addition of OH radical on unsaturated bonds, which produced oxygenated species leading ultimately to CO and CO2. Figure 9 presents the main reaction pathways of the thermal reaction of methane in the presence of water, in the conditions of figure 8 at 1573 K. Even in presence of a large amount of water, methane reacts mostly as in pyrolysis.

The presence of H2 in the blend inhibits slightly the successive dehydrogenations of the C2, but does not really change the main reaction route. Note that in this mixture, the inhibiting effect of H2 on methane conversion is due to its reactions with methyl radicals, as in pyrolysis, but also to the competition between reaction channels of OH radicals with methane and H2 (H2+OH=H2O+H). A major consequence of the pyrolysis-like reaction of methane is that the reforming is enabled by the formation of unsaturated hydrocarbons, which are in the same time soot precursors. Oxygenated compounds are produced by the addition of OH on ethylene, yielding formaldehyde and a methyl radical, and predominantly on acetylene, yielding ketene and an H-atom. These oxygenates, much more reactive than methane at this high temperature, react quickly to produce CO in few steps. Meanwhile, acetylene reacts with methyl radicals to produce propyne, which leads to propargyl radicals, and then to aromatic rings. There is then a competition between the fast pyrolysis process, which leads ultimately to soot and solid carbon and the somewhat slow reforming producing oxygenated species, which decompose into CO. The high temperature necessary to methane reforming by water in the reducing blend issued from gasification makes unavoidable the formation of heavy unsaturated compounds, soot and carbon deposits.

Conclusion

The pyrolysis and the steam reforming of methane has been studied at high temperature, as well as the behavior of complex mixtures representative of that produced by the gasification of the lignocellulosic biomass. It appears that near atmospheric pressure and for a residence time of 0.68 s, methane starts reacting at temperature below 1400 K and that it is necessary to heat up to more than 1700 K to achieve a complete reforming. The presence of carbon monoxide was found to have no kinetic effect, whereas carbon dioxide slightly promote the reaction by acting as an oxidizer. While hydrogen has a strong inhibiting effect, water does not influence dramatically the global rate of reaction but the product distribution. Since reaction of methyl radical with water or OH radicals are slow in comparison with the pyrolysis reactions, an important point is that the reforming process implies the formation of unsaturated compounds, on which OH radicals can add, but which are also soot precursors. A limitation of a temperature reforming process could then limit the energetic cost and the soot and carbon deposit formation.

Figure 1 :

 1 Figure 1: Temperature profiles in the reactor as a function of the oven set point.

CH4Figure 3 :

 3 Figure 3: Profiles of methane during its pyrolysis as a function of the fraction of H2 at 1523 K and 1673 K. Residence time is 0.68 s. Points represent experimental results and line simulations. The horizontal line gives the initial mole fraction of methane.

Figure 4 :

 4 Figure 4: Profiles of hydrogen and products during methane pyrolysis as a function of the fraction of H2 at 1523 K. Residence time is 0.68 s. Points represent experimental results and line simulations.

Figure 5 :

 5 Figure 5: Profiles of methane and hydrogen during methane pyrolysis as a function of the fraction of added CO at 1523 K and 1673 K. Residence time is 0.68 s. Points represent experimental results and line simulations.

Figure 6 :

 6 Figure 6: Profiles of methane, CO2 and products during methane pyrolysis as a function of the fraction of added CO2 at 1523 K. Residence time is 0.68 s. Points represent experimental results and line simulations.

Figure 7 :

 7 Figure 7: Profiles of species during the reaction of a mixture methane (1.26%), CO (5%), H2 (5%), H2O (5%) as a function of temperature. Residence time is 0.68 s. Points represent experimental results and line simulations.

Figure 8 :

 8 Figure 8: Profiles of species during the reaction of a mixture methane (1.26%), CO2 (5%), H2 (5%), H2O (5%) as a function of temperature. Residence time is 0.68 s. Points represent experimental results and line simulations.

Figure 9 :

 9 Figure 9: Reaction flux in steam reforming of methane in a mixture CH4/CO2/H2/H2O at 1573 K in condition of Figure 8. Analysis performed in the middle of the flow reactor, corresponding to a simulated conversion of 24%. Arrow thickness is proportional to the net flux. Red arrows represent the pyrolysis channel, blue arrows the reforming one.

  The analysis is made in the middle of the plug flow reactor, corresponding to a simulated methane conversion of around 24%. It appears that, even if a part of methyl radicals reacts with water or OH radicals yielding methanol or CH2OH, respectively, these reaction routes account for only 8% of the net flux of consumption of methyl radicals. The main reaction channel is the combination with another methyl radical yielding C2H6, C2H5+H or C2H4+H2. The formation of C2 accounts for 81% of methyl consumption.

	Remaining reactions consuming methyl radicals are H-atom abstractions on products,
	such as formaldehyde, which produces methane, and additions to unsaturated
	molecules, which yield heavier soot precursors. Ethane and ethylene dehydrogenate
	quickly by pericyclic H2 eliminations (concerted molecular elimination through a cyclic
	transition state) or by successive H-atom abstractions and radical decompositions. Large
	amounts of acetylene, which is more stable, are eventually produced. This latter can react
	by addition of an OH radical. This reaction is the main channel in these conditions and
	represents 61% of acetylene consumption. Formation of propyne, a precursor of aromatic
	rings, accounts for 23% of acetylene consumption, while H-atom abstractions yielding
	C2H radicals represents 10% of the flux. Remaining reactions include C4 formation. Note
	that in these conditions, very reactive C2H radicals react mainly with CO2 to produce CO
	and HCCO radicals.

Acknowledgement

This work was granted by the "Agence Nationale pour la Recherche" through the CINE-HT project.


Wazen M Shbair
email: shbair.wazen@loria.fr

Thibault Cholez
email: thibault.cholez@loria.fr

Jérôme Franc ¸ois † Isabelle Chrisment
email: isabelle.chrisment@loria.fr

Improving SNI-based HTTPS Security Monitoring

Recent surveys show that the proportion of encrypted web traffic is quickly increasing. On one side, it provides users with essential properties of security and privacy, but on the other side, it raises important challenges and issues for organizations, related to the security monitoring of encrypted traffic (filtering, anomaly detection, etc.). This paper proposes to improve a recent technique for HTTPS traffic monitoring that is based on the Server Name Indication (SNI) field of TLS and which has been implemented in many firewall solutions. This method currently has some weaknesses that can be used to bypass firewalls by overwriting the SNI value of new TLS connections. Our investigation shows that 92% of the HTTPS websites surveyed in this paper can be accessed with fake-SNI. Our approach verifies the coherence between the real destination server and the claimed value of SNI by relying on a trusted DNS service. Experimental results show the ability to overcome the shortage of SNI-based monitoring by detecting forged SNI values while having a very small false positive rate (1.7%). The overhead of our solution only adds negligible delays to access HTTPS websites. The proposed method opens the door to improve global HTTPS monitoring and firewall systems.

I. INTRODUCTION

Recent surveys show that the proportion of HTTPS traffic is quickly increasing. According to a recent report from French ISPs [START_REF]Report 2015-0832 from the French regulatory authority for telecommunications (ARCEP)[END_REF], the amount of encrypted traffic reached 50% of the Internet traffic in 2015 against only 5% back in 2012. Such steady increase is related to many factors such as: HTTPS adoption by giant websites like Facebook and Youtube [START_REF] Naylor | The cost of the S in HTTPS[END_REF], and the increasing computation power in network devices that makes complex encryption/decryption applicable to highspeed Internet [START_REF] Cao | A survey on encrypted traffic classification[END_REF]. This change in trends adds new challenges regarding network monitoring and management for security (filtering, anomaly detection, etc.) and related to the identification of encrypted traffic, especially HTTPS which supports many services.

HTTPS monitoring is related to network monitoring techniques, which provide knowledge about traffic flowing through a network. Thus, if appropriate data are monitored, it is possible to detect network attacks, security policy violations and malicious activities [START_REF] Mccarthy | An investigation on identifying SSL traffic[END_REF]. In this context, the terms Monitoring and Filtering are widely used when we need to have control over Internet usage. However, Monitoring is defined as recording the activities on the Internet (accessed websites, applications, etc.), while Filtering means restricting access to applications or websites that the administrator wants users to avoid by adding websites URLs to blacklists or using keyword filtering [START_REF]Internet filtering and monitoring in your business[END_REF], [START_REF] Xu | Internet censorship in china: Where does the filtering occur?[END_REF]. We need to monitor first to be able to filter. HTTPS (i.e. HTTP over SSL/TLS) is the most commonly used encrypted protocol [START_REF] Shbair | A multi-level framework to identify https services[END_REF], [START_REF] Husák | Network-based HTTPS client identification using SSL/TLS fingerprinting[END_REF], thus the accurate and efficient identification of HTTPS traffic is essential for managing and controlling current and future data networks [START_REF] Cao | Progress in study of encrypted traffic classification[END_REF]. The related work in this field can be classified into two branches: the Academic work and the more Practical solutions to which this work is more related. The scientific related work propose techniques like Website Fingerprinting, Machine Learning techniques and Markov Chain models to recognize the HTTPS traffic. A recent survey by Velan et al. [START_REF] Velan | A survey of methods for encrypted traffic classification and analysis[END_REF] presents these relevant work. However, these techniques are still under development and research and have many challenges related to applicability and accuracy. In the meantime, practical solutions have been implemented in firewall systems to monitor HTTPS traffic, such as HTTPS proxy [START_REF]Safety recommendations concerning the analysis HTTPS stream[END_REF] and monitoring based on SSL certificates. These methods still have many issues related either to privacy or efficiency [START_REF] Shbair | Efficiently bypassing SNI-based HTTPS filtering[END_REF].

In this paper, we explore one widely used practical technique to identify HTTPS traffic, named SNI-based monitoring that uses Server Name Indication (SNI), a field of the SSL/TLS handshake, to identify the accessed websites in HTTPS connections. The SNI is a clear string value from TLS ClientHello messages that provides a convenient way to know what service is accessed by a new HTTPS connection. SNI-based monitoring has been integrated in many firewall solutions (Sphirewall1 , Untangle NG2 , IPFire3 , etc.). In our previous work [START_REF] Shbair | Efficiently bypassing SNI-based HTTPS filtering[END_REF], we demonstrated that the reliability of SNI-based filtering is poor because it is easy to forge SNI values to bypass such protection. In this paper, we propose an improvement to SNI-based monitoring techniques to overcome such shortages, with the goal to improve HTTPS firewall systems. Our contribution is threefold: (1) we provide the first estimation of the percentage of HTTPS servers still accessible despite SNI-based filtering, [START_REF] Naylor | The cost of the S in HTTPS[END_REF] we propose a solution to detect fake-SNI and (3) we evaluate the efficiency of our solution regarding detection rates and overhead.

The rest of the paper is organized as follows: Section II and III respectively present the related work and an overview of SNI. Our solution to overcome fake-SNI is described in Section IV. Section V presents our investigation of HTTPS servers accessible with a fake-SNI. Section VI evaluates the proposed approach and related overhead. Finally, Section VII concludes the paper.

II. RELATED WORK

The wide spread of networks and services makes an increased need for efficient monitoring methods to ensure that network systems and users work within a predefined policy, to avoid some malicious Web applications or inappropriate content [START_REF] Dilman | Selected Areas in Communications[END_REF]. Enterprise networks statistics [START_REF]Internet filtering and monitoring in your business[END_REF] show that 66% of companies monitor their employees' network activity. For example almost 44% of online video is being viewed during work time and 29% of adult websites are accessed from work computers. Relevant work on HTTPS monitoring relies on the information exchanged during SSL/TLS handshake, such as monitoring the SSL certificate [START_REF] Holz | The SSL landscape: a thorough analysis of the x. 509 PKI using active and passive measurements[END_REF] [START_REF] Durumeric | Analysis of the HTTPS certificate ecosystem[END_REF], monitoring the handshake interactions sequence [START_REF] Korczynski | Markov chain fingerprinting to classify encrypted traffic[END_REF] and, more recently, monitoring SNI [START_REF] Shbair | Efficiently bypassing SNI-based HTTPS filtering[END_REF] (what we aim to improve). Identifying websites based on TLS interactions leverages machine learning techniques and is prone to insufficient training and identification errors while certificate monitoring is limited when certificates are shared among services or often changed.

However the most widespread approaches for HTTPS security monitoring rely on decryption. They are based on HTTPS Proxy Server, SSL/TLS encryption keys retrieval or cracking the encryption algorithms. In the first approach, a proxy server is placed between client and server and pretends to be the intended remote server. Then, the proxy establishes a secure connection to the correct server. As shown in Figure 1, when a client connects to the remote server via a HTTPS proxy, the client connects in reality to the proxy server, which plays the role of destination server by providing its own SSL certificate. Finally, the proxy establishes another secure connection with the real remote server. Thanks to this basic method, all encrypted web traffic is open to the proxy in clear but at the cost of trust and privacy. Fig. 1: HTTPS Proxy Server SSL/TLS encryption keys can be retrieved thanks to a "Key Recovery" mechanism or "Key escrow" [START_REF] Abelson | The risks of key recovery, key escrow, and trusted thirdparty encryption[END_REF]. All encryption keys are kept in a trusted third party, such as government or private entities, which have the right to access keys for authorized law enforcement purpose. As a result, a government may limit access to HTTPS websites that refuse to share their SSL/TLS key with the escrow system. Cracking the encryption algorithms is a very different approach which relies on flaws either in the encryption protocol or in the mathematical algorithm used to encrypt data. Using a full brute force attack is very unlikely because of the time needed to perform it (billions of years to break a 128-bit encryption).

The aforementioned approaches have many issues related to privacy (decryption on users' data) or computation complexity. We believe that improving SNI-based monitoring can make it a powerful method for managing HTTPS traffic. Indeed, SNI monitoring offers a better trade-off between security and privacy as private data stay fully encrypted while only the destination service is known.

A. Standard usage of SNI

In parallel with virtual hosting techniques that make multiple websites hosted on a single machine, appeared a new challenge to make it compatible with SSL/TLS. Virtual hosting can be either "IP-based" or "name-based". In the first type, each SSL certificate is mapped to a unique IP-address, so the server depends on the IP address to answer with the correct SSL certificate. But it is expensive to reserve an IP address for each website, therefore in a name-based method, all hosted websites share the same IP address and the server can easily retrieve the right certificate from the HTTP header, by reading the website's URL in the GET request. The problem with the named-based certificate identification used with HTTPS is that the HTTP header is sent encrypted after the SSL/TLS connection is established, and thus cannot be used to identify the certificate. Since each virtual website has its own SSL certificate, the problem is to select and expose the proper certificate corresponding to the intended website, but this was not envisioned when SSL/TLS was designed. Thus, an extension of the protocol was used to enable TLS to operate effectively with HTTP and overcome this limitation [START_REF] Blake-Wilson | RFC 3546: Transport layer security (TLS) extensions[END_REF].

The Server Name Indication is used during the SSL/TLS handshake protocol and specifies in plain text the requested hostname in Client Hello messages (i.e. Step 1 in Figure 2) of the SSL/TLS handshake [START_REF] Vlker | Can internet users protect themselves? challenges and techniques of automated protection of HTTP communication[END_REF]. More precisely, the behaviour of SNI is presented as follows: in order to provide the "servername", client may include an extension of type SNI in the extended Client Hello message. The "extension-data" field of SNI shall contain at most one hostname, thus a server is able to present the correct SSL certificate according to the hostname appearing in the SNI [START_REF] Eastlake | RFC 6066: The transport layer security (TLS) extensions: Extension definitions[END_REF].

B. Alternative usage: SNI-based HTTPS Monitoring

Monitoring based on SNI relies on checking the "servername" value in SNI. This value provides the DNS name of the HTTPS website to be accessed. It is a convenient way (i.e meaningful string) to know what service is accessed and Fig. 3: Escape Add-on interactions from [START_REF] Shbair | Efficiently bypassing SNI-based HTTPS filtering[END_REF] TABLE I: Example of Alternative Name values in Youtube's SSL certificate the "server-name" can be compared against a list to enforce HTTPS filtering. In [START_REF] Shbair | Efficiently bypassing SNI-based HTTPS filtering[END_REF], we designed and developed a tool, named Escape, which can be used to bypass such type of filtering by replacing the SNI value with a fake one. The tool can exploit two weaknesses: the first exploits backward compatibility and is based on the fact that, according to the RFC [START_REF] Eastlake | RFC 6066: The transport layer security (TLS) extensions: Extension definitions[END_REF], if a remote server does not understand the SNI, it should continue the handshake. Initially, this was done to ensure that HTTPS servers are still accessible to clients which do not support the SNI extension. However, the backward compatibility can be used to bypass a SNI-based firewall by changing the "server-name" in SNI with random values or with unblocked domain names. Figure 3 shows how the backward compatibility is exploited for bypassing firewalls thanks to Escape. In this example, a firewall restricts access to Facebook but Escape is able to change the SNI from "www.facebook.com" to "www.f@ceb00k.com", so that the client hello message will bypass the firewall as the fake domain "f@ceb00k.com" is not blocked. The second weakness is the Alternative Name field of the certificate standard X.509, which can holds multiple domain names, so one certificate for multiple domains. Table I gives a short list of the Certificate Subject Alternative Name fields appearing in the Youtube.com SSL certificate. In this context, shared certificates can be used to access a restricted website by sending SNI for a unrestricted one sharing the same SSL certificate. We consider that this weakness is less critical as it only allows a user to access services from the same provider or the same hosting cloud. The solution we propose in the next section aims to mitigate the "backward compatibility bypassing strategy" that can totally mask the accessed website.

IV. SNI-BASED MONITORING IMPROVEMENT

A. Architecture

In this work, we propose a robust and efficient solution to verify the content and the veracity of the SNI extension inserted by the client in order to overcome the aforementioned weakness. The main idea is to use the Domain Name System (DNS) information to validate the relation between the hostname appearing in the SNI and the real IP address of the destination server.

When a website is accessed, prior to any SSL/TLS handshake, a DNS query is sent to resolve the IP address of the domain name. Then, a ClientHello message is configured with the SNI value holding the domain name and sent to the destination server IP address previously retrieved from the DNS answer. We cannot simply monitor DNS requests issued by users because malicious users can either resolve the domain name locally [START_REF] Shbair | Efficiently bypassing SNI-based HTTPS filtering[END_REF], or contact a colluding DNS server to escape detection. However, we believe that the information carried in DNS messages issued from a trusted DNS server can still improve the SNI-based monitoring. Indeed, the DNS information can be used to validate the "server-name" in SNI because, according to the RFC [START_REF] Eastlake | RFC 6066: The transport layer security (TLS) extensions: Extension definitions[END_REF], SNI must only contain DNS-resolvable hostnames. Moreover, using DNS answers from a trusted DNS server is more stable and reliable than using a pre-configured list of IP addresses, which requires frequent updates. We are in line with other works that use reliable DNS information [START_REF] Foremski | DNS-Class: immediate classification of IP flows using DNS[END_REF] and remote server IP address [START_REF] Kim | A method for service identification of SSL/TLS encrypted traffic with the relation of session ID and Server IP[END_REF] to identify network traffic.

Figure 4 shows the verification procedure of our proposed solution that assesses SNI using a trusted DNS server. The steps are as follow:

1) Inspect the ClientHello message and extract the extensions list and the destination server IP address. 2) Search for SNI extension and extract the "servername" value. 3) Send a DNS request to a trustworthy server with the "server-name" to get the corresponding IP addresses related to the SNI hostname. 4) Check if information in the DNS response matches the destination server IP address.

B. Fake-SNI Detection

In this subsection, we describe how to perform the check to assess the SNI from the information in the DNS response. The fake-SNI can actually take different forms: (1) a random string, (2) an empty value, (3) another (unfiltered) valid domain.

In the first case, the random string will be easily detected because no valid answer will be returned by the DNS server (the "Answer RRs" field of the response will be set to 0, and the reply code will be 0x8183 "No such name"). In the second case, the firewall will detect the empty SNI and should block the connection since all modern systems add an SNI value by default. The only issue is when considering older hosts based on Windows-XP (and below) whose TLS implementation does not support SNI. However, those hosts should not be allowed to access the World-Wide-Web in a security-sensitive context because they miss security updates (since 04/2014 concerning Fig. 4: Client SNI verification using DNS WindowsXP) and are consequently very vulnerable. Moreover, their number is always decreasing and now accounts for only 4.24% of hosts 4 .

The third case will be illustrated by a real scenario. We assume the HTTPS website "twitter.com" is restricted by SNIbased filtering. To overcome this limitation, a client browser with Escape tool can overwrite the SNI of Twitter (twitter.com) with another valid domain (www.google.com). In the firewall, the processed ClientHello message destination IP-address is a Twitter's IP address (like 104.244.42.X), while the SNI contains "www.google.com". Our module then sends an independent DNS request with SNI value (i.e "www.google.com") to get the IP addresses related to this domain name. The DNS response contains some Google's IP addresses (like 66.102.1.X). Comparing the real destination IP address with those in the DNS response shows a clear mismatch between them, which means that the connection must be considered suspicious by the system and processed accordingly. For instance, the ClientHello message can be dropped to reset the HTTPS connection.

To check the correspondence between the destination server IP address and the ones included in the DNS response, different rules can by applied from the a rigorous check of the exact IP address match, to more loose rules checking the sub-network match on the first 24 bits. We will see in the evaluation section the detection strategy impacts detection rates.

V. EVALUATION OF HTTPS SERVERS PERMEABLE BY FAKE-SNI

A. Evaluation of HTTPS servers supporting SNI

Nowadays, the SNI extension is supported by most of browsers, such as Mozilla Firefox, Safari or Google Chrome, and by servers software, like Apache and Microsoft IIS 8. To assess how much SNI is used, we conducted the first investigation of SNI deployment with a large set of web servers accessed over a HTTPS connection. Our probing follows RFC6066 [START_REF] Eastlake | RFC 6066: The transport layer security (TLS) extensions: Extension definitions[END_REF], which describes how a server must interact in the case of receiving a Client Hello message with SNI extension. If the server supports SNI, it answers with a Server Hello message holding a SNI extension, with an empty value (i.e length equal 0), otherwise the server responds with the standard ServerHello message without extension. Based on this server behaviour, we implemented a SNI crawling module, where ServerHello messages are intercepted to verify if they hold an empty SNI extension.

Basically, SSL/TLS messages consist of two sub-layers, the first layer contains either SSL/TLS handshake, SSL/TLS Change Cipher Specification or SSL Alert protocol, while the second layer holds the SSL/TLS Record protocol, which is known as an envelop for application data. We intercept Server Hello messages to determine if the corresponding server support SNI or not. Server Hello messages are identified with a TLS Record type of 22 and an handshake type of 2. Once the Server hello message is detected, the list of extension is extracted and scanned for an extension with type zero and empty data. According to [START_REF] Eastlake | RFC 6066: The transport layer security (TLS) extensions: Extension definitions[END_REF], if this extension is present, the server supports SNI. Otherwise, it does not.

Our investigation looks at HTTPS domains taken from two lists. The first source is The Internet-Wide Scan Data Repository, which is managed and hosted by University of Michigan 5 . The selected dataset contains the Alexa Top 1 million domains that respond on port 443. Instead of using this large amount of domain, we use the top 500 HTTPS websites. The second list is provided by Wang et al. 6 and contains 120 sensitive sites blocked in China, the United Kingdom, and Saudi Arabia. We found 33 websites out of the 120 run over HTTPS and we added 14 of them which were not in the first list. In total, we consider 514 HTTPS websites for this study.

For each website, we accessed the index page with a Web browser supporting SNI (i.e Firefox). After initiating the SSL/TLS handshake, the ServerHello messages are dissected by our crawler. The results show that 230 websites out of 514 (i.e 44%) do not support SNI in the way recommended by the RFC, while the others perfectly support SNI. The websites that do not support SNI are probably hosted on a dedicated server, so that they do not need this extension to select the proper SSL certificate. Monitoring HTTPS websites that do not use SNI is a real challenge since fake-SNI values will be ignored on the server side. A client is then able to overwrite the SNI as the server do not care about the actual value. However, we will see in the next subsection that most servers supporting SNI can even be accessed with a fake-SNI.

B. Evaluation of the bypassing strategy

In this section, we study how HTTPS websites deal with a fake or unknown SNI, in particular if they implement the backward compatibility described in the RFC [START_REF] Eastlake | RFC 6066: The transport layer security (TLS) extensions: Extension definitions[END_REF]. We configured a SNI-based firewall system, named Sphirewall, to restrict the access to the 514 HTTPS websites and to reset the HTTPS connection if such websites are requested. The client browser (Firefox 33, on Ubuntu 14.04) is configured with Escape, which overwrites the SNI of the 514 websites with faked ones to bypass the firewall. As shown in Figure 3, the firewall checks the SNI value against a black list to take a decision. We assess the behaviour of the HTTPS servers by observing if the page loads or not despite the fake-SNI used to bypass the firewall.

The results show that 38 websites detect the fake-SNI by showing "HTTP hostname and TLS SNI hostname mismatch" message, while 3 websites show a "Bad Request" message. Overall, 8% of websites (41 out of 514) refuse to go further in the SSL/TLS handshake when facing a fake-SNI, while 92% of the contacted HTTPS servers ignore this inconsistency according to the backward compatibility principle explained above. For instance, websites like "www.change.org" and "www.uptobox.com" refuse the fake SNI, since both are hosted by CloudFlare, where a correct SNI is mandatory to return the correct SSL certificate.

The high success rate (92%) of the bypassing strategy motivates the need for a method able to check the SNI properly before forwarding the ClientHello to the remote destination server. In the next section, we evaluate our proposed method.

VI. EVALUATION OF SNI VERIFICATION BASED ON DNS

The proposed solution was evaluated against the same list of HTTPS websites used before. The evaluation consists in two parts; in the first one, we evaluate how the proposed technique behaves with a legit SNI to assess the false-positive rate, while in the second one, we evaluate the overhead of our solution.

A. Evaluation of the false-positive rate

All ClientHello messages are inspected for verification, which also includes the ones with a legit (i.e. unaltered) SNI. We need to evaluate how the proposed method deals with ClientHello messages holding a legit SNI, which, if not detected properly, will disturb the monitoring with a high falsepositive rate and may alter the global HTTPS connectivity in case of stronger ACL (i.e. filtering). For example, falsepositives can appear when the DNS is used for load balancing and configured to answer with different IP addresses for a same requested name. For more explanations, the authors of [START_REF] Mori | SFMap: Inferring services over encrypted web flows using dynamical domain name graphs[END_REF] investigate different scenario that lead to inconsistencies in DNS responses for hostnames related to HTTPS traffic. To assess the consistency between SNI values and DNS responses in a safe environment (i.e no fake-SNI is present), we calculate the false positive detection rate as the proportion of messages blocked despite a legit-SNI.

As shown in Figure 4, the original destination IP address of a ClientHello message is compared with the DNS answer that has been requested by using the "server-name" value written in SNI. Our solution is then flexible because we can use different rules to evaluate the ClientHello message destination IP address when comparing to the DNS response, for example:

• look for the full destination IP address (exact 32-bits match) in the DNS response

• look for a sub-network match (first 24-bits or 16-bits) of the destination IP address in the DNS response

B. Evaluation of the overhead

Our SNI verification can introduce an overhead related to computation and network latency. In our work, we neglect the computation overhead since the performed checks involve very trivial operations like the extraction of a header field or the comparison of IP addresses. We pay more attention to the increased latency introduced by the communications with a trusted DNS to get the information to make our decision. Thus, we study two cases: when the trusted DNS server is a global one (Google DNS) or a local one (LORIA DNS, installed in our laboratory network). These two DNS servers should exhibit different performances because one is closer in terms of network hops (less network delay), while the second should have a larger cache to answer directly to more requests (better cache hit). Of course, when clients already contact a trusted DNS server to resolve the HTTPS server name, the DNS response can be directly analysed without issuing an additional request.

Figure 5 shows the latency ranges of DNS queries towards both DNS servers. The x-axis shows the delay ranges in ms, and y-axis shows the amount of DNS responses. The first range [0-10[ ms shows the number of websites for which name resolution takes less that 10ms. We can notice that the local DNS performs a bit better in this range thanks to the reduced network delay to reach it. However, in the next ranges of delay, the global DNS performs a lot better, especially with a high value in the range [10,20[ ms which is probably due to a very large cache that allows it to answer directly for many names, while the local one needs to forward many requests to Root-Level Domain Servers explaining its high value in the worst delay range [40,2000[. According to these values, we notice that when using Google DNS, the average added delay is less than 15 ms. Moreover, according to [START_REF] Prandini | Splitting the HTTPS stream to attack secure web connections[END_REF], the average server response time (and standard deviation) is quite high with HTTPS 483 (± 44.1) ms. Our results show that we do not alter the browsing experience of users when accessing HTTPS websites. Currently, the proportion of encrypted traffic is quickly increasing. On one side, it provides users with essential properties of security and privacy, but also raises important challenges and issues related to the security monitoring of encrypted traffic. The contribution of this work is threefold. We conducted the first investigation of SNI deployment with a large set of web servers accessed over HTTPS connections, where the results show that 92% of the HTTPS websites included in the study can be accessed with a fake-SNI. To mitigate this issue, we proposed a novel DNS-based approach to validate SNI in the context of HTTPS security monitoring that consists in verifying the relation between the actual destination server and the claimed value of SNI based on a trusted DNS service. Finally, experimental results show the ability to overcome the shortage of SNI-based monitoring while having a small false positive rate (1.7%) and small overhead (15ms). We thus believe that this work represents an important development in the field of HTTPS monitoring, and can be applied to improve global HTTPS monitoring and firewall systems. Our future work will integrate the proposed method in a firewall system for HTTPS.

Fig. 2 :

 2 Fig. 2: SSL/TLS Handshake protocol

Fig. 5 :

 5 Fig. 5: DNS latency range per number of HTTPS websites

TABLE II :

 II Identification results regarding the IP address verification strategy render the complete web pages: load the website content, display advertisement, make statistics, etc. TableIIshows the score achieved by each strategy. With the exact match, our solution validates correctly 83.75% of the legit SNI, while focusing on 24-bits and 16-bits prefix allows us to validate up to 98.29% of legit SNI. This results in a overall 1.71% false positive rate. This low FP value should not alter web browsing and demonstrates the efficiency of using DNS information to validate client SNI.

	Detection Strategy	# Assessed Connections	True Negatives	False Positives
	Exact match	2501	83.75%	16.25%
	First 24-bits	2771	92.79%	7.21%
	First 16-bits	2935	98.29%	1.71%
	By using the aforementioned methods for comparison, the re-
	sults show that, all tested 514 HTTPS websites are accessible.
	But we investigated further. We then not only considered the
	main HTTPS connection of each web site, but also the large
	number related connections toward HTTPS servers (2986) that
	are used to

http://www.sphirewall.net

https://www.untangle.com

http://www.ipfire.org

http://www.w3counter.com/globalstats.php

https://scans.io/series/443-https-tls-alexa top1mil

https://cs.uwaterloo.ca/ t55wang/wf.html

ACKNOWLEDGEMENT

This work was partially funded by Flamingo, a Network of Excellence project (ICT-318488) supported by the European Commission under its FP7 Programme, and by HuMa, a project funded by Bpifrance and Region Lorraine under the FUI 19 framework.